Influence of mesna and cysteine on the systemic toxicity and therapeutic efficacy of activated cyclophosphamide
The experimental and pharmacokinetic basis for the local chemotherapy of body cavities with 4-(S-ethanol)-sulfido-cyclophosphamide (P1), a stable derivative of activated cyclophosphamide (CP), was evaluated on the S 180 ascites sarcoma in mice. The severe local toxicity of P1 observed after intraperitoneal administration was markedly reduced by increasing the injection volume (belly bath) without significant loss of cytotoxic activity on the S 180 tumor. Simultaneous application of L-cysteine as a "protector thiol" resulted in further reduction of toxicity without significantly decreasing the cytotoxic effect on tumor cells. Thus the therapeutic index was increased (2.5 fold) by the combination of belly bath and protection by L-cysteine, contrary to 2-mercaptoethane sulfonic acid sodium salt (Mesna) as protector thiol which reduced both the acute toxicity and the curative effectiveness of P1. Pharmacokinetic parameters were determined by measuring the concentrations of P1 and 4-hydroxycyclophosphamide (4-OH-CP), carboxyphosphamide and 4-ketocyclophosphamide in blood and peritoneal fluid. As a result of these measurements the reduction of toxicity of P1 after high volume i.p. administration is due to increased enzymatic detoxification of 4-OH-CP to 4-ketocyclophosphamide and carboxyphosphamide. The effect of L-cysteine on the toxicity of P1 is mainly the consequence of transmercaptalisation of P1 to 4-(S-cysteine)-sulfido-CP. By this reaction formation of the toxic 4-OH-CP in the peritoneal cavity is diminished, and the peritoneal clearance of "activated" CP reduced.