Kinetics and mechanism of hydrolysis of efavirenz

To determine the kinetics and mechanism of hydrolysis of efavirenz [(S)-6-chloro-4-(cyclopropylethynyl)-1,4-dihydro-4-(trifluoromethyl)-2H-3,1-benzoxazin-2-one] in aqueous solutions.

The solution stability was examined at 60 degrees C and an ionic strength of 0.3 M over the pH range of 0.6 to 12.8. The loss of efavirenz and the appearance of degradants were followed with a reverse-phase high-performance liquid chromatography assay. Characterization of the degradants was accomplished with liquid chromatography-mass spectrometry.

The degradation of efavirenz followed apparent first-order kinetics over the pH range of 0.6 to 12.8 at 60 degrees C. The catalytic effect of phosphate and borate buffers was negligible while acetate and citrate demonstrated buffer catalysis. The overall rate constant indicated a pH minimum (the pH of maximum stability) of approximately 4. Mass spectra data identified a degradant with a molecular weight consistent with hydrolysis of the cyclic carbamate to the corresponding amino alcohol. The degradation route was confirmed with spiking experiments with an authentic sample of the amino alcohol indicating that the carbamate hydrolysis pathway was the predominant reaction throughout the pH range studied. Subsequent degradation of the amino alcohol proceeded at the extremes of the pH range studied via rearrangement to the quinoline.

The pH-rate profile was consistent with a combination of a V-shaped profile in the pH range of 0-9 and a sigmoid-shaped profile in the pH range of 4-13. The plateau that began at pH 10-11 is a result of the ionization of the amine of the carbamate inhibiting the base-catalyzed hydrolysis of efavirenz, given that the ionized form of the carbamate is resonance-stabilized toward hydroxide-catalyzed degradation. Thus, increasing the pH resulted in a parallel decrease in the unionized fraction and increase in hydroxide ion concentration resulting in a constant k(obs) value.