VIRTUAL COFACTORS FOR AN ESCHERICHIA-COLI NITROREDUCTASE ENZYME - RELEVANCE TO REDUCTIVELY ACTIVATED PRODRUGS IN ANTIBODY-DIRECTED ENZYME PRODRUG THERAPY (ADEPT)

A nitroreductase enzyme has been isolated from Escherichia coil that h as the unusual property of being equally capable of using either NADH or NADPH as a cofactor for the reduction of its substrates which inclu de menadione as well as 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB 195 4). This property is shared with the mammalian enzyme, DT diaphorase. The nitroreductase can, like DT diaphorase, also use simple reduced py ridinium compounds as virtual cofactors. The intact NAD(P)H molecule i s not required and the simplest quaternary (and therefore reducible) d erivative of nicotinamide, 1-methylnicotinamide (reduced), is as effec tive as NAD(P)H in its ability to act as an electron donor for the nit roreductase. The structure-activity relationship is not identical to t hat of DT diaphorase and nicotinic acid riboside (reduced) is selectiv e, being active only for the nitroreductase. Irrespective of the virtu al cofactor used, the nitroreductase formed the same reduction product s of CB 1954 (the 2- and 4-hydroxylamino derivatives in equal proporti ons). Nicotinic acid riboside (reduced), unlike NADH, was stable to me tabolism by serum enzymes and had a plasma half-life of seven minutes in the mouse after an i.v. bolus administration. NADH had an unmeasura bly short half-life. Nicotinic acid riboside (reduced) could also be p roduced in vivo by administration of nicotinic acid 5'-O-benzoyl ribos ide (reduced). These results demonstrate that the requirement for a co factor need not be a limitation in the use of reductive enzymes in ant ibody directed enzyme prodrug therapy (ADEPT). It is proposed that the E. coli nitroreductase would be a suitable enzyme for ADEPT in combin ation with CB 1954 and a synthetic, enzyme-selective, virtual cofactor such as nicotinic acid riboside (reduced).