Congruent release of drug and polymer: A “sweet spot” in the dissolution of amorphous solid dispersions

Discovery of Cilnidipine Cocrystals with Enhanced Dissolution by the Use of Computational Tools and Semiautomatic High-Throughput Screening

Cocrystals are an attractive option for overcoming drug limitations, such as a low dissolution rate and absorption of poorly water-soluble compounds. Nevertheless, the discovery of new cocrystals remains a trial-and-error approach in which hundreds of coformers and several experimental methods are often tested. To streamline the cocrystal screening, computational methods can be used to select the coformers most likely to form a cocrystal, while high-throughput screening (HTS) approaches can rapidly screen them experimentally. In this manuscript, a new cocrystal of the extremely poorly soluble drug cilnidipine (solubility ≈30 ng/mL, 0.06 μM) was successfully discovered by applying HTS approaches. Only one cocrystal resulted from the screening with a total of 52 coformers, whereby the computational approach molecular complementarity successfully ranked this coformer (p-toluenesulfonamide) at the third position of the screening list. Dissolution studies conducted on the cocrystal in blank FaSSIF (fasted-state simulated intestinal fluid) and FaSSIF pH 6.5 revealed enhanced drug dissolution with a maximum achieved supersaturation equal to seven times the solubility of the crystalline drug. Dissolution rates of drug and coformer were compared for better mechanistic understanding of the cocrystal dissolution-supersaturation-precipitation behavior. The case of cilnidipine with a rare occurrence of cocrystals emphasized the importance of using joint computational and HTS approaches to enable successful cocrystal identification for pharmaceutical development.

Combining high throughput ASD screening with the rDCS to streamline development of poorly soluble drugs

Poor aqueous solubility and slow dissolution rate of active pharmaceutical ingredients (APIs) are often encountered challenges during oral drug development, leading to variable and insufficient bioavailability. To overcome these challenges, a so-called "enabling" formulation strategy is often pursued. Among these, amorphous solid dispersions (ASDs) are established as an effective means of improving drug absorption. However, evaluating the outcome of in vitro ASD screening approaches and relating this to the expected bioavailability increase can be difficult if not done systematically. Here we show, for the first time, how the combination of a high throughput ASD screening method with the refined Developability Classification System (rDCS) can streamline the formulation of poorly soluble APIs as ASDs. Using the Screening of Polymers for Amorphous Drug Stabilization (SPADS) approach to rapidly prepare ASD films, the improvement in dissolution performance of three APIs (befetupitant, celecoxib and itraconazole) was investigated with eight polymeric carriers. The results showed that the concentration of dissolved API was highly dependent on both the carrier and the drug load. For the APIs studied, Eudragit E, HPMC 100LV and Soluplus showed especially advantageous effects as carriers. Translating these results into the rDCS framework allowed for the visualization of the left-shift (more favorable for absorption) in classification. Several ASD films were classified as rDCS class I, showing a major improvement from the initial IIb classification of the pure API. This novel approach could be expanded to include a diverse set of screening methods for enabling formulation strategies, where the rDCS can allow for a direct comparison and support formulation selection.

Molar Ratio-Dependent Crystallization in Coamorphous Celecoxib-Carbamazepine Systems: The Interplay of Thermodynamics and Kinetics

Coamorphous drug delivery systems have emerged as a promising formulation strategy to enhance the solubility, oral bioavailability, and physical stability of poorly water-soluble drugs. The molar ratio of components in coamorphous systems plays a critical role in determining their physical stability. In this study, we investigated the crystallization behavior of coamorphous celecoxib-carbamazepine (CEL-CBZ) systems at different molar ratios. The growth rates of CEL crystals, CBZ crystals, and CEL-CBZ cocrystals were observed to exhibit distinct dependencies on the molar ratio of coamorphous systems, primarily due to their unique thermodynamic driving forces, despite sharing the same kinetic factor. The influence of the molar ratio on the crystallization of coamorphous systems arises from the interplay between its effects on molecular mobility and thermodynamic driving forces, leading to either cooperative or competing effects. Both the crystal growth and crystallization tendency results reveal that thermodynamics plays a more dominant role than kinetics in the crystallization of coamorphous CEL-CBZ systems across various molar ratios. This study provides fundamental insights into the mechanism by which the molar ratio influences the crystallization of coamorphous systems, highlighting the complex crystallization behavior of multicomponent amorphous systems as an interplay between kinetics and thermodynamics.

A review on design rules for formulating amorphous solid dispersions based on drug-polymer interactions in aqueous environment

Amorphous solid dispersions (ASDs) are multi-component formulations in which a drug is molecularly dispersed in a carrier. ASDs undergo complex dissolution mechanisms to generate and sustain a supersaturated state of poorly soluble drugs. The link between enhanced solubility, supersaturation stability and drug-polymer interaction (DPI) is critical for the rational design of ASDs. The key mechanism responsible for a high bioavailability is the evolution of supersaturation during the dissolution of ASDs which is also the driving force for drug precipitation. A critical determinant of robust supersaturation generation and stability during dissolution is the molecular interaction between the drug and polymer. Characterization of DPI in a solution state is, however, challenging because of the poor hydrodynamic resolution of the techniques, traditionally used in solid-state analysis. Further, the dissolution conditions, such as the choice of buffer, pH and ionic strength may complicate the analyses and predictions. The role of DPI is a poorly understood aspect of ASD dissolution and therefore is an active area of research. DPI is critical for understanding the design rules for formulating an optimal ASD formulation. The review focuses on different aspects of DPI to stabilize the supersaturated state of a drug during the dissolution of ASDs.

A comprehensive overview of the role of intermolecular interactions in amorphous solid dispersions

Many recent studies have indicated that drug-polymer intermolecular interactions are an important aspect of amorphous solid dispersions (ASDs) and determine many of the properties of this type of formulations. In this review, a comprehensive overview is given of the latest insights with respect to intermolecular interactions in ASDs. The thermodynamic properties and theoretical considerations of the interactions are discussed, followed by a detailed and critical overview of the various solid-state analysis techniques used to probe interactions at the disposal of the formulation scientist. As the physical stability and the pharmaceutical performance of the ASD are its most crucial properties, the most recent understanding of the influence of drug-polymer interactions on these aspects is addressed as well. It is clear that intermolecular interactions may provide many advantages for ASDs but need to be weighed against the possible disadvantages. Further investigation into the interplay and trade-off between physical stability and dissolution properties is necessary in order to be able to take full advantage of the possible benefits of the interactions.

Assessing the impacts of drug loading and polymer type on dissolution behavior and diffusive flux of GDC-6893 amorphous solid dispersions

It is desirable but remains challenging to develop high drug load amorphous solid dispersions (ASDs) without compromising their quality attributes and bio-performance. In this work, we investigated the impacts of formulation variables, such as drug loading (DL) and polymer type, on dissolution behavior, diffusive flux, and in vitro drug absorption of ASDs of a high Tg compound, GDC-6893. ASDs with two polymers (HPMCAS and PVPVA) and various DLs (20 - 80%) were produced by spray drying and their drug-polymer miscibility was evaluated using solid-state nuclear magnetic resonance (ssNMR). μFLUX™ apparatus was used to evaluate the dissolution and drug membrane transport of ASDs at target solution concentrations above the amorphous solubility. Polymer release was monitored using a high-performance liquid chromatography (HPLC) equipped with a charged aerosol detector (CAD). Subsequently, bio-accessibility (%BioA) profiles of the ASDs were evaluated using a benchtop Gastro-Intestinal Model with an advanced gastric compartment (Tiny-TIM), capable of simulating the GI transit as well as in vitro drug dissolution and absorption. Good miscibility and physical stability were observed in ASDs with both HPMCAS and PVPVA even at a high DL of 80%. All GDC-6893 ASDs exhibited dissolution profiles surpassing the amorphous solubility of 20 µg/mL, regardless of the DL and the type of polymer used. Glass-liquid phase separation (GLPS) was observed for ASDs, even at the DL of 80%, and all of these systems reached the maximum achievable diffusive flux. Tiny-TIM results showed an improvement in the %BioA of GDC-6893 ASD compared to its crystalline counterpart however the drug loading and polymer type had no significant impacts on %BioA profiles. Insights from this study suggest that although congruent drug and polymer release was not observed for both HMPCAS- and PVPVA-based ASDs at both 20% and 80% DLs, GDC-6893 and the polymer (HPMCAS or PVPVA) dissolved rapidly from high DL ASDs, followed by the occurrence of GLPS, resulting in the formation of nanosized colloidal species. The findings described herein highlight the importance of understanding both drug and polymer dissolution behavior, as well as in vitro drug absorption, which are essential for the rational design of optimal formulations with desired quality and bio-performance.

Development of a Melting Point Depression Method to Measure the Solubility of a Small-Molecule Drug in Poly-Lactic-co-Glycolic Acid (PLGA)

PURPOSE

The solubility of a crystalline drug in a polymer is commonly determined by measuring melting point depression with differential scanning calorimetry (DSC). The accuracy of this measurement depends on rapid dissolution of the drug into the molten polymer during the DSC heating scan. A preferred method of accelerating this dissolution process is to preblend the drug and polymer by cryo-milling. However, cryo-milling may be unsuitable for water-sensitive drugs or polymers such as poly(lactic-co-glycolic acid) (PLGA). The purpose of this study was to develop a PLGA-specific melting point depression method that did not require a cryo-milling operation.

METHODS

A three-step DSC method was used to measure the solubility of a small-molecule drug, voriconazole, in amorphous PLGA (Resomer ® RG 502H). First, drug/PLGA powder mixtures of multiple drug loadings were melted and rapidly cooled to form glassy solid solutions. Second, these solid solutions were heated above their Tg until the drug crystallized. Third, these crystallized samples were slowly heated to measure melting point depression (i.e., solubility temperatures).

RESULTS

The crystallization procedure generated the desired drug polymorph and likely generated small, well-mixed crystalline drug particles, as the drug dissolved rapidly into the molten polymer during melting point depression scans. Drug/PLGA solubility temperatures were determined with confidence between 40 - 100% drug loading. The solubility curve was extrapolated to lower drug loadings using the Flory-Huggins model.

CONCLUSION

This technique can assist product development of high-drug-loaded PLGA products, particularly those manufactured by melt extrusion.

Unveiling the impact of preparation methods, matrix/carrier type selection and drug loading on the supersaturation performance of amorphous solid dispersions

Amorphous solid dispersions (ASDs) are widely recognized for their potential to enhance the solubility of poorly water-soluble drugs, with factors such as molecular mobility, intermolecular interactions, and storage conditions playing critical roles in their performance. However, the influence of preparation methods on their performance remains underexplored, especially regarding their supersaturation . To address this gap, the present study systematically investigates ASDs of ibuprofen (IBU, used as a model drug) prepared using two widely utilized techniques (solvent evaporation, SE, and melt-quench cooling, M-QC). Three different matrices/carriers (Soluplus®, SOL, povidone, PVP, and copovidone, PVPVA) were employed to evaluate the combined influence of preparation method, matrix/carrier type, and drug loading on ASD performance. Supersaturation behavior during dissolution, particularly its dependence on the Sink Index (SI), was a key focus. All ASDs showed successful amorphization, but molecular near-order structures differed based on the preparation method. ATR-FTIR spectroscopy revealed stronger molecular interactions in M-QC ASDs (compared to SE). Dissolution studies under supersaturation conditions (SI = 0.1 and SI = 0.2) highlighted significant performance differences. M-QC ASDs consistently exhibited higher in vitro AUC(0→t) values under non-sink conditions compared to crystalline IBU. Conversely, SE ASDs showed improved supersaturation primarily under low SI conditions, especially with SOL at low drug loadings. The findings underscore the need for a systematic approach in developing ASDs, considering preparation method, matrix/carrier type, drug loading and dissolution study conditions collectively. These factors significantly influence dissolution behavior and supersaturation, emphasizing that they should not be independently studied but evaluated comprehensively to optimize ASD performance.

Roles of Supersaturation and Liquid-Liquid Phase Separation for Enhanced Oral Absorption of Poorly Soluble Drugs from Amorphous Solid Dispersions

Amorphous solid dispersion (ASD) is one of the most important enabling formulation technologies for the development of poorly soluble drugs. Because of its thermodynamically unstable nature in both solid and wet states, the evaluation and optimization of the formulation performance involves some difficulties. The dissolution process is sensitively influenced by various factors, including the applied dose, medium composition, and pH. Supersaturated solutions can cause liquid-liquid phase separation (LLPS) and/or crystallization, which complicates the comprehension of the dissolution process. However, LLPS should be evaluated carefully because it is closely related to oral absorption. As LLPS concentration is analogous to amorphous solubility, it can be a key factor in predicting oral absorption from ASDs, if absorption is limited by solubility. Moreover, LLPS droplets are expected to increase transmembrane flux by increasing the drug concentration near the epithelial cell membrane. In this review, recently updated knowledge on the dissolution, membrane permeation, and oral absorption behaviors of ASDs is discussed with an emphasis on LLPS behavior.

Developing Soluplus®-Based Microparticle Amorphous Solid Dispersions with High Drug Loading for Enhanced Celecoxib Dissolution via Electrospraying

Amorphous solid dispersion (ASD) is one of the most studied strategies for improving the dissolution performance of poorly water-soluble drugs, but ASDs often have low drug loadings, thereby necessitating larger dosage sizes. This study intended to create Soluplus® (SOL)-based microparticle ASDs with high drug loading (up to 60 w/w%) and long-term stability (at least 16 months) using electrospraying to enhance the dissolution of poorly water-soluble celecoxib (CEL). X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses showed that the electrosprayed SOL-CEL microparticles were amorphous, and Fourier transform infrared spectroscopy (FTIR) data indicated the presence of hydrogen bonding between SOL and CEL in the microparticles, which helped stabilize the ASDs. In vitro dissolution studies demonstrated that these ASDs improved the CEL dissolution rate by up to 8.2-fold compared to the crystalline form. Electrospraying presents a promising alternative to conventional methods like hot-melt extrusion (HME) and spraying drying (SD) for the production of ASDs, providing simplicity, high drug loading capability and long-term stability, thus catering to a variety of poorly water-soluble drugs.

Impact of HPMCAS Grade on the Release of Weakly Basic Drugs from Amorphous Solid Dispersions

Oppositely charged species can form electrostatic interactions in aqueous solution, and these may lead to reduced solubility of the interacting components. Herein, insoluble complex formation between the lipophilic weakly basic drugs, cinnarizine or loratadine, and the enteric polymer, hydroxypropyl methylcellulose acetate succinate (HPMCAS), was studied and used to better understand drug and polymer release from their corresponding amorphous solid dispersions (ASDs). Surface area normalized release experiments were performed at various pH conditions for three different grades of HPMCAS, LF, MF and HF, as well as their ASDs. Both polymer and drug release rates were measured for the ASDs. Complexation tendency was evaluated by measuring the extent of polymer loss from the aqueous phase in the presence of the drug. Results showed that release from ASDs with HPMCAS-LF was less impacted by the presence of a cationic form of the drug than ASDs prepared with the HF grade. Furthermore, an increase in pH, leading to a reduction in the extent of ionized drug also led to an improvement in release rate. These observations provide a baseline to understand the role of drug-polymer electrostatic interactions on release from ASDs formulated with HPMCAS. Future studies should focus on adding complexity to media conditions by employing simulated intestinal fluids with solubilizing components.

Long-term physical stability of amorphous solid dispersions: Comparison of detection powers of common evaluation methods for spray-dried and hot-melt extruded formulations

Although physical stability can be a critical issue during the development of amorphous solid dispersions (ASDs), there are no established protocols to predict/detect their physical stability. In this study, we have prepared fenofibrate ASDs using two representative manufacturing methods, hot-melt extrusion and spray-drying, to investigate their physical stability for one year. Intentionally unstable ASDs were designed to compare the detection power of each evaluation method, including X-ray powder diffraction (XRPD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and dissolution study. Each method did not provide the same judgment results on physical stability in some cases because of their different evaluation principles and sensitivity, which has been well-comprehended only for one-component glass. This study revealed that the detection powers of each evaluation method significantly depended on the manufacturing methods. DSC was an effective method to detect a small amount of crystals for both types of ASDs in a quantitative manner. Although the sensitivity of XRPD was always lower compared to that of DSC, interpretation of the data was the easiest. SEM was very effective for observing the crystallization of the small amount of drug for hot-melt extruded products, as the drug crystal vividly appeared on the large grains. The dissolution performance of spray-dried products could change even without any indication of physical change including crystallization. The advantage/disadvantage and complemental roles of each evaluation method are discussed for deeper understanding on the physical stability data of ASDs.

Counteracting the loss of release for indomethacin-copovidone ASDs

This work revisits the changing release behavior of indomethacin(IND)-copovidone amorphous solid dispersions (ASDs) when increasing their drug load (DL). While showing congruent release behavior at DL 0.1, ASDs with DLs of 0.3 and higher show incongruent release finally resulting in a complete loss of release. To study and explain this phenomenon, we modeled the release kinetics of these ASDs and looked into their phase behavior both experimentally and theoretically. We applied a diffusion model to accurately describe experimental release profiles for congruent release, incongruent release as well as for loss of release. Predicted concentration profiles for IND, copovidone, and water within the ASD revealed the formation of an ASD layer that almost exclusively contains amorphous IND. Our phase-diagram predictions and experimental data explain this phenomenon by water-induced phase separation in those parts of the ASD which did absorb water from the dissolution medium. Whereas the evolving copovidone-rich phase dissolved, the IND-rich phase remained undissolved and formed a super-hydrophobic cover of the remaining inner core of the ASD, thus finally completely preventing its dissolution. Higher DLs promote phase separation. This leads to the counterintuitive effect that the higher the DL, the lower the absolute amount of IND released. While the ASD containing 6 mg IND (DL 0.1) released 6 mg IND, the one containing 42 mg IND (DL 0.7) released only 1 mg IND. The theoretical approach applied in this work is for the first time able to quantitatively predict that reducing DL or tablet size could be used to overcome this problem.

Dissolution of copovidone-based amorphous solid dispersions: Influence of atomic layer coating, hydration kinetics, and formulation

Atomic layer coating (ALC) is an emerging, solvent-free technique to coat amorphous solid dispersion (ASD) particles with a nanolayer ceramic coating that has been shown to improve powder characteristics and limit drug crystallization. Herein, we evaluate the impact of aluminum oxide coatings with varying thickness and conformality on the release behavior of ritonavir/copovidone ASDs. Release performance of powders, neat tablets, and formulated tablets was studied. Confocal fluorescence microscopy (CFM) was used to visualize particle hydration and phase separation during immersion of the ASD in aqueous media. CFM revealed particle hydration requires defects for solvent penetration, but coatings, regardless of thickness, had minor impacts on powder dissolution provided defects were present. In tablets where less surface area is exposed to the dissolution media due to gel formation, slowed hydration kinetics resulted in phase separation of the drug from the polymer in coated samples, limiting release. Formulation with two superdisintegrants, crospovidone and croscarmellose sodium, as well as lactose achieved ∼90% release in less than 10 minutes, matching the uncoated ASD particles of the same formulation. This study highlights the importance of hydration rate, as well as the utility of confocal fluorescence microscopy to provide insight into release and phase behavior of ASDs.

Application of atomic force microscopy in the development of amorphous solid dispersion

Development of Amorphous Solid Dispersion (ASD) requires an in-depth characterization at different stages due to its structural and functional complexity. Various tools are conventionally used to investigate the processing, stability, and functionality of ASDs. However, many subtle features remain poorly understood due to lack of nano-scale characterization tools in routine practice. Atomic force microscopy (AFM) is a type of scanning probe microscopy, used for high resolution imaging and measuring features at the nano-scale. In recent years AFM has been used increasingly as a characterization tool in different areas of the development of ASD, including drug-polymer miscibility, localized characterization of the phase separated domains, lateral molecular diffusivity on ASD surface, crystallinity and crystallization kinetics in ASD, phase behavior of ASD during dissolution, and conformation of polymer during dissolution. In this review, we have highlighted the current applications of AFM in capturing critical aspects of stability and dissolution behavior of ASD. Potential areas of future development in this domain have been discussed.

Scale-up and clinical bioavailability assessment of a 45% drug loaded amorphous nanoparticle formulation of a BCS IV compound for oral delivery

A 45 % drug loaded (DL) amorphous nanoparticle (ANP) formulation for a BCS IV drug demonstrated promising pharmacokinetics in dogs (Purohit, et al., J. Pharm. Sci. 2023(113)1007-1019). This preclinical data enabled a human proof-of-concept assessment opportunity. The ANP freeze dried powder for oral suspension was prepared using solvent/antisolvent precipitation followed by organic solvent removal and freeze drying (FD). Challenges manifested during scale-up from 50 g to 280 g. Given the preclinical data, formulation change was restricted, therefore, process modifications were implemented. Cold collection after precipitation prevented particle growth but resulted in 75 nm particles at clinical scale (CS), compared to 150 nm at laboratory scale (LS). This size decrease rendered stabilizer amounts suboptimal for FD operation. Consequently, when FD powder was resuspended in water a smaller fraction of particles was below 450 nm (by filtration), ∼65 % for CS compared to ∼85 % for LS. Formulation was stable for > 6 months, evaluated by monitoring moisture content, assay, powder X-ray diffraction (PXRD), and redispersion time. Despite ∼65 % re-dispersibility, this 45 % DL formulation in humans had higher Cmax and AUC ∼73 % and ∼46 % respectively in fasted-state, and under fed-state it met bioequivalence criteria for AUC but Cmax was 20 % lower compared to reference (10 % DL ASD tablets) demonstrating advantage of ANP strategy over ASD approach.

Dissolution mechanisms of amorphous solid dispersions: Role of polymer molecular weight and identification of a new failure mode

The mechanisms of drug release from amorphous solid dispersions (ASDs) are complex and not fully explored, making it difficult to optimize for in vivo performance. A recurring behavior has been the limit of congruency (LoC), a drug loading above which the ASD surface forms an amorphous drug-rich barrier in the presence of water, which hinders release, especially in non-sink conditions. Drug-polymer interactions and drug glass transition temperature were reported to affect the LoC. However, the effect of polymer molecular weight has not been explored. ASDs of clotrimazole and different molecular weight grades of poly (vinylpyrrolidone) (PVP) were studied for their release to obtain their LoC drug loadings. Failure modes underpinning the LoC were investigated using fluorescence confocal microscopy to analyze the ASD/solution interface and phase behavior of ASD films at high relative humidity. ASDs with good release formed stable drug-rich nanodroplets at the ASD/solution interface, while ASDs with poor release were limited by one of two failure modes, depending on PVP molecular weight. In Failure Mode I the nanodroplets quickly agglomerated, while in Failure Mode II the system underwent phase inversion. This work highlights the importance of identifying the mechanisms underlying the LoC to improve the release of higher drug loading ASDs.

Exploring biorelevant conditions and release profiles of ritonavir from HPMCAS-based amorphous solid dispersions

Development of a release test for amorphous solid dispersions (ASDs) that is in vivo predictive is essential to identify optimally performing formulations early in development. For ASDs containing an enteric polymer, consideration of buffer properties is essential. Herein, release rates of hydroxypropyl methyl cellulose acetate succinate (HPMCAS) and ritonavir from ASDs with a 20% drug loading were compared in phosphate and bicarbonate buffers with different molarities, at pH 6.5. The bioaccessibility of ritonavir from the ASD in the tiny-TIM apparatus was also evaluated and compared to that of the crystalline drug. The surface pH at the dissolving solid: solution interface was evaluated using a pH-sensitive fluorescence probe for HPMCAS and ASD compacts in phosphate and bicarbonate buffers. Drug and polymer were found to release congruently in all buffer systems, indicating that the polymer controlled the drug release. Release was slowest in 10 mM bicarbonate buffer, and much faster in phosphate buffers with molarities typically used in release testing (20-50 mM). Release from the 10 mM bicarbonate buffer was matched in a 5 mM phosphate buffer. The surface pH of HPMCAS and HPMCAS:ritonavir ASDs was found to be lower than the bulk solution pH, where surface pH differences largely explained release rate differences seen in the different buffer systems. Ritonavir was highly bioaccessible from the ASD, as assessed by the tiny-TIM system, and much less bioaccessible when crystalline drug was used. The observations highlight the need for continued development of biorelevant assays tailored for ASD formulation assessment.

Amorphous solubility advantage: Theoretical considerations, experimental methods, and contemporary relevance

Twenty-five years ago, Hancock and Parks asked a provocative question: "what is the true solubility advantage for amorphous pharmaceuticals?" Difficulties in determining the amorphous solubility have since been overcome due to significant advances in theoretical understanding and experimental methods. The amorphous solubility is now understood to be the concentration after the drug undergoes liquid-liquid or liquid-glass phase separation, forming a water-saturated drug-rich phase in metastable equilibrium with an aqueous phase containing molecularly dissolved drug. While crystalline solubility is an essential parameter impacting the absorption of crystalline drug formulations, amorphous solubility is a vital factor for considering absorption from supersaturating formulations. However, the amorphous solubility of drugs is complex, especially in the presence of formulation additives and gastrointestinal components, and concentration-based measurements may not indicate the maximum drug thermodynamic activity. This review discusses the concept of the amorphous solubility advantage, including a historical perspective, theoretical considerations, experimental methods for amorphous solubility measurement, and the contribution of supersaturation and amorphous solubility to drug absorption. Leveraging amorphous solubility and understanding the associated physicochemical principles can lead to more effective development strategies for poorly water-soluble drugs, ultimately benefiting therapeutic outcomes.

Professor Lynne S. Taylor: Scientist, educator, and adventurer

This special edition of the Journal of Pharmaceutical Sciences is dedicated to Professor Lynne S. Taylor (Retter Distinguished Professor of Pharmacy, Department of Industrial and Molecular Pharmaceutics, Purdue University), to honor her distinguished career as a pharmaceutical scientist and educator. The goal of this commentary is to provide an overview of Professor Taylor's career path, summarize her key research contributions, and provide some insight into her personal and professional contributions as an educator, mentor, wife, mother, friend, and adventurer.

Vehicle effect on in-vitro and in-vivo performance of spray-dried dispersions

In early drug development, amorphous spray-dried dispersions (SDDs) applied to enhance the bioavailability of poorly water-soluble compounds are typically administered to preclinical species via oral gavage in the form of suspensions. The liquid formulations are usually prepared on the same day of dosing to minimize the exposure of the amorphous material to the aqueous vehicle, thereby reducing the risk of crystallization. Dose-ability (e.g. syringe-ability) of the suspensions is also a critical factor for the administration, particularly when high doses, thus concentrations, are required for toxicology studies. As a result, it is standard practice during early formulation screening to assess the stability and the maximum feasible concentration of SDDs in various vehicles. In this study, we evaluated the impact of different vehicles on the performance of a model SDD in-vitro and in-vivo settings, to mitigate the risks associated with its administration in liquid form. A poorly water-soluble compound (GEN-A) was selected to screen various SDDs and generate the SDD model at 30 % drug load with HPMCAS-MF polymer carrier. The SDD was suspended in selected aqueous vehicles after a careful vehicle components screening, that included suspending agents (HPC-SL), solubilizers (PEG400, Propylene glycol), surfactants (Vitamin E TPGS, SLS, Tween 80, Poloxamer 188), and complexing agents (HP-β-CD, SBE-β-CD). The suspensions were characterized for stability, dose-ability and dissolution in biorelevant media, prior administration in pre-clinical species. The SDD dissolution profile revealed that the drug's supersaturation level was positively impacted by the presence of a surfactant (SLS) and a complexing agent (SBE-β-CD) with respect to a suspending agents (HPC-SL) in the vehicle. Similarly, the pharmacokinetics profiles of the drug following the administration of the SDD in a vehicle with a complexing agent (SBE-β-CD) achieved greater exposure compare to the SDD in a vehicle with a suspending agent (HPC-SL). These findings confirm a synergistic effect between the SDD and the vehicles, suggesting that this combination could be leveraged to maximize the advantages of the amorphous approach.

Impact of dissolution medium pH and ionization state of the drug on the release performance of amorphous solid dispersions

Amorphous solid dispersions (ASDs) are widely employed as a strategy to improve oral bioavailability of poorly water soluble compounds. Typically, optimal dissolution performance from a polyvinylpyrrolidone vinyl acetate (PVPVA) based ASD is observed at relatively low drug loading limit. Above a certain drug load, termed limit of congruency (LoC), the release from ASDs significantly decreases. So far, the majority of the dissolution behavior has been tested in conditions where the drug primarily exists in unionized form. In this work, the impact of pH of the dissolution environment on the release performance of ASDs of an ionizable drug was studied. Atazanavir (ATZ), a weakly basic drug with a pKa of 4.5 was used as a model compound and PVPVA was used as a non-ionizable matrix polymer. Dissolution rate was measured using Wood's apparatus which normalizes the surface area of the dissolving tablet. The pH of the dissolution media was varied between 1 and 6.8, to cover a range where ATZ exists as >99 % ionized or unionized species. At pH 6.8, near complete release was observed only when the drug load was ≤ 6 %. Unlike typically observed drastic decline in release behavior for PVPVA based ASDs above LoC, ATZ ASDs underwent gradual decline in dissolution behavior when the DL was increased to 8 %. This was attributed to potential formation of an ATZ-PVPVA associated phase with dissolution rate slower than neat PVPVA. However, the 10 % DL ASD showed negligible ATZ release. On another extreme (pH 1) where ATZ is ∼100 % ionized, the dissolution rate of ATZ was faster than that of PVPVA. ASD dissolution rate was found to be slower than that of the neat drug but faster than PVPVA and interestingly, did not change with DL. This can be attributed to formation of an ionized ATZ-PVPVA phase which controls the dissolution rate of the ASD. At pH 3, where the drug is ∼97 % ionized, near complete release was observed for drug loads ≤ 8 %. To observe significant increase in drug loading with near complete release, >98 % ionization of ATZ was required. At pH 2 where ATZ is ∼99.7 % ionized, near complete release was observed for drug loads up to 30 %. Furthermore, the deterioration in dissolution performance with an increase in drug load continued to be gradual at pH 2. The enhancement in dissolution performance did not correlate with solubility enhancement of ATZ due to ionization. We theorize that the enhancement in the dissolution performance due to ionization is the result of formation of an ionized ATZ-PVPVA phase which increases the hydrophilicity and the miscibility of the ASD. This can help resist water induced phase separation during ASD dissolution and therefore, result in continuous, and congruent dissolution of the drug and polymer.

Mechanistic Insights into Amorphous Solid Dispersions: Bridging Theory and Practice in Drug Delivery

Improving the bioavailability  of poorly water-soluble drugs presents a significant challenge in pharmaceutical development. Amorphous solid dispersions (ASDs) have garnered substantial attention for their capability to augment the solubility and dissolution rate of poorly water-soluble drugs, thereby markedly enhancing their bioavailability. ASDs, characterized by a metastable equilibrium where the active pharmaceutical ingredient (API) is molecularly dispersed, offer enhanced absorption compared to crystalline forms. This review explores recent research advancements in ASD, emphasizing dissolution mechanisms, phase separation phenomena, and the importance of drug loading and congruency limits on ASD performance. Principal occurrences such as liquid-liquid phase separation (LLPS) and supersaturation are discussed, highlighting their impact on drug solubility, absorption and subsequent bioavailability. Additionally, it addresses the role of polymers in controlling supersaturation, stabilizing drug-rich nanodroplets, and inhibiting recrystallization. Recent advancements and emerging technologies offer new avenues for ASD characterization and production and demonstrate the potential of ASDs to enhance bioavailability and reduce variability, making possible for more effective and patient-friendly pharmaceutical formulations. Future research directions are proposed, focusing on advanced computational models for predicting ASD stability, use of novel polymeric carriers, and methods for successful preparations.

Correlation of surface properties with dissolution behavior of amorphous solid dispersion of Riluzole and its pharmacodynamic evaluation

Formulation of amorphous solid dispersion (ASD) of any poorly water-soluble drug is among the most promising techniques to increase the dissolution profile of drug and hence its bioavailability. Various literatures give evidences of the role of drug-polymer interactions in the ASD systems, very little information is available about the surface properties of the drug molecule and their ASDs which contributes to a higher dissolution profile. Current work focuses on exploring the surface behavior of a poorly water-soluble drug Riluzole (RLZ) and its ASDs prepared with two highly hydrophilic polymers, polyacrylic acid (PAA), and polyvinylpyrrolidone vinyl acetate (PVP VA). Initial characterization using X-ray diffraction (XRD) revealed about the weight fraction of drug required to prepare a single-phase homogenous system with both the polymers. The saturation solubility and the dissolution studies showed an increase in RLZ solubility as well as the dissolution profile due to the presence of polymers. The role of polymers in changing the surface properties in terms of wettability and polarity were explored using contact angle method and X-ray photon spectroscopy (XPS). Additionally, the neuroprotective efficacy and dose dependent hepatotoxicity were also evaluated in male wistar rats. These studies confirmed the increase in the surface polarity and hence the enhanced ability of ASD formulations to interact with water. The in vivo studies indicated that at the current recommended dose the efficacy as well as toxicity is increased for the ASD formulation. Hence, this formulation can be given at a lower dose to achieve same therapeutic effect with lower toxicity.

Parameterization of Physiologically Based Biopharmaceutics Models: Workshop Summary Report

This Article shares the proceedings from the August 29th, 2023 (day 1) workshop "Physiologically Based Biopharmaceutics Modeling (PBBM) Best Practices for Drug Product Quality: Regulatory and Industry Perspectives". The focus of the day was on model parametrization; regulatory authorities from Canada, the USA, Sweden, Belgium, and Norway presented their views on PBBM case studies submitted by industry members of the IQ consortium. The presentations shared key questions raised by regulators during the mock exercise, regarding the PBBM input parameters and their justification. These presentations also shed light on the regulatory assessment processes, content, and format requirements for future PBBM regulatory submissions. In addition, the day 1 breakout presentations and discussions gave the opportunity to share best practices around key questions faced by scientists when parametrizing PBBMs. Key questions included measurement and integration of drug substance solubility for crystalline vs amorphous drugs; impact of excipients on apparent drug solubility/supersaturation; modeling of acid-base reactions at the surface of the dissolving drug; choice of dissolution methods according to the formulation and drug properties with a view to predict the in vivo performance; mechanistic modeling of in vitro product dissolution data to predict in vivo dissolution for various patient populations/species; best practices for characterization of drug precipitation from simple or complex formulations and integration of the data in PBBM; incorporation of drug permeability into PBBM for various routes of uptake and prediction of permeability along the GI tract.

Accelerated and Biopredictive In Vitro Release Testing Strategy for Single Agent and Combination Long-Acting Injectables

The purpose of this study was to develop an in vitro release testing (IVRT) strategy to predict the pre-clinical performance of single agent and combination long acting injectable (LAI) suspension products. Two accelerated IVRT methods were developed using USP apparatus 2 to characterize initial, intermediate, and terminal phases of drug release. Initial and intermediate phases were captured using a suspension cup with moderate agitation to ensure a constant, low surface area exposure of the LAI suspension to the release media. The terminal phase was obtained by exposing the LAI suspension to a high initial paddle speed. This resulted in smaller suspension particulates with high cumulative surface area that were dispersed throughout the release media, enabling rapid drug release. The in vitro release profiles obtained with these two methods in 48 h or less were independently time scaled to reflect the in vivo time scale of approximately 1800 h. Level-A in vitro in vivo correlations (IVIVCs) were separately developed for each method and active pharmaceutical ingredient (API) using in vivo absorption profiles obtained by deconvolution of rat plasma concentration-time profiles. The IVIVCs were successfully validated for each API. This work provides a framework for evaluating individual phases of drug release of complex LAIs to ultimately predict their in vivo performance.

Generality of Enhancing the Dissolution Rates of Free Acid Amorphous Solid Dispersions by the Incorporation of Sodium Hydroxide

The incorporation of a counterion into an amorphous solid dispersion (ASD) has been proven to be an attractive strategy to improve the drug dissolution rate. In this work, the generality of enhancing the dissolution rates of free acid ASDs by incorporating sodium hydroxide (NaOH) was studied by surface-area-normalized dissolution. A set of diverse drug molecules, two common polymer carriers (copovidone or PVPVA and hydroxypropyl methylcellulose acetate succinate or HPMCAS), and two sample preparation methods (rotary evaporation and spray drying) were investigated. When PVPVA was used as the polymer carrier for the drugs in this study, enhancements of dissolution rates from 7 to 78 times were observed by the incorporation of NaOH into the ASDs at a 1:1 molar ratio with respect to the drug. The drugs having lower amorphous solubilities showed greater enhancement ratios, providing a promising path to improve the drug release performance from their ASDs. Samples generated by rotary evaporation and spray drying demonstrated comparable dissolution rates and enhancements when NaOH was added, establishing a theoretical foundation to bridge the ASD dissolution performance for samples prepared by different solvent-removal processes. In the comparison of polymer carriers, when HPMCAS was applied in the selected system (indomethacin ASD), a dissolution rate enhancement of 2.7 times by the incorporated NaOH was observed, significantly lower than the enhancement of 53 times from the PVPVA-based ASD. This was attributed to the combination of a lower dissolution rate of HPMCAS and the competition for NaOH between IMC and HPMCAS. By studying the generality of enhancing ASD dissolution rates by the incorporation of counterions, this study provides valuable insights into further improving drug release from ASD formulations of poorly water-soluble drugs.

An emerging terpolymeric nanoparticle pore former as an internal recrystallization inhibitor of celecoxib in controlled release amorphous solid dispersion beads: Experimental studies and molecular dynamics analysis

Solid oral controlled release formulations feature numerous clinical advantages for drug candidates with adequate solubility and dissolution rate. However, most new chemical entities exhibit poor water solubility, and hence are exempt from such benefits. Although combining drug amorphization with controlled release formulation is promising to elevate drug solubility, like other supersaturating systems, the problem of drug recrystallization has yet to be resolved, particularly within the dosage form. Here, we explored the potential of an emerging, non-leachable terpolymer nanoparticle (TPN) pore former as an internal recrystallization inhibitor within controlled release amorphous solid dispersion (CRASD) beads comprising a poorly soluble drug (celecoxib) reservoir and insoluble polymer (ethylcellulose) membrane. Compared to conventional pore former, polyvinylpyrrolidone (PVP), TPN-containing membranes exhibited superior structural integrity, less crystal formation at the CRASD bead surface, and greater extent of celecoxib release. All-atom molecular dynamics analyses revealed that in the presence of TPN, intra-molecular bonding, crystal formation tendency, diffusion coefficient, and molecular flexibility of celecoxib were reduced, while intermolecular H-bonding was increased as compared to PVP. This work suggests that selection of a pore former that promotes prolonged molecular separation within a nanoporous controlled release membrane structure may serve as an effective strategy to enhance amorphicity preservation inside CRASD.

The impact of diluents on the compaction, dissolution, and physical stability of amorphous solid dispersion tablets

Amorphous solid dispersion (ASD) is an effective approach for enhancing the solubility, dissolution, and bioavailability of poorly water-soluble drugs. However, these metastable forms can transform into more thermodynamically stable but less soluble crystalline forms. Despite this challenge, research on processing ASDs into solid dosage forms, such as tablets, is lacking. This work aims to fill this gap by investigating the impact of common diluents on the tableting behavior, dissolution, and physical stability of ASDs composed of itraconazole and hypromellose acetate succinate. Four widely used diluents found in commercially available ASD tablets were selected for the study: microcrystalline cellulose (MCC), anhydrous lactose, starch, and mannitol. The performance of ASD tablets varied significantly depending on the diluent used. Tablets prepared with MCC exhibited higher mechanical strength than those formulated using other diluents. ASD tablets containing mannitol and lactose revealed a faster release rate than those composed of MCC or starch. Notably, the study highlighted that the physical stability of ASDs within a tablet is not solely dependent on the amount of sorbed water; crystalline diluents like lactose and mannitol were found to facilitate ASD recrystallization within a tablet. In summary, the study underscores the importance of excipient selection, considering factors such as mechanical strength, dissolution rate, and physical stability of ASD tablets. These findings offer valuable insights into the selection of excipients for downstream ASD tablet development, leading to improved manufacturability, physical stability, and the overall quality of ASD drug products.

An Artificial Gut/Absorption Simulator: Understanding the Impact of Absorption on In Vitro Dissolution, Speciation, and Precipitation of Amorphous Solid Dispersions

Upon dissolution, amorphous solid dispersions (ASDs) of poorly water-soluble compounds can generate supersaturated solutions consisting of bound and free drug species that are in dynamic equilibrium with each other. Only free drug is available for absorption. Drug species bound to bile micelles, polymer excipients, and amorphous and crystalline precipitate can reduce the drug solute's activity to permeate, but they can also serve as reservoirs to replenish free drug in solution lost to absorption. However, with multiple processes of dissolution, absorption, and speciation occurring simultaneously, it may become challenging to understand which processes lead to an increase or decrease in drug solution concentration. Closed, nonsink dissolution testing methods used routinely, in the absence of drug removal, allow only for static equilibrium to exist and obscure the impact of each drug species on absorption. An artificial gut simulator (AGS) introduced recently consists of a hollow fiber-based absorption module and allows mass transfer of the drug from the dissolution media at a physiological rate after tuning the operating parameters. In the present work, ASDs of varying drug loadings were prepared with a BCS-II model compound, ketoconazole (KTZ), and hypromellose acetate succinate (HPMCAS) polymer. Simultaneous dissolution and absorption testing of the ASDs was conducted with the AGS, and simple analytical techniques were utilized to elucidate the impact of bound drug species on absorption. In all cases, a lower amount of crystalline precipitate was formed in the presence of absorption relative to the nonsink dissolution "control". However, formation of HPMCAS-bound drug species and crystalline precipitate significantly reduced KTZ absorption. Moreover, at high drug loading, inclusion of an absorption module was shown to enhance ASD dissolution. The rank ordering of the ASDs with respect to dissolution was significantly different when nonsink dissolution versus AGS was used, and this discrepancy could be mechanistically elucidated by understanding drug dissolution and speciation in the presence of absorption.

Proof-of-Concept in Developing a 45% Drug Loaded Amorphous Nanoparticle Formulation

Amorphous solid dispersion (ASD) is an enabling approach utilized to deliver poorly soluble compounds. ASDs can spontaneously generate drug-rich amorphous nanoparticles upon dissolution, which can act as a reservoir for maintaining supersaturation during oral absorption. But, conventional ASDs are often limited in drug loadings to < 20 %. For indications where the dose is high, this can translate into a significant pill burden. The aim of this research was to develop a high drug loading (DL) amorphous nanoparticle (ANP) formulation that can release the drug-rich nanoparticles into solution upon contact with aqueous environment. Nanoparticles were directly engineered using solvent/anti-solvent precipitation. The obtained nanoparticle suspension was then concentrated followed by solidification to a re-dispersible amorphous dosage form using spray drying or lyophilization. The impact of process variables was studied using dynamic light scattering (DLS), scanning electron microscopy (SEM), high performance liquid chromatography (HPLC), nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). It was observed that spray drying led to a non-re-dispersible formulation. Sucrose and trehalose containing lyocakes resulted in re-dispersible formulations. The trehalose containing lyocakes, in a dog study, gave comparable performance to the reference tablet in the fasted state but lower area under the curve (AUC) in fed state.

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.

Investigating the influence of protein secondary structure on the dissolution behavior of β-lactoglobulin-based amorphous solid dispersions

Proteins acting as carriers in amorphous solid dispersions (ASDs) demonstrate a notable sensitivity to the spray drying process, potentially leading to changes in their conformation. The main aim of this study was to investigate the dissolution performance of ASDs based on proteins with different content of secondary structures, specifically β-sheet and α-helix structures. We prepared β-sheet-rich and α-helix-rich β-lactoglobulin (BLG), along with corresponding ASDs containing 10 wt% and 30 wt% drug loadings, through spray drying using celecoxib as the model drug. Circular dichroism and Fourier Transform Infrared Spectroscopy results revealed that even though changes in secondary structure were obtained in the spray-dried powders, the BLGs exhibited reversibility upon re-dissolving in phosphate buffer with varying pH levels. Both β-sheet-rich BLG and α-helix-rich BLG exhibited enhanced dissolution rates and higher solubility in the media with pH values far from the isoelectric point (pI) of BLG (pH 2, 7, 8, and 9) compared to the pH closer to the pI (pH 3, 4, 5, and 6). Notably, the release rate and solubility of the drug and BLG from both types of BLG-based ASDs at 10 wt% drug loading were largely dependent on the solubility of pure SD-BLGs. α-helix-rich BLG-ASDs consistently exhibited equivalent or superior performance to β-sheet-rich BLG-ASDs in terms of drug release rate and solubility, regardless of drug loading. Moreover, both types of BLG-based ASDs at 10 wt% drug loading exhibited faster release rates and higher solubility, for both the drug and BLG, compared to the ASDs at 30 wt% drug loading in pHs 2, 7, and 9 media.

Reductive amination of oxidized hydroxypropyl cellulose with ω-aminoalkanoic acids as an efficient route to zwitterionic derivatives

Zwitterionic polymers, with their equal amounts of cationic and anionic functional groups, have found widespread utility including as non-fouling coatings, hydrogel materials, stabilizers, antifreeze materials, and drug carriers. Polysaccharide-derived zwitterionic polymers are attractive because of their sustainable origin, potential for lower toxicity, and possible biodegradability, but previous methods for synthesis of zwitterionic polysaccharide derivatives have been limited in terms of flexibility and attainable degree of substitution (DS) of charged entities. We report herein successful design and synthesis of zwitterionic polysaccharide derivatives, in this case based on cellulose, by reductive amination of oxidized 2-hydroxypropyl cellulose (Ox-HPC) with ω-aminoalkanoic acids. Reductive amination products could be readily obtained with DS(cation) (= DS(anion)) up to 1.6. Adduct hydrophilic/hydrophobic balance (amphiphilicity) can be influenced by selecting the appropriate chain length of the ω-aminoalkanoic acid. This strategy is shown to produce a range of amphiphilic, water-soluble, moderately high glass transition temperature (Tg) polysaccharide derivatives in just a couple of efficient steps from commercially available building blocks. The adducts were evaluated as crystallization inhibitors. They are strong inhibitors of crystallization even for the challenging, poorly soluble, fast-crystallizing prostate cancer drug enzalutamide, as supported by surface tension and Flory-Huggins interaction parameter results.

Poly(vinylpyridine-co-vinylpyridine N-oxide) Excipients Mediate Rapid Dissolution and Sustained Supersaturation of Posaconazole Amorphous Solid Dispersions

The chemical structure of excipients molecularly mixed in an amorphous solid dispersion (ASD) has a significant impact on properties of the ASD including dissolution behavior, physical stability, and bioavailability. Polymers used in ASDs require a balance between hydrophobic and hydrophilic functionalities to ensure rapid dissolution of the amorphous dispersion as well as sustained supersaturation of the drug in solution. This work demonstrates the use of postpolymerization functionalization of poly(vinylpyridine) excipients to elucidate the impact of polymer properties on the dissolution behavior of amorphous dispersions containing posaconazole. It was found that N-oxidation of pyridine functionalities increased the solubility of poly(vinylpyridine) derivatives in neutral aqueous conditions and allowed for nanoparticle formation which supplied posaconazole into solution at concentrations exceeding those achieved by more conventional excipients such as hydroxypropyl methylcellulose acetate succinate (HPMCAS) or Eudragit E PO. By leveraging these functional modifications of the parent poly(vinylpyridine) excipient to increase polymer hydrophilicity and minimize the effect of polymer on pH, a new polymeric excipient was optimized for rapid dissolution and supersaturation maintenance for a model compound.

Interplay of Drug-Polymer Interactions and Release Performance for HPMCAS-Based Amorphous Solid Dispersions

The interplay between drug and polymer chemistry and its impact on drug release from an amorphous solid dispersion (ASD) is a relatively underexplored area. Herein, the release rates of several drugs of diverse chemistry from hydroxypropyl methylcellulose acetate succinate (HPMCAS)-based ASDs were explored using surface area normalized dissolution. The tendency of the drug to form an insoluble complex with HPMCAS was determined through coprecipitation experiments. The role of pH and the extent of drug ionization were probed to evaluate the role of electrostatic interactions in complex formation. Relationships between the extent of complexation and the drug release rate from an ASD were observed, whereby the drugs could be divided into two groups. Drugs with a low extent of insoluble complex formation with HPMCAS tended to be neutral or anionic and showed reasonable release at pH 6.8 even at higher drug loadings. Cationic drugs formed insoluble complexes with HPMCAS and showed poor release when formulated as an ASD. Thus, and somewhat counterintuitively, a weakly basic drug showed a reduced release rate from an ASD at a bulk solution pH where it was ionized, relative to when unionized. The opposite trend was observed in the absence of polymer for the neat amorphous drug. In conclusion, electrostatic interactions between HPMCAS and lipophilic cationic drugs led to insoluble complex formation, which in turn resulted in ASDs with poor release performance.

A Commentary on Co-Processed API as a Promising Approach to Improve Sustainability for the Pharmaceutical Industry

Pharmaceutical products represent a meaningful target for sustainability improvement and emissions reduction. It is proposed here that rethinking the standard, and often linear, approach to the synthesis of Active Pharmaceutical Ingredients (API) and subsequent formulation and drug product processing will deliver transformational sustainability opportunities. The greatest potential arguably involves API that have challenging physico-chemical properties. These can require the addition of excipients that can significantly exceed the weight of the API in the final dosage unit, require multiple manufacturing steps to achieve materials amenable to delivering final dosage units, and need highly protective packaging for final product stability. Co-processed API are defined as materials generated via addition of non-covalently bonded, non-active components during drug substance manufacturing steps, differing from salts, solvates and co-crystals. They are an impactful example of provocative re-thinking of historical regulatory and quality precedents, blurring drug substance and drug product operations, with sustainability opportunities. Successful examples utilizing co-processed API can modify properties with use of less excipient, while simultaneously reducing processing requirements by delivering material amenable to continuous manufacturing. There are also opportunities for co-processed API to reduce the need for highly protective packaging. This commentary will detail the array of sustainability impacts that can be delivered, inclusive of business, regulatory, and quality considerations, with discussion on potential routes to more comprehensively commercialize co-processed API technologies.

Molecular complexes of drug combinations: A review of cocrystals, salts, coamorphous systems and amorphous solid dispersions

As the advancements in the medical technology and healthcare develop through the years, combinational therapy has evolved to be an important treatment modality in many disease settings, including cancer, cardiovascular disease and infectious diseases. In an effort to alleviate "pill burden" and improve patient compliance, fixed dose combinations (FDCs) have been developed to be used as effective therapeutics. Among all FDCs, the category of drug-drug molecular complexes has been proven an efficient methodology in designing and treating diseases, with many drugs being approved. Among all drug-drug molecular complexes, drug-drug cocrystals, salts, coamorphous systems and solid dispersions have been successfully developed and many have been approved by the FDA. In this review, we dwell deeply into the molecular mechanisms behind the different types of drug-drug molecular complexes, including the key functional groups involved in the intermolecular interactions, the applications of each category of molecular complexes, as well as the advantages and challenges thereof. This comprehensive review provides useful insights into the practical design and manufacture of drug-drug molecular complexes and points out the future direction for the development of new advantageous combinational therapies that benefit more patients.

Effect of Buffer pH and Concentration on the Dissolution Rates of Sodium Indomethacin-Copovidone and Indomethacin-Copovidone Amorphous Solid Dispersions

The incorporation of counterions into amorphous solid dispersions (ASDs) has been proven to be effective for improving the dissolution rates of ionizable drugs in ASDs. In this work, the effect of dissolution buffer pH and concentration on the dissolution rate of indomethacin-copovidone 40:60 (IMC-PVPVA, w/w) ASD with or without incorporated sodium hydroxide (NaOH) was studied by surface area-normalized dissolution to provide further mechanistic understanding of this phenomenon. Buffer pH from 4.7 to 7.2 and concentration from 20 to 100 mM at pH 5.5 were investigated. As the buffer pH decreased, the IMC dissolution rate from both ASDs decreased. Compared to IMC-PVPVA ASD, the dissolution rate decrease from IMCNa-PVPVA ASD was more resistant to the decrease of buffer pH. In contrast, while buffer concentration had a negligible impact on the IMC dissolution rate from IMC-PVPVA ASD, the increase of buffer concentration significantly reduced the IMC dissolution rate from IMCNa-PVPVA ASD. Surrogate evaluation of microenvironment pH modification by the dissolution of IMCNa-PVPVA ASD demonstrated the successful elevation of buffer microenvironment pH and the suppression of such pH elevation by the increase of buffer concentration. These results are consistent with the hypothesis that the dissolution rate enhancement by the incorporation of counterions originates from the enhanced drug solubility by ionization and the modification of diffusion layer pH in favor of drug dissolution. At the studied drug loading (∼40%), relatively congruent release between IMC and PVPVA was observed when IMC was ionized in ASD or in solution, highlighting the importance of studying the ionization effect on the congruent release of ASDs, especially when drug ionization is expected in vivo. Overall, this work further supports the application of incorporating counterions into ASDs for improving the dissolution rates of ionizable drugs when enabling formulation development is needed.

Solid-state analysis for pharmaceuticals: Pathways to feasible and meaningful analysis

The solid state of matter is the preferred starting point for designing a pharmaceutical product. This is driven by both patient preferences and the relative ease of supplying a solid pharmaceutical product with desired quality and performance. Solid form diversity is increasingly prevalent as a crucial element in designing these products, which underpins the importance of solid-state analytical methods. This paper provides a critical analysis of challenges related to solid-state analytics, as well as considerations and suggestions for feasible and meaningful pharmaceutical analysis.

Does Media Choice Matter When Evaluating the Performance of Hydroxypropyl Methylcellulose Acetate Succinate-Based Amorphous Solid Dispersions?

Hydroxypropyl methylcellulose acetate succinate (HPMCAS) is a weakly acidic polymer that is widely used in the formulation of amorphous solid dispersions (ASDs). While the pH-dependent solubility of HPMCAS is widely recognized, the role of other solution properties, including buffer capacity, is less well understood in the context of ASD dissolution. The goal of this study was to elucidate the rate-limiting steps for drug and HPMCAS release from ASDs formulated with two poorly water soluble model drugs, indomethacin and indomethacin methyl ester. The surface area normalized release rate of the drug and/or polymer in a variety of media was determined. The HPMCAS gel layer apparent pH was determined by incorporating pH sensitive dyes into the polymer matrix. Water uptake extent and rate into the ASDs were measured gravimetrically. For neat HPMCAS, the rate-limiting step for polymer dissolution was observed to be the polymer solubility at the polymer-solution interface. This, in turn, was impacted by the gel layer pH which was found to be substantially lower than the bulk solution pH, varying with medium buffer capacity. For the ASDs, the HPMCAS release rate was found to control the drug release rate. However, both drugs reduced the polymer release rate with indomethacin methyl ester having a larger impact. In low buffer capacity media, the presence of the drug had less impact on release rates when compared to observations in higher strength buffers, suggesting changes in the rate-limiting steps for HPMCAS dissolution. The observations made in this study can contribute to the fundamental understanding of acidic polymer dissolution in the presence and absence of a molecularly dispersed lipophilic drug and will help aid in the design of more in vivo relevant release testing experiments.

Design of Redispersible High-Drug-Load Amorphous Formulations: Impact of Ionic vs Nonionic Surfactants on Processing and Performance

Amorphous solid dispersions (ASDs) are an enabling formulation approach used to enhance bioavailability of poorly water-soluble molecules in oral drug products. Drug-rich amorphous nanoparticles generated in situ during ASD dissolution maintain supersaturation that drives enhanced absorption. However, in situ formation of nanoparticles requires large quantities of polymers to release drugs rapidly, resulting in an ASD drug load <25%. Delivering directly engineered drug-rich amorphous nanoparticles can reduce the quantities of polymers significantly without sacrificing bioavailability. Preparation of 90% drug-load amorphous nanoparticles (ANPs) of <300 nm diameter using solvent/antisolvent nanoprecipitation, organic solvent removal, and spray drying was demonstrated previously on model compound ABT-530 with Copovidone and sodium dodecyl sulfate (anionic). In this work, nonionic surfactant d-α-tocopheryl polyethylene glycol succinate (Vitamin E TPGS, or TPGS) was used to prepare ANPs as a comparison. Characterization of ANPs by dynamic light scattering, filtrate potency assay, scanning electron microscopy, and differential scanning calorimetry revealed differences in surface properties of nanoparticles afforded by surfactants. This work demonstrates the importance of understanding the impact of the stabilizing agents on nanoparticle behavior when designing a high-drug-load amorphous formulation for poorly water-soluble compounds as well as the impact on redispersion.

Evaluation of a Three-Fluid Nozzle Spraying Process for Facilitating Spray Drying of Hydrophilic Polymers for the Creation of Amorphous Solid Dispersions

Amorphous solid dispersions (ASDs) enable formulations to improve the solubility of poorly soluble active pharmaceutical ingredients (APIs). The amorphous state is reached through the disruption of the crystalline lattice of an API resulting in an increased apparent solubility with faster disintegration. Nevertheless, this form is characterized by a high-energy state which is prone to re-crystallization. To ensure a stable ASD, excipients, e.g., polymers that form a matrix in which an API is dispersed, are used. The applicable polymer range is usually linked to their solubility in the respective solvent, therefore limiting the use of hydrophilic polymers. In this work, we show the applicability of the hydrophilic polymer, polyvinyl alcohol (PVA), in spray-dried solid dispersions. Using a three-fluid nozzle approach, this polymer can be used to generate ASDs with a targeted dissolution profile that is characterized by a prominent spring and desired parachute effect showing both supersaturation and crystallization inhibition. For this purpose, the polymer was tested in formulations containing the weakly basic drug, ketoconazole, and the acidic drug, indomethacin, both classified as Biopharmaceutics Classification System (BSC) class II drugs, as well as the weakly basic drug ritonavir classified as BCS IV. Furthermore, ritonavir was used to show the enhanced drug-loading capacity of PVA derived from the advantageous viscosity profile that makes the polymer an interesting candidate for spray drying applications.

Development and Evaluation of Amorphous Solid Dispersion of Riluzole with PBPK Model to Simulate the Pharmacokinetic Profile

In the current work, screening of polymers viz. polyacrylic acid (PAA), polyvinyl pyrrolidone vinyl acetate (PVP VA), and hydroxypropyl methyl cellulose acetate succinate (HPMC AS) based on drug-polymer interaction and wetting property was done for the production of a stable amorphous solid dispersion (ASD) of a poorly water-soluble drug Riluzole (RLZ). PAA showed maximum interaction and wetting property hence, was selected for further studies. Solid state characterization studies confirmed the formation of ASD with PAA. Saturation solubility, dissolution profile, and in vivo pharmacokinetic data of the ASD formulation were generated in rats against its marketed tablet Rilutor. The RLZ:PAA ASD showed exponential enhancement in the dissolution of RLZ. Predicted and observed pharmacokinetic data in rats showed enhanced area under curve (AUC) and Cmax in plasma and brain with respect to Rilutor. Furthermore, a physiologically based pharmacokinetic (PBPK) model of rats for Rilutor and RLZ ASD was developed and then extrapolated to humans where physiological parameters were changed along with a biochemical parameter. The partition coefficient was kept similar in both species. The model was used to predict different exposure scenarios, and the simulated data was compared with observed data points. The PBPK model simulated Cmax and AUC was within two times the experimental data for plasma and brain. The Cmax and AUC in the brain increased with ASD compared to Rilutor for humans showing its potential in improving its biopharmaceutical performance and hence enhanced therapeutic efficacy. The model can predict the RLZ concentration in multiple compartments including plasma and liver.

Crystallinity: A Complex Critical Quality Attribute of Amorphous Solid Dispersions

Does the performance of an amorphous solid dispersion rely on having 100% amorphous content? What specifications are appropriate for crystalline content within an amorphous solid dispersion (ASD) drug product? In this Perspective, the origin and significance of crystallinity within amorphous solid dispersions will be considered. Crystallinity can be found within an ASD from one of two pathways: (1) incomplete amorphization, or (2) crystal creation (nucleation and crystal growth). While nucleation and crystal growth is the more commonly considered pathway, where crystals originate as a physical stability failure upon accelerated or prolonged storage, manufacturing-based origins of crystallinity are possible as well. Detecting trace levels of crystallinity is a significant analytical challenge, and orthogonal methods should be employed to develop a holistic assessment of sample properties. Probing the impact of crystallinity on release performance which may translate to meaningful clinical significance is inherently challenging, requiring optimization of dissolution test variables to address the complexity of ASD formulations, in terms of drug physicochemical properties (e.g., crystallization tendency), level of crystallinity, crystal reference material selection, and formulation characteristics. The complexity of risk presented by crystallinity to product performance will be illuminated through several case studies, highlighting that a one-size-fits-all approach cannot be used to set specification limits, as the risk of crystallinity can vary widely based on a multitude of factors. Risk assessment considerations surrounding drug physicochemical properties, formulation fundamentals, physical stability, dissolution, and crystal micromeritic properties will be discussed.

Tailoring the release of drugs having different water solubility by hybrid polymer-lipid microparticles with a biphasic structure

The aim of this study is to investigate the potential of hybrid polymer-lipid microparticles with a biphasic structure (b-MPs) as drug delivery system. Hybrid b-MPs of Compritol®888 ATO as main lipid constituent of the shell and polyethylene glycol 400 as core material were produced by an innovative solvent-free approach based on spray congealing. To assess the suitability of hybrid b-MPs to encapsulate various types of APIs, three model drugs (fluconazole, tolbutamide and nimesulide) with extremely different water solubility were loaded into the polymeric core. The hybrid systems were characterized in terms of particle size, morphology and physical state. Various techniques (e.g. optical, Confocal Raman and Scanning Electron Microscopy) were used to investigate the influence of the drugs on different aspects of the b-MPs, including external and internal morphology, properties at the lipid/polymer interface and drug distribution. Hybrid b-MPs were suitable for the encapsulation of all drugs (encapsulation efficiency > 90 %) regardless the drug hydrophobic/hydrophilic properties. Finally, the drug release behaviors from hybrid b-MPs were studied and compared with traditional solid lipid MPs (consisting of only the lipid carrier). Due to the combination of lipid and polymeric materials, hybrid b-MPs showed a wide array of release profiles that depends on their composition, the type of loaded drug, the drug loading amount and location, providing a versatile platform and allowing the formulators to finely balance the release performance of drugs intended for oral administration. Overall, the study demonstrates that hybrid, solvent-free b-MPs produced by spray congealing are an extremely versatile delivery platform able to efficiently encapsulate and release very different types of drug compounds.

Characterization of Channeling Effects Applied to Extended-Release Matrix Tablets Containing Pirfenidone

Pirfenidone (PRF) is an anti-fibrotic agent that has been approved by the Food and Drug Administration (FDA) for the treatment of mild to moderate idiopathic pulmonary fibrosis. However, the current oral administration dosing regimen of PRF is complex and requires high doses. Patients are instructed to take PRF three times daily, with each dose consisting of up to three capsules or tablets (600 mg/d or 1.8 g/d of PRF) taken with food. To improve the dosing regimen, efforts are being made to develop an extended-release tablet with a zero-order release pattern. In this study, two types of extended-release matrix tablets were compared: non-channeled extended-release matrix tablets (NChMT) and channeled extended-release matrix tablets (ChMT). In vitro release tests, swelling and erosion index, rheology studies, and X-ray microcomputed tomography (XRCT), were conducted. The results indicated that ChMT maintained a zero-order release pattern with a constant release rate, while NChMT exhibited a decreased release rate in the latter half of the dissolution. ChMT exhibited accelerated swelling and erosion compared to other formulations, and this was made possible by the presence of channels within the tablet. These channels allowed for thorough wetting and swelling throughout the entire depth of the tablet. The formation of channels was confirmed through XRCT images. In conclusion, the presence of channels in ChMT tablets increased the rate of swelling and erosion, resulting in a zero-order release pattern. This development offers the potential to improve the dosage of PRF and reduce its associated side effects.

In vitro dissolution/permeation tools for amorphous solid dispersions bioavailability forecasting II: Comparison and mechanistic insights

Along with the increasing demand for complex formulations comes the need for appropriate in vitro methodologies capable of predicting their corresponding in vivo performance and the mechanisms controlling the drug release which can impact on in vivo drug absorption. In vitro dissolution-permeation (D/P) methodologies that can account for the effects of enabling formulations on the permeability of drugs are increasingly being used in performance ranking during early development stages. This work comprised the application of two different cell-free in vitro D/P setups: BioFLUX™ and PermeaLoop™ to evaluate the dissolution-permeation interplay upon drug release from itraconazole (ITZ)- HPMCAS amorphous solid dispersions (ASDs) of different drug loads. A solvent-shift approach was employed, from a simulated gastric environment to a simulated intestinal environment in the donor compartment. PermeaLoop™ was then combined with microdialysis sampling to separate the dissolved (free) drug from other species present in solution, like micelle-bound drug and drug-rich colloids, in real time. This setup was applied to clarify the mechanisms for drug release and permeation from these ASDs. In parallel, a pharmacokinetic study (dog model) was conducted to assess the drug absorption from these ASDs and to compare the in vivo results with the data obtained from each in vitro D/P setup, allowing to infer which would be the most adequate setup for ASD ranking. Even though both D/P systems resulted in the same qualitative ranking, BioFLUX™ overpredicted the difference between the in vivo AUC of two ASDs, whereas PermeaLoop™ permeation flux resulted in a good correlation with the AUC observed in pharmacokinetic studies (dog model) (R2 ≈ 0.98). Also, PermeaLoop™ combined with a microdialysis sampling probe clarified the mechanisms for drug release and permeation from these ASDs. It demonstrated that the free drug was the only driving force for permeation, while the drug-rich colloids kept permeation active for longer periods by acting as drug reservoirs and maintaining constant high levels of free drug in solution, which are then immediately able to permeate. Hence, the data obtained points BioFLUX™ and PermeaLoop™ applications to different momentums in the drug product development pipeline: while BioFLUX™, an automated standardized method, poses as a valuable tool for initial ASD ranking during the early development stages, PermeaLoop™ combined with microdialysis sampling allows to gain mechanistic understanding of the dissolution-permeation interplay, being crucial to fine tune and identify leading ASD candidates prior to in vivo testing.

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.

Amorphous Solid Dispersion as Drug Delivery Vehicles in Cancer

Cancer treatment has improved over the past decades, but a major challenge lies in drug formulation, specifically for oral administration. Most anticancer drugs have poor water solubility which can affect their bioavailability. This causes suboptimal pharmacokinetic performance, resulting in limited efficacy and safety when administered orally. As a result, it is essential to develop a strategy to modify the solubility of anticancer drugs in oral formulations to improve their efficacy and safety. A promising approach that can be implemented is amorphous solid dispersion (ASD) which can enhance the aqueous solubility and bioavailability of poorly water-soluble drugs. The addition of a polymer can cause stability in the formulations and maintain a high supersaturation in bulk medium. Therefore, this study aimed to summarize and elucidate the mechanisms and impact of an amorphous solid dispersion system on cancer therapy. To gather relevant information, a comprehensive search was conducted using keywords such as "anticancer drug" and "amorphous solid dispersion" in the PubMed, Scopus, and Google Scholar databases. The review provides an overview and discussion of the issues related to the ASD system used to improve the bioavailability of anticancer drugs based on molecular pharmaceutics. A thorough understanding of anticancer drugs in this system at a molecular level is imperative for the rational design of the products.