The indications of tautomeric conversion in amorphous bicalutamide drug.

The investigation of tautomerization phenomenon in pharmaceutically relevant materials has important implications. The lack of knowledge about tautomeric preferences may negatively impact the formulation and manufacturing process as well as performance of drug product. In this paper we performed theoretical calculations to verify the occurrence of proton transfer in popular anti-androgen drug bicalutamide (BIC). Density functional theory (DFT) calculations determine the activation energy values for possible tautomeric paths providing a basis for comparison with experimental data. To find the indication of the presence of imidic acid and amide tautomers in amorphous BIC we applied infrared spectroscopy (IR). Finally, we performed isothermal broadband dielectric spectroscopy studies (BDS) to select the most likely mechanism of proton transfer in supercooled BIC. We found out that thermal processing applied during glass preparation via vitrification method results in the presence of amide and imidic acid forms in glassy BIC. Further heating leads to re-equilibration of supercooled BIC manifested by the growth of viscosity and effective dipole moment. Based on the value of activation energy determined in theoretical and experimental BDS studies we recognized that the observed time evolution of dielectric parameters likely reflects the increasing amount of more stable amide tautomer. The results presented herein indicate that in amorphous BIC in particular conditions the temperature-dependent changes in tautomeric composition due to intramolecular proton transfer are possible.