Polymorphs,
cocrystals, solvates, and hydrates have been reported for efavirenz
(EFV), which is part of high activity antiretroviral therapy (HAART),
and it is considered to be the best choice in the treatment of adults
and children. However, studies about thermodynamic stability and improvement
of dissolution properties have been rarely reported for the anhydrous
polymorphic forms. Therefore, the aim of this work was to characterize
the solid state of anhydrous polymorph I and polymorph II (herein
obtained), to study the thermodynamic stability and strategies to
improve the dissolution properties. In addition, techniques such as,
X-ray powder diffraction (XRPD), differential scanning calorimetry
(DSC), hot stage microscopy (HSM), scanning electron microscopy (SEM),
Fourier transform infrared (FT-IR), raman spectroscopy (RS), theoretical
calculations, and solid-state nuclear magnetic resonance (ss-NMR)
were used to complete this work. Thermodynamic studies showed that
polymorphs I and II are enantiotropically related with the isoenergetic
point between 35 and 40 °C. The EFV polymorph II showed itself
to be more stable and 10-fold more soluble than polymorph I, due to
modifications of morphology. Therefore, polymorph II could be an excellent
candidate with significant advantages for pharmaceutical formulations.
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