Pathways of reductive fragmentation of heterocyclic nitroarylmethyl quaternary ammonium prodrugs of mechlorethamine

Nitroarylmethyl quaternary (NMQ) ammonium salts have potential as prodrugs for enzymatic or radiolytic reduction to release amine effectors under hypoxia. Earlier studies demonstrated one-electron release of the cytotoxic amine mechlorethamine (HN2) from 4-nitroimidazolyl and 2-nitropyrrolyl NMQ prodrugs (but not from nitrobenzyl analogs) through intramolecular electron transfer. In this study we determined whether this is a general feature of heterocyclic NMQ prodrugs of HN2 and examined the reductive pathways in detail using pulse and steady-state radiolysis. The kinetics of radical fragmentation varied by more than four orders of magnitude, independently of the one-electron reduction potential, within the series of eight nitroheterocycles examined. In addition to the compounds identified previously, new 2-nitropyrrole and 3-nitrothiophene NMQ prodrugs were found to provide efficient HN2 release (G > 0.5 micromol/J in anoxic formate buffer). However, the nitrothiophene was sensitive to nucleophilic displacement of HN2, making it less promising. Product analysis by HPLC/mass spectrometry identified symmetrical dimers arising from benzyl-type radical intermediates but also demonstrated that these dimers are not reliable markers for the intramolecular fragmentation of the initial nitro radical anion. This study elucidated multiple competing pathways for reductive fragmentation of NMQ prodrugs and identified the preferred electron acceptors for use in the development of analogs that release more potent cytotoxins.