Chrono modulated multiple unit particulate systems (MUPS) via a continuous hot melt double extrusion technique: Investigation of the formulation and process suitability

Abstract

The current study is aimed at the development of chrono modulated multiple unit particulate systems (MUPS) of nifedipine (ND) by a continuous double extrusion process. ND, a poorly soluble drug, was formulated into an amorphous solid dispersion (ASD) to improve its solubility. Further, the ASD was converted into MUPS to control the drug release through a combination of pulsatile and sustained release portions. In the preparation of the ASD, the polymer HPMCAS LG was employed at different concentrations. MUPS were formulated by using Eudragit® FS100, Eudragit® RSPO, Klucel™ HF and lipids Geleol™, Compritol® ATO5. The differential scanning calorimetry and powder X-ray diffraction studies of MUPS revealed the amorphous nature of ND. Scanning electron microscopy (SEM) studies depicted the surface morphology of the ASD and the gradual change in the surface of the coated MUPS during in-vitro release studies. The in-vitro drug release profiles of ASD indicated significant improvement (p<0.05) of solubility of ND and MUPS demonstrated a combination of pulsatile and zero-order controlled release up to 12 h. Accelerated stability studies for MUPS at 40°C/75% RH revealed the formulations were stable. These findings suggest hot melt double extrusion as a potential alternative for conventional techniques to produce MUPS.

Graphical Abstract

1. Introduction

The oral route of drug administration is still the most convenient method for the treatment of patients, and solid dosage forms such as tablets/capsules are prescribed more often than the other dosage forms by physicians [1]. Novel drug delivery systems play a crucial role in addressing the drawbacks of conventional drug delivery systems and enhancing safety, convenience and patient compliance [2]. From a pharmacokinetic point of view, drug delivery systems should be optimally formulated for efficient therapy [3]. The concept of sustained drug delivery was first introduced in the 1960s [4–6]. The drug delivery systems developed in the initial days were proven safe via appropriate maintenance of drug concentrations within the therapeutic window. The performance of delivery systems is impacted by the interaction between the components of drug delivery systems and the surrounding environmental conditions, such as the temperature, pH and ionic concentration [7, 8].

Current controlled/modified drug delivery systems can be broadly classified into preprogrammed, targeted, and activation-modulated systems. In activation-modulated controlled delivery systems, drug release can be modified through the interaction between the excipients/polymers of the delivery system and conditions such as the microenvironment, enzymes, pH, temperature and chemical reactions [9, 10]. Modified/controlled release systems can be formulated into either single unit systems or multiple unit particulate systems (MUPS). In contrast to single unit systems, MUPS comprise of the dose which is divided into discrete particles that collectively form a single dosage form. These MUPS can exist as pellets, sugar spheres, powders, granules and ion exchange resin particles [11]. MUPS offer advantages over conventional single unit dosage forms such as reduced toxicity by potentially reducing the local drug concentration and inhibiting burst or premature release [12]. The significant advantages of MUPS include the avoidance of the all or none concept (dose dumping or incomplete release) and flexibility in dose adjustment. [13].

Hot melt extrusion (HME) is a well-known technology for producing matrix systems for modified/controlled drug delivery. An initial burst release is observed, releasing a large amount of the drug with polymers such as polymethacrylates, ethyl cellulose, and polyvinyl acetate with water soluble drugs. This could be an obstacle for the controlled release of drug at specific time intervals [14]. Additionally, there could be a considerable amount of dose trapped in the tablet matrix [15]. The common approach for modulating drug release from matrix extrudates is the selection of carriers and alteration of the drug/excipient ratio [16, 17]. Various researchers have studied the impact of formulation and process parameters on drug release in hot melt extrusion technology [18–22].

The current study aimed to formulate MUPS delivery systems of nifedipine (ND) via hot melt double extrusion method for the treatment of unstable angina, which can cause cardiac arrest in the early morning hours. The capsule dosage form developed in the current research work is aimed to administer to the patient before bed time in the night to avoid the risk of early morning cardiac arrest while the patient is in sleep. ND is a calcium channel blocker, a dihydropyridine derivative (1,4-dihydro-2,6-dimethyl-4-(2-nitrophenyl)-3,5-pyridine dicarboxylic acid dimethyl ester) widely used in the treatment of angina and hypertension [23]. ND is a photosensitive and poorly soluble drug with very low bioavailability [24]. Conventional methods such as fluid bed technology, extrusion spheronization or other coating systems are usually employed in the production of MUPS [11].

HPMCAS, an anionic polymer having a high Tg of 120°C and melt viscosity suitable for HME processing, makes the preparation of ASDs using HPMCAS via HME technically a good strategy [25]. The polymer is capable of improving the solubility of poorly soluble drugs and sustaining the super saturation levels of drug in gastrointestinal fluids for longer duration by inhibiting the recrystallization of drug [26]. HPMCAS, a cellulosic polymer has 4 types of substitutions on the hydroxyls: methoxy, with a mass percentage of 12–28 w %; succinate, with a mass percentage of 4–28 w %; hydroxypropyl, with a mass percentage of 4–23 w %; and acetate, with a mass percentage of 2–16 w % [27]. The high Tg and cellulosic backbone of the polymer reduces the molecular mobility of the drug which imparts excellent stability to ASDs [28]. HPMCAS is an amphiphilic polymer, with succinate groups with pK_a of about 5.0 and hydrophobic acetyl and methoxy groups. Due to this chemical composition HPMCAS has a unique property to remain unionized at pH less than 5.0 and mostly in colloidal state at pH 6.0–7.5 (intestinal pH) [29]. The amphiphilic nature of the polymer facilitates the interactions between insoluble drugs and its hydrophobic portions, while the negative charge of hydrophilic succinate portions provides the stability to these colloidal structures in aqueous media. The stable drug-polymer nanostructures formed can rapidly get dissolved, resulting in greater free drug concentration which is a super saturation condition compared to crystalline drug [30]. The novelty of the current investigation was to develop ND-MUPS in two stages via a double extrusion process. In the first step, an ASD of ND (ND-ASD) was produced to improve the solubility of the poorly soluble ND, and in the second step, the produced ND-ASD was milled and processed with the rate-controlling polymers to develop pulsatile or sustained release system (ND-MUPS). The pulsatile and sustained release portions of MUPS were combined to develop a single capsule dosage form contributing to the total dose, giving more flexibility for controlled release compared to tablet matrix systems. This MUPS formulation could provide the pulsatile drug release in early morning hours for the patient and also a sustained release up to 12h. The sustained release portion of the dosage form provides extra protection for the patient in case the patient needs to be rushed to the hospital in the morning. In the literature available, there have been no reports of formulations of MUPS coated via HME technology until now, and this exploratory study can provide valuable insights for the formulation of MUPS via HME technology.

2. Materials and methods

Nifedipine (API) and HPMCAS (Aquasolve®) LG, MG, and HG grades were kindly donated (Ashland Inc., Lexington, KY, USA). Klucel™ HF (hydroxyl propyl cellulose) was supplied by Ashland (DE, USA). Precirol® ATO 5 and Geleol™ were gift samples from Gattefosse (NJ, USA). Eudragit® FS 100 and Eudragit® RSPO were gift samples from Evonik (New York, USA). Sterotex® HM NF was a gift sample from Abitec (OH, USA). Ethocel™ (ethyl cellulose) 10 cps was purchased from Dow Chemicals, USA. Gelatin capsules were purchased from Total Pharmacy Supply (Arlington, TX, USA). All the other chemicals employed in this study were of analytical grade.

2.1. Selection of excipients and preparation of physical blend

The first step of the process involved the preparation of ND-ASD, employing HPMCAS LG to improve the aqueous solubility of ND. The second step, which was intended for controlled release, employed a pH-dependent polymer, i.e., Eudragit® FS100, for the development of a pulsatile release portion. To develop the sustained release portion, lipids of different hydrophilic lipophilic balances (HLBs), different high molecular weight hydrophilic polymers and water-insoluble polymers were employed. The ingredients were weighed accurately and passed through a #35 ASTM mesh. The physical mixture was prepared by mixing the ingredients using a V-shell blender (Maxiblend®, Globe Pharma, USA) for 15 min at 20 RPM. All the dispensing and blending processes were carried out with care to avoid light exposure to ND, and the physical mixtures were covered by black polybags.

2.2. Preparation of ND-ASD (step I) via HME

Physical mixture of ND and HPMCAS LG (500 g) was prepared at different ND loads of 20%, 40% and 60%. The physical mixtures were then sieved through a #25 ASTM mesh. Hot melt extrusion was carried out on a 16 mm co-rotating twin-screw extruder (16mm Prism Euro Lab, Thermo Fisher Scientific, Waltham, MA, USA) employing a Thermo Fisher Scientific standard screw configuration. The standard screw configuration had three mixing zones for facilitating the dispersive action between the drug and the polymer. The first mixing zone consisted of mixing elements at angles of 30° to 90° with each other, and the second mixing zone was relatively shorter, with elements at an angle of 60° with each other. The third mixing zone consisted of mixing elements at angles of 60° and 90° with each other (Fig. 1A). During the process, the feed rate of the physical mixture was 4.5–5 g/min through a gravimetric feeder, and the feed rate was calibrated every time before the start of the process. The process screw speed was 50 RPM. The residence time of the process was measured by introducing blue food color powder into the feeding zone after the barrel was totally occupied with blend. The time duration between the introduction of color and the first appearance of the color in the product was recorded as the residence time. The entire extrusion process was carried out with care to avoid light.

Figure 1.

A) Standard screw configuration for production of ND-ASD B) Modified screw configuration for production of ND-MUPS

The extrusion process was carried out at a temperature of 175°C in all zones using a 16 mm co-rotating twin-screw extruder (16mm Prism Euro Lab, Thermo Fisher Scientific, Waltham, MA, USA). The process temperature of 175°C was selected based on the melting point (M.P) of ND (172–174°C) and Tg of the polymer, HPMCAS LG (120°C). The extrudates were collected and allowed to cool at room temperature for 10–15 min. The extrudates were milled using a laboratory mixer and sifted through a #35 ASTM mesh. The milled extrudates (ND-ASD) were stored in an airtight HDPE container at ambient conditions until further use.

2.3. Preparation of MUPS by double extrusion (step II)

The preparation of ND-MUPS (step (II)) was carried out by a double extrusion process using a 16 mm twin-screw extruder (16mm Prism Euro Lab, Thermo Fisher Scientific, Waltham, MA, USA) and the feed rate was calibrated before extrusion. For this step, the milled ND-ASD product (passed through a #35 ASTM mesh) was mixed with the rate-controlling agent and blended. The process temperature was selected based on the Tg/M. P of the control release agent. The excipients tried for preparing MUPS (step II) are Eudragit® FS100, Sterotex®, Precirol® ATO 5, Geleol™, Klucel™ HF, Ethocel™ 10cps and Eudragit® RSPO. The feeding zone (zone 1) was maintained at ambient temperature and remaining zones were maintained at a temperature of above the Tg/M.P of the control release agent. The operational temperature range for production of ND-MUPS was 75–140°C and the details of process temperature with respect to formulations are mentioned in Table 2. A schematic representation of the screw configuration is presented in Fig. 1B. The modified screw configurations of one mixing zone at zone 7 (with 3–5 mixing elements making an angle of 30° with each other) were used. The extrusion process was carried out at a screw speed of 50 RPM. The die plate of the extruder was removed to ease the collection of the product. The collected product was allowed to cool at room temperature for approximately 15 min, sifted through a #8 ASTM mesh and filled in size 0 capsules. Pulsatile release doses and sustained release portions were produced in separate sub steps in this double extrusion step (step II). The final dose incorporated into the size 0 capsules comprises of 10 mg of pulsatile release dose of ND and 20 mg of sustained release dose of ND, for a total dose of 30 mg of ND (Table 1).

Table 2.

Formulation compositions and process parameters of twin-screw process.

FormulationCode	Functional agent coated over ND-ASD (40% drug load) of step I	% Weight gain of functional agent over ND-ASD (40% drug load) of step I	Process temperature (°C)	% Torque
F1	Eudragit® FS100	5.0	90	34‒48
F2	Eudragit® FS100	10.0	90	45‒52
F3	Eudragit® FS100	20.0	90	45‒65
F4	Sterotex®	2.5	75	3‒5
F5	Sterotex®	5.0	75	2‒5
F6	Sterotex®	10.0	75	2‒5
F7	Precirol® ATO5	5.0	75	3‒6
F8	Precirol® ATO5	10.0	75	2‒4
F9	Precirol® ATO5	20.0	75	2‒4
F10	Geleol™	5.0	75	5‒7
F11	Geleol™	10.0	75	4‒6
F12	Geleol™	20.0	75	3‒5
F13	Klucel™ HF	2.5	100	39‒42
F14	Klucel™ HF	5.0	100	70‒85
F15	Klucel™ HF	10.0	100	>100
F16	Eudragit® RSPO	5.0	95	72‒75
F17	Eudragit® RSPO	10.0	95	75‒78
F18	Eudragit® RSPO	20.0	95	85‒95
F19	Ethocel™ 10cps	5.0	110‒140	>100
F20	Ethocel™ 10cps	10.0	110‒140	>100
F21	Ethocel™ 10cps	20.0	110‒140	>100

Table 1.

Formulation compositions and process parameters of twin-screw process.

Weight of ND-ASD (step I) (mg) (with 40% drug load)		Weight of MUPS (step II) after different % of functional coat (mg)
		* % of Function al coat	2.5 %	5.0 %	10.0 %	20.0 %
A	25.0	C	25.62	26.25	27.50	30.00
B	50.0	D	51.25	52.50	55.00	60.00

^A)Weight of ND-ASD (mg) ≈ 10 mg of Nifedipine

^B)Weight of ND-ASD (mg) ≈ 20 mg of Nifedipine

^C)Weight of ND-MUPS (mg) ≈ 10 mg of Nifedipine

^D)Weight of ND-MUPS (mg) ≈ 20 mg of Nifedipine

^*% of functional coat mentioned is based on % weight gain of coating agent on the core weight of ND-ASD

2.4. High-performance liquid chromatography (HPLC) analysis

Quantitative high-performance liquid chromatography (HPLC) analysis was performed as reported in USP 43 on an isocratic HPLC (Waters Corp., Milford, MA, USA) equipped with an autosampler, UV/VIS detector and Empower software. The analytical column used was a Phenomenex Luna C₁₈ (5 µm, 250 mm x 4.6 mm) at a detection wavelength of 265 nm for ND. The composition of the mobile phase employed was 25:25:50 (% v/v/v) acetonitrile, methanol, and water. The calibration curve was plotted between concentrations of 1 – 100 µg/mL and found to be linear with a correlation coefficient (R²) of 0.999. The mobile phase was pumped at a flow rate of 1 mL/min from the solvent reservoir to the column. The samples collected were filtered through a 0.22 µm filter (Millex® GV, Durapore® PVDF) before being injected into the column, with an injection volume of 25 µL.

2.5. Particle size distribution (PSD) studies

The PSD of the milled granules was determined by sieve analysis using a vibratory sieve shaker (Performer III SS-3, Gilson Inc., USA). The analysis was conducted by using a pre-weighed sieve nest combination of #20, 30, 40, 60, 100 and 120 ASTM mesh series. An accurately weighed quantity of 5 g of sample was placed into the sieve nest, and the analysis was conducted for 5 min at an amplitude of 5 with a tapping rate of 60 taps/min. The amount of milled granules retained on each sieve was accurately weighed after the analysis. Similarly, the weight fraction was determined for the ND-MUPS by using the ASTM sieve combination of #8, 10, 12, 14, 16 and 20 for 5 min.

2.6. Flow characteristics

The flow properties of the physical mixture, the milled extrudates (ND-ASD) and coated ND-MUPS were evaluated by the angle of repose and Carr’s index. The angle of repose test was performed using a funnel placed on an even and stable platform with a funnel height of 2–4 cm from the top of the powder/extrudate pile. The angle of repose value was calculated based on the following equation.

$$\text{Angle of repose:}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}tan\left(\theta \right)=\frac{h}{r}$$ (1)

h = height of the pile

r = radius of pile at base

The Carr’s index measurement was performed until there was no change in the tapped volume. The number of taps tested were in the order of 500, 750, 1250 and 1500 to observe the change in the final volume. The Carr’s index value was calculated by using the following equation.

$$\text{Carr's index (CI):}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}CI=\left(\frac{D_T-D_B}{D_T}\right)\times 100$$ (2)

D_T = tapped density

D_B = bulk density

2.7. Fourier transform infrared spectroscopy (FTIR)

FTIR studies were performed with an Agilent Cary 660 FTIR Spectrometer (Agilent Technologies, Santa Clara, CA) in the range of 4000 – 650 cm⁻¹. The study was performed to assess the interaction between the ND and the excipients. The bench included an ATR (Pike Technologies MIRacle ATR, Madison, WI) fitted with a single-bounce, diamond-coated ZnSe internal reflection element.

2.8. Differential scanning calorimetry (DSC)

DSC analysis was performed for the ND, ND-ASD and coated ND-MUPS (Discovery DSC25; TA Instruments, New Castle, DE, USA). The M.P of the ND and the crystallinity of the ND-ASD /ND-MUPS was determined with a temperature ramp of 10°C/min. The Tg of the ND-ASD was calculated via a heat-cool-heat cycle. In the initial heating cycle, the ND-ASD samples were heated up to 180°C at a heating ramp of 10°C/min followed by cooling to −10°C, and then, the samples were heated to 130°C at a heating rate of 1°C/min to identify the glass transition temperature.

2.9. Powder X-ray diffraction (PXRD) analysis

XRD analysis was carried out using Rigaku X-ray equipment (D/MAX-2500PC, Rigaku Corporation, Tokyo, Japan) armed with a copper tube anode with a step width of 0.02°/s over a range of 5° to 40° on a 2θ scale at room temperature. The operating conditions for the experiment were 40 kV, current of 100 mA and scanning speed of 10°/min.

2.10. Scanning electron microscopy (SEM)

The surface morphology of the ND, physical mixtures and milled ND-ASD and ND-MUPS was studied through SEM studies. The samples (n=3) for each study were taken from the respective formulation to be tested for surface morphology. The surface morphology of the ND-MUPS during the in-vitro release studies was examined by collecting samples at predetermined intervals using a JSM-7200FLV field-emission scanning electron microscope (JOEL, Peabody, MA, USA) at an acceleration voltage of 5 kV.

The samples collected from in-vitro release studies at different time points were placed in weighing boats and dried for 24 h at 40°C using an incubator without shaking mode (BioShaker, M. BR-024, USA) to remove any water pertaining to the surface. Prior to imaging, the samples were sputter coated with platinum using a fully automated Denton Desk V TSC sputter coater (Denton Vacuum, Moorestown, NJ, USA). The measurements were performed under argon atmosphere.

2.11. In-vitro drug release studies

The in-vitro drug release studies were performed for ND, ND-ASD and the coated ND-MUPS using a USP apparatus type II (SR8-plus™, Hanson) at a temperature of 37± 0.5°C and paddle speed of 50 RPM (n = 6). The study was conducted in 750 mL of 0.1 N HCl (pH 1.2) for the first 2 h. The study period of 2–6 h was conducted in phosphate buffer at a pH 6.8 followed by pH 7.4 phosphate buffer for the rest of the study. The method followed here was the addition of the required volume of 0.2 M sodium phosphate buffer to 0.1N HCl to adjust the pH accordingly. In the case of the ND-MUPS, studies were separately conducted for the pulsatile portion, sustained release portion and combination portions in single capsules. The total dose was 30 mg in the capsules, which included 10 mg of pulsatile dose intended to be released between 6–8 h and 20 mg of sustained release dose intended for an extended release of up to 12 h. Aliquots of 2 mL were collected at predetermined time intervals (2, 4, 6, 8, 10 and 12 h) and replenished with an equivalent volume of fresh medium. The collected samples were filtered through a 0.45 µm membrane filter and analyzed for ND content at 340 nm using a UV-Visible spectrophotometer (Genesys 6; Thermo Scientific., USA).

2.12. Drug release kinetics

The in-vitro drug release data of the ND-MUPS was subjected to various mathematical models (zero order, first order, Higuchi and Korsmeyer-Peppas) to evaluate the mechanism of drug release.

$$\text{First order model:}\hspace{0.5em}\hspace{0.5em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}Q=Q_0\hspace{0.17em}{e}^{kt}$$ (3)

$$\text{Zero order model:}\hspace{0.5em}\hspace{0.5em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}Q=Q_0+kt$$ (4)

$$\text{Higuchi model:}\hspace{0.5em}\hspace{0.5em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}Q=k{t}^{1/2}$$ (5)

$$\text{Korsmeyer-Peppas model:}\hspace{1em}\hspace{1em}Q=k{t}^n$$ (6)

In the above equations, Q denotes the drug released in time t, Q₀ represents the initial drug amount, and k stands for the rate constant. Here, n is the diffusion exponent, which describes the mechanism of drug release.

2.13. Stability studies

The stability studies of the ND-ASD of step I were carried out at 40°C and 75% RH for 3 months, and for ND-MUPS (step II), the studies were conducted for up to 45 days. The product was stored in a tightly closed HDPE container and placed in a stability chamber (Caron, 6030) for the study period. After the specific time, the samples were collected from the chamber, and their crystallinity, drug content and in-vitro drug release were evaluated. For the in-vitro drug release studies, the similarity factor (ƒ₂) was calculated per the implementations of the U.S. Food and Drug Administration by comparing the in-vitro drug release profiles of the fresh and stability samples [31]. The ƒ₂ value is a logarithmic transformation of the sum-squared error of the differences between the fresh and stability samples. The ƒ₂ value is calculated using the following equation:

$$f_2=50log\left\{{\left[1+\left(\frac{1}{n}\right)\sum _{t=1}^n{\left(R_t-T_t\right)}^2\right]}^{-1/2}\times 100\right\}$$ (7)

where T_t is the cumulative release rate of the stable sample and R_t represents the cumulative drug release rate of the fresh sample at predetermined time points. The term n denotes the number of time points. An f₂ value ≥50 indicates similarity between the in-vitro drug release profiles of the fresh and stable samples.

2.14. Statistical analysis

The in-vitro drug release data were subjected to statistical analysis by using GraphPad Prism (Version 8.1.1, GraphPad Software Inc., La Jolla, CA, USA). A student unpaired t-test was performed for the % drug release from the in-vitro drug release profiles of pure drug and formulations, and values of p ≤ 0.05 were considered as statistically significant.

3. Results and discussion

3.1. Rationale for selection of excipients

Excipient selection was made based on the two steps in the double extrusion process. For the first step, to prepare the ND-ASD, the polymer selected was HPMCAS LG. The reason for this selection is the higher Tg of the HPMCAS LG (120°C), which could impart stability to the ASD and inhibit the phase separation of the drug in the ND-ASD. The solubility of HPMCAS LG above pH 5.5 could help delay drug release compared to other hydrophilic polymers, thus assisting the polymers in coating in the 2^nd step for controlled drug release. This pH dependent solubility of HPMCAS LG can help to avoid the drug release in the initial 2 hours and aid the functional coat polymer to release the drug efficiently in the later hours of study. The excipients for the second step were selected based on their ability to control drug release in a timely manner, i.e., pulsatile release and sustained release up to a period of 12 h. The excipients for the second step were hydrophobic lipids, high molecular weight hydrophilic polymers, polymers with pH-dependent solubility and pH-independent water-insoluble polymers (Table 2).

3.2. Preparation of ND-ASD: selection of drug load (Step I)

The ND-ASD was prepared with 3 different drug loads, i.e., 20%, 40% and 60% w/w, and the process torques were observed to be between 50–55%, 42–48% and 24–27%, respectively. During the extrusion process the torque was well within manageable levels and with increasing drug load, the % torque decreased, indicating the plasticizing effect of ND. At 60%w/w drug load, the extrudates appeared as liquefied molten masses, which were not suitable for collection and milling, whereas the extrudates of 20%w/w and 40%w/w drug loads produced extrudates that were better suited for downstream processing such as collection and milling. Therefore, based on process feasibility, % drug load and rate of drug release (further discussed in the in-vitro drug release studies), formulation of 40% w/w drug-loaded extrudates (ND-ASD) was finalized for further studies.

3.3. Preparation of ND-MUPS (Step II)

In the second step, ND-MUPS were produced by melting/plasticizing the functional/rate-controlling excipient, which controls drug release by encapsulating the milled ND-ASD granules. The concentration of the rate-controlling/functional agent (mentioned in Table 1 and Table 2) mentioned in terms of the % weight gain over the original weight of the milled ND-ASD. The process parameters are presented in Table 2. ND-MUPS were formulated targeting the biphasic drug release, i.e., for a pulsatile release of an immediate release dose (10mg) after 6 h of administration and a sustained release dose release of up to 12 h. Both these pulsatile ND-MUPS (10mg) and sustained release ND-MUPS (20mg) were manufactured as separate steps and filled into the capsules as a single combined dose (total 30mg). Thus, a single capsule of 30mg contains both pulsatile release and sustained release portions. The % torque during the extrusion process was lower with lipids than with polymers because the molten lipids provided lubrication for the passage of ND-MUPS through the barrel [32]. Formulation trials with ethyl cellulose (F19-F21) were not practically possible, as the operating temperature was 140°C (above the Tg of ethyl cellulose (127°C)), which further increased the resultant viscosity of the formulation, causing equipment shutdown due to higher % torque. The remaining formulations were operated within the acceptable torque range of the equipment.

3.4. PSD and flow properties

Initially, the mixing zone employed had a length of 5 mixing elements with an angle of 30° with each other. The cohesion between the particles depends on the binding nature of the formulation and distributive force applied through the mixing zone, which controls the physical bonds formed between the particles and eventually affects the particle size [33]. Therefore, based on the % torque values and size of ND-MUPS, the length of the mixing zone was reduced to 3 elements to avoid higher torque conditions with polymers (used in step II) and to control the size of the product. The observed results are in accordance with the reported results [34, 35].

The PSD studies of the ND-MUPS produced with configurations with 5 mixing zones and 3 mixing zones revealed differences. With a 5 mixing zone configuration, the particles were widely distributed between 500–5000 µm, with approximately 50% of the particles between 3000–5000 µm and an occasional % torque above 80%. With the 3 mixing zone configuration, 80–90% of the product was between 1500–2000 µm, and mostly exhibited an acceptable range of % torque depending on the composition. The ND-MUPS tested revealed excellent flow properties, with angle of repose values ranging from 25.34–28.61 and CI values between 12.6–14.8.

3.5. DSC analysis

The DSC thermograms of the ND, ND-ASD and ND-MUPS are presented in Fig. 2. The DSC thermogram of nifedipine showed a sharp peak at 174°C, indicating the crystalline nature of the API. The prepared ASDs and MUPS did not show any signs of crystallinity, which indicates the successful formation of ASD. In further thermal analysis at a slower rate of temperature change via a heat-cool-heat cycle, the ASD exhibited a single Tg of 60.3°C. This confirms the molecular-level dispersion of nifedipine with HPMCAS LG. According to the Gordan-Taylor equation, if the API and polymer have different Tg values, the Tg of the resultant ASD depends on the individual glass transition temperatures of the ND and polymer [36]. The Tg of the ND-ASD was found to be 60.3°C, which is dependent on the Tg of ND (47°C) and the Tg of HPMCAS LG (122°C). This suggests and indicates the amorphous nature of ND in the developed dosage form.

Figure 2.

DSC thermograms of A) Tg of ND-ASD B) Tg of HPMCAS LG C) Fresh and stability samples of ND-ASD and ND-MUPS

3.6. PXRD analysis

In the PXRD analysis, crystalline ND had characteristic peaks at 8.1°, 11.9°, 16.2°, 18.6°, 19.8°, 24.7°, 26.7° 35.9°, 42.4° and 48.6° (Fig. 3). The prepared ND-ASD and ND-MUPS did not show any characteristic crystalline peaks of nifedipine, confirming the crystalline to amorphous conversion of the nifedipine. These results are in accordance with the DSC results and confirm the miscibility of nifedipine and HPMCAS LG at the molecular level. The amorphous nature of the product could also be attributed to the cellulosic backbone of HPMCAS LG, which inhibited the interactions that could lead to the recrystallization of the API [28].

Figure 3.

XRD spectra of Nifedipine, fresh and stability samples of ND-ASD and ND-MUPS

3.7. FTIR

In the FTIR spectra (Fig. 4), the bands at 3332, 1677 and 1222 cm⁻¹ suggest NH stretching vibrations, carbonyl (C=O) stretching and C-O ester stretching. C-H aromatic stretching was observed at 686–792 cm^−1. The respective peaks of NH stretching vibrations, C=O stretching, C-O ester stretching and C-H aromatic stretching were slightly shifted or broadened to 3328, 1681, 1225 and 686–793, respectively. The results suggest that there may be a slight interaction between ND and HPMCAS LG. However, overall the solubility improvement can be attributed to the hydrophilic groups present in the polymer structure and precipitation inhibitory effect of HPMCAS LG during in-vitro drug release studies [28, 37].

Figure 4.

FTIR spectra of Nifedipine, HPMCAS LG, Eudragit® RSPO, Eudragit® FS100, Physical mixture, ND-ASD and ND-MUPS (combination of F2 and F16)

3.8. SEM

The SEM images (Fig. 5 and Fig. 6) were depicted on a scale of 10–100µ. SEM images of the ND and physical mixture demonstrated the crystalline nature of pure ND, which was not observed in the milled ND-ASD and ND-MUPS, suggesting that the product was amorphous. The pulsatile release ND-MUPS showed an intact surface morphology up to 6 h of dissolution study and further showed cracks in the samples collected after 7 h owing to the solubility of Eudragit® FS 100 at pH >7.0. The sustained release ND-MUPS showed an intact surface up to 2 h (0.1 N HCl stage), whereas the cracks were slightly evident from 6 h onwards and were intensified up to 12 h. This indicates the combinatory behavior of the pH-dependent solubility of HPMCAS in the ND-ASD and the pH-independent and water-insoluble nature of Eudragit® RSPO, which covered the outer surface of the ND-ASD. The results of in-vitro release drug studies are in accordance with the morphological observations of the SEM study.

Figure 5.

SEM images of A) Nifedipine B) Physical mixture C) Milled ND-ASD D) ND-MUPS (Eudragit® FS100 coated) E) ND-MUPS (Eudragit® RSPO coated)

Figure 6.

SEM images of in-vitro release studies of ND- MUPS: Eudragit® FS100 coated (A-C) MUPS A) after 2h of study B) after 6h of study C) after 7h of study: Eudragit® RSPO coated (D-F) MUPS D) after 2h of study E) after 6h of study F) after 12h of study

3.9. In-vitro drug release studies

The in-vitro drug release studies were conducted in 0.1 N HCl for the first 2 h followed by pH 6.8 phosphate buffer for the next 4 h, and the rest of the study was conducted in pH 7.4 phosphate buffer. For the ND-ASD and ND-MUPS (pulsatile portion) the in-vitro release study was carried out for a time period of 8h and samples were collected for each hour to observe the rate of drug release and whether the ND-ASD can maintain the drug concentration level without any drop in the concentration of drug for the tested period. This is to demonstrate the precipitation inhibitory effect of the formulations. This property will further help in controlling the drug release when the ND-ASD is further coated with rate controlling agents to produce ND-MUPS. The ND-MUPS (sustained release portion) was tested for a period of 12h to check whether the formulation can sustain the drug release up to 12h. The statistical comparison of % drug release between 2–8h of pure drug and ND-ASD of different % drug loads showed significant improvement in % drug release of ND (p <0.05) indicating the improvement in solubility of ND via formation of ASD (Fig. 7A). At the higher drug load (60% w/w), the % drug release between 2–8h was significantly (p<0.05) less compared to formulations with 10% and 20% (w/w) drug load due to the higher ND/polymer ratio. As there was no statistically significant (p <0.05) difference between the % drug release from drug release profiles of the 20% and 40% w/w extrudates, the 40%w/w extrudate formulation (ND-ASD) was finalized for further studies. The drug concentration did not drop (for the 20% and 40% w/w drug loads) for the tested duration, indicating the inhibition of ND precipitation in the presence of HPMCAS LG and the proper balance between the drug load and the hydrophilic succinyl groups, avoiding phase separation/precipitation [38].

Figure 7.

In-vitro drug release studies of A) ND-ASD of different drug loads B) ND-MUPS of (Eudragit® FS100 coated) different % of functional coat (5–20%) (mean ±SD: n=6)

The ND-MUPS for pulsatile release were formulated with the polymer Eudragit® FS100 due to its solubility above pH 7.0 [39]. This polymer has been reported as a successful thermoplastic polymer in melt extrusion for controlled release. The melt extruded matrix is generally reported to be less porous, which could provide more control over drug release. The sustained release ND-MUPS portion was formulated with different lipids, hydrophilic polymers of high molecular weight and hydrophobic polymers and pH-independent polymers [18, 40]. The ND-MUPS for pulsatile release was designed with different % Eudragit® FS 100 ranging from 5–20% weight gain based on the original weight of the ND-ASD granules (F1, F2 and F3: Fig. 7B). The polymer Eudragit® FS 100 is an acidic polyelectrolyte that is solubilized above pH 7.0. With 5% weight gain of Eudragit® FS 100 (F1), burst release was observed after exposure to aqueous media in 2 h. This may be attributed to insufficient polymer concentration to cover the ND-ASD particles. With 10% (F2) and 20% (F3), the weight gains of polymer the drug release was controlled up to 6 h, and when the pH was above 7.0 (after 6 h), a sigmoidal pulsatile drug release pattern was observed. This is because the drug is already dispersed/miscible in FS 100 in amorphous form, and along with FS 100, the drug is solubilized in media above pH 7.0. As the 10% polymer (F2) proportion could control the drug release up to 6 h, this concentration was finalized for the pulsatile release portion of MUPS.

The dissolution profiles of lipids and Klucel™ HF-based sustained release MUPS are presented in Fig. 8. The lipid excipients selected are molten at the operated temperatures and have the property of recrystallization upon cooling. This could possibly provide good control over drug release by means of the lipids encapsulating the ND-ASD particles. However, with lipid-based excipients, the release was incomplete; there have been similar reports with methods that include melt processing techniques [41]. This might be because all the particles encapsulated/coated by hydrophobic lipids had limited drug diffusion due to the reduced wettability of the ND-MUPS, resulting in incomplete drug release. This could also have made the diffusion of the remaining amount of drug difficult, as the drug should diffuse from the inner layers of the coated ND-MUPS [42]. As Sterotex® (HLB 1) was the most hydrophobic lipid of the tested lipids, the release was observed to be relatively slow (F5 and F6) (Fig. 8C). F4 and F7 showed a burst release in the initial hours, which might be due to insufficient concentrations of lipid agents covering the ND-ASD particles. Geleol™-based MUPS could not control the drug release due to the relatively higher HLB (3) of Geleol™, which resulted in burst release, as approximately half of the dose was released in the initial hours irrespective of the Geleol™ concentration (Fig. 8 D). However, the Precirol® encapsulated formulations (F8 and F9) showed controlled release due to the balance in the HLB value (2) and medium chain length (C16-C18) of the lipid. There was no statistically significant difference (p >0.05) observed between the % drug release of 10% and 20% Precirol® coated formulations, and the end release was only approximately 70% in the coated MUPS (Fig. 8A). This might be due to the saturated drug concentration in the microenvironment of the MUPS, which might inhibit complete release. Similar observations with the lipid excipients were previously observed for single unit (tablet) dosage forms [43].

Figure 8.

In-vitro drug release studies of A) Precirol® coated ND-MUPS B) Klucel™ HF coated ND-MUPS C) Sterotex® coated ND-MUPS D) Geleol™ coated ND-MUPS (mean ±SD: n=6)

The experiments with Klucel™ HF (Fig. 8 B) were performed at 100°C to avoid high torque values at temperatures above 120°C. High molecular weight polymers, due to their highly viscous nature, form a gel layer around ND-MUPS, which controls drug diffusion [44]. With a 10% (F15) concentration of Klucel™ HF, the torque went out of the range of the maximum equipment capacity. A granulated product of 2.5% (F13) and 5% (F14) Klucel™ HF was obtained, and initial burst release was observed (Fig. 8 B). This might be due to the hydrophilicity of Klucel™ HF, which could have allowed a higher quantity of dissolution medium to encounter the swollen polymer layer of ASD [45], The processing temperature below the Tg of Klucel™ HF might have produced a granular product rather than an encapsulated product which eventually could not control the drug release.

The Eudragit® RSPO-coated product showed a sustained drug release profile up to 12 h (Fig. 9 A). This is due to the pH-independent nature of Eudragit® RSPO (which is polyethyl acrylate, methyl methacrylate, and trimethyl ammonium ethyl methacrylate chloride in a ratio of 1:2:0.1) and the swelling characteristic of polymer on hydration, facilitating sustained release [46]. At 10% weight gain, the product (F17) showed incomplete drug release (data not shown), which might be due to the greater amount of the water-insoluble polymer encapsulating the product. The 20% weight gain formulation (F18) showed higher torque under the operating conditions due to the higher concentration of the polymer. Thus, formulation F18 was not considered for in-vitro drug release evaluation. The drug release profile of formulation F16 sustained drug release for up to 12 h due to appropriate concentrate of functional polymer around the milled ND-ASD units. Based on these results, a final formulation of MUPS (dose of 30 mg) was selected as a combination of MUPS equivalent to 10 mg of pulsatile dose (F2) (Eudragit® FS 100 coated) and a 20 mg dose of SR dose (F16) (coated with Eudragit® RSPO). In the total dose of MUPS (30 mg) filled in a single capsule, the pulsatile portion was 33.3%, and the sustained release portion was 66.7%. The dissolution profile of the combinatory MUPS (Fig. 9 B) depicts the pulsatile release of 10 mg after 6 h and a sustained release profile over 12 h. Approximately 35% of the total dose was released between 6–8 h, ensuring the availability of pulsatile dose after 6 h. The sigmoidal curve between 6–8 h in the release profile indicates that the release is predominantly from pulsatile portion of ND-MUPS. The SR portion was able to maintain drug release over 12 h, indicating the ability of the formulation to provide sustained drug release up to 12 h.

Figure 9.

In-vitro drug release studies of A) Eudragit® RSPO coated ND-MUPS (5 % coated) B) ND-MUPS -Combination of F2 and F16 (mean ±SD: n=6)

3.10. Drug release kinetics

The drug release kinetics data for formulations F9, F14 and F16 (applied for data after 2 h of study, as HPMCAS LG is insoluble at pH 1.2) are presented in Table 3.

Table 3.

Release kinetics of MUPS formulations

Formulation code	Zero order	First order	Higuchi	Korsmeyer-Peppas	Diffusional Exponent (n)
	Correlation coefficient (R2 )
F9	0.9693	0.9155	0.992	0.9850	0.632
F14	0.8581	0.8257	0.914	0.9911	0.414
F16	0.9957	0.957	0.9816	0.9828	1.099

The release kinetics and mechanism of drug release were determined by using equations of zero order, first order, the Higuchi model and the Korsmeyer-Peppas model. The Precirol® and Klucel™ HF-based formulations showed good correlation with the Higuchi model. The n values of the Peppas equation indicated Fickian diffusion for the Klucel™ HF-based formulation (F14). This might be due to the high viscosity/gelling nature of Klucel HF. In this formulation system, the release is dependent on the rate of water diffusion into the polymer and the relaxation/expansion of the polymer chains [14]. Klucel™ HF is a highly viscous polymer with a heterogeneous polymer structure and a combination of high- and low-density regions. The initial burst release might be caused by penetration of aqueous media into low-density regions of the polymer, and further sustained release might be due to high-density regions of the polymer [47]. In the Precirol®-based formulation (F9), the non-Fickian diffusion might be due to the probable lipid relaxation and non-gelling nature of lipids, as the manufacturing process involves melting, mixing and coating around the ASD particles [48]. The Eudragit® RSPO-based formulations (F16) showed zero-order release, which indicates the pH-independent and water-insoluble nature of Eudragit® RSPO, producing controlled drug release.

3.11. Stability studies

The stability studies were performed at 40°C/75%RH for ND-ASD for 3 months and 45 days for MUPS. The assay values of the stability samples were in the acceptable range (97.4 ±2.04–103.8±1.11%), and there was no variation in the physical characteristics. The amorphous nature of the drug was preserved in both the ASD and MUPS in the stability studies (Fig. 2 and Fig. 3). The in-vitro release profiles of the stability samples of the MUPS formulation (combination of F2 and F16) were similar to those of the fresh samples (f2 =57). These results indicate the stability of ASD and MUPS under accelerated conditions. The stability of formulations may be due to the Tg of the ASD is above the conditions of stability storage and the cellulosic backbone of HPMCAS. These factors restrict the molecular mobility between the dispersed API and polymer, inhibiting phase separation and thus providing stability to the formulation [30].

4. Conclusion

A MUPS-type delivery system was successfully developed for ND via an HME double extrusion process. In the first step, the % drug release data from in-vitro drug release studies of pure drug and ASD indicated the significant enhancement of rate of dissolution (p<0.05) indicating the improvement of solubility of poorly water-soluble ND through preparation of ASD, and in the second step, ND-ASD was converted into pulsatile release and sustained release MUPS. The final capsule dose form contained a combination of pulsatile dose and a sustained release dose to provide an immediate pulsatile release dose after 6 h along with sustained release for up to 12 h. The SEM study results were in accordance with the in-vitro study results. Eudragit® FS 100 provided successful pulsatile release due to its pH-dependent solubility above a pH of 7.0. Eudragit® RSPO produced a zero-order controlled release for up to 12 h due to its pH-independent and water-insoluble nature. The prepared ND-ASD was still amorphous after 3 months at 40°C/75% RH, and the prepared ND-MUPS showed a drug release profile similar to that of the freshly prepared MUPS after 45 days of stability study at 40°C/75% RH. As the preparation of ND-MUPS via HME technology is not an explored area, this study can provide valuable insights in production of MUPS for controlled drug release. This novel double extrusion step could provide a potential alternative to conventional techniques for producing MUPS for controlled release.