In vivo antitumor activity and host toxicity of methoxymorpholinyl doxorubicin: Role of cytochrome P450 3A

Methoxymorpholinyl doxorubicin (MMDX; PNU 152243) is a promising doxorubici n derivative currently undergoing clinical evaluation. Previous in vitro st udies suggested that the compound undergoes hepatic biotransformation by cy tochrome p450 (CYP) 3A into a more cytotoxic metabolite(s). The present stu dy examined the role of CYP3A-mediated metabolism in the in vivo antitumor activity and host toxicity of MMDX in the mouse model and investigated the potential for increasing the therapeutic effectiveness of the drug by induc ing its hepatic CYP-catalyzed activation, We Found that MMDX cytotoxicity f or cultured M5076 tumor cells was potentiated 22-fold by preincubating the drug with NADPH-supplemented liver microsomes from untreated C57BL/6 female mice. A greater (50-fold) potentiation of MMDX cytotoxicity was observed a fter its preincubation with liver microsomes isolated from animals pretreat ed with the prototypical CYP3A inducer pregnenolone-16 alpha-carbonitrile. In contrast, in vivo administration of the selective CYP3A inhibitor trolea ndomycin (TAO) reduced both potentiation of MMDX cytotoxicity and the rate of CYP3A-catalyzed,N-demethylation of erythromycin by isolated liver micros omes (55.5 and 49% reduction, respectively). In vivo antitumor activity exp eriments revealed that TAO completely suppressed the ability of 90 mu g/kg MMDX i.v,. a dose close to the LD10, to delay growth of s,c, M5076 rumors i n C57BL/6 mice and to prolong survival of DBA/2 mice with disseminated L121 0 leukemia. Moreover, TAO administration markedly, inhibited the therapeuti c efficacy of 90 mu g/kg MMDX i.v. in mice bearing experimental M5076 liver metastases; a complete loss of MMDX activity was observed in Liver metasta ses-bearing animals receiving 40 mu g/kg MMDX i,v. plus TAO, However, pregn enolone-16 alpha-carbonitrile pretreatment failed to enhance MMDX activity in mice bearing either s.c. M5076 tumors or experimental M5076 liver metast ases. Additional experiments tarried out in healthy C57BL/6 mice showed tha t TAO markedly inhibited MMDX-induced myelosuppression and protected the an imals against lethal doses of MMDX. Taken together, these findings demonstr ate that an active metabolite(s) of MMDX synthesized via CYP3A contributes significantly to its in vivo antitumor activity and host toxicity.