Influence of variation in molar ratio on co-amorphous drug-amino acid systems

Advances in coamorphous drug delivery systems

In recent years, the coamorphous drug delivery system has been established as a promising formulation approach for delivering poorly water-soluble drugs. The coamorphous solid is a single-phase system containing an active pharmaceutical ingredient (API) and other low molecular weight molecules that might be pharmacologically relevant APIs or excipients. These formulations exhibit considerable advantages over neat crystalline or amorphous material, including improved physical stability, dissolution profiles, and potentially enhanced therapeutic efficacy. This review provides a comprehensive overview of coamorphous drug delivery systems from the perspectives of preparation, physicochemical characteristics, physical stability, in vitro and in vivo performance. Furthermore, the challenges and strategies in developing robust coamorphous drug products of high quality and performance are briefly discussed.

Co-amorphous solid dispersion systems of lacidipine-spironolactone with improved dissolution rate and enhanced physical stability

Co-amorphous solid dispersion (C-ASD) systems have attracted great attention to improve the solubility of poorly soluble drugs, but the selection of an appropriate stabilizer to stabilize amorphous forms is still a huge challenge. Herein, C-ASD system of two clinical combined used drugs (lacidipine (LCDP) and spironolactone (SPL)) as stabilizers to each other, was prepared by solvent evaporation method. The effects of variation in molar ratio of LCDP and SPL (3:1, 1:1, 1:3, 1:6, and 1:9) on the drug release characteristics were explored. Polarized light microscopy (PLM), powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were employed to evaluate the solid states. Prepared C-ASDs were further studied for their stability under the high humidity (RH 92.5%). Further analysis of C-ASDs via Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy confirmed that hydrogen bond interactions between the two drugs played a significant role in maintaining the stability of the C-ASDs systems. Moreover, molecular dynamic (MD) simulations provided a clear insight into the stability mechanism at the molecular level. This study demonstrated the novel drug-drug C-ASDs systems is a promising formulation strategy for improved dissolution rate and enhanced physical stability of poorly soluble drugs.

The Role of Glass Transition Temperatures in Coamorphous Drug-Amino Acid Formulations

The improved physical stability associated with coamorphous drug-amino acid (AA) formulations may indicate a decrease in mobility of the amorphous drug molecules, compared to the neat amorphous form of the drug. Since the characteristic glass transition temperatures ( T_gα and T_gβ) represent molecular mobility in amorphous systems, we aimed to characterize T_gα and T_gβ and to determine their role in physical stability as well as their potential usefulness to determine the presence of an excess component (either drug or AA) in coamorphous systems. Indomethacin (IND)-tryptophan (TRP) and carvedilol (CAR)-TRP were used as model coamorphous systems. The analytical techniques used were X-ray powder diffractometry (XRPD) to determine the solid-state form, dynamic mechanical analysis (DMA) to probe T_gα and T_gβ, and differential scanning calorimetry (DSC) to probe thermal behavior of the coamorphous systems. T_gα analysis showed a gradual monotonous increase in T_gα values with increasing AA concentration, and this increase in the T_gα value is not the cause of the improved physical stability. The T_gβ analysis for the IND-TRP sample with 10% drug had a T_gβ of 226.8 K, and samples with 20-90% drug had similar T_gβ values around 212.5 K. For CAR-TRP, samples with 10-40% drug had similar T_gβ values around 230.5 K, and samples with 50-90% drug had similar T_gβ values around 223.3 K. The similar T_gβ values in coamorphous systems at different drug ratios indicate that they in fact are the T_gβ of the component that is in excess to an ideal drug-AA coamorphous mixture. DSC and XRPD analysis showed that for IND-TRP, IND is in excess if the drug concentration is 30% or above and will eventually recrystallize. For CAR-TRP, CAR is in excess and recrystallizes when the drug concentration is 50% or above. We have proposed a means of estimating, on the basis of T_gβ, which drug to AA ratios will lead to optimally physically stable coamorphous systems that can be considered for further development.

Co-Amorphous Solid Dispersions for Solubility and Absorption Improvement of Drugs: Composition, Preparation, Characterization and Formulations for Oral Delivery

The amorphous solid state offers an improved apparent solubility and dissolution rate. However, due to thermodynamic instability and recrystallization tendencies during processing, storage and dissolution, their potential application is limited. For this reason, the production of amorphous drugs with adequate stability remains a major challenge and formulation strategies based on solid molecular dispersions are being exploited. Co-amorphous systems are a new formulation approach where the amorphous drug is stabilized through strong intermolecular interactions by a low molecular co-former. This review covers several topics applicable to co-amorphous drug delivery systems. In particular, it describes recent advances in the co-amorphous composition, preparation and solid-state characterization, as well as improvements of dissolution performance and absorption are detailed. Examples of drug-drug, drug-carboxylic acid and drug-amino acid co-amorphous dispersions interacting via hydrogen bonding, π−π interactions and ionic forces, are presented together with corresponding final dosage forms.

Undesired co-amorphisation of indomethacin and arginine during combined storage at high humidity conditions

The use of co-amorphous systems for solubility enhancement of poorly water-soluble drugs has recently gained interest in the field of pharmaceutical technology. However, undesired co-amorphisation of a drug may lead to an alteration of the performance of the drug product, e.g. the previously observed co-amorphisation of indomethacin and arginine upon storage of tablets containing both components in an initially crystalline form at room temperature (RT) and 75% relative humidity (RH). Therefore, the aim of the present study was to further investigate this unintended co-amorphisation by storing plain crystalline γ-indomethacin and arginine as well as physical mixtures of both components at RT and three different RH levels (28, 58, and 75% RH). After storage for up to 101 days, their properties were analysed by X-ray powder diffraction, infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and HPLC. Results showed that the solid state of plain γ-indomethacin did not change during storage at all three storage conditions. In contrast, arginine was found to form a dihydrate upon storage at RT/58% RH and RT/75% RH. The physical mixtures, stored at RT/28% RH and RT/58% RH, remained crystalline and were chemically stable, while the formation of a co-amorphous salt between indomethacin and arginine as well as basic hydrolysis of indomethacin started already 1 day after exposure to RT/75% RH. Moreover, formation of a crystalline salt of indomethacin and arginine upon storage at RT/75% RH was observed. As neither of these instabilities occurred, if indomethacin was stored separately, the simultaneous effects of arginine and moisture on the solid state properties and chemical stability of indomethacin should be taken into account, if selecting arginine as excipient.

Characterization of systems with amino-acids and oligosaccharides as modifiers of biopharmaceutical properties of furosemide

Furosemide is the most commonly prescribed diuretic drug in spite of its suboptimal biopharmaceutical properties. In this work, the addition of different amino-acids was studied with the aim of selecting an enhancer of the furosemide solubility. The best results were obtained with arginine. Also, binary (furosemide:arginine) and ternary (furosemide:arginine:β-cyclodextrin and furosemide:arginine:maltodextrin) systems were prepared by the kneading method and they were compared with their corresponding physical mixtures. These new systems were characterized by Fourier transform infrared and Raman spectroscopy, X-ray powder diffractometry, scanning electron microscopy, thermogravimetric analysis, and differential scanning calorimetry. In addition, dissolution studies were performed in simulated gastric fluid. The best results in relation to improving biopharmaceutical properties were obtained with a binary combination of furosemide and arginine, demonstrating that this system could result in a suitable candidate for the development of a promising pharmaceutical formulation of the drug.

On the role of salt formation and structural similarity of co-formers in co-amorphous drug delivery systems

Co-amorphous drug delivery systems based on amino acids as co-formers have shown promising potential to improve the solubility and bioavailability of poorly water-soluble drugs. Potential salt formation is assumed to be a key molecular interaction responsible for amorphous stability and increased solubility. However, little is known about the importance of the overall structure of the co-former. In this study, the structurally related amino acids arginine (basic) and citrulline (neutral) were chosen together with four model drugs (acidic furosemide and nitrofurantoin; basic cimetidine and mebendazole) to investigate the importance of salt formation versus structural similarity of co-formers. Drug-amino acid mixtures were ball milled at a molar ratio of 1:1. Generally, arginine showed a higher tendency to successfully form co-amorphous systems with the model drugs compared with citrulline, irrespective of assumed salt formation. Salt forming mixtures showed much higher T_gs, faster dissolution rates, higher solubility and physical stability compared to the corresponding non-salt forming mixtures. In conclusion, structural similarity of the co-formers does not lead to similar co-former performance for a given drug. Salt formation is not a prerequisite for the formation of a co-amorphous system, but if a co-amorphous salt system is formed, improved dissolution rate and physical stability are observed.

Coamorphous Active Pharmaceutical Ingredient-Small Molecule Mixtures: Considerations in the Choice of Coformers for Enhancing Dissolution and Oral Bioavailability

In the recent years, coamorphous systems, containing an active pharmaceutical ingredient (API) and a small molecule coformer have appeared as alternatives to the use of either amorphous solid dispersions containing polymer or cocrystals of API and small molecule coformers, to improve the dissolution and oral bioavailability of poorly soluble crystalline API. This Commentary article considers the relative properties of amorphous solid dispersions and coamorphous systems in terms of methods of preparation; miscibility; glass transition temperature; physical stability; hygroscopicity; and aqueous dissolution. It also considers important questions concerning the fundamental criteria to be used for the proper selection of a small molecule coformer regarding its ability to form either coamorphous or cocrystal systems. Finally, we consider various aspects of product development that are specifically associated with the formulation of commercial coamorphous systems as solid oral dosage forms. These include coformer selection; screening; methods of preparation; preformulation; physical stability; bioavailability; and final formulation. Through such an analysis of coamorphous API-small molecule coformer systems, against the more widely studied API-polymer dispersions and cocrystals, it is believed that the strengths and weaknesses of coamorphous systems can be better understood, leading to more efficient formulation and manufacture of such systems for enhancing oral bioavailability.

Studying of crystal growth and overall crystallization of naproxen from binary mixtures

Broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) were applied to investigate the molecular dynamics and phase transitions in binary mixtures composed of naproxen (NAP) and acetylated saccharides: maltose (acMAL) and sucrose (acSUC). Moreover, the application of BDS method and optical microscopy enabled us to study both crystallization kinetics and crystal growth of naproxen from the solid dispersions with the highest content of modified carbohydrates (1:5wt ratio). It was found that the activation barriers of crystallization estimated from dielectric measurements are completely different for both studied herein mixtures. Much higher E_a (=205kJ/mol) was obtained for NAP-acMAL solid dispersion. It is probably due to simultaneous crystallization of both components of the mixture. On the other hand, lower value of E_a in the case of NAP-acSUC solid dispersion (81kJ/mol) indicated, that naproxen is the only crystallizing compound. This hypothesis was confirmed by X-ray diffraction studies. We also suggested that specific intermolecular dipole-dipole interactions between active substance and excipient may be an alternative explanation for the difference between activation barrier obtained for NAP-acMAL and NAP-acSUC binary mixtures. Furthermore, optical measurements showed that the activation energy for crystal growth of naproxen increases in binary mixtures. They also revealed that both excipients: acMAL and acSUC move the temperature of the maximum of crystal growth towards lower temperatures. Interestingly, this maximum occurs for nearly the same structural relaxation time, which is a good approximation of viscosity, for all samples. Finally, it was also noticed that although naproxen crystallizes to the same polymorphic form in both systems, there are some differences in morphology of obtained crystals. Thus, the observed behavior may have a significant impact on the bioavailability and dissolution rate of API produced in that way.