Design and Synthesis of 2-Nitroimidazoles with Variable Alkylating and Acylating Functionality

Abstract

The synthesis of a small series of 2-nitroimidazoles in which the β-amino alcohol side chain was amidated with a range of alkylating/acylating functionality is described. Synthetic methodologies were developed that generally provided for selective N-acyl versus N,O-bisacyl products. In vitro, target analogs showed minimal radiosensitization activity, with only a few exhibiting a sensitizer enhancement ratio (SER) >2.0 and C 1.6 values comparable to reference agents RB-6145 and RSU-1069. In an assay to determine potential to alkylate biomolecules, representative analogs showed <1% of the alkylating activity of RSU-1069. In vivo, one analog showed an enhancement ratio of 1.6 relative to vehicle control when tested in B6C3F1 mice with an implanted KHT sarcoma. The data reinforce prior findings that there is a correlation between alkylation potential and in vivo activity.Key words 2-nitroimidazole; radiation sensitizer; alkylation potential; soft drug Several years ago, the dual function 2-nitroimidazole RB-6145 (1a, Fig. 1) was reported to demonstrate excellent radiosensitizing activity in preclinical models of tumor hypoxia.1) Compound 1a is a prodrug of RSU-1069 (2), which has an aziridine moiety in the side chain and was shown to have a high differential toxicity toward hypoxic cells compared to oxic cells under in vitro and in vivo experimental conditions. Additional studies showed that 1a was considerably less emetic than 2 (which had been briefly evaluated in a Phase I clinical trial 2) ) and thus was chosen for further development as its R-enantiomer (1b, CI-1010). Toward this end, we reported on a scaleable synthesis of 1b.3) Selective, bioreductive metabolism of CI-1010 in hypoxic cells was shown to increase DNA alkylation, and form cross links and strand breaks. 4) Despite its efficacy in killing tumors, CI-1010 produced a selective and irreversible degeneration of the outer nuclear and photoreceptor layers of the retina in relevant preclinical animal models. Mechanistic studies showed that CI-1010 induced cell death in photoreceptors and provided evidence in support of a p53-linked activation of caspase-3 in response to DNA damage caused by CI-1010.5) This toxicity was manifested at concentrations required for therapeutic efficacy, thus leading to the termination of the preclinical development of CI-1010.The high reactivity of the aziridinyl moiety of 2 toward biological nucleophiles led us to consider the installation of other alkylation/acylation functionality off the 2-nitroimidazole core, which might be manifested in lower toxicity. In this paper, we report on the synthesis of a small series of 2-nitroimidazoles in which the amino alcohol side chain is amidated with a range of head pieces (Michael acceptors, chloromethyl amide, β-lactam) representing a spectrum of predicted alkylating/acylating reactivity, based on the concept of "soft" drug design developed by Bodor. 6) Results and DiscussionChemistry The synthetic methodologies utilized to make our 2-nitroimidazole analogs are shown in Fig. 2. The ...