Purpose: The aim of the work is to study the crystallization of efavirenz to understand the preferential formation of various polymorphic forms, to establish their identity, to study the transformation between the polymorphic forms on heating and to determine their free energy.
Methods: Slow crystallization from different solvents under controlled conditions was employed to prepare various crystalline forms. The TGA and DSC were used to study their thermal behavior and inter-conversion of these forms. The calorimetrically determined enthalpies of solution and solubility data are utilized to determine the transition temperatures.
Results: Six polymorphic forms of efavirenz are identified and characterized completely. The TGA scans of all the forms did not show any mass loss indicating absence of hydrate or solvate. The thermally induced transformations are observed in the DSC scans of five forms II-VI indicating them to be metastable which are converted to stable higher melting forms. The melting temperature and enthalpy of fusion of lower melting (FormL) and higher melting forms (FormH) reveal that four of these polymorphic pairs are monotropically related. The enthalpies of solution of FormL are found to be more exothermic as compared to corresponding FormH. The transition temperature (Tt) determined using enthalpy of solution and solubility data was found to be higher than the melting of both the forms except for polymorphic pair VIL/VIH. The effect of ΔCp on transition temperature is also reported.
Conclusions: The form I is found to be thermodymanically most stable but least soluble. The forms II-V are metastable and are converted irreversibly to stable forms. The enthalpy of fusion rule and virtual transition temperature provided complementary evidence for the existence of monotropy in these polymorphic pairs. However, enantiotropy is demonstrated in VIL/VLH pair and is well established in our study.
Novelty: The present study reveals the thermodynamic aspects of various isolated polymorphic forms of efavirenz. Solution calorimetry along with other techniques is used to study the transformation of one form to another. The emphasis is laid on determination of transition temperature of various polymorphic pairs which has not been reported earlier.