Nitroreductase-based GDEPT.

Nitroreductases that metabolise aromatic nitro groups to hydroxylamines are attractive as enzymes for GDEPT because of the very large electronic change that this metabolism generates, providing an efficient switch that can be exploited to generate potent cytotoxins. While nitroreductase enzymes are widespread, nearly all the work using these in GDEPT has been with the nfsB gene product of Escherichia coli, an oxygen-insensitive flavin mononucleotide nitroreductase (NTR). Four classes of prodrugs for NTR have been described; dinitroaziridinylbenzamides, dinitrobenzamide mustards, 4-nitrobenzylcarbamates and nitroindolines. While some quinones are excellent substrates for NTR, none have been identified as potential GDEPT prodrugs. The most widely studied prodrug used for GDEPT in conjunction with NTR is the dinitroaziridinylbenzamide CB 1954. This shows high selectivity (>1000-fold) in cell lines transfected with NTR, has potent and long-lasting inhibition of NTR-transfected tumours in mice, and is in Phase I trial in conjunction with virally-delivered NTR enzyme. The related mustard SN 23862 has similar selectivity and superior bystander effects in animal models. Nitrobenzyl carbamates of a variety of cytotoxic amines (including aniline mustards, enediynes, duocarmycin analogues, pyrrolobenzodiazepines and the antitumour antibiotics doxorubicin, actinomycin D and mitomcyin C) are metabolised efficiently by NTR to the hydroxylamines, that fragment to release the amines. Nitroindoline derivatives of duocarmycins also show moderate selectivity for NTR-transfected cell lines in culture.