The medical potential of catalytic antibodies

Abstract

Catalytic antibodies are relatively slow catalysts with turnover numbers some 106 less than is common for enzymes. However, they have the advantage of high affinity for a pre-selected substrate and the ability to carry out a pre-determined chemical transformation with an efficiency that is adequate for medical application. In a feasibility study, we have chosen to investigate antibody catalysis of carbamate ester cleavage and apply it to achieve cell-kill in a system that is a paradigm for ADAPT: Antibody Directed Abzyme Prodrug Therapy. We have differentiated the two alternative pathways for hydrolysis of an aryl carbamate ester by the synthesis of a tetrahedral phosphonamidate ester transition state analogue for the disfavoured BAc2 pathway and its use as a hapten to generate antibody catalysts. Such abzymes can lower the activation energy of the BAC2 pathway relative to that for the normal ElcB hydrolysis. Of the antibodies thus elicited, DF8-D5 proved to be the best catalyst, showing good Michaelis-Menten kinetics and strong inhibition by the hapten. Hammett analysis with a range of substrates gave p = +0.53 for the DF8-D5 hydrolysis and p = 2.63 for the hydroxide mediated reaction of a range of p-substituted carbamates, which confirms the mechanistic switch. When a carbamate ester of a phenolic mustard is used as substrate, a cognate antibody EA11-D7 can cause cell kill of human colorectal carcinoma cells in tissue culture as a result of the same catalytic cleavage process to release a cytotoxic phenolic mustard.