Does acrolein contribute to the cytotoxicity of cyclophosphamide?

To determine whether the release of acrolein from oxazaphosphorinane-cytostatics contributes to their cytotoxic action, the effect of 4-hydroperoxycyclophosphamide, 4-hydroperoxy-semi-cyclophosphamide, 4-hydroperoxy-dechloro-cyclophosphamide, and acrolein on murine L 1210 leukemia cells in vitro was compared by measuring the median survival time (MST) after transplantation of the tumor cells in DBA2/Han mice. We found that only 4-hydroperoxycyclophosphamide, which is able to release both acrolein and the alkylating metabolite phosphoramide-mustard, decreased the transplantability of L 1210 cells, while the structurally analogous 4-hydroperoxy-dechloro-cyclophosphamide and 4-hydroperoxy-semi-cyclophosphamide, which under physiological conditions only release acrolein but no alkylating split products showed no cytotoxicity. Acrolein itself showed only a marginal effect, when administered in concentrations equivalent to the release of acrolein from the oxazaphosphorinane-derivatives in test. In this case, however, significant lysis of the L 1210 cells was observed by estimating dye exclusion, while acrolein released intracellularly from 4-hydroperoxy-oxazaphosphorinane-compounds did not. This points to a different mechanism of the cytotoxic action of extracellular acrolein and acrolein released intracellularly from activated oxazaphosphorinane-compounds. The results suggest that the cytotoxic effect of activated cyclophosphamide is based on the alkylating moiety of the molecule. Neither the 4-hydroperoxy-group nor the activated oxazaphosphorinane-ring itself, nor acrolein released intracellularly during toxification of activated cyclophosphamide exert a direct cytotoxic effect. Thus, the release of acrolein from activated CP apparently does not contribute to the cytotoxicity of CP in vivo.