Pharmaceutical amorphous solid dispersion: A review of manufacturing strategies

Abstract

Amorphous solid dispersions (ASDs) are popular for enhancing the solubility and bioavailability of poorly water-soluble drugs. Various approaches have been employed to produce ASDs and novel techniques are emerging. This review provides an updated overview of manufacturing techniques for preparing ASDs. As physical stability is a critical quality attribute for ASD, the impact of formulation, equipment, and process variables, together with the downstream processing on physical stability of ASDs have been discussed. Selection strategies are proposed to identify suitable manufacturing methods, which may aid in the development of ASDs with satisfactory physical stability.

Graphical abstract

The review provides an updated overview of amorphous solid dispersion (ASD) manufacturing techniques. The impact of manufacturing variables of each method and downstream processing on the critical physical stability of ASDs are discussed.

1. Introduction

1.1. Amorphous solid dispersions (ASDs) for oral drug delivery

Drug solubilization is an essential step for orally administered medications to be absorbed systemically. Unfortunately, a large percentage of marketed drugs (~40%) and those in the R&D pipeline (~90%) are poorly water-soluble1, 2, 3. Therefore, various formulation strategies have been employed to overcome the solubility and/or dissolution challenges of these drugs^4,5. Drug solubility and dissolution rate of poorly water-soluble drugs can be successfully improved by formulating them as ASDs6, 7, 8, 9, 10, 11. ASD is a solid dispersion in which the active ingredient is dispersed within an excipient matrix in a substantially amorphous form¹². The amorphous state of the drug in ASDs is critical for increasing their solubility^13,14. With the drug in an amorphous form, no energy is required to break the drug crystal lattice. For this reason, relative to the crystalline form, the amorphous form of many poorly water-soluble drugs can achieve substantially higher apparent solubility and markedly faster dissolution¹⁵. ASDs are also known to result in higher membrane flux due to a higher supersaturation¹⁶ and thus, improve bioavailability^17,18. ASDs also have a higher wettability due to the presence of hydrophilic polymers⁷.

Based on formulation composition, solid dispersions are classified as first, second, or third generation¹⁹. Solid dispersions prepared using crystalline carriers are the first generation. Their drug release rate is generally slower than the other two generations of solid dispersions²⁰. ASDs, which consist of an amorphous drug in combination with an amorphous polymer, constitute the second generation^7,21. ASD formulations could also contain additional excipients, such as additional polymer and/or surfactants to further enhance drug release and stability²². Such ASDs are known as the third generation. Due to their solubility and dissolution advantages, ASD formulations have drawn increasing interest over the last decade, in both academia^21,23, 24, 25, 26, 27, 28, 29 and industry. Table 1 summarizes the US Food and Drug Administration (FDA)-approved pharmaceutical products based on ASDs^28,30. There has been an increase in the number of ASDs under development and reaching the market over the last decade^7,22. Currently, there are numerous methods available for ASD preparation, each of which has its advantages as well as limitations. This review aims to discuss the different manufacturing approaches to preparing ASDs.

Table 1.

Examples of FDA-approved products that are based on amorphous solid dispersions.

Trade name	Chemical name	Manufacturing technique	Company	Year of Approval
Cesamet®	Nabilone	Solvent evaporation	Meda Pharmaceuticals	1985
Isoptin® SR	Verapamil	Melt extrusion	Ranbaxy Laboratories	1987
Sporanox®	Itraconazole	Fluid bed bead layering	Janssen	1992
Prograf®	Tacrolimus	Spray drying	Astellas Pharma	1994
NuvaRing®	Etonogestrel/Ethinyl Estradiol	Melt extrusion	Merck	2001
Kaletra®	Lopinavir/Ritonavir	Melt extrusion	AbbVie	2007
Intelence®	Etravirine	Spray drying	Janssen	2008
Modigraf®	Tacrolimus	Spray drying	Astellas Pharma	2009
Zortress®	Everolimus	Spray drying	Novartis	2010
Norvir® Tablet	Ritonavir	Melt extrusion	AbbVie	2010
Onmel®	Itraconazole	Melt extrusion	Merz Pharma	2010
Incivek®	Telaprevir	Spray drying	Vertex	2011
Zelboraf®	Vemurafenib	Solvent/anti-solvent precipitation	Roche	2011
Kalydeco®	Ivacaftor	Spray drying	Vertex	2012
Noxafil®	Posaconazole	Melt extrusion	Merck	2013
Astagraf XL®	Tacrolimus	Wet granulation	Astellas Pharma	2013
Belsomra®	Suvorexant	Melt extrusion	Merck	2014
Harvoni®	Ledipasvir/Sofosbuvir	Spray drying	Gilead Sciences	2014
Viekira XR™	Dasabuvir/Ombitasvir/Paritaprevir/Ritonavir	Melt extrusion	AbbVie	2014
Epclusa®	Sofosbuvir/Velpatasvir	Spray drying	Gilead Sciences	2016
Orkambi®	Lumacaftor/Ivacaftor	Spray drying	Vertex	2016
Venclexta®	Venetoclax	Melt extrusion	AbbVie	2016
Zepatier®	Elbasvir/Grazoprevir	Spray drying	Merck	2016
Stivarga®	Regorafenib	-	Bayer	2017
Mavyret™	Glecaprevir/Pibrentasvir	Melt extrusion	AbbVie	2017
Lynparza®	Olaparib	Melt extrusion	AstraZeneca	2018
Orilissa®	Elagolix	Wet granulation	AbbVie	2018
Erleada®	Apalutamide	Spray drying	Janssen	2018
Trikafta®	Elexacaftor (Crystalline)/Ivacaftor/Tezacaftor	Spray drying	Vertex	2019
Symdeko®	Tezacaftor/Ivacaftor and Ivacaftor	Spray drying	Vertex	2019
Braftovi®	Encorafenib	Melt extrusion	Pfizer	2020
Oriahnn™	Elagolix/estradiol/norethindrone acetate	Melt extrusion	AbbVie	2020

1.2. Physical stability challenges of ASDs

Below the melting point, the liquid/amorphous form of a drug has higher free energy than the crystalline form, hence there is a thermodynamic drive for crystallization. The solid-state physical instabilities associated with ASDs include amorphous–amorphous phase separation (AAPS) and/or the conversion of the amorphous drug to a crystalline form (crystallization), both of which negate the solubility advantage of ASDs. AAPS is a phenomenon wherein distinct drug-rich and polymer-rich amorphous phases are formed throughout the ASD matrix, which initially consisted of molecularly mixed drug and polymer³¹. These drug-rich phases are more prone to crystallization due to the reduction in the inhibitory effect of the polymer. Therefore, the drug within an ASD system exhibiting AAPS crystallizes faster than a drug within a completely miscible system. Since AAPS and the subsequent decrease in the inhibitory effect of the polymer occur prior to crystallization, increasing the polymer concentration in the ASD does not necessarily lead to improved stability to crystallization. This is contrary to the trend typically observed for ASDs that remain miscible³². Janssens et al.³³ demonstrated a varying degree of drug-polymer miscibility in itraconazole/Eudragit® E100 solid dispersions prepared by film casting and spray drying. The miscibility limit for itraconazole into Eudragit® E100 was 15% when prepared by film casting and 27.5% when prepared by spray drying. This highlighted the influence of processing parameters such as solvent evaporation rate on AAPS³³. Exposure of ASDs to moisture upon storage can also result in AAPS³².

Crystallization, in general, can be viewed as consisting of two steps, viz. nucleation and crystal growth. Nucleation is the formation of small nuclei where the term nucleus refers to the minimum amount of a new crystalline phase that is capable of independent existence. Nucleation is then followed by nuclei crystal growth^34,35. An ideal manufacturing process should be able to produce homogenous ASDs which can retain their amorphous form for the required duration of time. In the following discussions, ‘stability’ of an ASD would refer to the solid-state physical stability of the ASD—particularly its tendency not to crystallize unless otherwise specified.

Ideally, the molecularly dispersed polymer in an ASD offers protection against drug crystallization by altering the thermodynamics and kinetics of the system. The ease of drug crystallization from its amorphous state depends on the driving force for crystallization. This force is governed by the free energy difference between the amorphous and crystalline states and the molecular interactions. The presence of functional groups that are either hydrogen bond donors or acceptors results in energetically favorable drug-polymer intermolecular interactions. These interactions, together with a higher entropy result in lower chemical potential of the drug in a miscible drug–polymer ASD system than that of a pure amorphous drug. This lowered drug chemical potential translates to a reduction in the thermodynamic driving force for the drug to convert to its crystalline form36, 37, 38. However, since the drug in the ASD is often supersaturated, it is also important to reduce the mobility of the drug molecules to delay phase separation and crystallization. A viscous polymer matrix can help provide this kinetic stabilization³⁹. Hence, although the glass transition temperature (T_g) of the amorphous drug is usually lower than that of the polymer, an ASD system would typically have a T_g somewhere between the T_g of the drug and the polymer. This increase in T_g increases the kinetic barrier to crystallization^40,41. This is also the primary reason for the ‘T_g −50 °C’ rule. According to this rule, the molecular mobility of an amorphous solid becomes negligible 50 °C below its T_g⁴². Therefore, a polymer with high T_g is generally crucial for an ASD. When the drug-polymer system is miscible and contains drug amounts lower than the saturation solubility of the drug in the polymer, the ASD will be thermodynamically stable. Therefore, amorphous formulations benefit from the presence of a polymeric carrier, which serves as an amorphous form stabilizer. Also, because a majority of polymers used for ASD preparation are hydrophilic, they enhance drug dissolution by increasing formulation wettability⁴³. Further, in some drug loading regimens, the dissolution of the drug is controlled by the dissolution of the polymer⁴⁴. The polymer properties for generating successful ASDs have been comprehensively reviewed in the past^45,46 along with a description of approaches and methods for a rational polymer selection²³. Briefly, besides an acceptable toxicological profile, a polymer capable of generating a homogenous dispersion with a single amorphous drug-polymer phase is preferred. For this, the polymer should provide a certain degree of drug solubility and kinetic stabilization³⁸. The typically-used polymers for ASDs are often utilized across different manufacturing platforms and include povidone derivatives such as polyvinylpyrrolidone (PVP) and polyvinylpyrrolidone/vinyl acetate (PVPVA)⁴⁷, polymethacrylates derivatives (Eudragit® series)48, 49, 50, hydroxypropyl methylcellulose (HPMC)⁵¹, hydroxypropyl methylcellulose acetate succinate (HPMCAS)⁵², and polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft copolymer (Soluplus®)⁵³. Their T_g and solubility in organic solvents have been listed in Table 2.

Table 2.

Commonly used polymers for ASD preparation.

Polymer	Tg (°C)	Solubility in solvents
Hydroxypropyl methylcellulose	175–185	Water, ethanol:dichloromethane (1:1, 2:1), methyl acetate:methanol (1:1)
Hydroxypropyl methylcellulose acetate succinate	100–110	Caustic water, acetone, methanol, dichloromethane, chloroform
Hydroxypropyl methylcellulose phthalate	133–137	Water, acetone, ethyl acetate, methyl ethyl ketone, ethanol:dichloromethane (1:1) methanol, dichloromethane, tetrahydrofuran
Polyvinylpyrrolidone	175–180	Water, acetone, ethanol, methanol, ethyl acetate, methyl ethyl ketone, dichloromethane, tetrahydrofuran
Polyvinylpyrrolidone/vinyl acetate	70–110	Water, acetone, ethanol, methanol, ethylacetate, methyl ethyl ketone, dichloromethane, tetrahydrofuran
Polymethacrylates derivatives (Eudragit®-L100, S100)	>150	Water (only L100), acetone, ethanol, methanol, ethanol:dichloromethane (1:1)
Cellulose acetate phthalate	160–170	Acetone, ethyl acetate, methyl ethyl ketone
Soluplus®	72	water, acetone, ethanol, methanol, dichloromethane

Additives such as secondary stabilizers or surfactants are often added to ASDs to augment the product. The inclusion of surfactants can favor nanodroplet formation⁵⁴. The formation of nanodroplets increases apparent drug solubility^55,56. However, surfactants can cause drug leaching from ASDs and enhance nucleation, promoting drug crystallization during dissolution^49,57,58. Therefore, the impact on ASD stability is an important selection criterion for surfactant. Since the choice of excipients could have a significant effect on ASD stability as well as other material properties that affect processing, formulation optimization is crucial before downstream processing of ASDs. However, despite the selection of an optimal formulation, exposure of ASDs to thermal, environmental humidity, and mechanical stresses during manufacturing, storage, and dissolution have been known to cause issues of instability^32,59, 60, 61, 62. As the temperature increases, there could be a large increase in molecular mobility in the ASD that can accelerate phase separation and crystallization of the drug⁶³. Ambient humidity can introduce moisture into the hygroscopic ASD systems. Moisture reduces the system T_g and causes a plasticizing effect which increases the molecular mobility of ASD and the risk of crystallization. Absorbed water can also potentially disrupt the drug-polymer interactions by competing with hydrophilic polymers for hydrogen bond formation⁶⁴. For certain ASD systems, excess moisture can also reduce the drug-polymer solubility^65,66. During the formulation process, mechanical stress such as grinding, crushing, or compressing can promote deformation-induced molecular mobility in ASDs^67,68. In fact, the generation and maintenance of an amorphous drug form remains a primary challenge associated with many ASD systems and is a limiting factor for their wider application69, 70, 71, 72, 73. Therefore, while reviewing ASD preparation methods, it is important to also focus on the impact of manufacturing variables on the physical stability of ASDs. Common factors affecting ASD stability are listed in Table 3. The impact of parameters of specific manufacturing processes is discussed in their relevant sections.

Table 3.

General factors impacting the stability of amorphous solid dispersions.

Factor (increase)	Stability	Cause	Ref.
Glass transition temperature (Tg)	Increases	Antiplasticization effect by polymers	74
Molecular mobility	Decreases	Molecular mobility is directly responsible for drug recrystallization	75
Configurational entropy	Increases	Low configurational entropy will favor crystallization	76
Configurational enthalpy	Decreases	The greater thermodynamic driving force for crystallization causes an increased rate of nucleation	77
Drug chemical potential	Decreases	Systems with lower drug chemical potential are generally more stable	78
Humidity, mechanical stress, and temperature	Decreases	These factors can significantly increase molecular mobility and may plasticize the material	79
Polymer concentration	Increases	Kinetic stabilization	11
Surfactant concentration	Decreases	Enhance nucleation, accelerate solution-mediated crystallization	57

2. Manufacturing methods for preparing amorphous solid dispersions

ASDs can be manufactured by several methods but the underlying principle for their formation is the same. Firstly, the lattice structure of a crystalline drug is broken and converted into a liquid state by applying heat or dissolving it in a solvent. The system is then rapidly cooled (if using heat) or dried (if using solvent), causing it to fall out of the equilibrium at the T_g. This results in the generation of a solid drug in an amorphous state. To generate an amorphous state, the liquefied drug should be cooled or dried at a sufficiently fast rate. ASD manufacturing methods can be broadly classified into solvent-based methods and melting or fusion methods. Solvent evaporation-based methods include spray drying (SD), electrospraying, and rotary evaporation, wherein the drug and polymer are dissolved in a solvent which is then evaporated to form an ASD^7,80. These are suitable methods for thermolabile drugs. In melting methods, the physical mixture of the drug and the polymer is melted and solidified rapidly to form the ASD⁸¹. Although some of the methods to produce ASDs have been well-established, researchers have made consistent efforts over the past decade to further improve and understand them. In addition, novel manufacturing techniques are constantly emerging. Therefore, the purpose of this review is to provide an updated overview of manufacturing techniques for ASDs.

Different manufacturing processes will generate ASD products with different physical and functional properties^53,82. Therefore, an adequate understanding of manufacturing processes and their impact on product properties is crucial for obtaining a successful ASD product. To aid the development of robust ASDs, we will discuss the impact of formulation, equipment, and process variables together with downstream processing on the critical physical stability of ASDs for each method. In addition, the advantages and limitations of each of the processes have been evaluated. Lastly, in consideration of the several variables mentioned above, selection strategies have been proposed to identify suitable manufacturing methods.

2.1. Solvent evaporation-based methods

The solvent evaporation processes for preparing ASDs essentially involve drug–polymer dissolution in organic solvent systems and their subsequent evaporation. Aqueous solvents can also be used in conjunction with organic systems to enhance polymer solubility, and/or reduce the extent of organic solvent usage. Amorphous drug–polymer dispersion is generated by rapidly evaporating the solvent from the drug–polymer solution. Because organic solvent evaporation is usually performed at temperatures well below the drug melting point, solvent evaporation is particularly suitable for thermolabile formulation systems⁸³.

An important consideration when developing an ASD using a solvent evaporation process is the choice of a solvent system. The most challenging aspect of this method is obtaining a solvent system that can solubilize the drug–polymer system and be compatible with the formulation⁸⁴, and has a low residue in the product. Poor or partial solubility of the constituents may lead to longer processing times and non-homogenous ASDs. In order to obtain the desired solvent parameters, often a combination of solvents is used. In such cases, azeotrope forming solvents such as water with ethanol (95.5%, w/w) or isopropanol (87.7%, w/w) are preferred. This is because binary solvents with different evaporation rates can cause a variable degree of supersaturation that can result in rapid precipitation of selective components at some point in the evaporation process. Such an event generates a strong potential for phase separation⁸⁵. Similarly, significant differences in the solubility of components can result in a faster rate of drying and selective precipitation of the component with lower solubility on the droplet surface^86,87. This in turn could further affect ASD stability. The solvent should not affect the physical or chemical stability of the formulation constituents during the manufacturing process before being evaporated from the system. The amount of residual solvent(s) in the final ASD products must be within the acceptable values of the International Council for Harmonization Q3C (R6) guideline^88,89. This guideline defines three different classes of solvents: Classes I, II, and III. Of these, the Class I solvents are to be avoided and Class III are the most preferable. However, Class II solvents can also be used to a limited extent if Class III solvents fail to provide adequate manufacturing conditions. Adequate removal of the residual solvent from the final product is particularly crucial when toxic organic solvents have been employed^90,91. Therefore, ASD preparation by solvent evaporation-based method is usually followed by secondary drying92, 93, 94, 95. Other important considerations for a solvent–evaporation based method are operator exposure to harmful organic solvents and the environmental impact of solvent waste.

Additional important solvent properties include drying efficiency, combustibility, viscosity, and toxicity⁹⁶. The drying efficiency is governed by the extent of heat and mass transfer which in turn depends on heat supply and solvent vapor removal. Thus, in order to obtain adequate drying efficiency, solvent parameters affecting its evaporation rate such as vapor pressure, boiling point, specific heat, the heat of vaporization, and viscosity need to be assessed⁸⁴. A high feed solution viscosity can also cause sticking of the solution to the processing equipment, which can result in low product yields⁹⁷. Typical solvents used for solvent evaporation methods are water⁹⁸, alcohols (methanol, ethanol or isopropanol)^98,99 or other organic solvents such as dichloromethane¹⁰⁰, acetone¹⁰¹, ethyl acetate⁹⁹, and methyl ethyl ketone⁹⁹ (Table 4).

Table 4.

Commonly used solvents for ASD preparation.

Solvent	Boiling point (°C)	Solubility in water (g/100 g)	Density (at 25 °C, g/mL)	Viscosity (at 25 °C, cP)	Dielectric constant	ICH Class (limit ppm)
Acetone	56.2	Miscible	1.049	0.295	20.7	Class 3
Butanone	79.6	29	0.805	0.4	18.51	Class 3
Butyl acetate	126.1	0.68	0.882	0.685	5.07	Class 3
Chloroform	61.7	0.795	1.498	0.536	4.81	Class 2 (60)
Dichloromethane	39.6	1.32	1.326	0.413	9.08	Class 2 (600)
Dimethyl acetamide	165	Miscible	0.937	0.92	37.78	Class 2 (1090)
Dimethyl formamide	153	Miscible	0.944	0.97	36.7	Class 2 (880)
Dimethyl sulfoxide	189	25.3	1.092	1.987	47	Class 3
Ethanol	78.5	Miscible	0.789	1.04	24.6	Class 3
Ethyl acetate	77	8.7	0.895	0.428	6	Class 3
Glycerin	290	Miscible	1.261	954	42.5	–
Isopropanol	82.6	Miscible	0.786	1.96	18.2	Class 3
Methanol	64.6	Miscible	0.791	0.543	32.6	Class 2 (3000)
Tetrahydrofuran	66	Miscible	0.889	0.48	7.52	Class 2 (720)
Water	100	–	0.998	1	78.5	–

–, not applicable.

There are several methods based on the principle of solvent evaporation like spray drying, electrospraying, fluidized bed drying, supercritical fluid methods, spray freeze-drying, as well as many laboratory-scale methods. Each method has unique processing procedures and parameters, which may impact ASD product properties. Small changes in the processing conditions can lead to substantial differences in product characteristics and performance³⁶. Therefore, a fundamental understanding of different processes is essential for selecting the most appropriate manufacturing method.

2.1.1. Spray drying

Spray drying is one of the widely used processes for manufacturing ASDs^102,103 because it is a continuous and commercially scalable drying process¹⁰⁴. The schematic set-up of the process is presented in Fig. 1a. The spray drying process constitutes several steps. First, the feed solution/suspension containing the drug and the polymer (and possibly other additives) is pumped into the drying chamber through a spray-nozzle. The different types of commonly used nozzles are shown in Fig. 1 (b, c, d). The two-fluid nozzle has been the most commonly used nozzle for preparing spray-dried ASDs, particularly on a laboratory-scale¹⁰⁵. The energy required to atomize the liquid is primarily provided by a gas. Liquid fed into the nozzle under low pressure can be mixed either internally or externally with the gas¹⁰⁶. Another widely used nozzle in the pharmaceutical industry is the pressurized nozzle^107,108, which solely uses the feed liquid pressure for atomization. The potential energy of the liquid is converted into kinetic energy within the pressure-swirl nozzle. Due to internal instability as well as instability arising from the interaction with the surrounding air, the annular liquid lamella disintegrates¹⁰⁹. The pressure-swirl nozzle does not atomize highly viscous liquids effectively¹¹⁰. A higher solution viscosity decreases the swirl intensity and leads to a higher liquid throughput as the cross-sectional area of the liquid increases¹⁰⁵. Pressurized nozzles provide the ability to produce larger particles with better flow properties and are easy to scale-up. This is particularly advantageous for downstream processing, as it can improve powder flow, die filling, compression, and tablet uniformity. Sildenafil was spray-dried with poly (lactide-co-glycolide) using a pressurized nozzle to form microparticulates (4–8 μm) of ASDs¹¹¹. No significant changes in physicochemical properties or in vitro drug release were observed during a scaled-up manufacturing process using the same procedure.

Figure 1.

A schematic of (a) spray dryer set-up and the manufacturing variables affecting product properties and performance; (b) external mixing two-fluid nozzle; (c) internal mixing two-fluid nozzle; (d) pressure-swirl nozzle.

The choice of the feed pump depends on the feed material viscosity, the type of atomization nozzle, and the drying capacity¹⁰⁴. The droplets atomized by the nozzle come in contact with the heated gas, which causes evaporation of the solvent in the drying chamber. The duration of particle residence in the drying chamber will vary depending on the equipment and process parameters, however, it is usually in the range of a few milliseconds^112,113. Industrial spray dryers are equipped to have a gas flow rate as high as 5000 kg/h that can result in a solvent evaporation capacity of up to 400 kg/h⁸⁴. The dried material is carried to the cyclone separator, where the heavier particles are separated from the drying gas and collected¹¹⁴. The finer particles are removed with the exhaust gases which are collected via a filter. Particles can deposit at the bottom of the drying chamber in some cases and may be scraped. Scraping can be done with the aid of vibratory devices, and/or compressed air¹¹⁵. Although mechanical brushes can also be used, they might result in additional stresses^45,116, 117, 118. One of the concerns in using spray drying is the amount of residual solvent. Therefore, spray drying is usually followed by secondary drying.

ASD product characteristics and performance can vary significantly by fine-tuning the formulation and process parameters⁸⁴. Relevant manufacturing parameters are shown in Fig. 1a 119, 120, 121. Of these, two of the crucial processing variables are the inlet temperature and the feed rate. Optimization of these factors is essential to obtain a homogenous amorphous dispersion. Selection of the inlet temperature is dictated by the physical and chemical stability of formulation constituents and the boiling point of the solvent(s)¹²². An operating and processing space for generating stable ASDs is shown in Fig. 2¹²³. During rapid solvent evaporation from the atomized droplet, if the surface film formed is permeable, a porous particle is formed. A hollow particle with a thicker shell is formed if the initial film is impermeable^124,125. Slower rates of evaporation can provide adequate time for molecular rearrangement. This can cause phase separation or even crystallization. The extent of phase separation/crystallization is dependent upon the strength of the drug-polymer interactions¹²⁶. Studies have shown that the extent of drug-polymer miscibility varies depending on the location in the spray dryer from which it has been collected¹²⁷. Naproxen-PVP-VA ASDs collected from the cyclone of the (Pro-C-epT Micro) spray dryer showed the narrow glass transition width, indicating a higher degree of drug-polymer miscibility relative to the ASDs sampled from the collector¹²⁷.

Figure 2.

Spray drying design space for generating stable ASDs. Figure adapted from Ref. 123.

Recently, solvent composition during spray drying has also been shown to have a significant impact on ASDs despite the complete solubilization of drug and polymer in the selected solvent system. In their work, Li et al.⁸⁵ found that water addition to the solvent system can lead to phase separation of ASDs during spray drying even for an initially one-phase feed solution. For the investigated ritonavir–PVPVA system, phase separation appeared to be subtle at a 25% drug loading with the co-solvents of water and methanol (10:90 ratio). However, a significant reduction in drug release rate was noted for this batch. Both experimental and modeling results indicated that the extent of phase separation increased when a higher amount of water (60:40 water–methanol ratio) was added to the spray solvent. When comparing systems prepared from the same solvent composition but with varying drug-loads, higher-drug loaded ASDs were more prone to phase separation than the low-drug-loading system. However, the impact of phase separation on drug release rates of high-drug loaded ASDs was minimal, likely due to the already compromised release often seen at high drug loadings⁸⁵. A change in the co-solvent ratio has also been shown to alter the surface composition of spray-dried ASDs, likely due to the varying evaporation and diffusion kinetics^128,129. These observations are particularly significant, considering a higher surface ratio of drug to polymer is known to result in an increased tendency of the amorphous drug to recrystallize¹³⁰. For an in-depth reading regarding the fundamentals of ASD particle engineering by spray drying, readers are referred to some previously published reviews^{84,102,124,131}.

The solubility of the drug and excipients in the feed solvent limits the output of a traditional spray drying process. If the solubility of the solute and excipient in the solvent is very different, spray-dried ASDs obtained from such solutions are often not homogeneous^128,129,132, 133, 134. Recently, modified spray drying techniques have been developed for ASD production in which the aqueous, organic, or combination feed solution is heated by a heat exchanger before being atomized and spray-dried^135,136. This process generally leads to spray-dried ASDs that are more homogeneous¹³⁶. The increase in temperature of the feed solution increases the solubility of the drug and other excipients. Such temperature increase of feed solution can reduce its viscosity and enable improved uniformity of atomization¹³⁶. In addition, this modification allows for the rapid evaporation of the solvent and shorter times of particle solidification than conventional spray drying¹³⁶. However, operation safety and formulation stability concerns must be carefully evaluated before employing this method for preparing ASDs.

Spray drying has some limitations. A major concern in spray-dried ASDs is solvent residue^137,138. The low bulk density ASD powder prepared by this method often needs densification to improve its flow properties for further processing into the final dosage forms. Additionally, adhesion of the material to the equipment walls reduces product yield which can be a limiting factor early in development where the developers have a limited amount of the active ingredient, especially for those expensive drugs. Adding silicon dioxide into the feed has been shown to increase bulk density, and therefore the recovery of ASD product^139,140. Nevertheless, spray drying remains one of the most popular methods due to its applicability to a wide variety of compounds and its ability to obtain a product with a high drug load and the desired particle properties by fine-tuning multiple processing variables. For example, albendazole was found to be degrading up to 97.4% during hot-melt extrusion, despite the precautions of earlier forced chemical and thermal degradation tests¹⁴¹. However, no degradation was seen for spray-dried ASDs. The capability of spray drying to prepare high drug-loaded indomethacin-PVP ASDs was compared with co-milling and supercritical anti-solvent process in another report¹⁴². The spray drying method could prepare stable formulations up to a higher drug load (80%, w/w) relative to the other processes (60%, w/w)¹⁴². In a very short timeframe, spray drying can generate particles with a size range from nano-to micro-meter scales. Nanoparticles of celecoxib-phospholipoid E80 and trehalose were prepared via spray drying¹⁴³. Microspheres (3–10 μm) of caffeine ASDs were generated when it was spray-dried with poly (lactic-co-glycolic acid) and polylactic acid¹⁴⁴. It is also a process that can be scaled up from laboratory to industrial manufacturing. For example, Sawicki et al.⁹⁸ showed that spray drying is more suited to scale-up than freeze-drying. For a Phase one clinical trial of docetaxel or paclitaxel, spray drying was a method of choice over freeze-drying since the values of both saturation solubility and precipitation onset time of spray-dried ASDs were either similar or better to the freeze-dried ASD⁹⁸.

2.1.2. Electrospraying

In electrospraying, electrical forces atomize feed solution (containing drug and other additives) into small droplets in the range of a few nanometers or micrometers. Similar to spray drying, the rapid rate of solvent evaporation contributes to the formation of the amorphous drug state within the ASD. With atomization of the drug-containing solvent and rapid drying, the method is somewhat similar to that of spray drying^124,125,137. However, one of the key advantages of electrospraying over spray drying and many other techniques is its capability to produce small particles, with a narrow particle size distribution145, 146, 147. The ‘free-fall’ of the droplets with subsequent rapid solvent evaporation allows for minimal to no agglomeration of the electrosprayed particles¹⁴⁸. This technique additionally offers adaptability with basic equipment designs to generate particles with the desired size, shape, and morphology^149,150.

The standard electrospraying set-up is comprised of four significant parts: a siphoning system (usually a pump), a spray nozzle set-up with a variable high voltage, and a grounded substrate¹⁵¹. The electrically conductive feed is pumped gradually into the spray nozzle, which has an applied electrical potential difference. When the feed solution is ejected from the nozzle, at an adequately high applied voltage, the free charges on the solution surface create an electrical pressure. This results in the generation of the ‘Taylor cone’, where the meniscus at the nozzle tip is shaped like a cone (Fig. 3)¹⁵². The solution at the tip of the cone has a high free charge and is pulled away rapidly towards the collector, forming a highly charged solution jet¹⁵¹. During the flight to the collector, solvent evaporation on the primary droplet surfaces causes them to shrink. This increases charge concentration causing the droplet to undergo Coulomb fission and break into even smaller droplets¹⁵¹. These nano- or micro-sized droplets allow for instant solvent evaporation so that only solidified particles reach the grounded substrate^153,154. Several studies have also used an additional assembly of a corona neutralizer. The neutralizer is placed opposite to or concentrically around the nozzle and is used to prevent further Coulomb fission and disruption of charged droplets to obtain monodispersed particles155, 156, 157, 158, 159, 160, 161. Besides the Taylor cone-jet mode, many other electrospraying modes can be achieved by adjusting the electrical potential. However, the Taylor-cone mode, which emits a steady stream of microscopic jet and breaks up periodically into uniformly sized droplets is the preferred mode for the generation of monodisperse particles.

Figure 3.

Formulation and process variables of electrospraying affecting ASD performance (left diagram) and the mechanism of particle formation by electrospraying (right diagram).

For electrospraying, the processing conditions are optimized such that the solution kinetic energy and surface tension are overcome by the electrostatic repulsion, allowing the jet to break into smaller droplets¹⁵¹. However, if the kinetic energy in the Taylor cone and the surface tension exceeds the electrostatic repulsion, usually due to the presence of high molecular weight polymers, the charged solution jet will not break into droplets¹⁶². This forms fine polymeric fibers (instead of particles) with their diameter ranging from a couple of nanometers to a few micrometers¹⁶³. This process is known as electrospinning. Although ASDs can be prepared using electrospinning164, 165, 166, 167, electrospraying is preferred due to its ability to generate spherical and monodisperse particles with better flow characteristics relative to electrospun fibers.

Bohr et al.^168,169 formulated celecoxib-poly (lactic-co-glycolic acid) (PLGA) microspheres of ASDs with electrospraying. Not only was the drug release from ASDs faster than that of a pure crystalline drug, but optimization of electrospraying allowed for varying ASD properties like particle size and porosity which helped alter drug release profiles. Yu et al.¹⁷⁰ prepared ketoprofen-PVP ASD nanoparticles using electrospraying with drug-polymer ratios of 1:10, 2:10, and 5:10. Almost complete drug release was achieved for all the ratios within 1 min at a 50 mg drug dose in phosphate buffer solution. Similar rapid release (<1 min) was exhibited for acetaminophen/PVP K25 prepared by electrospraying¹⁷¹. Electrospraying can also be useful for preparing ASDs of drugs that are poorly soluble in common solvents or have a high melting point, which makes using traditional heat or solvent-based methods challenging. For instance, quercetin is one such drug with a high melting point of 326 °C. Li et al.¹⁷² formulated a quercetin-PVP ASD by electrospraying their dimethylacetamide and ethanol-based solution, which exhibited rapid release (<10 s) and 10-fold higher permeation rates across porcine sublingual mucosae than crystalline quercetin.

Although there are studies indicating successful ASD formation by electrospraying, there are also reports highlighting the complexity of electrospraying in preparing ASDs. When clarithromycin ASDs were prepared by electrospraying, Mohammadi et al.¹⁷³ observed an incomplete amorphization of the drug. Similarly, even though the -Eudragit® L100 ASDs prepared by Zhang et al.¹⁷⁴ had a significantly enhanced drug release, incomplete amorphization (<5% crystallization) was observed in the formulation. Besides the nature of the drug^175,176, ambient pressure is one of the factors that results in incomplete amorphization during electrospraying. For example, Nyström et al.^157,158,177 reported the varying effect of ambient pressure on drug amorphization during electrospraying. Electrospraying the solutions of budesonide and piroxicam at low pressure (0.3 bar) led to powders exhibiting higher degree of amorphization compared to solutions electrosprayed at atmospheric pressure. On the other hand, indomethacin solutions electrosprayed at lowered pressure (0.3 bar) prompted the formation of more crystalline drug than that produced at atmospheric pressure.

An important parameter impacting the crystallinity of electrosprayed solids is the electrical field, especially for dipolar compounds. Increased interaction of the compound dipole moments with a strong electrical field, can cause molecular rearrangements that promote crystallization¹⁷⁸. Also, it is important for the feed solution to have sufficient conductivity^179,180 and low viscosity so that the solution jet can be broken into smaller droplets^181,182. Fig. 3 shows some other equipment and process variables that can affect the drug amorphization during electrospraying.

Since even a small trace of crystalline phase can induce subsequent crystallization of the ASD, residual crystallinity is a parameter that needs to be monitored while manufacturing ASDs, especially via electrospraying. Although there is some advancement in scale-up approaches of electrospraying like multi-spray nozzle¹⁸³, nozzle-free¹⁸⁴, and high-speed electrospinning¹⁶⁷, the use of electrospinning for ASD preparation is restricted due to its low production rates and complex process design. For in-depth reading regarding the fundamentals of electrospraying, readers can refer to some previously published reviews151, 152, 153, 154^,185.

2.1.3. Fluidized bed technology

Fluid bed technology is used for various pharmaceutical unit operations including granulation (fluidized bed granulator), coating (fluidized bed coater), drying (fluidized bed dryer), and cooling186, 187, 188. Along similar lines, fluidized bed coaters and granulators are also used to manufacture ASDs^189,190. For ASD preparation, the drug-polymer solutions are sprayed onto inert excipient cores, with the solvent evaporation and ASDs layering occurring simultaneously. The organic solvent can be recovered and recycled¹⁹¹. This method has been used to formulate both controlled- and immediate-release solid dispersions^192,193. Direct formation of ASD granules by this method reduces additional downstream processing steps, which aids in avoiding potential stability issues during these processes^45,194. In addition, such granules allow an additional coating of suitable excipients which can control the release profiles or enhance the ASD stability¹⁹⁵. For example, indomethacin–PVP ASDs prepared by coating onto sugar spheres were further coated with various polymers to achieve the desired drug release and diffusion rates¹⁹⁴.

Based on the nozzle location, there are four types of setups available for fluidization as shown in Fig. 4¹⁹⁶. In both the bottom and tangential spray process, the drying air and feed solution are introduced in the same direction. This presents a more controlled particle movement and allows for a uniform ASD coating. Therefore, these two configurations are usually the preferred fluid bed technology methods to prepare ASDs¹⁹⁷. The typical formulation and process parameters of fluidized bed manufacturing which affect product properties are shown in Fig. 4¹⁹⁸. Dipyridamole controlled release ASD pellets were formulated by Beten et al.¹⁹² using the bottom spray fluidized bed process. When Ho et al.¹⁹³ prepared nifedipine-HPMC ASDs by layering them on sugar spheres using a fluidized bed coater, they observed varying rates of drug release with varying drug-polymer ratios. Similarly, Sun et al.¹⁹⁹ employed a fluidized bed coating method to prepare silymarin-PVPP ASDs. Additionally, Zhang et al.²⁰⁰ demonstrated the success of this process to prepare ASDs at low air temperatures of <30 °C for thermolabile drugs. This technique was also utilized to produce wax-based (floating) ASD pellets which gave a sustained release for 12 h²⁰¹. The wax-based core was coated with protocatechuic acid and ethylcellulose solution in a single-step fluidized bed coating method. Fluid bed layering has also been used to prepare an amorphous complex of drug and cyclodextrins to enhance the drug dissolution profile²⁰². For ASD preparation, fluid bed layering can be challenging on a larger scale due to stickiness of the material and over-agglomeration; though there are still a few marketed products manufactured using the fluid bed process^203,204, probably because the fluidized bed offers a one-step granulation-coating-drying approach with the possibility of relatively lower operating temperatures than spray drying.

Figure 4.

Fluidized bed manufacturing variables affecting product properties and the different types of fluidized bed dryers (a) top spray; (b) bottom spray; (c) Wurster; (d) tangential or side rotor spray.

2.1.4. Supercritical fluids

Supercritical fluids (SCFs) have also been used to produce ASDs. SCFs are simply gases that simultaneously present gaseous and liquid state properties under specific pressure and temperature conditions205, 206, 207. The liquid property of SCFs is useful for drug-polymer solubilization whereas the gaseous property aids in solid diffusion and solvent evaporation²⁰⁶. Although theoretically nearly all gases can be SCFs, practically only a few are used due to the limitation of attainable temperature and pressure conditions. In fact, >98% of all SCF applications have been developed with carbon dioxide (CO₂)²⁰⁵. This is because CO₂ has a low critical temperature (31 °C) and pressure (7.4 MPa), making it easier to achieve adequate conditions for an SCF process. In addition, it is non-flammable, reusable, non-toxic, and inexpensive^206,208,209. Another gas used as an SCF in the pharmaceutical industry is trifluoromethane²⁰⁹. The major advantage of SCF-based methods is that they are relatively greener in nature and have lower production costs compared to other solvent-evaporation processes. The solvent evaporation process can also be controlled more by adjusting the temperature and pressure conditions. The low viscosity of SCFs result in a high diffusivity and rapid solvent evaporation with faster and higher yields²¹⁰. Drawbacks of using SCF-based methods for ASD preparation include the difficulty in removing residual organic solvents (if used), and the high capital investment²⁰⁵.

SCF-based processes can be divided into three groups²⁰⁶. The first includes processes that use SCFs as solvents. An example of such a process is the rapid expansion of a supercritical solution (RESS). In RESS the drug and polymer/s are dissolved in an SCF and the SCF is then rapidly expanded by sudden decompression. This is usually done by passing the SCF solution through an orifice at low pressure²¹⁰. RESS is an advantageous single-step process requiring minimal to no organic solvent. However, a typical issue with this process is particle agglomeration. Many poorly water-soluble compounds have inadequate solubility in SCFs under moderate conditions of temperature (<60 °C) and pressure (<300 bar)^206,210. Cosolvents such as methanol can be added to CO₂ to aid drug solubility. However, this necessitates an additional step for removing the residual solvent which increases process complexity and expense211, 212, 213.

The second group consists of processes that use SCFs as antisolvents. In these processes, the drug-polymer system is solubilized in an organic solvent and then mixed with an SCF which acts as an antisolvent. The solubilization capability of the organic solvent reduces as SCF is being added to the organic solvent. Thus, the new liquid mixture now becomes supersaturated with respect to the drug (and the polymer) causing them to precipitate as ASDs. To successfully produce an ASD by this method, the drug and polymer should possess high solubility in the selected organic solvent and limited/no solubility in the SCF. Also, the selected organic solvent should be miscible with the selected SCF (antisolvent). The precipitated particles are subsequently filtered. There are different techniques that have the same underlying principle but differ in the mixing method of the solvent and antisolvent^206,210. The examples include precipitation with compressed fluid antisolvent (PCA), supercritical antisolvent (SAS) precipitation, aerosol solvent extraction system (ASES), gas antisolvent (GAS) precipitation, and solution-enhanced dispersion by SCF (SEDS). For the PCA method, the organic solvent solution is introduced via capillaries into the controlled chamber containing the SCF²¹⁴, whereas in SAS the solvent solution is introduced via a nozzle^215,216. These methods have been applied to provide ASDs on an industrial-scale²¹⁷. Atorvastatin²¹⁸, megestrol acetate²¹⁹, and valsartan²²⁰ are ASDs produced by the SAS approach and have exhibited improved solubility and bioavailability. Indomethacin²²¹, cefdinir²²², and glycyrrhizic acid ASDs²²³ produced by this process have shown enhanced solubility whereby the formulated powder also exhibited uniform particle size. Glibenclamide ASDs prepared by the SAS were shown to have similar solubility as those prepared by solvent evaporation using a rotary evaporator²⁰⁸. A recent study further demonstrated improved permeability of zidovudine-poly (l-lactic acid) solid dispersions relative to the pure crystalline drug when tested in an ex vivo everted rat intestinal sac model²²⁴. The observed effect was attributed to the enhanced poly (l-lactic acid) (polymer) plasticization which increased the extent of drug diffusion in the polymer matrix. In the ASES method, the organic solvent solution and SCF are sprayed at the same time (using different nozzles) into the chamber²¹⁶. Itraconazole-HPMC ASD particles (100–500 nm) produced by ASES showed >609-fold increase in the amount of drug released during dissolution compared to the pure crystalline drug. Also, the bioavailability of ASDs as determined in rat models was similar to that of the marketed ASD drug product Sporanox®²²⁵.

As an alternative to the above methods, the SCF can be added to the organic solution in the GAS technique^206,226. A key unique feature of this method is that the SCF can be used as an antisolvent in the supercritical condition for the entire time of processing. The pressure applied varies continually from one Bar to the final pressure. With an increase in pressure, the concentration of the gas employed increases, causing the ASD to precipitate. Compared to other SCF methods, GAS is a slow process that can allow for molecular rearrangement and is therefore not ideal for ASDs. SEDS, another process of the second group uses a unique patented nozzle that allows SCF to function both as an antisolvent and a dispersing agent for the organic solution²²⁷. Thus, the organic solution and SCF are atomized simultaneously²²⁸. There are two types of a nozzle in the SEDS: one with two channels (one each for SCF and organic solution) and another with three channels. The nozzle with three channels provides more choices in operating variables. For example, one channel can be for the organic solution with a drug, a second for polymer in aqueous solution, and a third for SCF²²⁹. Puerarin microparticles produced by SEDS were amorphous whereas the ones prepared by GAS were crystalline²³⁰, likely due to the slow process of GAS.

The third group includes methods that use SCFs as solutes, including precipitation from gas-saturated solutions (PGSS). It utilizes the ability of supercritical CO₂ (scCO₂) to diffuse into organic compounds like polymers. When scCO₂ diffuses into the polymer, it lowers polymer T_g and viscosity. In the PGSS process, the drug-polymer physical mixtures are first introduced into SCF. Elevated pressure and SCF cause the mixture to melt. This non-viscous solution is sprayed into a receiving chamber with controlled pressure. Due to rapid decompression, SCF escapes the solid matrix, and ASDs are formed. This method is particularly suitable for materials like PVP and PLGA which easily absorb SCF. PGSS is an advantageous SCF-based process since it does not require organic solvents and usually operates at low pressures with lower gas consumption relative to other processes such as RESS. Composite solid dispersions of fenofibrate and stearoyl macrogol-32 glycerides (Gelucire®) have been prepared using PGSS by Pestieau et al.²³¹ Biphasic dissolution of the formulation indicated a significantly higher concentration of the drug in the aqueous (0.1 mol/L HCl) as well as organic (octanol) layer relative to the physical mixture. In another study, progesterone solid dispersions were prepared with PGSS²³². Amongst the tested variables, a yield of 94.7% and the highest extent of progesterone dissolution after 20 min (85.6%) was observed for lower progesterone-to-excipient ratio (1:10) and process values of higher pressure (186 bar), higher temperature (60 °C), and a longer processing time (30 min).

SCFs have also been used as processing aids in combination with melt extrusion (usually in RESS or PGSS mode)²³³. A schematic for the selection of an appropriate SCF-based process for ASD preparation is shown in Fig. 5. Among SCF-based processes, SAS-based processes are generally favored for ASD preparation due to easier scale-up and more tunable variables²³⁴. Besides the formulation composition (discussed in previous methods), the key variables that affect product attributes in the SCF-based methods are pre-expansion conditions (temperature and pressure of vessel), nozzle type (atomization, dimension) and angle of impact of the jet stream, feed rate of solution, flow rate of the SCF, and final drying/extraction time²³⁴.

Figure 5.

Schematic for selection of SCF-based process for ASD preparation.

2.1.5. Spray-freeze-drying

Spray-freeze-drying is one of the cryogenic technology for preparing ASD powders (Fig. 6)^235,236. During the spray-freeze-drying process, the feed solution or suspension comprising of the drug, polymer, and possibly other excipients are atomized and sprayed directly into a cryogenic liquid^237,238. The frozen particles are then transferred to a freeze-dryer to generate a flowable ASD powder. This process can be further categorized depending on the type of injection devices (capillary, rotary, pneumatic, ultrasonic, two-fluid/three fluid nozzle), location of the nozzle, and the composition of the cryogenic liquid (liquid nitrogen, liquid argon, compressed CO₂)²⁰⁷. However, generally, spray-freeze-dried powders are amorphous and porous, which results in a high surface area and a high dissolution rate239, 240, 241, 242, 243, 244. Also, relative to spray drying, spray-freeze-drying offers a higher control over particle size, compatibility with more excipients, less thermal stress, and a higher yield²⁴⁵. The spray-freeze-drying process has been successfully used to enhance the dissolution and bioavailability of poorly soluble drugs like carbamazepine^239,240, and danazol^241,243,244,246, 247, 248. He et al.²⁴⁹ demonstrated that this technology could be employed for producing stable and free-flowing baicalein powers that could not be obtained by a conventional solvent evaporation method (i.e., rotavapor). Pluronic F68 was used as a carrier and inhibited baicalein from crystallizing. In comparison with the ASDs prepared by rotary evaporation, the spray-freeze-dried ASDs exhibited significantly enhanced baicalein dissolution rate and improved oral bioavailability in rats²⁴⁹, likely as a result of their porous structure and higher specific surface area.

Figure 6.

Preparation of ASDs by spray-freeze-drying. The feed solution is atomized and sprayed directly into a cryogenic liquid and the frozen particles are subsequently transferred to a freeze-dryer to generate dried ASD powder.

Since spray-freeze-drying is a combination of spraying and freeze-drying processes, the manufacturing considerations for preparing stable ASD systems using this method are a combination of those two as well (discussed in their respective sections). Nevertheless, an important distinction is the solvents used for preparing the feed solution. Spray drying solvents may have a high ratio of organic components. However, for spray-freeze-drying, it is preferable that the feed solution have a low organic component since freeze-drying usually does not permit high organic solvent content. Another concern with spray-freeze-drying is that the porous and low-density structures of the ASDs may make them fragile and difficult for secondary processing.

2.1.6. Other solvent-based methods

Solvent casting is a commonly used laboratory process for preparing ASDs250, 251, 252, 253. A drug–polymer solution (mostly organic) is spread onto a substrate and then the solvent is evaporated at room temperature under normal pressure²⁵⁴. The solvent evaporation can also be sped up using a hot plate²⁵⁵ or by placing it in an oven (at low temperatures and pressure) and followed by cooling²⁵⁰. The films that form are usually milled to obtain powders. However, this method is limited to drugs that can be solubilized in solvents with low boiling points like ethanol, chloroform, dichloromethane, or their mixtures256, 257, 258. Also, it might be challenging to completely evaporate the residual solvent.

Solvent evaporation using a rotary evaporator is another frequently used process for a small-scale ASD preparation^48,259. In this process, the drug–polymer solution, typically using an organic solvent, is evaporated under vacuum and at slightly elevated temperatures. Simultaneous application of vacuum and heat increases the rate of solvent evaporation and allows the use of a solvent with higher boiling points if required. In the rotary evaporator process, solvents like tetrahydrofuran or dimethylformamide that could not be used in a solvent casting process can be used. The final product is collected from the flask and, if necessary, can be further milled.

Sandhu et al.¹²³ reported rotating jet-spinning as an alternative to electrospinning for ASD production with particles in the nano- or micro-meter size range. The typical set-up consists of a rotating reservoir containing drug-polymer solutions attached to a motor (Fig. 7). The reservoir can either be perforated, or equipped with a side nozzle, or possibly placed over metal plates with perforations for ejecting the solution. The drug-polymer solution is placed into a preheated or room temperature rotating metal container (spinning reservoir) which rotates at high speed (generally in the range of 2000–13,000 rpm). When the reservoir is rotated (about its axis) at a rate that overcomes the capillary and centrifugal forces, the solution jet is ejected from a side nozzle/perforations on the reservoir or a gap between metal plates holding the reservoir²⁶⁰. This jet is propelled along a long spiral trajectory extended by the centrifugal forces. This generates a higher surface area. Solvent evaporation also occurs during this step. The solvent evaporation rate is dependent upon the solvent diffusion coefficient in the polymer²⁶¹. Relative to solvent evaporation methods like spray drying or electrospinning, challenges posed by this method include its lower ability to remove the residual organic solvent(s) to a satisfactory level, and the necessity of a batch mode²⁶². Further study on the scale-up capability of this method for producing ASDs is warranted for the feasibility of industrial manufacturing²⁶².

Figure 7.

A typical set-up of the rotating jet-spinning process. The reservoir containing feed solution is rotated such that the solution jet is ejected with a higher surface area, resulting in rapid solvent evaporation.

2.2. Melting-based methods

In melt-based methods, the formulation components are heated to form dispersions, followed by cooling. Solvent avoidance is a significant benefit of the melting techniques²⁶³. However, a major drawback of these methods is that the high temperatures may induce drug degradation⁸⁴. Melting methods also require sufficient solubility/miscibility of drugs in the polymer melt, which can be very difficult for certain molecules to achieve^7,264,265.

An efficient ASD production by melting depends on the operational as well as compositional variables. The use of a polymer in the production of an ASD is of course to stabilize the amorphous form, but it is also important for processing because the polymer provides a molten medium for drug solubilization or dispersion. Therefore, the polymers used in melting processes are usually polymers or waxes with a low melting point or T_g^52,266. Commonly used carriers include PVP⁵², PVPVA267, 268, 269, cellulose esters and acrylates^47,52, and polymethacrylate derivatives²⁶⁸. There are some commercial polymers specifically designed for melting processes such as HPMCAS270, 271, 272, Soluplus®^273,274, and Affinisol® HPMC HME (modified HPMC)²⁷⁵. These carriers can also be used in combination to achieve improved amorphization, stability, dissolution, and bioavailability^47,276.

Sufficient plasticization is essential to form an ASD during the melting process. Although the drug itself can provide some plasticizing effect²⁷⁷, additional plasticizers can be added to aid the mixing of drug and polymer. Melting-based ASD preparation processes are also often characterized by high shear stress, which can be reduced by the incorporation of a plasticizer²⁶⁵. Plasticizers reduce the viscosity of the mixture and therefore can lower the processing temperature. Some examples of frequently used plasticizers are d-α-tocopheryl^270,278, poloxamers^273,279, low molecular-weight poly (ethylene glycol)s (PEGs)²⁶⁵, and surfactants^280,281. Plasticizer selection is dependent on its intended formulation functionality, such as lowering the processing temperature or lowering the melt viscosity. Added plasticizers that remain in the product, can affect its properties and performance (physicochemical stability, dissolution, T_g, hygroscopicity, or appearance). In addition, conventional plasticizers are used in a concentration range of 5%–30%, w/w^282,283 which increases the total weight of the formulation and may result in large, unacceptable dosage forms. Therefore, an ideal plasticizer should be capable of providing the desired plasticizing effect and then be removed from the formulation to mitigate its possible negative effects before final processing. For this reason, scCO₂, low boiling solvents, or reagents that can evaporate or sublime are being used^123,233,284.

2.2.1. Hot-melt extrusion (HME)

For commercial-scale production, only two types of melting processes are available, hot-melt extrusion (HME) and melt agglomeration (discussed later in the ‘Granulation’ subsection). HME, and especially the twin-screw melt extrusion with Meltrex™ as a representative example, is one of the most widely used techniques for producing ASDs^263,265,285. The drug and the polymer are mixed, melted, dispersed, and then extruded under specific processing conditions^265,286. Fig. 8 shows the HME process, which can be divided theoretically into five steps, namely: feeding, melting and plasticizing, mixing/kneading and conveying, discharging, and cooling for further downstream processing. Important parts of a hot-melt extruder include a feed hopper, barrel, extrusion screws, torque sensors, heating-cooling system and dies.

Figure 8.

Twin-screw hot-melt extrusion and the associated manufacturing variables impacting product properties.

The HME equipment has a flexible design, which enables processes to be tailored to achieve the desired outcomes and to accommodate varying raw materials, by adjusting the modular design elements, namely the screws and barrels²⁸⁷. It is also possible to apply HME technology to drugs liable to oxidation and hydrolysis by excluding oxygen and moisture from the mixture^265,287,288. The barrels can be flanged together or linked via internal tie rods. The most critical component of the extruder is the screw, which determines the quality and quantity (per unit time) of the extruded material^52,287. Based on the screw configuration, the process may be categorized into single-screw or twin-screw extrusion. A twin-screw provides many benefits over the single screw model, with decreased drug residence time in the extruder and facilitated continuous mass flow with better mixing^278,289. It can also be designed with two separate hoppers, both of which can vary in temperature over a wide range and are equipped with the self-cleaning function^265,266,290. In addition, twin-screw extruders can limit thermal stress on the materials by reducing the ‘non-motion’ and preventing localized overheating^265,266. It also presents easier feeding of materials and less propensity to overheat²⁸⁸.

Depending on the desired shear level and operating speed, two screws can be designed in different configurations^52,291 of co-rotating (rotating in the same direction) or counter-rotating (rotating in the opposite direction), depending on the desired mixing intensity. Co-rotating screws are usually used in pharmaceutical manufacturing because they generate relatively lower shear forces than the counter-rotating²⁶⁶. Nakamichi et al.²⁸⁹ demonstrated that the physicochemical properties of the extruded material were substantially impacted by the machine operating conditions. The study reported that since the kneading screws kept the material in the machine for a prolonged period under shear, stable ASDs exhibiting super-saturation upon dissolution could be prepared irrespective of changes in operating conditions such as the rate of revolution of the screws (20–100 rpm) and the amount of water (0%–50%) introduced in the feed²⁸⁹. When the kneading screws were detached from the screws and only the feed screw elements were used for ASD preparation, the extent of amorphization and dissolution profiles of the extruded material was substantially impacted by the machine operating conditions. Although partial crystallinity was observed in all the batches, the extent of crystallinity decreased with a reduction in rotation rate and amount of water. Slow screw rotation and the addition of a sufficient amount of water to the mixture increased the rate of dissolution of the drug, although no super-saturation occurred in any of the batches tested²⁸⁹.

The kneading paddles play a vital role in drug amorphization. Verhoeven et al.²⁹² reported that at least one kneading zone was necessary for the homogeneous distribution of metoprolol tartrate in ethyl cellulose matrices, even though the homogeneity of distribution and drug release rate were not significantly altered by the number of kneading zones or their location along the extruder barrel. The impact of screw configuration on the dissolution behavior was studied by Liu et al.²⁹³ with indomethacin-Eudragit® EPO ASDs. They observed that indomethacin dissolution into the polymer melt was accelerated by the kneading blocks. Adequate shear stress, shear rate and mean residence time (which is linked to lower screw speed and feeding rate), are required to ensure complete drug amorphization^{263,266,294,295}. However, excessively high shear stress, shear rate, and residence times pose the risk of material degradation.

The molten mass can be conveyed to the dies of a variety of shapes and sizes depending on the desired product properties. For example, the production of films²⁹⁶ and patches²⁹⁷ use flat dies, whereas pelletization and spheronization use circular dies^298,299. Injection molding can also be utilized to fill the molten drug-polymer mass into molds^233,300. Tablets, granules, capsules, or ear inserts can be produced in these molds^{263,265,295,301}. After cooling at room temperature, the resulting product is collected and can be further milled to obtain the required particle sizes^265,302. Apart from these important variables, other parameters such as temperature, die geometry, barrel design, and screw speed can also impact the final product properties²⁶⁵. Recent work has shown that the degassing process can remove volatile substances and subsequent air bubbles, which facilitates consistent production of extrudates with improved cross-sectional uniformity³⁰³. In order to reduce torque during screw rotation, certain minimum temperatures capable of plasticizing the material are required in HME^263,265. Material flow properties are also critical to ensure a consistent feed rate from the hopper. For poorly flowing material, flow aids such as spray-dried mannitol, maltodextrin, and colloidal silica¹²³ can be used. As some of these flow aids are crystalline, the miscibility of the drug in the polymer may be affected, further complicating the solid-state and chemical analysis.

HME is continuous, single-step, solvent-free, and capable of scaling up²⁶³. The HME method can be equipped with on-line and in-line quality-control analysis such as near-infrared, Raman, and dielectric spectroscopies³⁰⁴, which facilitate quality by design and continuous manufacturing. Some drawbacks include the higher energy usage and exclusion of thermolabile compounds^263,266,305. Changes in the design of the equipment (e.g., presence of kneading elements), as well as the addition of plasticizers, can lead to a reduction in processing temperatures and/or residence time, and thus minimize the potential of thermal degradation of drugs during the processing²⁶⁵.

2.2.2. KinetiSol®

KinetiSol® is a modern fusion-based method that uses high shear force combined with heat to melt the drug-polymer blend and generate an ASD³⁰⁶. A rapid temperature rise is generated by the combined mechanical forces involved in the process. This creates a molten mass that is immediately quenched and processed further. The schematic representation of KinetiSol® technology is shown in Fig. 9. A series of paddles rotating in a cylindrical vessel and shaft with high-speed mixing blades produce a large amount of frictional and shear energy³⁰⁷. This mechanical force results in the generation of heat, and the material temperature increases without applying external heating. Computer software controls the real-time temperature of the composition inside the KinetiSol® chamber, and the molten mass is quickly ejected from the processing vessel upon reaching the user-defined temperature endpoint. The entire process duration is normally <20 s, and the material is generally exposed to high temperatures for <5 s before the product is discharged and cooled³⁰⁸. The method is designed to operate in a batch mode at a laboratory-scale, whereas it can be run semi-continuously at an industrial-scale with output as high as 1000 kg/h^309,310.

Figure 9.

Schematic of KinetiSol® technology. Mechanical forces involved in the process result in a rapid temperature increase. This creates a molten mass which is immediately quenched to generate ASDs.

Being a short process in which desired temperature is reached within seconds, and material is ejected rapidly when the target temperature is reached, it lowers the exposure of the material to thermal stress, which is beneficial to ASD regarding degradation. Hydrocortisone-HMPC and -PVPVA matrices were found to be chemically ‘preserved’ during KinetiSol® processing (160 °C, <30 s); while heating the mixture at higher temperatures of >180 °C reduced drug potency due to decomposition³¹¹. Griseofulvin ASDs prepared with this method did not exhibit any sign of crystallinity or increased impurity, even after storage at 40 °C and 43% relative humidity (RH) for 6 months³⁰⁹. The utilization of heat generated by the process (with no added external heating) in KinetiSol® makes it one of the few methods applicable to drugs with high melting points as well as low solubility in organic solvents. This also enables the elimination of plasticizers and reduces the risk of extended heat exposure³¹². Even viscous polymers (e.g., PVP K30, PVP K90, HPMC K15 M) can be applied in KinetiSol® processing without the aid of plasticizers.

Another benefit of KinetiSol® is that it permits high drug loading without high torque and degradation, unlike other thermal processes³⁰⁶. Itraconazole dispersion with Methocel E50LV was prepared using KinetiSol® at a temperature range below the drug melting point without excessive shear³¹². T_g values of itraconazole-Eudragit® L100-55 ASDs prepared by KinetiSol® (at 1:2 ratio) were higher than those prepared using other thermal techniques, suggesting improved stability. This improved stability was attributed to the ability of KinetiSol® to offer a short processing time of <10 s, a processing temperature lower than the polymer degradation temperature, and the use of low torque (even without plasticizers)³⁰⁶.

Hughey et al.³¹² conducted a design of experiment matrix to examine the effect of KinetiSol® process variables on drug stability. Meloxicam-Soluplus® ASDs were prepared at varying processing temperatures (110–140 °C) and processing speeds (2250–3000 rpm). The results indicated that the residence time was inversely related to process speed; when the processing speed was increased from 2250 to 3000 rpm, the residence time decreased from 22 s to less than 3 s. No definite relationship between the processing speed and drug degradation was observed. Ejection temperature and degradation, however, were found to be correlated. The samples ejected at 110 and 118 °C showed a drug content of >95%, but when the ejection temperature was raised above 125 °C it decreased to <90%, and further decreased to 79% when the ejection temperature was raised to >140 °C.

Similar effects of KinetiSol® processing conditions have been reported on the stability of ritonavir-PVPVA ASDs³⁰⁸. The processing time was decreased, but the amount of impurity increased with an increase in mechanical energy (processing speed), indicating that exceeding the mechanical energy over a certain limit can result in drug degradation. Of important note was that even though the ejection temperature was maintained at 80–100 °C, which was far below the drug degradation temperature of 160 °C, a high processing speed of 2000 rpm still caused the drug to degrade. The above case studies illustrate the capacity of KinetiSol® for generating ASDs after adequate process optimization.

2.2.3. Three-dimensional (3D) printing

Three-dimensional printing (3DP) is a relatively innovative technology that can transform 3D computer models into physical objects by additive manufacturing^313,314. The significance and relevance of 3DP for pharmaceutical applications have been discussed in depth elsewhere315, 316, 317, 318, 319, 320, 321, 322. It is a technique to manufacture customized medicines for patients³²³ by delivering multiple drugs324, 325, 326, 327 with varying print settings, allowing for better control over the drug dissolution kinetics328, 329, 330, 331. There are several techniques and variations in 3DP, such as material jetting, binder jetting, and material extrusion³³². The most widely used technology in the pharmaceutical sector is material extrusion and fused deposition modeling (FDM)^333,334. In vivo experiments in animals as well as humans have demonstrated good tolerance of FDM 3D printed formulations335, 336, 337.

For ASD preparation, 3DP has often been used in conjunction with HME³³⁸. HME + FDM 3DP involves heating a drug–polymer mix and extruding it with the HME technique, followed by 3DP, and cooling-solidification of the molten mass into the desired form on the build plate^339,340. Although many polymers have been studied for HME + FDM 3DP³⁴¹, hydroxypropyl cellulose (HPC) remains the most commonly used one due to its suitable properties of particle size, viscosity, and T_g which allow for ease of extrusion, shape retention, and dimensional accuracy while generating a stable product^323,342, 343, 344. Friability of the tablets greatly depends on the particle size of the employed polymer with smaller polymer particles improving the friability, whereas disintegration time and dissolution properties mainly depend on the viscosity of the employed polymers³⁴⁵. The polymers with higher viscosity usually lead to slower disintegration and dissolution³⁴⁵. One obvious drawback of FDM 3DP is the need for drug dispersions to be prepared by HME, which increases the probability of thermal drug degradation^346,347. Another significant downside is the constraint on the use of excipients and drugs because they need to generate filaments with the necessary mechanical and physical characteristics for a successful 3DP^348,349. Nowadays many 3DP studies in pharmaceutical applications have focused on the selection and optimization of excipient filaments appropriate for 3DP. The drug loading potential of the process is typically limited since a significant amount of polymer is required to impart adequate rheological properties to the extrudate. Therefore, the possibility of eliminating the HME step in FDM 3DP would be of great value in pharmaceutical product development³¹⁶. In the plastics industry, direct pellet extrusion, a new 3DP material extrusion process, has recently been introduced as a possible alternative to FDM 3DP³⁵⁰. This technology uses a single screw extruder with a printer nozzle to directly print material in the form of pellets/powder. Since this technology does not require the preparation of filaments using HME, it may potentially permit the extrusion of mixtures which, due to the insufficient mechanical properties of the filaments, would not be printed by conventional FDM (e.g., being too brittle or too flexible). Similar technology was employed by Goyanes et al.³⁵¹ to prepare cylindrical ‘printlets’ or 3D printed tablets of ASDs. This new, single-step 3DP technique was used to prepare itraconazole ASD printlets directly from powdered materials using four different grades of HPC (HPC-UL, SSL, SL, and L). All the generated printlets exhibited acceptable mechanical and physical characteristics complying to pharmacopeial limits. In addition, no drug degradation was detected in the printlets and they exhibited a sustained drug release profile with drug concentrations exceeding the crystalline drug solubility. This research demonstrated the ability of this technology to resolve one of the major drawbacks of HME FDM 3DP, i.e., the need for filament preparation by HME.

Recently another single-step 3DP process, selective laser sintering (SLS), was used to produce ritonavir–copovidone ASDs³⁵². A complete conversion of the crystalline drug to its amorphous form was obtained as a function of laser-assisted selective fusion. In this study, powder flow properties, surface temperature, chamber temperature, laser speed, and hatch spacing were found to be crucial for successful ASD formation³⁵². Moreover, ritonavir in the SLS 3D printed tablets exhibited a 20-fold increase in solubility relative to the crystalline drug. These novel, single-step technologies could be advantageous for the preparation of ASDs for preclinical studies where the quantity of drugs is limited or the use of traditional HME is challenging.

2.2.4. Microwave heating

Microwave-induced in situ amorphization is being investigated, wherein the crystalline drug in the final dosage form is amorphized in situ inside the final dosage type using a microwave oven³⁵³. Several experiments have demonstrated the feasibility of using microwaves to prepare ASDs. Doreth et al.³⁵⁴ obtained indomethacin–PVP ASDs generated in situ by microwave heating. The indomethacin-PVP ASDs exhibited no microwave-induced drug degradation and the dissolution rate of the microwaved amorphous tablets was 6-fold higher than that of the physical mixture containing crystalline indomethacin-PVP³⁵⁴.

The microwave-induced amorphization process consists of three main steps. First, preparing the drug-polymer physical mixture. Second, inducing drug amorphization with continuous or intermittent microwave heating. The last step is cooling, followed by possible pulverizing and sieving. In order to achieve a higher degree of drug amorphization, the formulation should be optimized such that the drugs have high solubility in the polymer, and the polymers can be melted or softened at relatively low temperatures355, 356, 357. To date, only a few polymers have been studied for this technology, with a primary focus on PVP K12. Therefore, it is important to identify more potential carriers to be used with the microwave heating method to expand its applicability in pharmaceutical applications.

High input of microwave energy is needed for the amorphization³⁵⁴ because unlike water, polymers and drug molecules are weak absorbers of microwave^358,359. Usually, the following two techniques are applied to generate sufficient heat: i) convective heating, i.e., a microwave absorbing reactor or a sample holder is heated and this heat is then indirectly transferred to the sample^353,360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370; and ii) The addition of microwave absorbing solvent³⁷¹ or solvent slurry^372,373 to the drug-polymer solution mix and followed by solvent evaporation due to heating. For example, Moneghini et al.³⁵³ placed the physical mixture of ibuprofen and PVP-VA in a glass sample holder, and materials were heated due to the convective heating from glass, whereas Abreu-Villela et al.³⁷² manufactured ASDs by microwaving suspensions prepared by adding defined amounts of water to the drug-polymer physical mixture.

The microwave heating time is typically <15 min, which has a relatively short exposure time to heat and thus potentially shields drug degradation374, 375, 376. The potential of the microwave heating method is its applicability to introduce drug amorphization in the final dosage form without downstream processing^354,377, 378, 379. Of a particular note, such in situ amorphization process by microwave heating can take place in the final dosage form. Doreth et al.³⁵⁴ demonstrated an in situ amorphization process where a drug can be amorphized within its final dosage form (tablets) through microwave irradiation. The authors also noted that increasing moisture content resulted in a higher fraction of amorphous drug. For the tablets in which the absorbed water evaporated rapidly from the surface after storage, the microwaves could not be absorbed, leaving a shell of the crystalline drug at the surface. In regard to the drug chemical stability, there is potential for drug degradation during microwave processing due to the microwave energy applied³⁷⁶. But even before microwaving, the presence of absorbed water, which is a prerequisite for facilitating in situ amorphization by microwave irradiation, can potentially lead to drug hydrolysis and degradation. A potential explanation why Doreth et al.³⁵⁴ did not detect drug degradation during their in situ amorphization process may be due to the short storage time (RT and 54% RH for two weeks) prior to microwaving at 1000 W for 90 s. It is possible that long-term storage can have a substantial effect on the drug stability profile, especially considering the large quantity of absorbed water (approx. 10%) present in such systems. Therefore, it is important to perform more systematic studies to evaluate the drug degradation propensity during formation as well as storage of ASDs prepared by microwave heating. There are presently only a few other publications on microwave-induced in situ amorphization in the final dosage form377, 378, 379.

2.2.5. Other melt-based methods

On a laboratory-scale, ASDs can be produced by the melt-quench method, where formulation ingredients are melted, and followed by a natural or forced rapid cooling³⁸⁰. A water bath³⁸¹ or a hot plate³⁸² can be used to melt the formulation components while the molten mass can then be solidified either by allowing it to cool naturally to room temperature383, 384, 385 or by immersing in an ice bath386, 387, 388 or liquid nitrogen³⁸⁹. However, during early drug discovery and development of ASD formulations there is a limited amount of drug to work with. Therefore, a high throughput method that can prepare ASDs in small batches is particularly useful and efficient. Guo et al.³⁹⁰ proposed a novel acoustic fusion method, which can provide a fast process with only a small amount of drugs. In this method, a Labram resonant acoustic mixer was used for acoustic fusion. The heating metal plate attached to the mixer was preheated to 80–160 °C and 10 mg of solid load (2 mg drug and 8 mg polymer) was filled into each glass vial of the 24-vial plate. The vials were then sealed and placed in the acoustic fusion heating block. The powder was mixed at an intensity of 50–80 G. The heating/mixing time varied between 15 and 60 min depending on the formulation. The samples were then removed and cooled to room temperature, which resulted in a dense glassy solid. The batch size limit for this process can vary from 10 mg to 2000 mg depending on the powder density. Torcetrapib, itraconazole and lopinavir of ASDs were prepared using a variety of polymer systems, including HPMCAS (L, M, and H), copovidone, Soluplus®, Vitamin-E TPGS, Kolliphor EL, and Eudragit®³⁹⁰. These ASDs demonstrated significantly higher drug solubility compared to the crystalline form. Thus, this approach can be used to prepare and optimize ASDs during early screening.

Another potential method for early-stage pre-clinical investigations of ASDs is ultrasonic-assisted or ultrasound compaction (USC). USC is a modified tableting process that provides heat, pressure, and shear to melt the powders during compaction with ultrasonic energy. The USC method is a relatively new fusion-based technique in pharmaceutical research391, 392, 393, 394, 395, 396, 397, 398. Fini et al.³⁹⁵ pioneered the use of USC for drug amorphization. In this earlier study, the effectiveness of USC in preparing indomethacin and β-cyclodextrin SDs was examined³⁹⁵. Indomethacin melted during processing and eventually was dispersed in β-cyclodextrin. Due to the presence of amorphous indomethacin, the USC formulations displayed enhanced dissolution properties over the physical mixture or kneaded compositions. Sancin et al.³⁹³ also prepared a ketoprofen–Eudragit® S100 ASD system sing USC. The findings showed that the dispersed ketoprofen crystallized at a much lower rate (>6 months) than the pure ketoprofen produced using USC (<1 day). Similarly, Fini et al.³⁹⁴ prepared PVP-based indomethacin systems using USC. The use of a PVP matrix allowed a faster dissolution rate than the formulations prepared with β-cyclodextrin. Fini et al.³⁹² also investigated PEG-based compositions of indomethacin prepared by USC. It was hypothesized that, due to the low melting point of the material, the USC process would soften or melt each material, enabling a solid dispersion to be formed inside the die cavity. However, despite the observed increased dissolution rate, the formulations prepared by USC were not able to reach the dissolution rate of the ASD control samples prepared by the melt-quench method, suggesting, incomplete amorphization³⁹². This is perhaps the reason for the observed increase in indomethacin dissolution rate with an increase in the input of ultrasonic energy, suggesting that higher energy input leads to a higher extent of amorphization³⁹².

When monitoring the USC process, Ueda et al.³⁹¹ noted changes in both the punch position and the die pressure with respect to the phase transition process of the polymer. Their study inferred that a sudden increase in the pressure on the lower punch can detect the optimum ultrasound energy for complete transformation. This is why the polymer as well as system T_g are important considerations during the USC process.

The primary advantages of using this method are the low quantities of material required and elimination of downstream processing. However, a crucial limitation of this method is the incomplete amorphization of the drug due to the lack of distributive mixing with ultrasonic energy. The product may show some inhomogeneity, and phase separation or presence of a small amount of crystalline phase which can be detrimental to the stability of an ASD system, as discussed in the earlier sections. For wider applicability of this method, more in-depth studies are required to understand the requisite physicochemical properties of the drug and the polymer, stability of the materials during and after processing, as well the method reproducibility³⁹⁹.

2.3. Others miscellaneous techniques

2.3.1. Granulation

Preparation of ASDs by the granulation approach enables the direct generation of ASD granules with a reduced number of manufacturing unit operations and minimized cost for investing in new equipment^400,401. Depending on the nature of the binder fluid, granulation can be categorized into solvent evaporation or melting process categories. Several granulation techniques can be utilized to produce ASDs including fluidized bed granulation, tumbling granulation, and mixing granulation^190,402, 403, 404, 405, 406, 407. Fig. 10 shows the commonly used high-speed granulator and the associated manufacturing variables affecting ASD properties.

Figure 10.

Preparation of ASDs using a high-speed mixer granulator. Molten drug–polymer mass can be added in the form of binder into the excipient mixture to form granules (melt agglomeration). Depending on the availability of suitable solvent system, alternate route includes using an organic feed solution as a binder fluid.

The principle of granulation for ASD preparation by solvent evaporation is the same as discussed in the fluidized bed technique section earlier (fluidized bed granulators). In this case, the binder usually consists of a drug dissolved in appropriate solvent(s) which is then added to the remaining excipients of the formulation. There are several examples in the literature which have utilized solvent evaporation-based granulation for the successful preparation of ASDs^{190,402,403,406,408,409}. In addition, if it is critical to avoid drug exposure to the organic and/or aqueous solvents, the process of melt agglomeration can be employed wherein a molten mass of the drug and the polymer serves as a binder for forming granules^405,410. This further reduces the drying steps associated with the solvent-based wet granulation process. Melt agglomeration also employs standard granulation equipment such as high shear granulators^411,412 and fluidized bed granulators^81,413. The melted drug-polymer mass serves as a binding liquid for granulation, ensuring sufficient homogeneity. Polymers for melt agglomeration can be liquids such as PEG 300 and caprylocaproyl macrogol-8 glycerides (Labrasol®) or semi-solids like stearoyl polyoxyl-32 glycerides (Gelucire® 50/13)^411,414. Solid polymers with low melting temperature/T_g, such as PEG (3000⁴¹² and 6000⁸¹ grades) and poloxamer 188^412,415 can also be used. However, similar to other melting processes, the use of high temperatures limits its application to thermally sensitive drugs. The limited choice in carriers is another drawback of this method because high T_g polymers are not suitable⁴⁰⁵.

A modified method of granulation is thermal adhesion granulation (TAG)⁴¹⁶. This process can be loosely considered as a combination of solvent- and melt-based processes. In TAG, little to no solvent is added to the drug and excipient mixture relative to the traditional wet granulation methods. The mixture is heated (30–130 °C) to promote the formation of the adhesive binder, and then mixed by tumble rotation until granules are formed. Drying is not needed in most instances because little to no solvent is added in the process. Following cooling, granules of the specific particle size can be collected by sieving. The method imparts good flow properties and binding ability to granules to form tablets with low friability and sufficient hardness⁴¹⁶. Lin et al.⁴¹⁷ converted hydrophilic polymers such as PVP and HPMC into the matrix materials while processing them with other diluents using TAG. Chen et al.⁴⁰⁴ developed cilostazol ASDs by TAG using two adsorbents dibasic calcium phosphate anhydrous (Fujicalin®) and microcrystalline cellulose (Microcell®) for granule formation.

However, a major drawback of TAG is that it is challenging to formulate tablets of high-dose drugs since it necessitates the use of a larger amount of other excipients for inert core⁴¹⁸. To overcome this challenge, Theismann et al.⁴¹⁹ developed an alternative process of spray granulation capable of preparing high drug-loading (80%, w/w) nicotinamide (NAM) granules by wet extrusion and spheronization. However, it would be a challenge to employ this method for stable ASD preparation due to the higher extent of water/solvent exposure. Continuous-spray granulation (CSG) may overcome the above limitations in solvent exposure⁴²⁰. This is a one-step method generating granules from solution or suspension. In the CSG method, the solution or suspension containing the drug and polymer (and possibly other excipients) is spray-dried generating small particles. These are then further layered by continuous spray. A typical set-up for CSG is a kind of a combination of spray dryer and fluid bed granulator. It consists of a two-fluid spray nozzle which is placed vertically at the bottom of the drying chamber. Additional air nozzles are present on the inside wall of the drying chamber which remove adhered powder off and maintain a continuous flowable state of the particles during drying/layering. Recently, Tanaka et al.⁴²⁰ prepared ASD granules of rebamipide and PVPVA using a continuous-spray granulator. Dense and smooth granules with satisfactory physical stability (20 °C/75% RH for up to 6 months) and improved dissolution properties relative to the pure crystalline drug were obtained with the optimized polymer concentration.

2.3.2. Co-precipitation

Antisolvent co-precipitation is another widely employed approach to produce ASDs^44,421,422 at both small-scale423, 424, 425, 426, 427, 428 and industrial-scale¹⁸. Compared to the spray-dried ASDs, the ASDs produced by the solvent exchange process have a larger particle size and often do not need further densification. These result in a superior compaction profile, limiting the need for added excipients such as compaction aids¹²³. In addition, it is possible to maximize drug loading as high as 70% for certain compounds¹²³.

Co-precipitation amorphize drugs by the rapid precipitation of the ASD in the anti-solvent. The low processing temperatures circumvent the stability issues for the thermal-liable drugs. The co-precipitation process, as shown in Fig. 11, begins with the dissolution of the drug and an ionic polymer in the nonvolatile solvents that are usually dimethylacetamide, dimethylformamide, or dimethylsulfoxide. The solution is then transferred gradually into a pH-controlled and chilled anti-solvent (usually water). It is crucial that both the drug and the polymer are insoluble in anti-solvent and that the common solvent is miscible with the anti-solvent to cause rapid co-precipitation⁴²³. An additional condition for the anti-solvent environment is that in the precipitated suspension state, the drug-polymer ASD remains in an amorphous state, i.e., the T_g of the amorphous composite is above the precipitation temperature as plasticized by the anti-solvent environment. After the controlled co-precipitation process, the precipitated wet solid mass can still contain a relatively significant amount of the organic solvent, which needs further removal. The organic solvent is generally removed by washing the solid precipitate with appropriate aqueous solvent until the residual organic solvent is <0.1%, w/w¹⁰. The washing is followed by a drying process to remove the aqueous solvent from the precipitate with the forced-air oven or fluid bed dryer.

Figure 11.

Schematic of co-precipitation process to prepare ASDs.

Commonly used polymers for this process are HPMCAS (L, M, H grades), cellulose acetate phthalate (CAP), cellulose acetate butyrate, polyvinyl phthalate, hypromellose phthalate (HPMCP), and polymethacrylates derivatives/Eudragit® (L100–55, L100, S-100, and EPO grades)¹²³. The use of low temperature, low solvent/antisolvent ratio, and adequate shear helps in improving the operational efficiency of co-precipitation. The time required for drug transition between the two solvents must be less than that associated with drug diffusion and crystallization in the solvent. The method therefore can benefit from agitation which rapidly mixes the solvent and antisolvent⁴²⁹. Rotor-stator devices are often incorporated into the co-precipitation process for adequate mixing. The introduction of higher shear often provides greater volumetric efficiency and enables the formation of more homogenous products⁴²⁹. Other process variables that need to be monitored and optimized for preparing a stable ASD are shown in Fig. 12.

Figure 12.

Co-precipitation and its processing variables for preparing ASDs.

In order to generate ASD, many variations of solvent-mediated precipitation have been explored, e.g., evaporative aqueous solution precipitation (EPAS), flash nanoprecipitation, and controlled precipitation. In EPAS, the drug–polymer solution is atomized into a heated aqueous solution, where the heated antisolvent evaporates the solvent (generally dichloromethane)⁴³⁰. However, this method is limited to low boiling solvents such as dichloromethane that can be readily evaporated due to the use of heated aqueous fluid as an antisolvent. Also, since this process occurs at elevated temperatures, it may not be ideal for ASD stability, particularly for the thermal-liable compounds. A modification to the EPAS process is controlled precipitation, which involves in-line extraction of solvent by vacuum distillation⁴³¹. The controlled precipitation method also utilizes solvents with low boiling points such as methanol. Mann et al.⁴³² used a slightly altered co-precipitation system in which nonionic polymers were used and the precipitation was induced by organic antisolvents as opposed to pH modified aqueous antisolvents. The integration of surfactants into these ASDs was also possible due to the use of aqueous solvents. Moreover, using solvents and antisolvents with relatively lower boiling points makes the downstream isolation and drying of the precipitates easier.

A major limitation of the co-precipitation method is that due to pH-dependent solubility and stability, some pH sensitive compounds may not have sufficient time for adequate precipitation in the anti-solvent. Furthermore, heat and moisture may cause stability concerns during the washing and drying processes.

2.3.3. Milling-based methods

On a laboratory-scale, milling/cryogrinding has been employed for preparing ASDs of certain compounds433, 434, 435, 436, 437, 438, 439. The reduction in particle size has long been known to decrease crystallinity and achieve amorphization440, 441, 442. A partially amorphous anti-inflammatory product has been manufactured using SoluMatrix®, a technique that entails dry milling the crystalline drug with a hydrophilic carrier⁴⁴³. However, the method is not widely popular in the pharmaceutical industry⁴³³ as there is often a risk of residual crystallinity, which can act as a seed and induce nucleation/crystallization during the shelf life, even with some stabilizers (e.g., magnesium aluminometasilicate, crospovidone, sugar)^441,442.

3. Downstream processing

It is clear from the discussion so far that variations in manufacturing techniques have a substantial impact on ASD properties such as particle size, particle porosity, and density, flowability, tabletability, and stability. However, it is important to note that these properties are not only linked with the manufacturing process but also to the formulation composition. Obviously, even slight changes in the process parameters and formulation can alter these physicochemical properties. During the development of a specific ASD product, the process parameters need to be carefully evaluated and optimized to obtain an optimal product. In many cases, downstream processing is necessary to obtain such desired properties.

During the downstream processing of ASDs, there is often a risk of drug crystallization, especially during operations that expose the ASDs to moisture, thermal or mechanical stress^{59,60,68,386,444,445}. The crystallization of a drug from the ASD during transport and storage due to the mechanical activation by grinding, crushing, or even scratching has also been discussed by Német et al⁶⁸. Therefore, ideal ASDs require minimal downstream unit operations. However, in reality, certain downstream processes might be inevitable. Depending on the manufacturing method, the ASDs obtained can be in the form of a fine powder, granule, extrudate, or fiber. Processes like spray drying, electrospraying, super-critical fluid technology, cryogrinding, and milling usually generate fine ASD powders that require further densification (or granulation) to enhance their density and flow properties for efficiently manufacturing their final dosage forms446, 447, 448. For ASDs, dry granulation and direct compaction are usually preferred over wet granulation due to the moisture sensitivity of ASDs. Fu et al.⁴⁴⁹ observed that the amorphous drug is more inclined to crystallize in a tablet prepared from a wet granulated method than in tablets prepared by direct compression. Leane et al.⁴⁴⁴ also noted a significantly higher crystallinity in the tablets with roller-compacted granules, in contrast to directly compressed tablets under accelerated storage conditions (40 °C/75% RH).

HME generates extrudates (usually spaghetti-shaped) while processes like electrospinning and rotating-jet spinning result in ASD fibers, which need to be milled into granules. The granules of ASDs can be further combined with additives such as disintegrants, flow aids, compression aids, lubricants, etc. for efficient encapsulation or tableting⁴⁵⁰. HME granules, however, usually have limited compaction properties due to the reduced porosity of the extrudates and the ductile properties of the polymers used. Recently, two types of post-die melt extrudate processing milling techniques were evaluated for itraconazole-HPMC ASDs, viz. hammer mill and air classifier mill⁴⁵⁰. The product was prepared by using the hammer mill melted at higher mill loadings. In contrast, this problem did not emerge when an air classifier mill was used. The addition of an elastic substance (e.g., carrageenan or polyglycolic glyceride) has been shown to prevent an amorphous drug from crystallization during compression and storage^59,451, possibly due to the cushioning effect of elastic materials. However, the addition of elastic substances during compression can generate other undesirable effects on product properties and performance such as physicochemical stability, dissolution, T_g, hygroscopicity, or appearance.

Large surface area can contribute to the lower stability of amorphous drug because T_g of ASD is typically lower on the surface than in the bulk particle region^452,453. Increased crystallization rates are often noticed on the particle surface because of greater molecular mobility⁴⁵⁴. Coating celecoxib-PVP ASDs prepared by the fluidized bed (Wurster) process with coating excipients (PVA, inulin, and polyvinyl acetate were tested) formed a barrier layer⁴⁵⁵. This led to lower molecular mobility at the surface and increased protection against moisture. Fig. 13 shows a schematic of possible downstream processes to prepare a solid dosage form of an ASD (tablet/capsule).

Figure 13.

A schematic of downstream processing routes to prepare ASD solid dosage forms.

4. Selection criteria of an ASD manufacturing method

Besides the drug thermal stability and solubility, other important considerations for selecting a manufacturing method are batch size (scale), and equipment availability. If the drug can melt <150 °C without degradation, melting methods can be considered. Amongst melting methods, if the purpose of preparation is screening, even DSC pans can be used for ASD preparation by melt-quenching. For a larger laboratory-scale batch, melt-quenching by using a hot-plate/oil bath can be considered. However, a molten mass with a viscosity of >300 cP may make ASD processing with these methods challenging. In such cases, HME or solvent evaporation techniques can be considered.

The key factors to consider when choosing a solvent evaporation-based manufacturing method are the properties of the organic solvents used to solubilize the drug and polymer(s). For a screening study, if the boiling point of the solvent/s is < 50 °C, the solvent cast method can be used. For solvent/s boiling at higher temperatures or for a larger batch, rotavapor or spray drying can be the choice. For intermediate or industrial-scale production, the choice of processes for ASD preparation is limited. This is because, at a larger scale, additional criteria such as the process efficiency, process yield, particle properties, and if applicable, solvent toxicity, environmental impact, the safety of the operator, and flammability/explosion risk need to be considered. Based on formulation properties, a straightforward method selection decision tree for preparing ASDs is shown in Fig. 14.

Figure 14.

A method selection decision tree with the commonly used manufacturing processes for preparing ASDs³⁰.

5. Conclusions

A successful development of amorphous solid dispersion formulations depends on three primary factors: active pharmaceutical ingredient properties, stabilizing polymer, and processing technology. Polymers provide the basic and essential foundation for a stable drug amorphization and the process supplies the energy required to transform the system to an amorphous form. The effectiveness of the process is critical to generate, capture, and preserve the amorphous form. The success of these processes is dependent on the processing time and the supersaturation conditions that are being generated during the formation of the solid dispersion.

Despite the discovery of solid dispersions in the early 1960s, the application of the solid dispersion concept to solve solubility challenges had been limited for several decades, partly due to the lack of commercially viable processing technologies. However, the past two decades have seen remarkable progress in developing pharmaceutical ASD products as our understanding of ASD systems and their manufacturing technologies have evolved considerably, leading to several commercial products in addition to numerous in development. Spray drying and HME have become the mainstay of ASD preparation in the pharmaceutical industry, while newer methods are constantly being added into the toolbox that promise to improve the quality, productivity, and/or better performance of the products.

Acknowledgments

This work was supported, in whole or in part, by the Bill & Melinda Gates Foundation [No.OPP1159809, USA]. Qi (Tony) Zhou and Sonal Bhujbal are also supported by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health under Award (No. R01AI132681, USA). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of Health. The authors would like to thank Bristol Myers Squibb and Oral Product Development for supporting this work as a part of Sonal Bhujbal's summer internship. The Bristol Myers Squibb authors acknowledge that one or more patent applications, related to the work disclosed in this paper, may be planned, pending, and/or has been granted.

Author contributions

Sonal V. Bhujbal, Qi (Tony) Zhou, Sumit Kumar, Biplob Mitra, and Uday Jain were responsible for the conception and design of the review. Sonal Bhujbal wrote the manuscript. Tony (Qi) Zhou, Sumit Kumar, Biplob Mitra, Uday Jain, Yuchuan Gong, Lynne S. Taylor, Anjali Agrawal, and Shyam Karki supervised and revised the manuscript. All of the authors have read and approved the final manuscript.

Conflicts of interest

The authors have no conflicts of interest to declare.