Nitrobenzocyclophosphamides as potential prodrugs for bioreductive activation: Synthesis, stability, enzymatic reduction, and antiproliferative activity in cell culture

Abstract In efforts to obtain potential anticancer prodrugs for gene-directed enzyme prodrug therapy using Eschericia coli nitroreductase, a series of four benzocyclophosphamide analogues were designed and synthesized incorporating a strategically placed nitro group in a position para to the benzylic carbon for reductive activation. All four analogues were found to be stable in phosphate buffer at pH 7.4 and 37 °C and were good substrates of E. coli nitroreductase with half lives between 7 and 24 min at pH 7.0 and 37 °C. However, only two analogues 6a and 6c , both with a benzylic oxygen in the phosphorinane ring para to the nitro group, showed a modest 33–36-fold enhanced cytotoxicity in E. coli nitroreductase-expressing cells. These results suggest that good substrate activity and the para benzylic oxygen are required for activation by E. coli nitroreductase. Compounds 6a and 6c represent a new structure type for reductive activation and a lead for further modification in the development of better analogues with improved selective toxicity to be used in gene-directed enzyme prodrug therapy.