Water-induced phase separation of miconazole-poly (vinylpyrrolidone-co-vinyl acetate) amorphous solid dispersions: Insights with confocal fluorescence microscopy

Dissolution Mechanisms of Amorphous Solid Dispersions: Application of Ternary Phase Diagrams To Explain Release Behavior

The use of amorphous solid dispersions (ASDs) in commercial drug products has increased in recent years due to the large number of poorly soluble drugs in the pharmaceutical pipeline. However, the release behavior of ASDs is complex and remains not well understood. Often, the drug release from ASDs is rapid and complete at lower drug loadings (DLs) but becomes slow and incomplete at higher DLs. The DL where release becomes hindered is termed the limit of congruency (LoC). Currently, there are no approaches to predict the LoC. However, recent findings show that one potential cause leading to the LoC is a change in phase morphology after water-induced phase separation at the ASD/solution interface. In this study, the phase behavior of ASDs in contact with aqueous solutions was described thermodynamically by constructing experimental and computational ternary phase diagrams, and these were used to predict morphology changes and ultimately the LoC. Experimental ternary phase diagrams were obtained by equilibrating ASD/water mixtures over time. Computational ternary phase diagrams were obtained by Perturbed Chain Statistical Associating Fluid Theory (PC-SAFT). The morphology of the hydrophobic phase was studied with fluorescence confocal microscopy. It was demonstrated that critical point (plait point) composition approximately corresponded to the ASD DL, where the hydrophobic phase, formed during phase separation, became interconnected and hindered ASD release. This work provides mechanistic insights into the ASD release behavior and highlights the potential of in silico ASD design using phase diagrams.

Simultaneous Water Sorption and Crystallization in ASDs 1: Stability Studies Lasting for Two Years

One way to increase the slow dissolution rate and the associated low bioavailability of newly developed active pharmaceutical ingredients (APIs) is to dissolve the API in a polymer, leading to a so-called amorphous solid dispersion (ASD). However, APIs are often supersaturated in ASDs and thus tend to crystallize during storage. The kinetics of the crystallization process is determined by the amount of water the ASD absorbs during storage at relative humidity (RH), storage temperature, polymer type, and the drug load of the ASD. Here, the crystallization kinetics and shelf life of spray-dried ASDs were investigated for ASDs consisting of nifedipine (NIF) or celecoxib (CCX) as the APIs and of poly(vinylpyrrolidone-co-vinyl acetate) or hydroxypropyl methylcellulose acetate succinate as polymers. Samples were stored over 2 years at different RHs covering conditions above and below the glass transition of the wet ASDs. Crystallization kinetics and onset time of the crystallization were qualitatively studied by using powder X-ray diffraction and microscopic inspection and were quantitatively determined by using differential scanning calorimetry. It was found that the NIF ASDs crystallize much faster than CCX ASDs at the same drug load and at the same storage conditions due to both higher supersaturation and higher molecular mobility in the NIF ASDs. Experimental data on crystallization kinetics were correlated using the Johnson-Mehl-Avrami-Kolmogorov equation. A detailed thermodynamic and kinetic modeling will be performed in Part 2 of this paper series.

Effects of Additives on the Physical Stability and Dissolution of Polymeric Amorphous Solid Dispersions: a Review

Polymeric amorphous solid dispersion (ASD) is a popular approach for enhancing the solubility of poorly water-soluble drugs. However, achieving both physical stability and dissolution performance in an ASD prepared with a single polymer can be challenging. Therefore, a secondary excipient can be added. In this paper, we review three classes of additives that can be added internally to ASDs: (i) a second polymer, to form a ternary drug-polymer-polymer ASD, (ii) counterions, to facilitate in situ salt formation, and (iii) surfactants. In an ASD prepared with a combination of polymers, each polymer exerts a unique function, such as a stabilizer in the solid state and a crystallization inhibitor during dissolution. In situ salt formation in ASD usually leads to substantial increases in the glass transition temperature, contributing to improved physical stability. Surfactants can enhance the wettability of ASD particles, thereby promoting rapid drug release. However, their potential adverse effects on physical stability and dissolution, resulting from enhanced molecular mobility and competitive molecular interaction with the polymer, respectively, warrant careful consideration. Finally, we discuss the impact of magnesium stearate and inorganic salts, excipients added externally upon downstream processing, on the solid-state stability as well as the dissolution of ASD tablets.

Kinetics of Water-Induced Amorphous Phase Separation in Amorphous Solid Dispersions via Raman Mapping

The poor bioavailability of an active pharmaceutical ingredient (API) can be enhanced by dissolving it in a polymeric matrix. This formulation strategy is commonly known as amorphous solid dispersion (ASD). API crystallization and/or amorphous phase separation can be detrimental to the bioavailability. Our previous work (Pharmaceutics 2022, 14(9), 1904) provided analysis of the thermodynamics underpinning the collapse of ritonavir (RIT) release from RIT/poly(vinylpyrrolidone-co-vinyl acetate) (PVPVA) ASDs due to water-induced amorphous phase separation. This work aimed for the first time to quantify the kinetics of water-induced amorphous phase separation in ASDs and the compositions of the two evolving amorphous phases. Investigations were performed via confocal Raman spectroscopy, and spectra were evaluated using so-called Indirect Hard Modeling. The kinetics of amorphous phase separation were quantified for 20 wt% and 25 wt% drug load (DL) RIT/PVPVA ASDs at 25 °C and 94% relative humidity (RH). The in situ measured compositions of the evolving phases showed excellent agreement with the ternary phase diagram of the RIT/PVPVA/water system predicted by PC-SAFT in our previous study (Pharmaceutics 2022, 14(9), 1904).

Dissolution Mechanisms of Amorphous Solid Dispersions: A Close Look at the Dissolution Interface

Despite the recent success of amorphous solid dispersions (ASDs) at enabling the delivery of poorly soluble small molecule drugs, ASD-based dosage forms are limited by low drug loading. This is partially due to a sharp decline in drug release from the ASD at drug loadings surpassing the 'limit of congruency' (LoC). In some cases, the LoC is as low as 5% drug loading, significantly increasing the risk of pill burden. Despite efforts to understand the mechanism responsible for the LoC, a clear picture of the molecular processes occurring at the ASD/solution interface remains elusive. In this study, the ASD/solution interface was studied for two model compounds formulated as ASDs with copovidone. The evolution of a gel layer and its phase behavior was captured in situ with fluorescence confocal microscopy, where fluorescent probes were added to label the hydrophobic and hydrophilic phases. Phase separation was detected in the gel layer for most of the ASDs. The morphology of the hydrophobic phase was found to correlate with the release behavior, where a discrete phase resulted in good release and a continuous phase formed a barrier leading to poor release. The continuous phase formed at a lower drug loading for the system with stronger drug-polymer interactions. This was due to incorporation of the polymer into the hydrophobic phase. The study highlights the complex molecular and phase behavior at the ASD/solution interface of copovidone-based ASDs and provides a thermodynamic argument for qualitatively predicting the release behavior based on drug-polymer interactions.

Supersaturation and phase behavior during dissolution of amorphous solid dispersions

Amorphous solid dispersion (ASD) is a promising strategy to enhance solubility and bioavailability of poorly water-soluble drugs. Due to higher free energy of ASD, supersaturated drug solution could be generated during dissolution. When amorphous solubility of a drug is exceeded, drug-rich nanodroplets could form and act as a reservoir to maintain the maximum free drug concentration in solution, facilitating the absorption of the drug in vivo. Dissolution behavior of ASD has received increasing interests. This review will focus on the recent advances in ASD dissolution, including the generation and maintenance of supersaturated drug solution in absence or presence of liquid-liquid phase separation. Mechanism of drug release from ASD including polymer-controlled dissolution and drug-controlled dissolution will be introduced. Formation of amorphous drug-rich nanodroplets during dissolution and the underlying mechanism will be discussed. Phase separation morphology of hydrated ASD plays a critical role in dissolution behavior of ASD, which will be highlighted. Supersaturated drug solution shows poor physical stability and tends to crystallize. The effect of polymer and surfactant on supersaturated drug solution will be demonstrated and some unexpected results will be shown. Physicochemical properties of drug and polymer could impact ASD dissolution and some of them even show opposite effect on dissolution and physical stability of ASD in solid state, respectively. This review will contribute to a better understanding of ASD dissolution and facilitate a rational design of ASD formulation.

Formulation and Processing Strategies which Underpin Susceptibility to Matrix Crystallization in Amorphous Solid Dispersions

Through matrix crystallization, an amorphous solid may transform directly into its more stable crystalline state, reducing the driving force for dissolution. Herein, the mechanism of matrix crystallization in an amorphous solid dispersion (ASD) was probed. ASDs of bicalutamide/copovidone were prepared by solvent evaporation and hot melt extrusion, and sized by mortar and pestle or cryomilling techniques, modulating the level of mechanical activation experienced by the sample. Drug loading (DL) of the binary ASD was varied from 5-50%, and ternary systems were formulated at 30% DL with two surfactants (sodium dodecyl sulfate, Vitamin E TPGS). Imaging of partially dissolved or crystallized compacts by scanning electron microscopy with energy-dispersive X-ray analysis and confocal fluorescence microscopy was performed to investigate pathways of hydration, phase separation, and crystallization. Monitoring drug and polymer release of ASD powder under non-sink conditions provided insight into supersaturation and desupersaturation profiles. Systems at the greatest risk of matrix crystallization had high DLs, underwent mechanical activation, and/or contained surfactant. Systems having greatest resistance to matrix crystallization had rapid and congruent drug and polymer release. This study has implications for formulation and process design of ASDs and risk assessment of matrix crystallization.

Optimization of mechanically assisted coamorphous dispersion extraction of hydrophobic compounds from plant tea (Citri Reticulatae Pericarpium) using water

This study aimed to establish a novel mechanically assisted coamorphous dispersion extraction (MADE) method for the extraction of hydrophobic compounds (hesperidin, nobiletin and tangeretin) from Citri Reticulatae Pericarpium using water. The surface morphology, particle size distributions, phase states and functional groups of the coground product surface were characterized by Scanning Electron Microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. The parameters affecting the efficiency of extraction method were optimized by single-factor experiments and response surface methodology. The method showed good linear relationships in the range of 1-500 μg/mL with correlation coefficients (R²) ≥ 0.9990, low limits of detection ranging from 3.0 to 28.3 ng/mL, and acceptable recoveries ranging from 87.0 to 91.0%. Therefore, the proposed MADE method is a promising, efficient and organic solvent-free method for the extraction of hydrophobic compounds from plant tea.

Phase separation in surfactant-containing amorphous solid dispersions: Orthogonal analytical methods to probe the effects of surfactants on morphology and phase composition

Amorphous-amorphous phase separation (AAPS) is an important phase transition process for amorphous solid dispersion (ASD) performance both in terms of drug release as well as physical and chemical stability during storage. Addition of surfactants to ASD systems can impact both of these processes. One possible mechanism through which surfactants affect ASD performance is via their impact on AAPS. Unfortunately, despite their increasing usage in ASD formulations, the effect of surfactant on AAPS is still poorly understood, and there are limited analytical techniques that provide microstructural and composition information about phase separated ASDs. In this study, the impact of four surfactants (sodium dodecyl sulfate, Tween 80, Span 20 and Span 85) on water-induced phase separation in ASDs formulated with ritonavir and polyvinylpyrrolidone/vinyl acetate (PVPVA) was investigated using a variety of orthogonal analytical methods. Transparent films of ASDs with different compositions were prepared by spin coating. Fluorescence confocal microscopy in combination with an in situ humidity chamber was used to monitor the kinetics and morphology of phase separation following exposure to high relative humidity. Optical photothermal IR analysis of phase separated films enabled characterization of domain composition and surfactant distribution. Liquid-liquid phase separation concentration, zeta potential and solution nuclear magnetic resonance spectroscopy measurements enabled interpretation of interaction with and partition of surfactants into the drug-rich phase. It was found that phase separation kinetics and morphology were notably changed by the surfactants. Further, the surfactants showed different affinities for the drug-rich versus the aqueous/polymer-rich phases. The employed analytical techniques were found to be complementary in providing insight into surfactant location in phase separated systems. This study highlights the complexity of phase separation, especially in the presence of surfactants, and provides a foundation to understand the impact of AAPS on ASD performance.

Advanced structural characterisation of pharmaceuticals using nano-thermal analysis (nano-TA)

The production of drug delivery systems fabricated at the nano scale comes with the challenges of identifying reliable characterisation tools, especially for solid dosage forms. A full understanding of physicochemical properties of solid-state systems at a high spatial resolution is essential to monitor their manufacturability, processability, performance (dissolution) and stability. Nano-thermal analysis (nano-TA), a hybrid of atomic force microscopy (AFM) and thermal analysis, has emerged as a solution to address the need for complete characterisation of samples with surface heterogeneity. Nano-TA provides not only physical information using conventional AFM but also the thermal behaviour of these systems as an additional chemical dimension. In this review, the principles and techniques of nano-TA are discussed with emphasis on recent pharmaceutical applications. Building on nano-TA, the combination of this approach with infrared spectroscopic analysis is briefly introduced. The challenges and considerations for future development of nano-TA characterisation are also outlined.

Amorphous Solid Dispersions: Utilization and Challenges in Preclinical Drug Development within AstraZeneca

The poor aqueous solubility of many active pharmaceutical ingredients (APIs) dominates much of the early drug development portfolio and poses a major challenge in pharmaceutical development. Polymer-based amorphous solid dispersions (ASDs) are becoming increasingly common and offer a promising formulation strategy to tackle the solubility and oral absorption issues of these APIs. This review discusses the design, manufacture, and utilisation of ASD formulations in preclinical drug development, with a key focus on the pre-formulation assessments and workflows employed at AstraZeneca.

Analytical and Computational Methods for the Determination of Drug-Polymer Solubility and Miscibility

In the pharmaceutical industry, poorly water-soluble drugs require enabling technologies to increase apparent solubility in the biological environment. Amorphous solid dispersion (ASD) has emerged as an attractive strategy that has been used to market more than 20 oral pharmaceutical products. The amorphous form is inherently unstable and exhibits phase separation and crystallization during shelf life storage. Polymers stabilize the amorphous drug by antiplasticization, reducing molecular mobility, reducing chemical potential of drug, and increasing glass transition temperature in ASD. Here, drug-polymer miscibility is an important contributor to the physical stability of ASDs. The current Review discusses the basics of drug-polymer interactions with the major focus on the methods for the evaluation of solubility and miscibility of the drug in the polymer. Methods for the evaluation of drug-polymer solubility and miscibility have been classified as thermal, spectroscopic, microscopic, solid-liquid equilibrium-based, rheological, and computational methods. Thermal methods have been commonly used to determine the solubility of the drug in the polymer, while other methods provide qualitative information about drug-polymer miscibility. Despite advancements, the majority of these methods are still inadequate to provide the value of drug-polymer miscibility at room temperature. There is still a need for methods that can accurately determine drug-polymer miscibility at pharmaceutically relevant temperatures.

Exploring the Role of Surfactants in Enhancing Drug Release from Amorphous Solid Dispersions at Higher Drug Loadings

To reduce the dosage size of amorphous solid dispersion (ASD)-based formulations, it is of interest to devise formulation strategies that allow increased drug loading (DL) without compromising dissolution performance. The aim of this study was to explore how surfactant addition impacts drug release as a function of drug loading from a ternary ASD, using felodipine as a model poorly soluble compound. The addition of 5% TPGS (d-α-tocopheryl polyethylene glycol 1000 succinate, a surfactant) to felodipine-polyvinylpyrrolidone/vinyl acetate ASDs was found to facilitate rapid and congruent (i.e., simultaneous) release of drug and polymer at higher DLs relative to binary ASDs (drug and polymer only). For binary ASDs, good release was observed for DLs up to <20% DL; this increased to 35% DL with surfactant. Microstructure evolution in ASD films following exposure to 100% relative humidity was studied using atomic force microscopy coupled with nanoscale infrared imaging. The formation of discrete, spherical drug-rich domains in the presence of surfactant appeared to be linked to systems showing congruent and rapid release of drug and polymer. In contrast, a contiguous drug-rich phase was formed for systems without surfactant at higher DLs. This study supports the addition of surfactant to ASD formulations as a strategy to increase DL without compromising release. Furthermore, insights into the potential role of surfactant in altering ASD release mechanisms are provided.

Effectively reducing antibiotic contamination and resistance in fishery by efficient gastrointestine-blood delivering dietary millispheres

The overuse of antibiotics during the medication treatment is inevitable in the extensively-applied intensive and semi-intensive aquaculture mode; the accompanied antibiotic contamination and antimicrobial resistance pose threats to the ecosystems and cause great loss to the aquaculture industry. To solve the problem, this work introduced the antibiotic-laden dietary millispheres (DMSs) with internal porous structure for the high availability, attractiveness and digestibility to fish. Two types of antibiotics with distinct solubilities - tetracycline chloride (TCH) and sulfadiazine (SDZ) were made into the DMSs, individually, which were then directly adopted in the feeding of fish. Carassius auratus was chosen as the target fish in this work. The mesocosm study demonstrate that, compared with the regular way of oral administration (feeding the mixture of antibiotics and commercial feed pellets), the DMSs could use much less (i.e. one order of magnitude lower) antibiotic dose to reach the equivalent antibiotic concentration in gastrointestine and blood. As a robust alternative, either TCH- or SDZ-laden DMSs achieved efficient drug delivery in vivo, which importantly facilitated the source reduction of antibiotics, the alleviation of antibiotic contamination in fishery and the control of antibiotic resistance especially in sediments.

Drug Release and Nanodroplet Formation from Amorphous Solid Dispersions: Insight into the Roles of Drug Physicochemical Properties and Polymer Selection

Dissolution of amorphous solid dispersions (ASD) can lead to the formation of amorphous drug-rich nano species (nanodroplets) via liquid-liquid phase separation or glass-liquid phase separation when the drug concentration exceeds the amorphous solubility. These nanodroplets have been shown to be beneficial for ASD performance both in vitro and in vivo. Thus, understanding the generation and stability of nanodroplets from ASD formulations is important. In this study, the impacts of polymer selection and active pharmaceutical ingredient (API) physicochemical properties (wet glass transition temperature (T_g) and log P) on nanodroplet release were studied. Six APIs with different physicochemical properties were formulated as ASDs with two polymers, polyvinylpyrrolidone/vinyl acetate (PVPVA) and hydroxypropyl methylcellulose acetate succinate (HPMCAS). Their release performance was evaluated using both powder and surface normalized dissolution of compacts. In general, HPMCAS-based dispersions resulted in higher drug release compared to PVPVA-based dispersions. The two polymers also exhibited different trends in nanodroplet formation as a function of drug loading (DL). PVPVA ASDs exhibited a "falling-off-the-cliff" effect, with a dramatic decline in release performance with a small increase in drug loading, while HPMCAS ASDs exhibited a negative "slope" in the release rate as a function of drug loading. For both polymers, low T_g compounds achieved higher levels of nanodroplet formation compared to high T_g compounds. The nanodroplets generated from ASD dissolution were also monitored with dynamic light scattering, and HPMCAS was found to be more effective at stabilizing nanodroplets against size increase. Insights from this study may be used to guide formulation design and selection of excipients based on API physicochemical properties.

Influence of Particle Size and Drug Load on Amorphous Solid Dispersions Containing pH-Dependent Soluble Polymers and the Weak Base Ketoconazole

Among the great number of poorly soluble drugs in pharmaceutical development, most of them are weak bases. Typically, they readily dissolve in an acidic environment but are prone to precipitation at elevated pH. This was aimed to be counteracted by the preparation of amorphous solid dispersions (ASDs) using the pH-dependent soluble polymers methacrylic acid ethylacrylate copolymer (Eudragit L100-55) and hydroxypropylmethylcellulose acetate succinate (HPMCAS) via hot-melt extrusion. The hot-melt extruded ASDs were of amorphous nature and single phased with the presence of specific interactions between drug and polymer as revealed by X-ray powder diffraction (XRPD), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FT-IR). The ASDs were milled and classified into six particle size fractions. We investigated the influence of particle size, drug load, and polymer type on the dissolution performance. The best dissolution performance was achieved for the ASD made from Eudragit L100-55 at a drug load of 10%, whereby the dissolution rate was inversely proportional to the particle size. Within a pH-shift dissolution experiment (from pH 1 to pH 6.8), amorphous-amorphous phase separation occurred as a result of exposure to acidic medium which caused markedly reduced dissolution rates at subsequent higher pH values. Phase separation could be prevented by using enteric capsules (Vcaps Enteric®), which provided optimal dissolution profiles for the Eudragit L100-55 ASD at a drug load of 10%.

Phase behavior of ASDs based on hydroxypropyl cellulose

Novel polymeric carriers for amorphous solid dispersions (ASDs) are highly demanded in pharmaceutical industry to improve the bioavailability of poorly-soluble drug candidates. Besides established polymer candidates, hydroxypropyl celluloses (HPC) comes more and more into the focus of ASD production since they have the availability to stabilize drug molecules in aqueous media against crystallization. The thermodynamic long-term stability of HPC ASDs with itraconazole and fenofibrate was predicted in this work with PC-SAFT and compared to three-months enduring long-term stability studies. The glass-transition temperature is a crucial attribute of a polymer, but in case of HPC hardly detectable by differential scanning calorimetry. By investigating the glass transition of HPC blends with a miscible polymer, we were for the first time able to estimate the HPC glass transition. Although both, fenofibrate and itraconazole reveal a very low crystalline solubility in HPC regardless of the HPC molecular weight, we observed that low-molecular weight HPC grades such as HPC-UL prevent fenofibrate crystallization for a longer period than the higher molecular weight HPC grades. As predicted, the ASDs with higher drug load underwent amorphous phase separation according to the differential scanning calorimetry thermograms. This work thus showed that it is possible to predict critical drug loads above which amorphous phase separation and/or crystallization occurs in HPC ASDs.

Water-Induced Phase Separation of Spray-Dried Amorphous Solid Dispersions

Spray drying is widely used in the manufacturing of amorphous solid dispersion (ASD) systems due to its fast drying rate, enabling kinetic trapping of the drug in amorphous form. Spray-drying conditions, such as solvent composition, can have a profound impact on the properties of spray-dried dispersions. In this study, the phase behavior of spray-dried dispersions from methanol and methanol–water mixtures was assessed using ritonavir and copovidone [poly(vinylpyrrolidone-co-vinyl acetate) (PVPVA)] as dispersion components. The resultant ASDs were characterized using differential scanning calorimetry (DSC), fluorescence spectroscopy, X-ray photoelectron spectroscopy (XPS), as well as surface-normalized dissolution rate (SNDR) measurements. Quaternary phase diagrams were calculated using a four-component Flory–Huggins model. It was found that the addition of water to the solvent system can lead to phase separation during the spray-drying process. A 10:90 H₂O/MeOH solvent system caused a minor extent of phase separation. Phase heterogeneity in the 50 and 75% drug loading ASDs prepared from this spray solvent can be detected using DSC but not with other techniques used. The 25% drug loading system did not show phase heterogeneity in solid-state characterization but exhibited a compromised dissolution rate compared to that of the miscible ASD prepared from H₂O-free solvent. This is possibly due to the formation of slow-releasing drug-rich phases upon phase separation. ASDs prepared with a 60:40 H₂O/MeOH solvent mixture showed phase heterogeneity with all analytical methods used. The surface composition of dispersion particles as measured by fluorescence spectroscopy and XPS showed good agreement, suggesting surface drug enrichment of the spray-dried ASD particles prepared from this solvent system. Calculated phase diagrams and drying trajectories were consistent with experimental observations, suggesting that small variations in solvent composition may cause significant changes in ASD phase behavior during drying. These findings should aid in spray-drying process development for ASD manufacturing and can be applied broadly to assess the risk of phase separation for spray-drying systems using mixed organic solvents or other solvent-based processes.

Myth or Truth: The Glass Forming Ability Class III Drugs Will Always Form Single-Phase Homogenous Amorphous Solid Dispersion Formulations

The physical stability of amorphous solid dispersions (ASD) of active pharmaceutical ingredients (APIs) of high glass forming ability (GFA class III) is generally expected to be high among the scientific community. In this study, the ASD of ten-selected class III APIs with the two polymers, PVPVA 64 and HPMC-E5, have been prepared by spray-drying, film-casting, and their amorphicity at T0 was investigated by modulated differential scanning calorimetry and powder X-ray diffraction. It was witnessed that only five out of ten APIs form good quality amorphous solid dispersions with no phase separation and zero crystalline content, immediately after the preparation and drying process. Hence, it was further established that the classification of an API as GFA class III does not guarantee the formulation of single phase amorphous solid dispersions.

Recent Advances in Understanding the Micro- and Nanoscale Phenomena of Amorphous Solid Dispersions

Many pharmaceutical drugs in the marketplace and discovery pipeline suffer from poor aqueous solubility, thereby limiting their effectiveness for oral delivery. The use of an amorphous solid dispersion (ASD), a mixture of an active pharmaceutical ingredient and a polymer excipient, greatly enhances the aqueous dissolution performance of a drug without the need for chemical modification. Although this method is versatile and scalable, deficient understanding of the interactions between drugs and polymers inhibits ASD rational design. This current Review details recent progress in understanding the mechanisms that control ASD performance. In the solid-state, the use of high-resolution theoretical, computational, and experimental tools resolved the influence of drug/polymer phase behavior and dynamics on stability during storage. During dissolution in aqueous media, novel characterization methods revealed that ASDs can form complex nanostructures, which maintain and improve supersaturation of the drug. The studies discussed here illustrate that nanoscale phenomena, which have been directly observed and quantified, strongly affect the stability and bioavailability of ASD systems, and provide a promising direction for optimizing drug/polymer formulations.

Influence of Polymers on the Physical and Chemical Stability of Spray-dried Amorphous Solid Dispersion: Dipyridamole Degradation Induced by Enteric Polymers

Amorphous solid dispersions (ASDs) are inherently unstable because of high internal energy. Evaluating physical and chemical stability during the process and storage is essential. Numerous researches have demonstrated how polymers influence the drug precipitation and physical stability of ASDs, while the influence of polymers on the chemical stability of ASDs is often overlooked. Therefore, this study aimed to investigate the effect of polymers on the physical and chemical stability of spray-dried ASDs using dipyridamole (DP) as a model drug. Proper polymers were selected by assessing their abilities to inhibit drug recrystallization in supersaturated solutions. HPMC E5, Soluplus®, HPMCP-55, and HPMCAS-LP were shown to be effective stabilizers. The optimized formulations were further stored at a high temperature (60 °C) and high humidity (40 °C, 75% RH) for 2 months, and their physical and chemical stability was evaluated using polarizing optical microscopy, FTIR, HPLC, and mass spectrometry (MS). In general, crystallization was observed in all samples, which indicated the physical instability under stressed storage conditions. Also, it was noted that the polymers in ASDs rather than physical mixtures, induced a dramatic drug degradation after being exposed to a high temperature (HPMCP-55 > 80% and HPMCAS-LP > 50%) and high humidity (HPMCP-55 > 40% and HPMCAS-LP > 10%). The MS analysis further confirmed the degradation products, which might be generated from the reaction between dipyridamole and phthalic anhydride decomposed from HPMCP-55 and HPMCAS-LP. Overall, the exposure of ASDs to stressed conditions resulted in recrystallization and even the chemical degradation induced by polymers.