Methoxymorpholinyl doxorubicin (
MMDX;
PNU 152243) is a promising
doxorubicin derivative currently undergoing clinical evaluation. Previous in vitro studies suggested that the compound undergoes hepatic biotransformation by
cytochrome P450 (CYP) 3A into a more cytotoxic metabolite(s). The present study 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 inducing its hepatic CYP-catalyzed activation. We found that
MMDX cytotoxicity for 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 after its preincubation with liver microsomes isolated from animals pretreated with the prototypical
CYP3A inducer pregnenolone-16alpha-carbonitrile. In contrast, in vivo administration of the selective
CYP3A inhibitor troleandomycin (
TAO) reduced both potentiation of
MMDX cytotoxicity and the rate of CYP3A-catalyzed N-demethylation of
erythromycin by isolated liver microsomes (55.5 and 49% reduction, respectively). In vivo antitumor activity experiments revealed that
TAO completely suppressed the ability of 90 microg/kg
MMDX i.v., a dose close to the LD10, to delay growth of s.c. M5076
tumors in C57BL/6 mice and to prolong survival of DBA/2 mice with disseminated
L1210 leukemia. Moreover,
TAO administration markedly inhibited the therapeutic efficacy of 90 microg/kg
MMDX i.v. in mice bearing experimental M5076 liver
metastases; a complete loss of
MMDX activity was observed in liver
metastases-bearing animals receiving 40 microg/kg
MMDX i.v. plus
TAO. However, pregnenolone-16alpha-carbonitrile pretreatment failed to enhance
MMDX activity in mice bearing either s.c. M5076
tumors or experimental M5076 liver
metastases. Additional experiments carried out in healthy C57BL/6 mice showed that
TAO markedly inhibited
MMDX-induced myelosuppression and protected the animals against lethal doses of
MMDX. Taken together, these findings demonstrate that an active metabolite(s) of
MMDX synthesized via
CYP3A contributes significantly to its in vivo antitumor activity and host toxicity.