Preparation and characterization of a .beta.-lactamase-Fab' conjugate for the site-specific activation of oncolytic agents

Antibody-directed catalysis (ADC) is a two-step method for the targeted delivery of chemotherapeutic agents in which enzyme-antibody conjugates, prelocalized to antigen-bearing cells, activate prodrugs designed to be substrates for the enzyme. An enzyme-Fab' conjugate exhibiting both native beta-lactamase activity and immunoreactivity toward carcinoembryonic antigen (CEA) was constructed. Treatment of CEA-expressing LS174T cells with this conjugate imparted beta-lactamase activity to the cells; beta-lactamase activity was not imparted by treatment with unconjugated beta-lactamase and not to CEA negative cells treated with conjugate. Cephalosporin-based prodrugs, and other substrates synthesized as model compounds, were found to have wide variations in their kinetic parameters toward the conjugate, with kcat values ranging from 16 to 3300 s-1 and KM values ranging from 5 to 160 microM. The prodrug derived from desacetylvinblastine-3-carboxylic acid hydrazide (DAVLBHYD) was studied in vitro and found to be 5-fold less cytotoxic to LS174T cells than the parent DAVLBHYD. For antigen-positive cells preincubated with conjugate, however, the prodrug showed the same potency as the parent drug. Thus, the combination of conjugate and prodrug appears to provide antigen-dependent toxicity to tumor cells.