Stable amorphous solid dispersions of fenofibrate using hot melt extrusion technology: Effect of formulation and process parameters for a low glass transition temperature drug

Abstract

Development of stable amorphous solid dispersions (ASDs) for a low glass transition temperature (Tg) drug is a challenging task. The physico-chemical properties of the drug and excipients play a critical role in developing stable ASDs. In this study, ASDs of poorly soluble fenofibrate, a drug with a low Tg, were formulated using hydroxy propyl methylcellulose acetate succinate (HPMCAS) via hot melt extrusion (HME). The feasible processing conditions were established at varying drug loads and processing temperatures. The prepared ASDs were characterized for crystallinity using differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD). Fourier transform-infrared spectroscopy was performed to study the potential interactions. DSC and PXRD studies confirmed the amorphous state of fenofibrate in the prepared ASDs. A discriminative in vitro dissolution method was established to study the impact of HPMCAS grades on dissolution profile. The dissolution parameters such as dissolution efficiency, initial dissolution rate and mean dissolution rate, suggested improved dissolution characteristics compared to pure fenofibrate. Accelerated stability studies at 40 °C/75% RH showed preservation of the amorphous nature of fenofibrate in formulations with 15% drug load and in vitro drug release studies indicated similar release profiles (f2 >50). This study provides an insight into the formulation and processing of ASDs for poorly soluble drugs with low Tg.

1. Introduction

One of the biggest challenges faced by pharmaceutical companies is the production of stable, effective, and economic dosage forms for molecules with poor aqueous solubility. Over 50% of new drug molecules continuously discovered through high throughput screening and combinatorial chemistry, are crystalline and hydrophobic in nature [1 ]. Drugs with poor gastrointestinal fluid solubility have poor bioavailability. Various solubilization techniques have therefore been developed as potential solutions, based on approaches like prodrugs [2], complexation [3], micellar systems like self-emulsifying drug delivery systems (SEDDS) [4,5], salt formation for ionizable molecules [6], size reduced systems such as nano-crystalline forms [7], and amorphous solid dispersions (ASDs) [8]. The formation of ASDs is one of the reliable, robust, and reproducible techniques for improving solubility and bioavailability [1]. This technique involves dispersing the poorly soluble drug into a polymeric matrix, preferably at the molecular level [9].

ASDs can be prepared by employing either the solvent based techniques or hot melt technique. In solvent dependent methods, the drug and polymer are dissolved in a common organic solvent followed by rapid solvent removal using an anti-solvent or spray drying [10,11]. The hot melt extrusion (HME) technique involves mixing the drug and polymer in a molten state, then shearing and extruding the mixture into different shapes [12]. HME is preferred to solvent techniques as it is continuous, economical, and solventfree [8,13].

HPMCAS is an anionic polymer with a high Tg (120 °C) and HME-suitable melt viscosity. It is also capable of improving the solubility of poorly soluble drugs and maintaining their super saturation levels in gastrointestinal fluids over a long period [14]. Moreover, HPMCAS constrains the nucleation and crystal growth upon storage and inhibits recrystallization after super saturation state in the gastrointestinal fluid [15,16]. The choice of HPMCAS to prepare ASDs via the HME process is a good strategy because of the reported polymer stability at higher temperatures and shear conditions [17]. HPMCAS as a cellulosic polymer contains 4 types of substitutions on the hydroxyls: methoxy, with a mass proportion of 12–28 w %; hydroxypropyl, with a mass proportion of 4–23 w %; succinate, with a mass proportion of 4–28 w %; and acetate, with a mass proportion of 2–16 w % [18]. Apart from its high Tg, HPMCAS has the unique ability to exist in an un-ionized state. The high Tg is the controlling factor in reducing the drug mobility leading to excellent physical stability of ASDs. Even in its partially ionized state also it minimizes the formation of big polymeric aggregates ensuring the stability of drug-polymer colloidal structures. HPMCAS is amphiphilic, allows the interactions between insoluble drugs and its hydrophobic portions, while the hydrophilic portions contribute to the stability of these colloids in aqueous solutions [19].

In ASDs, API crystallization is prevented by carriers that reduce API molecular mobility by raising the energy barrier required for phase transition (crystallization) [20,21]. The temperature for final API crystal dissolution in the carrier (at a specific ratio of API and polymer) is called the solubility temperature, below which the system is over saturated with API and considered unstable. In this condition, API rich domains tend to recrystallize over a period of time [22]. The drug and polymer ratio is therefore crucial to formulate stable ASDs, especially at specific temperature and humidity conditions [23]. Recently, a few reports have been made on improvement of bioavailability [24] stabilizing such formulations and crystal growth patterns [25].

Fenofibrate, is a prodrug of fenofibric acid, which is used in cholesterolemia, hypertriglyceridemia, mixed dislipidmia as it can lower the triglyceride levels and low density lipoprotein cholesterol levels. Fenofibrate is a BCS class II, neutral lipophilic and water insoluble molecule (log P 5.2) with oral bioavailability of 30% in humans [26]. This study was aimed to develop stable ASDs of fenofibrate which has a low Tg, and confirm the effect of drug load and polymer grade on recrystallization behavior on storage. In the case of ASDs there is a possibility of polymer and drug phase separation due to reduced viscosity and molecular mobility. This phase separation kinetics depends on the resultant Tg of the ASD and storage conditions [27,28]. The process feasibility was verified using 3 commercial grades of HPMCAS i.e. LG, MG, and HG. The feasibility of the extrusion process was established over a wide range of process temperatures and at different drug loads. A discriminative dissolution media was developed to observe the difference between the in-vitro performance of formulations of different grades of the polymer. The effect of proportion of the components on resultant Tg of ASDs was observed and its impact on the stability of ASDs under accelerated conditions was also investigated. This novel study provides valuable insights on the stability of formulations containing drugs with low Tg’s and possible phase transitions during accelerated testing.

2. Materials and methods

2.1. Materials

HPMCAS (Aquasolve®) LG, MG, and HG grades, and fenofibrate (API) were kindly donated (Ashland Inc., Lexington, KY, USA). Gelatin capsules were purchased from Total pharmacy supply (Arlington, TX, USA). All other chemicals used were of analytical grade.

2.2. Preformulation studies

2.2.1. Solubility determination

Fenofibrate solubility studies were performed in purified water, pH 6.8 phosphate buffer, 0.05 M sodium lauryl sulfate (SLS) in purified water, and 0.05 M SLS in pH 6.8 phosphate buffer. Excess fenofibrate was added to 10 mL of each medium and shaken (Taitec Bioshaker., Saitama-ken, Japan) for 48 h at the speed of 100 rpm at 37 °C. The samples were centrifuged (Eppendorf., NY, USA) at 15000 rpm for 15 min and the supernatant filtered and diluted with the respective medium. The quantity of fenofibrate was estimated using a UV–Vis spectrophotometer (Genesys 6; Thermo Scientific., USA) at a maximum absorption wavelength of 288 nm. All experiments were performed in triplicate.

2.2.2. Thermal analysis

Differential scanning calorimetry (DSC) analysis studies were performed to determine the melting point (Tm) and glass transition temperatures (Tg) of fenofibrate and Tg of HPMCAS LG, MG, and HG. DSC was carried out using Discovery the DSC-25 cartridge (TA Instruments, New Castle, DE, USA), coupled with an RCS90 cooling device. Prior to use, the instrument was calibrated for temperature and heat capacity using the indium and sapphire standards. Approximately 5 mg of each sample was accurately weighed and sealed in an aluminum pan. An empty aluminum pan was used as a reference. The samples were equilibrated under nitrogen gas for 1 min at 25 °C and then heated to 120 °C for fenofibrate and 220 °C for HPMCAS polymer at a heating rate of 10°C/min under an inert nitrogen purge of 50 mL/min. A heat-cool-heat cycle was performed to determine the Tg of HPMCAS. The thermograms were analyzed to detect the melting point of API and Tg of different grades of the polymer.

2.2.3. Extrusion of pure polymer

About 100 g of HPMCAS (LG, MG, and HG grades) was sieved through #25 mesh ASTM. The different pure polymer grades were extruded on an 11 mm co-rotating twin-screw extruder (Thermo Fisher Scientific, Waltham, MA, USA) using a Thermo Fisher Scientific standard screw configuration. The screw configuration consists of the 3 mixing zones with first mixing zone with elements making angles 30°–90° with each other followed by relatively shorter second mixing zone with elements making an angle of 60° with each other. The third mixing zone contains elements making angles 60° and 90° with each other to assist the dispersive action. A feed rate of 4–5 g/min was maintained with the aid of a gravimetric feeder and screw speed was 50 rpm.

2.3. Preparation of fenofibrate ASDs

2.3.1. Preparation of the physical mixture

Fenofibrate and HPMCAS were sieved through a #25 mesh ASTM and used for preparing the physical mixtures for extrusion. Binary mixtures of fenofibrate with different HPMCAS (LG, MG, and HG) grades were prepared at drug loads of 15 and 30% w/w. Blending was performed on a V-shell blender (Maxiblend; Globe- Pharma, New Jersey, USA) at 20 rpm for 15 min.

2.3.2. Melt extrusion process

The prepared physical mixtures were extruded in the temperature range of 90–160 °C using an 11mm twin-screw extruder (Process 11; Thermo Fisher Scientific). A thermo standard screw configuration consisting of 3 mixing zones was used. Zones 2–8 and the die were maintained at the same temperature while the feeding zone was kept at ambient temperature. Different process temperatures were selected for checking the feasibility of the extrusion process. Three types of process temperatures were selected i.e. just above the melting point (Tm) of drug (90 °C), just above the Tg of the polymer (130 °C), and 40 °C above the Tg of the polymer (160 °C). The process screw speed was 50rpm, and feed rate was 3.5–4.0g/min. The resulting extrudates were allowed to cool at ambient temperature. The extrudates were then milled using a laboratory mixer, passed through a #30 ASTM mesh and stored in an air tight HDPE container at ambient conditions till further use.

2.4. Fourier transform-infrared spectroscopy (FTIR)

FTIR studies were performed using Agilent Cary 660 FTIR Spectrometer (Agilent Technologies, Santa Clara, CA, USA) in a 4000–600 cm⁻¹ spectral range. The analysis was carried out to examine interactions between the different polymer grades, with API in the physical mixture and the extrudates. The bench equipment was equipped with an ATR (Pike Technologies MIRacle ATR, Madison, WI, USA), arranged with a single-bounce, diamond-coated ZnSe internal reflection element.

2.5. Solid state characterizations

Solid State Characterization was performed to detect the crystalline nature of the drug and to confirm the formation of ASDs by HME.

2.5.1. DSC studies

DSC analysis was performed for the extrudates (Discovery DSC25; TA Instruments, New Castle, DE, USA) via a heat-cool-heat cycle. In the first heating cycle, the samples were heated to 175 °C at a heating ramp of 10°C/min followed by cooling to −40 °C. Finally, the samples were heated to 130 °C at a heating rate of 1 °C/ min to identify the Tg of the extrudates.

2.5.2. Powder X-ray diffraction measurement (PXRD)

PXRD studies were performed for fenofibrate and milled extrudates using the Rigaku X-ray system (D/MAX-2500PC, Rigaku Corporation, Tokyo, Japan) equipped with a copper tube anode and a standard sample holder. The diffraction measurements were performed under the following conditions: CuKa radiation, 40 kV voltage, and 40 mA current. The 20 scanning range was 2–50°, with a step width of 0.02°/S at a scanning speed of 2°/min. Samples placed on a sample holder were gently compressed with a clean metal bar and diffractograms were collected at room temperature (20–25 °C).

2.6. In vitro drug release studies

In vitro drug release studies were performed for extrudates and pure API (dose equivalent to 67 mg) filled in 00 size capsules using sinkers. The study was performed using USP apparatus type II (SR8- plus™; Hanson, CA, USA) at a paddle speed of 50 rpm and 37 ± 0.5 °C for 2 h (n = 6). The medium used for this study was a pH phosphate buffer +0.005 M SLS and the volume of the media was 1000 mL. Sample aliquots (3 mL) were collected at 15, 30, 45, 60, 90 and 120 min, and an equivalent volume of fresh medium maintained at 37 ± 0.5 °C was replaced. The collected samples were filtered through a 0.45 mm membrane (Durapore®; Millpor Sigma, MA, USA) filter, diluted suitably, and estimated for fenofibrate content at 288 nm using a UV–visible spectrophotometer (Genesys 6; Thermo Scientific., USA).

2.7. Stability studies

The milled formulations passed through a #30 ASTM mesh were placed in HDPE containers. lg of desiccant was placed in each container and the container was closed with lid. Then the formulations were subjected to accelerated stability studies for 45 days at 40 °C/75% RH using a stability chamber (Caron, 6030). Samples were observed for physical appearance and analyzed for crystallinity, drug content, and in vitro drug release. The similarity factor f₂) for drug release was calculated using the following equation:

$$f_2=50\log \left\{{\left[1+\left(\frac{1}{n}\right)\sum _{t=1}^n{\left(R_t-T_t\right)}^2\right]}^{-1/2}\times 100\right\}$$ (1)

where.

R_t = cumulative drug release of initial samples,

T_t = cumulative release of the test sample at predetermined time points,

N = number of time points

An f₂ value of >50 indicates the similarity between the samples.

2.8. Statistical analysis

The statistical analysis of the in-vitro drug release data was performed by using GraphPad Prism (Version 8.1.1, GraphPad Software Inc., La Jolla, CA, USA). Unpaired t-test was performed and values of p < 0.05 the differences were considered statistically significant.

3. Results and discussion

3.1. Preformulation studies

3.1.1. Solubility studies

Fenofibrate is reported to have poor aqueous solubility with a log P value of 5.2 [26]. Hence, solubility studies need to be conducted using a dissolution medium that can provide sink conditions. Given that fenofibrate has no ionizable groups, its solubility in aqueous media is low, and it therefore requires a surfactant to ensure solubility in the aqueous medium [29]. Fenofibrate exhibited much higher solubility in the aqueous medium in the presence of surfactants. SLS was included in the medium because the official USP 40 medium for fenofibrate dissolution is 0.05 M SLS in water. Moreover, HPMCAS is an enteric pH soluble polymer; hence, the 6.8 phosphate buffer was combined with SLS. Fenofibrate showed poor solubility in pH 6.8 phosphate buffer (0.097 ± 0.004 mg/L) and purified water (0.284 ± 0.042 mg/L). However, the addition of SLS increased the solubility in water and pH 6.8 phosphate buffer as the solubility values in 0.05 M SLS in water and 0.05 M SLS in pH 6.8 phosphate buffer were 573.619 ± 16.54 mg/L and 490.432 ± 9.45 mg/L, respectively. Hence, solubility studies confirmed that a single dose of 67 mg of fenofibrate (67 mg is one of the marketed doses of fenofibrate) would require a suitable medium to test the dissolution rate.

3.1.2. Thermal analysis

After DSC analysis, fenofibrate showed an endothermic peak at 82.4 °C, corresponding to its melting point. In the heat-cool-heat cycle, fenofibrate showed a Tg of −19.9 °C (Fig. 1A and B). HPMCAS LG, MG, and HG grades had no thermal transitions in the 25–220 °C range during the heating cycle, confirming the amorphous nature of the HPMCAS polymer. However, in the heat-cool-heat cycle, a transition was observed at the 119–122 °C range, which corresponds to the Tg of the polymer grades (Fig. 1C).

Fig. 1.

A) M.P of fenofibrate B) Glass transition temperature of fenofibrate C) Glass transition temperature of different grades of HPMCAS.

3.1.3. Pure polymer extrusion

Pure polymer extrusion indicated that all 3 polymer grades were extrudable in the 160–200 °C range. Based on our results, the ease of extrusion of pure polymers was in the order HG > MG > LG. At temperatures <180 °C the extrudates appeared light yellow/light brown in color whereas, at >180 °C, the MG and HG grades were observed to be darker in appearance than the LG grade extrudates. The LG grade extrudates appeared dark when the processing temperature was >190 °C. This indicates the relatively greater thermal stability of the LG grade compared with the MG and HG grades. These observations are in accordance with previous reports where the change in polymer physicochemical properties may be attributed to the release of free succinic acid and acetic acid [17,30,31]. The higher temperature associated with greater torque may have led to this change. Based on these observations and previous literature reports, 160°C was selected as the maximum processing temperature for the extrusion process.

3.2. Effect of drug load and processing conditions on the preparation of fenofibrate ASDs

Fenofibrate ASDs formulations were prepared using API loads of 15 and 30% (w/w). The extrusion was carried out at temperatures of 90 °C, 130 °C and 160 °C. Process temperatures were selected based on API melting point and polymer Tg. The main aim of selecting the range of process temperatures was to examine the feasibility of preparation of fenofibrate ASDs with HPMCAS. The process parameters are presented in Table 1.

Table 1.

Process parameters of hot melt extrusion process.

Run no	% Drug load	HPMCAS grade	Process temperature (C°)	Screw RPM	% Torque	Appearance of extrudates
1	15	LG	90	50	>100	N/A
2	15	MG	90	50	>100	N/A
3	15	HG	90	50	>100	N/A
4	15	LG	130	50	54–60	Yellowish smooth strands
5	15	MG	130	50	52–57	Yellowish smooth strands
6	15	HG	130	50	60–63	Yellowish smooth strands
7	15	LG	160	50	26–33	Cream color smooth strands
8	15	MG	160	50	31–33	Cream color smooth strands
9	15	HG	160	50	32–35	Cream color smooth strands
10	30	LG	90	50	81–88	Yellowish smooth strands
11	30	MG	90	50	79–85	Yellowish smooth strands
12	30	HG	90	50	76–84	Yellowish smooth strands
13	30	LG	130	50	29–32	Yellowish smooth strands
14	30	MG	130	50	28–33	Yellowish smooth strands
15	30	HG	130	50	26–28	Yellowish smooth strands
16	30	LG	160	50	13–16	Cream color smooth strands
17	30	MG	160	50	14–17	Cream color smooth strands
18	30	HG	160	50	16–19	Cream color smooth strands

Based on the process parameters and data from pure polymer extrusion, fenofibrate clearly showed a plasticizer effect on the extrusion process provided the feasibility of extrusion at 130 °C. In the trials of lower (15%) fenofibrate load, at a processing temperature (at 90 °C) below polymer Tg, were not successful, indicating that the proportion of fenofibrate was insufficient to exhibit the required plasticizing effect for the extrusion of the product. However, formulations with 30% drug load at 90 °C had successful extrusion but resulted in higher torque. This may be attributed to the higher Tg (199–122 °C) of HPMCAS polymers. Therefore, trials conducted at 90 °C were considered practically non-feasible and hence not considered for further studies.

3.3. Solid state characterizations of ASDs

3.3.1. DSC studies

The thermal characteristics are shown in Figs. 2 and 3. DSC thermograms exhibited a melting point of 82.4 °C for fenofibrate, while all the formulations with different drug loads showed no signs of crystallinity, indicating successful formation of ASDs (Fig. 2).

Fig. 2.

DSC thermograms of fenofibrate extrudates with A) HPMCAS LG B) HPMCAS MG C)HPMCAS HG.

Fig. 3.

Glass transition temperature of I) Extrudates of 30% drug load and II) Extrudates of 15% drug load: A) LG extrudates B) MG extrudates C) HG extrudates.

Further ASDs were analyzed using DSC at a lower rate exhibited a single Tg, confirming the molecular level dispersions of the drug in the polymer (Fig. 3). Given that the drug and polymer have different Tg values, their resulting ASDs had different Tg values from the pure drug and polymer. According to the Gordan-Taylor equation, the resultant Tg depends on the Tg of individual components [32].

The formulation Tg was dependent on drug load concentration, and a higher drug load resulted in a lower overall Tg because the drug’s Tg was much lower (−19.9 °C) than that of the polymer (119–122 °C) (Fig. 1B and C) [19]. This may be the reason why Tg was lowered with increased drug plasticizing effect [33]. The formulations with 30% drug showed a Tg of 34–39 °C (Fig. 3 I), and that with 15% drug load showed a Tg of 70–80 °C, depending on the grade of the polymer used (Fig. 3 II). The Tg of formulations did not vary much with the polymer grade used. This is evident from the relatively similar Tg (119–122 °C) for all 3 polymer grades. These findings are in accordance with previous reports [14].

3.3.2. PXRD analysis

In PXRD analysis, the crystalline molecule fenofibrate showed characteristic peaks at 12.1°,14.5°,16.9°, 21.0°, 22.4°, 24.2°, and 24.9° (Fig. 4). The prepared ASDs exhibited no characteristic peaks of API, confirming the amorphous nature of ASDs. These results support the DSC results and infer that the API-polymers were miscible at molecular levels. They reason for this amorphous nature may also be attributed to the cellulosic backbone of the polymer which prevents interactions leading to recrystallization [34].

Fig. 4.

XRD diffractograms of fenofibrate extrudates with all grades of HPMCAS processed at different temperatures.

3.4. FTIR studies

The FTIR spectra of the fenofibrate, physical mixtures, and formulations are presented in Fig. 5. The fenofibrate FTIR spectra showed characteristic peaks at 1725 cm⁻¹ (indicating a C=O stretching of the ester group), 2984 and 2934 cm⁻¹ (represented the benzene ring; aromatic stretching), 165ûcm⁻¹(C=O of the ketone group), 1598 cm⁻¹ (lactone carbonyl functional group), and 1303 cm⁻¹ (ether group) [35,36]. The physical mixtures and formulations showed characteristic absorption bands of fenofibrate, indicating the absence of interaction between fenofibrate and the polymers studied.

Fig. 5.

FTIR spectra of fenofibrate and extrudates of different grades of HPMCAS.

3.5. In vitro drug release studies

3.5.1. Establishing a discriminating medium for dissolution

Fenofibrate is a drug with no ionizable groups, hence there is a need to develop a suitable medium for in-vitro drug release testing. Employing surfactants is one of the approaches to mimic GI tract conditions and proper surfactant concentration is necessary to test drug release and discriminate between the formulations [29]. The dissolution medium was developed based on the USP dissolution method for fenofibrate (0.05 M SLS + water) and pH dependent solubility of HPMCAS. To develop the discriminating medium, formulations containing higher drug loads (30%) processed at 160 °C were selected and the dissolution testing was done in pH 6.8 phosphate buffer, with different concentrations of SLS and different paddle speeds (50 and 75 rpm). With pH 6.8 phosphate buffer +0.05 M SLS medium there was no discrimination observed between the drug release profiles of extrudates of different polymer grades (Fig. 6A). This is probably due to the greater solubility of fenofibrate in the SLS medium, overshadowing the effect of polymer grades on solubility enhancement. Based on these results the concentration of SLS was reduced to develop the discriminating effect between the formulations of different grades of HPMCAS. pH phosphate buffer +0.005 M SLS medium with a paddle speed of 50 rpm was selected because of its discriminating effect on the dissolution of ASDs prepared with different polymer grades (Fig. 6B).

Fig. 6.

In-vitro drug release profiles of fenofibrate ASD in A) pH 6.8 phosphate buffer +0.05 M SLS B) pH 6.8 phosphate buffer +0.005 M SLS (mean ± SD; n = 6).

3.5.2. In vitro drug release studies in 0.005 M SLS + pH 6.8 phosphate buffer

The in vitro drug release studies done in pH 6.8 phosphate buffer +0.005 M SLS showed discrimination between the release profiles of fenofibrate and HPMCAS ASDs. In this study, the ASDs drug release profiles at different drug loads and processing temperature showed significant improvement (p < 0.05), compared with the drug release profiles of the pure drug (Fig. 7). The summary of dissolution parameters was represented in Table 2.

Fig. 7.

In-vitro drug release profiles of fenofibrate ASD in pH 6.8 phosphate buffer 0.005M SLS (mean ± SD; n = 6).

Table 2.

Summary of dissolution parameters of API and formulations (mean ± SD; n = 6); D/PG denotes % drug load and polymer grade; Q₁₅ and Q₆₀% drug released in 15 and 60 min; DE₁₅ and DE_{60 are} dissolution efficiency at 30 and 60 min; t_50% time taken (minutes) to release 50% of the drug; MDR and IDR are mean dissolution rate and initial dissolution rate respectively.

Run	D/PG	Q15	Q60	DE15	DE60	MDR	IDR	t50% (minutes)
Fenofibrate (pure drug)	N/A	0.74	1.08	0.37	0.79	0.02	0.05	N/A
7	15/LG	36.49	98.61	18.24	58.73	1.15	2.43	30
16	30/LG	40.23	67.62	20.11	46.56	1.08	2.68	30
8	15/MG	41.55	92.36	20.77	56.28	1.19	2.77	30
17	30/MG	34.58	75.69	17.49	45.44	1.01	2.31	45
9	15/HG	14.53	30.25	7.26	17.79	0.42	0.97	N/A
18	30/HG	4.59	15.10	2.29	7.48	0.19	0.31	N/A

These ASDs added to aqueous medium may be in the form of a) free or solvated drug, b) free or solvated polymer, c) polymeric colloids, d) amorphous drug-polymer nano structures, e) large amorphous structures, and f) small drug-polymer nano aggregates (20–300 nm), depending on their size and composition [19]. Introduction of ASDs into the aqueous medium enhances free drug concentration and maintains long-term free drug concentration, to facilitate drug absorption and prevent drug recrystallization. According to previous reports on the rapid formation of drug-polymer nano structures, high energy amorphous forms improve drug solubility and absorption. The main mechanism reported for HPMCAS based ASDs was the formation of nano aggregates which enhance dissolution and prevent precipitation of high energy amorphous forms [37].

In in vitro drug release studies, the formulations of the LG and MG grade showed a 60–100% release within 1 h of the study (Fig. 7A and B). In both LG and MG grade polymers, the effect of drug load and processing temperature on dissolution was observed. The effect of drug load was further evident from dissolution efficiency (DE60) and mean dissolution rate (MDR) values shown in Table 2. At lower drug loads and high processing temperature, greater polymer fluidization might have facilitated improvement in the dissolution rate. This effect was more visible in LG grade formulations than MG grade formulations. This may be due to the greater rate of solubility of LG grade polymer than that of the MG grade polymer in the dissolution media. This characteristic property can be attributed to the higher succinyl content of LG grade than MG grade. The LG grade formulations with lower drug loads and higher processing temperatures showed a clear variation compared to other LG grade formulations at the 45 min time point. This may be due to the dispersion of a lower proportion of molten drug load in a completely fluidized polymer. Formulations of MG grades followed the same trend but the dissolution rate was relatively lower than that of the LG formulations which was evident from t₅₀% values (Table 2). This variation in LG and MG grades may be attributed to the differences in their succinyl and acetyl contents. The HPMCAS succinyl groups have a pKa of approximately 5. At pH ≤ 4.0, <10% of polymer is ionized, and at pH ≥ 5.0, 50% of polymer is ionized. Therefore, HPMCAS LG is soluble above 5.0, and due to a greater proportion of succinyl groups, its ionization maybe at more ease, leading to faster drug release compared with the MG grade formulations [14]. Although the drug is amorphous in MG formulations, the polymer solubility in the aqueous medium and succinyl/acetyl ratio of the polymer skeleton may be the rate controlling factor. The DE15 values of LG and MG formulations (Table 2) were about 45–55 fold higher than the pure drug. In HG formulations, although solubility was improved by 20–40 folds compared with the pure drug, drug release was incomplete (Fig. 7C). This may be attributed to the solubility of the HG grade at higher pH (>7.0) and a higher proportion of acetate and hydroxypropoxy substituents [17]. The % dissolution efficiency (DE), mean dissolution rate (MDR) and initial dissolution rate values (IDR) of the formulations LG, MG and HG were significantly (p < 0.05) improved, compared with the pure drug.

3.6. Stability studies

The drug content of all the formulations was found to be between 97.34 ± 1.26%−103.69 ± 2.05%, indicating stability of the formulations under accelerated conditions. Data from DSC analysis suggest that in accelerated stability studies (40 °C/75% RH) done for 45 days, the formulations with 15% drug load (Tg 74–77 °C) manufactured at different processing temperatures were stable and preserved the amorphous nature of fenofibrate (Fig. 8A). The formulations with 30% drug load were found to be unstable, as drug recrystallization was observed (Fig. 8B and C).

Fig. 8.

DSC thermograms of fenofibrate extrudates with A) 15% drug load processed at 130 °C and 160 °C B) 30% drug load processed at 160 °C and C) 30% drug load processed at 130 °C after 45 days at 40°C/75%RH.

Fenofibrate recrystallization in formulations with 30% drug load maybe attributed to the lower Tg (35–39 °C) of extrudates which is below the storage temperature (Tstorage; 40°C/75% RH). This destabilizing factor of 30% drug loaded extrudates compared to 15% drug loaded extrudates led fenofibrate separation from the polymer. This is evident from the Tg of 30% drug load extrudates, which is relatively closer to the Tg of the drug than that of formulations with 15% drug load. Another factor may be polymer saturation, thus, additional drug was phase separated out in 30% extrudates [38]. Storing amorphous dispersions at a temperature above/near their Tg increases crystallization/phase separation tendency. This is due to the molecular mobility and more specifically, reduced viscosity of the system [27,28]. In this study, the 30% drug load formulations are expected to have a much lower viscosity due to lesser HPMCAS content, than formulations with 15% drug load. Literature reports the importance of the Tm/Tg ratio to the stability of amorphous dispersions. However, a high Tm/Tg ratio increases the chances of physical destabilization like phase separation and recrystallization [39]. The Tm/Tg ratio was between 2.2 and 2.3 for 30% formulations, and 1.1–1.6 for 15% formulations. It is therefore evident that the formulations with a high Tm/Tg ratio were unstable. As a thumb rule by Hancock et al., a 50 °C difference between sample Tg and storage conditions ensures thermodynamic stability of amorphous dispersions [40]. According to previous reports, the time for 5% phase separation in ASDs increases by approximately 10 folds for any 10 °C increment in the difference between the formulation’s Tstorage and Tg. In this study, a difference of about 35–40 °C between Tg and Tstorage provided stability to the 15% drug load formulations. These results are in accordance with a previous report where it was shown that a 5–30 °C difference between Tstorage and Tg showed <5% of phase separation over a period of 2 years [19]. This may confirm the superior capacity of HPMCAS to hold the drug in an amorphous state and the importance of optimized drug loading in the formulation. Based on DSC results, it is evident that the formulation Tg affected its stability under accelerated conditions, and a higher Tg may have been the preventing factor for molecular mobility and imparting stability to the formulation. The in vitro drug release profiles of the stability samples of LG (f₂ value: 63–68) and MG grades (f₂ value: 65–70) were similar (f₂ > 50) to fresh samples in the case of 15% drug load formulations, while they were dissimilar (f₂ < 50) in 30% drug load formulations. This may be due to recrystallization of the drug in formulations of 30% drug load, which affected the rate of drug release. Similarly, HG grade formulations with 30% drug load showed fenofibrate recrystallization. However, at both drug loads, HG stability samples showed similar drug release (f₂ value: 69–81) compared with initial samples. This may be due to incomplete drug release induced by the insolubility of the HG grade polymer in the release media. From these results, it can be concluded that drug loads in ASDs should be optimized especially for drugs with lower Tg [17].

4. Conclusion

In this study, a feasible processing window was established to produce fenofibrate ASDs with all 3 grades of HPMCAS. The amorphous nature of the formulations was confirmed by DSC and XRD analysis. A discriminating medium for dissolution studies was established for formulations of LG, MG, and HG grade polymers. The in vitro dissolution data showed a significant improvement in solubility of fenofibrate by approximately 20–80 folds, compared with that of pure fenofibrate, owing to the different polymer grades of the formulations and processing conditions employed. The lower drug load and higher processing temperature resulted in higher drug release rates, indicating the effect of formulation and process parameters. The accelerated stability studies at 40 °C/75% RH for 45 days showed no signs of crystallinity with the 15% drug load formulations, indicating the effect of drug load on the stability of formulations containing low Tg drugs. Based on our experimental data, it can be concluded that the 15% drug load formulations performed well in terms of drug release and stability.