Degradation pathways of a peptide boronic acid derivative, 2-Pyz-(CO)-Phe-Leu-B(OH)(2)

The peptide boronic acid derivative 2-Pyz-(CO)-Phe-Leu-B(OH)(2) is a potent inhibitor of 20S proteasome and a proposed anticancer agent. During prefor mulation studies, the compound presented erratic stability behavior. Effort s were made to isolate and identify the degradation products, thereby helpi ng to identify possible mechanisms for the degradation. The reaction of 2-P yz-(CO)-Phe-Leu-B(OH)(2) with hydrogen peroxide not only provided a conveni ent way to isolate the initial degradation products seen from hydrolysis in aqueous buffers but also showed that the major, initial degradation pathwa y was probably oxidative in nature. The isolated degradation products were characterized by nuclear magnetic resonance spectroscopy, mass spectrometry , and optical rotation dispersion. In the presence of hydrogen peroxide, th e boronic acid group was cleaved from 2-Pyz-(CO)-Phe-Leu-B(OH)(2) to give a n alcohol with an apparent retention of the original stereochemistry. Subse quent isomerization and further hydrolysis were then seen. Surprisingly, ad ded ascorbate and EDTA accelerated rather than inhibited degradation. Degra dation of 2-Pyz-(CO)-Phe-Leu-B(OH)(2) under acidic and basic conditions see med to be mediated by an initial oxidative degradation pathway similar to t hat seen with the peroxide. (C) 2000 Wiley-Liss, Inc.