Structure-activity relationships for 4-nitrobenzyl carbamates of 5-aminobenz[e]indoline minor groove alkylating agents as prodrugs for GDEPT in conjunction with E. coli nitroreductase

Twelve substituted 4-nitrobenzyl carbamate prodrugs of the 5-aminobenz[e]indoline class of DNA minor groove alkylating agents were prepared and tested as prodrugs for gene-directed enzyme prodrug therapy (GDEPT) using a two-electron nitroreductase (NTR) from E. coli B. The prodrugs and effectors were tested in a cell line panel comprising parental and transfected human (SKOV/Skov-NTR(neo), WiDr/WiDr-NTR(neo)), Chinese hamster (V79(puro)/V79-NTR(puro)), and murine (EMT6/EMT6-NTR(puro)) cell line pairs. In the human cell line pairs, several analogues bearing neutral methoxyethoxy-, 2-hydroxyethoxy-, or 3-hydroxypropoxy-substituted side chains were good substrates for NTR as measured by cytotoxicity ratios, with NTR-ve/NTR+ve ratios similar to the established NTR substrates CB1954 (an aziridinyl dinitrobenzamide) and the analogous bromomustard. Selectivity for NTR decreased with increasing side-chain size or the presence of a basic amine group. Low to modest selectivity was observed in the Chinese hamster-derived cell line pair; however, in the murine EMT6/EMT6-NTR(puro) cell line pair, the above hydroxyalkoxy analogues again showed significant selectivity for NTR. The activity of the 2-hydroxyethoxy analogue was evaluated against NTR-expressing EMT6 tumors comprising ca. 10% NTR+ve cells at the time of tumor treatment. A small decrease in NTR+ve cells was observed after treatment, with a lesser effect against NTR-ve target cells, but these effects were not statistically significant and were much less than for the dinitrobenzamides. These results suggest that useful GDEPT prodrugs based on the 4-nitrobenzyl carbamate and 5-aminobenz[e]indoline motifs may be developed if optimization of pharmacokinetics can be addressed.