Causes and possibilities to circumvent cyclophosphamide toxicity

Cyclophosphamide is an inert prodrug converted into 4-hydroxycyclophosphamide (OHCP) by hepatic hydroxylation. OHCP is in equilibrium with its tautomeric aldophosphamide (ALDO). From ALDO, the cytotoxic active metabolites are formed enzymatically by phosphodiesterases; these are the alkylating metabolite phosphoramide mustard (PAM) and the proapoptotic aldehyde 3-hydroxypropanal (HPA). PAM damages the DNA by alkylation; HPA amplifies the thereby induced apoptosis. The generally accepted view that acrolein, which is believed to be formed in the formation of PAM by β-elimination from ALDO would be mainly responsible for the toxicity of cyclophosphamide, has to be revised because no acrolein is formed in the systemic circulation of patients after cyclophosphamide administration. It is shown that not acrolein, but OHCP itself is the true toxic metabolite of cyclophosphamide. Toxicity tests with OHCP and PAM were carried out, which demonstrated that OHCP unfolds its toxicity, not as a carrier of PAM but is toxic itself by reacting with nucleophilic groups of macromolecules, for example, thiol groups of membrane proteins. Further experiments demonstrate that the toxicity of oxazaphosphorine cytostatics may be drastically reduced if the formation of the pharmacologically active metabolite ALDO bypasses the formation of OHCP. Toxicity experiments in mice with S-ethanol-cyclophosphamide (SECP) that hydrolyzes to OHCP show that SECP is as toxic as OHCP, whereas the thiazolidine of ALDO, which hydrolyzes to ALDO bypassing OHCP is 7-9 times less toxic without loss of antitumor activity.