THE MEDICAL POTENTIAL OF CATALYTIC ANTIBODIES

Catalytic antibodies are relatively slow catalysts with turnover numbe rs some 106 less than is common for enzymes. However, they have the ad vantage of high affinity for a pre-selected substrate and the ability to carry out a predetermined chemical transformation with an efficienc y that is adequate for medical application. In a feasibility study, we have chosen to investigate antibody catalysis of carbamate ester clea vage and apply it to achieve cell-kill in a system that is a paradigm for ADAPT: Antibody Directed Abzyme Prodrug Therapy. We have different iated the two alternative pathways for hydrolysis of an aryl carbamate ester by the synthesis of a tetrahedral phosphonamidate eater transit ion state analogue for the disfavoured BAc2 pathway and its use as a h apten to generate antibody catalysts. Such abzymes can lower the activ ation energy of the BAC2 pathway relative to that for the normal ElcB hydrolysis. Of the antibodies thus elicited, DF8-D5 proved to be the b est catalyst, showing good Michaelis-Menten kinetics and strong inhibi tion by the hapten. Hammett analysis with a range of substrates gave r ho = +0.53 for the DF8-D5 hydrolysis and rho = 2.63 for the hydroxide mediated reaction of a range of p-substituted carbamates, which confir ms the mechanistic switch. When a carbamate ester of a phenolic mustar d 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 mu stard.