Testing double mutants of the enzyme nitroreductase for enhanced cell sensitisation to prodrugs: effects of combining beneficial single mutations

Overview & Synopsis

Prodrug activation gene therapy for cancer involves expressing prodrug-activating enzymes in tumour cells, so they can be selectively killed by systemically administered prodrug. For example, nitroreductase (E.C. 1.6.99.7)(NTR), sensitises cells to the prodrug CB1954 (5-[aziridin-1-yl]-2,4-dinitrobenzamide), which it converts to a potent DNA-crosslinking agent. However, low catalytic efficiency with this non-natural substrate appears to limit the efficacy of this enzyme-prodrug combination for eliminating the target cancer cells. To improve this, we aim to engineer NTR for improved prodrug activation. Previously, a number of single amino acid substitutions at 6 positions around the active site of the enzyme were found to increase activity, resulting in up to ∼5 fold enhanced cell sensitisation to CB1954. In this study we have made pairwise combinations among some of the best mutants at each of these 6 sites. A total of 53 double mutants were initially screened in , then the 7 most promising were inserted into an adenovirus vector and compared in SKOV3 human ovarian carcinoma cells for sensitisation to CB1954 and two alternative prodrugs. The most effective mutants, T41L/N71S and T41L/F70A, were 14–17-fold more potent than WT NTR at sensitising the cancer cells to CB1954. The best mutant for activation of the dinitrobenzamide mustard prodrug SN23862 was T41L/F70A (4.8-fold improvement); and S40A/F124M showed 1.7-fold improvement over WT with the nitrobenzylphosphoramide mustard prodrug LH7. In two tumour xenograft models using SKOV3 or human prostate carcinoma PC3, T41L/N71S NTR demonstrated greater CB1954-dependent anti-tumour activity than WT NTR.