Solubility Advantage of Amorphous Pharmaceuticals: II. Application of Quantitative Thermodynamic Relationships for Prediction of Solubility Enhancement in Structurally Diverse Insoluble Pharmaceuticals

• Published in: Pharmaceutical research 2010-12, Vol.27 (12), p.2704-2714
• Main Authors: Murdande, Sharad B, Pikal, Michael J, Shanker, Ravi M, Bogner, Robin H
• Format: Article
• Language: English
• Subjects: amorphous; Biochemistry; Biological and medical sciences; Biomedical and Life Sciences; Biomedical Engineering and Bioengineering; Biomedical materials; Biomedicine; Chromatography, High Pressure Liquid; crystal; Crystallization; dissolution; General pharmacology; Medical Law; Medical sciences; Models, Chemical; Molecular Structure; Pharmaceutical Preparations - chemistry; Pharmaceutical sciences; Pharmaceutical technology. Pharmaceutical industry; Pharmacology. Drug treatments; Pharmacology/Toxicology; Pharmacy; re-crystallization; Research Paper; Solubility; solubility enhancement; solute activity in amorphous; Thermodynamics
• ISSN: 0724-8741; 1573-904X
• DOI: 10.1007/s11095-010-0269-5

Purpose To quantitatively assess the solubility advantage of amorphous forms of nine insoluble drugs with a wide range of physico-chemical properties utilizing a previously reported thermodynamic approach. Methods Thermal properties of amorphous and crystalline forms of drugs were measured using modulated differential calorimetry. Equilibrium moisture sorption uptake by amorphous drugs was measured by a gravimetric moisture sorption analyzer, and ionization constants were determined from the pH-solubility profiles. Solubilities of crystalline and amorphous forms of drugs were measured in de-ionized water at 25°C. Polarized microscopy was used to provide qualitative information about the crystallization of amorphous drug in solution during solubility measurement. Result For three out the nine compounds, the estimated solubility based on thermodynamic considerations was within two-fold of the experimental measurement. For one compound, estimated solubility enhancement was lower than experimental value, likely due to extensive ionization in solution and hence its sensitivity to error in pKa measurement. For the remaining five compounds, estimated solubility was about 4- to 53-fold higher than experimental results. In all cases where the theoretical solubility estimates were significantly higher, it was observed that the amorphous drug crystallized rapidly during the experimental determination of solubility, thus preventing an accurate experimental assessment of solubility advantage. Conclusion It has been demonstrated that the theoretical approach does provide an accurate estimate of the maximum solubility enhancement by an amorphous drug relative to its crystalline form for structurally diverse insoluble drugs when recrystallization during dissolution is minimal.