Insights into Dissolution and Solution Chemistry of Multidrug Formulations of Antihypertensive Drugs

Physicochemical Properties and Molecular Insights of Favipiravir and Roflumilast Solid Dispersions for COVID-19 Treatment

Background/Objectives: Fixed-dose combinations (FDCs) offer significant advantages for patients and healthcare systems by improving adherence and reducing pill burden. However, developing multi-drug formulations remains challenging due to complexities in drug compatibility, stability, and dissolution behavior. The COVID-19 pandemic has necessitated innovative therapeutic approaches. This study aims to develop and evaluate an FDC containing FR (an antiviral drug) and RT (a PDE4 inhibitor) for potential COVID-19 treatment. Methods: The proposed dual-layer FDC was formulated to achieve immediate release of RT using Klucel EXF and controlled release of FR using a combination of Klucel HXF and Compritol ATO888. Critical quality attributes, including drug-excipient compatibility, solid-state properties, tablet uniformity, and dissolution kinetics, were assessed. RT and FR quantification methods were developed and validated per international guidelines. Compatibility studies were conducted by combining excipients in fixed ratios with APIs, followed by stability testing. Results: No degradation or adverse interactions were observed between APIs and excipients. RT exhibited rapid dissolution within 30 min, while FR release was effectively controlled through a gel-forming matrix and lipid barrier. Bulk powder and tablet physical parameters met pharmacopeial standards, and content uniformity between layers was maintained. The formulation demonstrated a stable dissolution profile for both drugs, ensuring consistent drug release. Conclusions: The novel FDC of RT and FR exhibits favorable physicochemical properties, a stable dissolution profile, and potential for improved treatment efficacy in COVID-19 patients. By optimizing drug release mechanisms and ensuring formulation stability, this FDC could serve as a pharmaco-economically viable alternative to existing therapies, enhancing patient compliance and treatment outcomes.

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.

Impact of surfactants on solution behavior and membrane transport of amorphous solid dispersions

The purpose of the study was to develop an amorphous solid dispersion (ASD) of a poorly soluble compound (AK100) and investigate the impact of different surfactants on its dissolution, supersaturation and membrane transport. The solubility of the AK100 was determined in crystalline and amorphous form in the absence and presence of three surfactants at different concentrations: sodium dodecyl sulphate (SDS), polysorbate 80 (PS80) and D-α-tocopherol polyethylene glycol succinate (TPGS). The relation between solubility and surfactant solubilization was evaluated using a computational model. The ASD powder was prepared by solvent evaporation for non-sink dissolution experiments with and without the pre-dissolved surfactants. A transport study with Caco-2 cells was conducted to evaluate the impact of surfactants-based formulation on membrane transport. Both the corresponding crystalline and amorphous solubility of AK100 increased linearly as a function of the surfactant concentrations. The supersaturation was maintained for at least three hours in absence of surfactant and in presence of TPGS, whereas supersaturation declined with SDS and PS80. As expected, the membrane flux of the AK100 was higher for the ASD than for the crystalline powder, and further increased with increased concentration of TPGS. The supersaturation ratio based on the activity-based calculation from Caco-2 cells study was always higher than that of the concentration-based one for the amorphous and crystalline forms of AK100. This study shows how additional solubilizing excipients during formulation development can improve the resulting dissolution and phase behavior of supersaturated drug solution.

Excipient effects on supersaturation, particle size dynamics, and thermodynamic activity of multidrug amorphous formulations

Multidrug formulations enhance patient compliance and extend the life cycle of pharmaceutical products. To overcome solubility challenges for multidrug combinations, amorphous formulations are commonly used. However, the excipients for creating amorphous formulations are often selected without an understanding of their effects on the bioavailability of the drugs. In this context, we investigated the impact of three types of excipients (polymers, surfactants and amino acids) on the supersaturation and thermodynamic activity of multidrug amorphous formulations. Additionally, we studied the particle size dynamics of the colloidal phase formed as a result of liquid-liquid phase separation. The amorphous solubility of two drugs, atazanavir and ritonavir, was determined in solutions containing predissolved excipients and the particle size dynamics of the colloidal particles was measured by dynamic light scattering. Dissolution experiments of atazanavir and ritonavir were conducted in predissolved sodium dodecyl sulfate (SDS), an anionic surfactant, and alanine solutions under non-sink conditions. Membrane transport of the drugs was evaluated using a MicroFLUX setup. The polymers had only minor effects on the amorphous solubility, but SDS significantly increased the solubilities of both drugs. In contrast, the other non-ionic surfactants and amino acids reduced the solubility of atazanavir but had no negative effect on ritonavir. Polymers were effective in maintaining supersaturation and preventing the coarsening of the colloidal particles. Conversely, alanine was neither able to inhibit the solution crystallization nor increase the flux of either drug. Despite the increase in the amorphous solubility of both drugs in SDS, flux was reduced. These results highlight the importance of properly selecting excipients for supersaturating amorphous formulations. The choice of excipient impacts the thermodynamic activity, the phase behaviour of the drugs and hence, the resulting absorption after oral intake.

Recent Advances in Enhancement of Dissolution and Supersaturation of Poorly Water-Soluble Drug in Amorphous Pharmaceutical Solids: A Review

Amorphization is one of the most effective pharmaceutical approaches to enhance the dissolution and oral bioavailability of poorly water-soluble drugs. In recent years, amorphous formulations have been experiencing rapid development both in theoretical and practical application. Based on using different types of stabilizing agents, amorphous formulations can be mainly classified as polymer-based amorphous solid dispersion, coamorphous formulation, mesoporous silica-based amorphous formulation, etc. This paper summarizes recent advances in the dissolution and supersaturation of these amorphous formulations. Moreover, we also highlight the roles of stabilizing agents such as polymers, low molecular weight co-formers, and mesoporous silica. Maintaining supersaturation in solution is a key factor for the enhancement of dissolution profile and oral bioavailability, and thus, the strategies and challenges for maintaining supersaturation are also discussed. With an in-depth understanding of the inherent mechanisms of dissolution behaviors, the design of amorphous pharmaceutical formulations will become more scientific and reasonable, leading to vigorous development of commercial amorphous drug products.

Impact of Simulated Intestinal Fluids on Dissolution, Solution Chemistry, and Membrane Transport of Amorphous Multidrug Formulations

The solution behavior and membrane transport of multidrug formulations were herein investigated in a biorelevant medium simulating fasted conditions. Amorphous multidrug formulations were prepared by the solvent evaporation method. Combinations of atazanavir (ATV) and ritonavir (RTV) and felodipine (FDN) and indapamide (IPM) were prepared and stabilized by a polymer for studying their dissolution (under non-sink conditions) and membrane transport in fasted state simulated intestinal fluid (FaSSIF). The micellar solubilization by FaSSIF enhanced the amorphous solubility of the drugs to different extents. Similar to buffer, the maximum achievable concentration of drugs in combination was reduced in FaSSIF, but the extent of reduction was affected by the degree of FaSSIF solubilization. Dissolution studies of ATV and IPM revealed that the amorphous solubility of these two drugs was not affected by FaSSIF solubilization. In contrast, RTV was significantly affected by FaSSIF solubilization with a 30% reduction in the maximum achievable concentration upon combination to ATV, compared to 50% reduction in buffer. This positive deviation by FaSSIF solubilization was not reflected in the mass transport–time profiles. Interestingly, FDN concentrations remain constant until the amount of IPM added was over 1000 μg/mL. No decrease in the membrane transport of FDN was observed for a 1:1 M ratio of FDN-IPM combination. This study demonstrates the importance of studying amorphous multidrug formulations under physiologically relevant conditions to obtain insights into the performance of these formulations after oral administration.