Influence of mesna and cysteine on the systemic toxicity and therapeutic efficacy of activated cyclophosphamide

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Abstract
Presumably the coadministration of the uroprotector mesna in cyclophosphamide treatment does not influence the systemic activity of its activated metabolite. This was newly investigated in a mouse model. The LD50 values of i.p. administered mafosfamide, a derivative of act. CP, were increased by the simultaneous i.p. administration of mesna (mafosfamide: mesna 1: 2 on a molar weight basis) from 590 mg/kg to 750 mg/kg, and after i.v. injection of cytostatic and thiol from 505 mg/kg to 810 mg/kg. Administration of 2 × molar cysteine i.v. or i.p. to mafosfamidetreated animals was even more effective against its lethal toxicity (LD50 i.p. 1800 mg/kg and i.v. 1130 mg/kg). Bone marrow toxicity (severe leukocytopenia) was partially abolished by both thiols. Also the therapeutic efficacy of act. CP against L1210 leukemia in DBA2 mice was reduced by 50% in the presence of cysteine and of mesna. Compared with mesna the higher detoxification effect of cysteine is attributed to its longer half-life (t1/2 20 min vs 12 min of mesna) and presumably an accumulation of cysteine in some cell systems (distribution coefficient 1.20 ml/g vs 0.68 ml/g of mesna). Nevertheless, our study clearly demonstrates a distinct systemic deactivation of act. CP by mesna, which might be of clinical relevance.

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