Metabolism and pharmacokinetics of the 2ME2 Analog, ENMD-1198, an orally bioavailable microtubule destabilizing agent, in rats, dogs, and humans

B99 Background: Clinical studies of 2-methoxyestradiol (2ME2, Panzem ® ), a microtubule destabilizing agent, demonstrate an excellent safety profile for 2ME2 with evidence of clinical benefit, including complete and partial responses and prolonged stable disease across a range of patients with different advanced cancers. 2ME2 is metabolized by conjugation at the 3- and 17-positions as well as oxidation at the 17-position. A series of analogs of 2ME2 modified at positions 3 and 17 was generated. ENMD-1198 (2-methoxyestra-1,3,5,(10),16-tetraene-3-carboxamide), was selected as the lead molecule for further clinical study due to improved metabolic stability and more potent antiproliferative properties in preclinical models compared to 2ME2. Preclinical toxicity studies evaluating daily oral dosing of ENMD-1198 for 28 days indicated a 10-fold difference in the maximum tolerated dose (MTD) between rats and dogs. In female rats the MTD is 60 mg/m2/d, which results in a Cmax of 927 ng/mL and an AUC of 3724 ng·h/mL, whereas the MTD in female dogs is 600 mg/m2/d, which results in a Cmax of 4396 ng/mL and an AUC of 29235 ng·h/mL. The organs affected by ENMD-1198 in both species were those with cell populations of high mitotic potential, consistent with its mechanism of action: bone marrow, lymph nodes, spleen, thymus, GI tract, and testes. Methods: To assess metabolic differences between rats and dogs, radiolabeled compound was synthesized and metabolite profiles were evaluated. A first-in-man Phase 1 clinical trial of ENMD-1198 in patients with advanced cancers is ongoing with a starting dose based on data from the most sensitive species (rats). Metabolite profiles from the ENMD-1198 dosed rats and dogs were compared to the plasma samples from patients dosed with 30 mg/m2 ENMD-1198. Results: While there were quantitative differences between rats and dogs, overall, metabolism of ENMD-1198 appears to be formation of metabolites via hydroxylation of the B or D ring or des-methylation of the O-methyl group followed by conjugation with glucuronide or sulfate. Consistent with preclinical data from both the rat and dog, once daily oral dosing of ENMD-1198 in humans demonstrates dose proportionality across the dose range 5-30 mg/m2. At 5, 10, 20, and 30 mg/m2, mean peak plasma exposures are 68.8, 141.2, 418.7, and 677.8 ng/mL, respectively. The plasma concentration time curves have resulted in AUCs of 390.0, 1373.8, 2515.4 and 5880.8 ng·h/mL, respectively and a half life of approximately 13 h (range 10.6 to 19.5). Similar to dogs, ENMD-1198 was the major component in the human plasma at all time points. There were no dominant metabolites (>20% of total area of the identified peak) found in human, rat or dog plasma. M7a/b and M10, hydroxylated ENMD-1198, are the most abundant plasma metabolites in all three species. Conclusions: ENMD-1198 is a 2ME2 analog with improved preclinical pharmacokinetics and antitumor activity compared to 2ME2. There is a significant species difference in sensitivity, but no differences in dominant metabolites. A Phase 1 dose escalation study in humans is ongoing, which to date has demonstrated dose proportional PK exposure from 5 to 30mg/m2, and a metabolite profile that appears similar to both rats and dogs.