A method is described for screening compounds that inhibit crystallization in solution to enable more accurate measurement of amorphous drug solubility. Three polymers [polyvinylpyrrolidone, hydroxypropyl methylcellulose, and hydroxypropyl methylcellulose acetate succinate (HPMCAS)] were screened for their ability to inhibit the crystallization of neat amorphous drugs during measurement of solubility of the amorphous form in water. Among the polymers evaluated, HPMCAS was found to be most promising. The use of HPMCAS provided an "apparent solubility" of amorphous drugs that was closer to the theoretically calculated values. With danazol, agreement was essentially quantitative, and for griseofulvin and iopanoic acid, agreement was within a factor of two; these maximum concentrations were sustained for a period of 40-90 min. Dynamic light scattering of filtered samples (0.22 µ) revealed the presence of colloidal drug-polymer assemblies in solution (100-150 nm). The supernatant resulting from this centrifugation gradually decreased in concentration, but remained supersaturated with respect to crystalline drug for several hours. Thus, HPMCAS has been shown to be a useful additive in dissolution media to allow a more accurate determination of the solubility of fast crystallizing neat amorphous drugs, at least for the drugs studied, and it should also serve to retard crystallization in vivo and therefore, facilitate improved bioavailability.
Keywords: amorphous; cellulosic polymers; crystal; polymers; recrystallization; solubility; supersaturation.
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