Phase separation of supersaturated solution created from amorphous solid dispersions: Relevance to oral absorption

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.

Biorelevant Dissolution Method Considerations for the Appropriate Evaluation of Amorphous Solid Dispersions: are Two Stages Necessary?

Biorelevant dissolution testing has been widely used to better understand a drug or formulation's behavior in the human gastrointestinal (GI) tract. The successful evaluation of biorelevant dissolution behavior requires recognizing the importance of utilizing suitable biorelevant media in conjunction with an appropriate dissolution method, especially for supersaturating drug delivery systems, such as amorphous solid dispersions (ASDs). However, most conventional biorelevant dissolution testing methods are not able to accurately reflect the dissolution, supersaturation, and precipitation tendencies of a drug or formulation, which could misinform ASD formulation screening and optimization. In this study, we developed a single compartment 2-stage pH-shift dissolution testing method to simulate the changes in pH, media composition, and transit time in the GI tract, and results were compared against the conventional single compartment 1-stage dissolution method. Nine model drugs were selected based on their ionization properties (i.e. acid, base or neutral) and precipitation tendency (i.e. moderate or slow crystallizer). The dissolution results confirmed that 2-stage pH-shift dissolution is the preferred biorelevant dissolution method to assess non-ionized weak base (nifedipine) and neutral (griseofulvin) compounds exhibiting a moderate precipitation rate from solution when formulated as ASDs. Finally, we designed a flowchart guidance for the appropriate biorelevant dissolution performance characterization of different categories of ASD formulations.

Inhibition of Liquid-Liquid Phase Separation for Breaking the Solubility Barrier of Amorphous Solid Dispersions to Improve Oral Absorption of Naftopidil

Amorphous solid dispersion (ASD) is one of the most promising technologies for improving the oral absorption of poorly soluble compounds. In this study, naftopidil (NFT) ASDs were prepared using vinylpyrrolidone-vinyl acetate copolymer (PVPVA), hydroxypropyl methylcellulose acetate succinate (HPMCAS), and poly(methacrylic acid-co-methyl methacrylate) L100-55 (Eudragit) to improve the dissolution and oral absorption behaviors of NFT. During the dissolution process of ASD, liquid-liquid phase separation (LLPS) may occur when certain requirements are met for providing a maximum quasi-stable concentration achievable by amorphization. The occurrence of LLPS was confirmed in the presence of PVPVA and HPMCAS; however, Eudragit inhibited LLPS owing to its molecular interaction with NFT. Although the dissolution behavior of the Eudragit ASD was found to be markedly poorer than that of other ASDs, it offered the best oral absorption in rats. The findings of the current study highlight the possibility for improving the oral absorption of poorly soluble drugs by this ASD, which should be eliminated from candidate formulations based on the conventional in vitro tests.

Preparation of Solid Self-Nanoemulsifying Drug Delivery Systems (S-SNEDDS) by Co-Extrusion of Liquid SNEDDS and Polymeric Carriers-A New and Promising Formulation Approach to Improve the Solubility of Poorly Water-Soluble Drugs

The present study focused on a new formulation approach to improving the solubility of drugs with poor aqueous solubility. A hot melt extrusion (HME) process was applied to prepare drug-loaded solid self-nanoemulsifying drug delivery systems (S-SNEDDS) by co-extrusion of liquid SNEDDS (L-SNEDDS) and different polymeric carriers. Experiments were performed with L-SNEDDS formulations containing celecoxib, efavirenz or fenofibrate as model drugs. A major objective was to identify a polymeric carrier and process parameters that would enable the preparation of stable S-SNEDDS without impairing the release behavior and storage stability of the L-SNEDDS used and, if possible, even improving them further. In addition to commercially available (co)polymers already used in the field of HME, a particular focus was on the evaluation of different variants of a recently developed aminomethacrylate-based copolymer (ModE) that differed in M_w. Immediately after preparation, the L-SNEDDS and S-SNEDDS formulations were tested for amorphicity by differential scanning calorimetry. Furthermore, solubility and dissolution tests were performed. In addition, the storage stability was investigated at 30 °C/65% RH over a period of three and six months, respectively. In all cases, amorphous formulations were obtained and, especially for the model drug celecoxib, S-SNEDDS were developed that maintained the rapid and complete drug release of the underlying L-SNEDDS even over an extended storage period. Overall, the data obtained in this study suggest that the presented S-SNEDDS approach is very promising, provided that drug-loaded L-SNEDDS are co-processed with a suitable polymeric carrier. In the case of celecoxib, the E-173 variant of the novel ModE copolymer proved to be a novel polymeric carrier with great potential for application in S-SNEDDS. The presented approach will, therefore, be pursued in future studies to establish S-SNEDDS as an alternative formulation to other amorphous systems.

A Novel Aminomethacrylate-Based Copolymer for Solubility Enhancement-From Radical Polymer Synthesis to Manufacture and Characterization of Amorphous Solid Dispersions

The present study covers the synthesis, purification and evaluation of a novel aminomethacrylate-based copolymer in terms of its suitability for improving the solubility and in vitro release of poorly water-soluble drug compounds. The new copolymer was synthesized by solvent polymerization with radical initiation and by use of a chain transfer agent. Based on its composition, it can be considered as a modified type of dimethylaminoethyl methacrylate-butyl methacrylate-methyl methacrylate "EUDRAGIT^® E PO" (ModE). ModE was specifically developed to provide a copolymer with processing and application properties that exceed those of commercially available (co-)polymers in solubility enhancement technologies where possible. By varying the concentration of the chain transfer agent in the radical polymerization process, the molecular weight of ModE was varied in a range of 173-305 kDa. To evaluate the solubility-enhancing properties of ModE, a series of drug-loaded extrudates were prepared by hot melt extrusion using the novel-as well as several commercially available-(co-)polymers. These extrudates were then subjected to comparative tests for amorphousness, solubility-enhancing properties, storage stability, and drug release. Celecoxib, efavirenz, and fenofibrate were used as model drugs in all experiments. Of all the (co-)polymers included in the study, ModE with a molecular weight of 173 kDa showed the best performance in terms of desired properties and was shown to be particularly suitable for preparing amorphous solid dispersions (ASDs) of the three model drugs, which in a first set of dissolution experiments showed better release behavior under pH conditions of the fasting stomach than higher molecular weight ModE types, as well as a variety of commercially available (co-)polymers. Therefore, the results demonstrate the successful synthesis of a new copolymer, which in future studies will be investigated in more detail for universal application in the field of solubility enhancement.

Hot-melt extruded hydroxypropyl methylcellulose acetate succinate based amorphous solid dispersions: Impact of polymeric combinations on supersaturation kinetics and dissolution performance

Nucleation inhibition and maintenance of drug supersaturation over a prolonged period are desirable for improving oral absorption of amorphous solid dispersions. The present study investigates the impact of binary and ternary amorphous solid dispersions on the supersaturation kinetics of nifedipine using the polymers hydroxypropylmethylcellulose acetate succinate (HPMCAS) LG, and HG, Eudragit® RSPO, Eudragit® FS100, Kollidon® VA64 and Plasdone™ K-29/32. The amorphous solubility, nucleation induction time, and particle size analysis of nifedipine in a supersaturated solution were performed with and without the presence of polymers, alone or in combination. The HPMCAS-HG and HPMCAS-HG + LG combinations showed the highest nifedipine amorphous solubility of 169.47, 149.151 µg/mL, respectively and delay in nucleation induction time up to 120 min compared to other polymeric combinations. The solid dispersions prepared via hot melt extrusion showed the transformation of crystalline nifedipine to amorphous form. The in-vitro non-sink dissolution study revealed that although the binary nifedipine/HPMCAS-LG system had shown the greater supersaturation concentration of 66.1 µg/mL but could not maintain a supersaturation level up to 360 min. A synergistic effect emerged for ternary nifedipine/HPMCAS-LG/HPMCAS-HG, and nifedipine/HPMCAS-LG/Eudragit®FS100 systems maintained the supersaturation level with enhanced dissolution performance, demonstrating the potential of polymeric combinations for improved amorphous solid dispersion performance.

Amorphous solid dispersions of cyclosporine A with improved bioavailability prepared via hot melt extrusion: Formulation, physicochemical characterization, and in vivo evaluation

In this study, the amorphous solid dispersions of cyclosporine A (CsA-ASDs) were prepared by hot melt extrusion (HME) with PVP K12 as carrier to improve the oral bioavailability of CsA. The polymers were screened by solubilization and recrystallization inhibition experiments, then the CsA-ASDs were prepared with optimized technological parameters and characterized on thermodynamics and morphology. The results showed that CsA was dispersed among PVP K12 as amorphous form in CsA-ASDs, and the infrared spectrum testified that there was possible hydrogen bond interaction between CsA and PVP K12. The in vivo pharmacokinetics of CsA formulations in rats were analyzed via LC-MS. The AUC of CsA-ASD tablets increased by 7.3 times compared to CsA bulk powder and 3.1 times in contrast to CsA-PM tablets, respectively. The experiment proved that CsA-ASD tablets significantly improved the dissolution and absorption of the drug. This study had a reference value for the bioavailability improvement of oral CsA preparations.

Application of UV dissolution imaging to pharmaceutical systems

UV-vis spectrometry is widely used in the pharmaceutical sciences for compound quantification, alone or in conjunction with separation techniques, due to most drug entities possessing a chromophore absorbing light in the range 190-800 nm. UV dissolution imaging, the scope of this review, generates spatially and temporally resolved absorbance maps by exploiting the UV absorbance of the analyte. This review aims to give an introduction to UV dissolution imaging and its use in the determination of intrinsic dissolution rates and drug release from whole dosage forms. Applications of UV imaging to non-oral formulations have started to emerge and are reviewed together with the possibility of utilizing UV imaging for physical chemical characterisation of drug substances. The benefits of imaging drug diffusion and transport processes are also discussed.

Role of Permeability on the Biopredictive Dissolution of Amorphous Solid Dispersions

An ideal dissolution test for amorphous solid dispersions (ASDs) should reflect physicochemical, physiological, and hydrodynamic conditions which accurately represent in vivo dissolution. However, this is confounded by the evolution of different molecular and colloidal species during dissolution, generating a supersaturated state of the drug. The supersaturated state of a drug is thermodynamically unstable which drives the process of precipitation resulting in a loss of solubility advantage. Maintaining a supersaturated state of the drug with the help of precipitation inhibiting excipients is a key component in the design of ASDs. Therefore, a biopredictive dissolution test is critical for proper risk assessment during the development of an optimal ASD formulation. One of the overlooked components of biopredictive dissolution is the role of drug permeability. The kinetic changes in the phase behavior of a drug during dissolution of ASDs are influenced by drug permeability across a membrane. Conventionally, drug dissolution and permeation are analyzed separately although they occur simultaneously in vivo. The kinetic phase changes occurring during dissolution of ASDs can influence the thermodynamic activity and membrane flux of a drug. The present review evaluates the feasibility, predictability, and practicability of permeability/dissolution for the optimal development and risk assessment of ASD formulations.

Biorelevant dissolution testing and physiologically based absorption modeling to predict in vivo performance of supersaturating drug delivery systems

Supersaturating drug delivery systems (SDDS) enhance the oral absorption of poorly water-soluble drugs by achieving a supersaturated state in the gastrointestinal tract. The maintenance of a supersaturated state is decided by the complex interplay among inherent properties of drug, excipients and physiological conditions of gastrointestinal tract. The biopharmaceutical advantage through SDDS can be mechanistically investigated by coupling biopredictive dissolution testing with physiologically based absorption modeling (PBAM). However, the development of biopredictive dissolution methods possess challenges due to concurrent dissolution, supersaturation, precipitation, and possible redissolution of precipitates during gastrointestinal transit of SDDS. In this comprehensive review, our effort is to critically assess the current state-of-knowledge and provide future directions for PBAM of SDDS. The review outlines various methods used to retrieve physiologically relevant values for input parameters like solubility, dissolution, precipitation, lipid-digestion and permeability of SDDS. SDDS-specific parameterization includes solubility values corresponding to apparent physical form, dissolution in physiologically relevant volumes with biorelevant media, and transfer experiments to incorporate precipitation kinetics. Interestingly, the lack of experimental permeability values and modification of absorption flux through SDDS possess the additional challenge for its PBAM. Supersaturation triggered permeability modifications are reported to fit the observed plasma concentration-time profile. Hence, the experimental insights on good fitting with modified permeability can be potential area of future research for the development of in vitro methods to reliably predict oral absorption of SDDS.

Relevance of Liquid-Liquid Phase Separation of Supersaturated Solution in Oral Absorption of Albendazole from Amorphous Solid Dispersions

Amorphous solid dispersion (ASD) is one of the most promising formulation technologies for improving the oral absorption of poorly soluble drugs, where the maintenance of supersaturation plays a key role in enhancing the absorption process. However, quantitative prediction of oral absorption from ASDs is still difficult. Supersaturated solutions can cause liquid-liquid phase separation through the spinodal decomposition mechanism, which must be adequately comprehended to understand the oral absorption of drugs quantitatively. In this study, albendazole (ALZ) was formulated into ASDs using three types of polymers, poly(methacrylic acid-co-methyl methacrylate) (Eudragit) L100, Vinylpyrrolidone-vinyl acetate copolymer (PVPVA), and hydroxypropyl methylcellulose acetate succinate (HPMCAS). The oral absorption of ALZ in rats administered as ASD suspensions was not explained by dissolution study but was predicted using liquid-liquid phase separation concentration, which suggested that the absorption of ALZ was solubility-limited. The oral administration study in dogs performed using solid capsules demonstrated the low efficacy of ASDs because the absorption was likely to be limited by dissolution rate, which indicated the importance of designing the final dosage form of the ASDs.

Amorphous Solid Dispersions and the Contribution of Nanoparticles to In Vitro Dissolution and In Vivo Testing: Niclosamide as a Case Study

We developed an amorphous solid dispersion (ASD) of the poorly water-soluble molecule niclosamide that achieved a more than two-fold increase in bioavailability. Notably, this niclosamide ASD formulation increased the apparent drug solubility about 60-fold relative to the crystalline material due to the generation of nanoparticles. Niclosamide is a weakly acidic drug, Biopharmaceutics Classification System (BCS) class II, and a poor glass former with low bioavailability in vivo. Hot-melt extrusion is a high-throughput manufacturing method commonly used in the development of ASDs for increasing the apparent solubility and bioavailability of poorly water-soluble compounds. We utilized the polymer poly(1-vinylpyrrolidone-co-vinyl acetate) (PVP–VA) to manufacture niclosamide ASDs by extrusion. Samples were analyzed based on their microscopic and macroscopic behavior and their intermolecular interactions, using differential scanning calorimetry (DSC), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR), and dynamic light scattering (DLS). The niclosamide ASD generated nanoparticles with a mean particle size of about 100 nm in FaSSIF media. In a side-by-side diffusion test, these nanoparticles produced a four-fold increase in niclosamide diffusion. We successfully manufactured amorphous extrudates of the poor glass former niclosamide that showed remarkable in vitro dissolution and diffusion performance. These in vitro tests were translated to a rat model that also showed an increase in oral bioavailability.

Crystallization Tendency of Pharmaceutical Glasses: Relevance to Compound Properties, Impact of Formulation Process, and Implications for Design of Amorphous Solid Dispersions

Amorphous solid dispersions (ASDs) are important formulation strategies for improving the dissolution process and oral bioavailability of poorly soluble drugs. Physical stability of a candidate drug must be clearly understood to design ASDs with superior properties. The crystallization tendency of small organics is frequently estimated by applying rapid cooling or a cooling/reheating cycle to their melt using differential scanning calorimetry. The crystallization tendency determined in this way does not directly correlate with the physical stability during isothermal storage, which is of great interest to pharmaceutical researchers. Nevertheless, it provides important insights into strategy for the formulation design and the crystallization mechanism of the drug molecules. The initiation time for isothermal crystallization can be explained using the ratio of the glass transition and storage temperatures (T_g/T). Although some formulation processes such as milling and compaction can enhance nucleation, the T_g/T ratio still works for roughly predicting the crystallization behavior. Thus, design of accelerated physical stability test may be possible for ASDs. The crystallization tendency during the formulation process and the supersaturation ability of ASDs may also be related to the crystallization tendency determined by thermal analysis. In this review, the assessment of the crystallization tendency of pharmaceutical glasses and its relevance to developmental studies of ASDs are discussed.

Effect of preparation method on the surface properties and UV imaging of indomethacin solid dispersions

This work explores the use of UV imaging in solid dispersion systems. Solid dispersions are one of the common strategies used in improving the dissolution of poorly soluble drugs. Three manufacturing techniques (spray drying (SD), freeze drying (FD) and homogenising (HG)) are investigated. Differential Scanning Calorimetry (DSC) and X-Ray Powder Diffraction (XRPD) was used in characterising the solid dispersions. Advanced imaging was implemented to give an insight into how these solid dispersions performed. The DSC and XRPD results showed that all three methods and the various ratios studied produced amorphous solid dispersions. Ultra-Violet (UV) imaging of the pseudo Intrinsic Dissolution Rate (IDR) deduced only two samples to have superior pseudo IDR values to the IDR of the parent drug indomethacin (INDO). The whole dose imaging of the capsule formulation however showed all the samples (SD, FD and HG) to have superior dissolution to that of INDO which was in contrast to the IDR results. The UV images obtained from the determination of the pseudo IDR also showed a phenomenon the authors are reporting for the first time where increased polymer (Soluplus) content produced "web-like" strands that migrated to the top of the quartz cell which may have been responsible for the low pseudo IDR values. The authors also report for the first time using this UV imaging technique, the tip of a capsule coming off for drug to go into solution. The area under the curve suggested the best five samples dissolution wise to be 1:3 SD > 1:1 HG > 1:1 SD > 1:3 FD > 1:3 HG meaning a ratio of INDO to SOL in these dispersion of up to 1:3 being sufficient to produce significant dissolution increases. The developed interfacial (surface) area ratio (Sdr) highlighted how the surface area of the IDR compacts varied between the batches, in particular highlighting larger surface area gains for the FD and HG compacts. A choice of instrumentation/techniques to use in making solid dispersions may well come down to cost or instrument availability for a formulator as all three techniques were successful in improving the dissolution of indomethacin. This work thus highlights the importance of having both complimentary IDR and whole dosage imaging techniques in giving a better understanding of solid dispersion systems.