Inhibition of intracellular esterases by antitumour chloroethylnitrosoureas: Measurement by flow cytometry and correlation with molecular carbamoylation activity

Abstract

Antitumour chloroethylnitrosoureas (Cnus) decompose in physiological conditions yielding alkylating species and organic isocyanates. While antitumour activity is mainly attributed to the alkylation of DNA, carbamoylation of intracellular proteins by isocyanates may also have pharmacological and toxicological relevance. We previously reported a novel dynamic flow cytoenzymological assay for esterase inhibition in intact murine cells by BCNU and related isocyanates, and proposed that this might form the basis of an assay for intracellular carbamoylation. We have now examined a wide range of Cnus, isocyanates, and alkylating agents for their ability to inhibit cellular esterases. BCNU, CCNU, their derived isocyanates, and the 4-OH metabolites of CCNU exhibited potent inhibitory activity ( i 50 values 5.5 × 10 −5−7.3 × 10 −4M). Chlorozotocin and GANU were relatively inactive ( i 50⪢10 −2M). ACNU, TCNU and the 2-OH metabolites of CCNU exhibited intermediate activity ( i 50 values 1.1 × 10 −32.3 × 10 −2M). Compounds not able to form isocyanates were essentially inactive. Poor membrane permeability was also implicated in the weak activity of chlorozotocin and GANU. There was overall a good correlation between esterase inhibition and chemical carbamoylating activity, but some particular differences were identified. We concluded that flow cytoenzymological assay appears to have the potential to provide useful measurement of intracellular protein carbamoylation by existing Cnus and novel derivatives, and also offers the advantage of cell subpopulation identification for in vivo evaluation of these agents.