Aqueous solubility of crystalline and amorphous drugs: Challenges in measurement

Measurement of drug solubility is one of the key elements of active pharmaceutical ingredient (API) characterization during the drug discovery and development process. This report is a critical review of experimental methods reported in the literature for the measurement of aqueous solubility of amorphous, partially crystalline and crystalline organic compounds. A summary of high-throughput automated methods used in early drug discovery research is also provided in this report. This review summarizes the challenges that are encountered during solubility measurement and the complexities that are often overlooked. Even though there is an advantage in using the amorphous form of a drug due to its higher solubility, measurement of its solubility with useful accuracy is still a practical problem. Therefore, this review provides recommendations of preferred methods and precautions in using these methods to determine the aqueous solubility of amorphous and crystalline new molecular entities, with emphasis on the physico-chemical characterization of the solid state of the test substance.