Mechanism of Degradation of an alpha- Keto-Epoxide, PIV-PR-020: A Model for the Warhead for Various Proteasome Inhibitor Anticancer Agents

Abstract

Proteasome inhibitors are clinically effective treatment of multiple myeloma and mantle cell lymphoma. One proteasome inhibitor already on the market and one under clinical investigation are epoxy-ketones that potently and irreversibly bind to the proteasome. Kinetics were studied by loss of an fÑ-keto-epoxide, PR-020 Pivoyl (PIV-PR-020), a model for the warhead of a number of proteasome inhibitor anticancer agents, namely, carfilzomib and oprozomib, in aqueous solutions as a function of pH, temperature and solvent composition. The loss of PIV-PR-020 followed pseudo first order kinetics and provided a U-shaped pH-rate profile with an apparent pH-independent region between pH values of approximately 4 to 7. The degradation of PIV-PR-020 in aqueous solution was found to be both acid catalyzed below pH 4 and base catalyzed above pH 7. During the course of the study, the temperature, ionic strength, buffer concentration, and halide effect were evaluated while keeping the concentration of the reactant constant. Under acidic conditions, as you increase the halide nucleophilicity, the reactivity increases. The fÑ-keto-epoxide ring opening in an SN2 reaction under acidic conditions and is undefined under basic conditions. It has also been shown that there is some scrambling at the carbon at the C1 position at higher pH values due to base catalyzed isomerization, and there is evidence that this diastereomer further degrades to a diol. At pH 8, the rate equations are complex due to reversible and parallel reactions. The study of PIV-PR-020 provides insight into the likely routes and rates of degradation at the warhead of carfilzomib and oprozomib.