Pharmacokinetics of "activated" cyclophosphamide and therapeutic efficacies.

Estimation of "activated" cyclophosphamide (4-OH-CP) in blood of cancer patients and laboratory animals has revealed significant differences between pharmacokinetics of cyclophosphamide (CP) in man and laboratory animals after CP treatment. Whereas in blood of mice and rats relatively high concentrations of 4-OH-CP were found to exist for a relatively short time, in blood of humans only low, but longer-lasting, blood levels were detected after administration of comparable CP doses. In order to examine whether these different pharmacokinetic behaviors might account at least in part for the known differences of antitumor activity and toxicity of CP between humans and laboratory animals, the authors studied the influence of pharmacokinetics of activated CP on therapeutic efficacy and toxicity after injection of 4-(S-ethanol)-sulfido-cyclophosphamide (P1), a pro drug of activated CP, into nude mice bearing heterotransplanted human bladder sarcoma. With P1, which hydrolyzes quickly in blood to yield 4-OH-CP, different blood level shapes of 4-OH-CP could be established either by single bolus injection of P1 or by repetitive injection of a loading dose followed by several maintenance doses which caused nearly constant levels of activated CP for a longer time period. With these models it was found that 4-OH-CP showed more therapeutic efficacy when present in blood at relatively low levels for longer times than after bolus injection of the same dose resulting in a sharp peak level of activated CP. So after single intraperitoneal (IP) injection of 300 mg/kg P1 which caused a bioavailability of 36 mumol/ml-1/minute a 67% inhibition of tumor growth was achieved, whereas a tumor growth reduction of 83% was obtained after injection of the same dose in 6 fractions resulting in constant blood levels with a bioavailability of only 17 mumol/ml-1/minute. In contrast to the significant influence on antitumor efficacy of activated CP, practically no effect of pharmacokinetics on toxicity of 4-OH-CP could be observed. Therefore, the bioavailability of activated CP, which killed 50% of the animals, was determined to be approximately 89 mumol/ml-1/minute after adjustment of pharmacokinetics to yield constant levels and approximately 79 mumol/ml-1/minute after single bolus injection. The experiments presented show that by adjustment of pharmacokinetics the therapeutic index of P1, defined as bioavailability causing 50% of animals to die, referred to bioavailability causing 90% tumor growth inhibition, could be more than doubled.

The influence of the protector thiol L-cystein on the toxic and therapeutic responses of stabilized "activated" cyclophosphamide (4-(S-ethanol)-sulfido-cyclophosphamide).

The influence of L-cystein on the toxic and therapeutic responses of 4-(S-ethanol)-sulfido-cyclophosphamide (P1), a stabilized "activated" cyclophosphamide, was investigated. Stabilized "activated" cyclophosphamides hydrolyze under physiological conditions to 4-hydroxycyclophosphamide (4-OH-CP). The antitumor activity of P1 was investigated on a heterotransplanted human bladder sarcoma in nude mice and in perfusion experiments carried out on the isolated tumor bearing limb in rats. Due to its rapid hydrolysis to 4-OH-CP, P1 exhibits severe local toxicity which is decreased by the protector thiol L-cystein. Simultaneous application of double molar amounts of L-cystein reduces toxicity in nude mice to approximately one-third. Therapeutic activity is not affected by this ratio of L-cystein so that the protector thiol increases the therapeutic efficacy of P1. Higher amounts of L-cystein reduce both the acute toxicity in nude mice and the therapeutic efficacy against the human xenograft. The perfusion experiments demonstrate that a P1 concentration necessary to cure rats with tumor bearing limb is only tolerated in combination with L-cystein.