Omacetaxine mepesuccinate

The mechanism of action of omacetaxine mepesuccinate has not been fully elucidated but includes inhibition of protein synthesis and is independent of direct Bcr-Abl binding. Omacetaxine mepesuccinate binds to the A-site cleft in the peptidyl-transferase center of the large ribosomal subunit from a strain of archaeabacteria. In vitro, omacetaxine mepesuccinate reduced protein levels of the Bcr-Abl oncoprotein and Mcl-1, an anti-apoptotic Bcl-2 family member. Omacetaxine mepesuccinate showed activity in mouse models of wild-type and T315I mutated Bcr-Abl CML.

Cardiac Electrophysiology

In an uncontrolled pharmacokinetic study there were no reports of QTcF >480 ms or ΔQTcF >60 ms in 21 treated patients who received omacetaxine mepesuccinate 1.25 mg/m² BID for 14 consecutive days. There was no evidence for concentration-dependent increases in QTc for omacetaxine mepesuccinate or 4'-DMHHT. Although the mean effect on QTc was 4.2 ms (upper 95% CI: 9.5 ms), QTc effects less than 10 ms cannot be verified due to the absence of a placebo and positive controls.

The dose proportionality of omacetaxine mepesuccinate is unknown. A 90% increase in systemic exposure to omacetaxine mepesuccinate was observed between the first dose and steady state.

Absorption

The absolute bioavailability of omacetaxine mepesuccinate has not been determined. Omacetaxine mepesuccinate is absorbed following subcutaneous administration, and maximum concentrations are achieved after approximately 30 minutes.

Distribution

The steady-state (mean ± SD) volume of distribution of omacetaxine mepesuccinate is approximately 141 ± 93.4 L following subcutaneous administration of 1.25 mg/m² twice daily for 11 days. The plasma protein binding of omacetaxine mepesuccinate is less than or equal to 50%.

Elimination

The terminal elimination half-life of omacetaxine mepesuccinate in plasma is 14.6 hours.

Metabolism

Omacetaxine mepesuccinate is primarily hydrolyzed to 4′-DMHHT via plasma esterases with little hepatic microsomal oxidative and/or esterase-mediated metabolism in vitro.

Excretion

Following a single subcutaneous dose of radiolabeled omacetaxine mepesuccinate, the mean total recovery of radioactivity in excreta was approximately 81% of the radioactive dose. Approximately 37% of the radioactivity was recovered in urine and approximately 44% in feces.

Drug Interaction Studies

Cytochrome P450 (CYP) Enzymes

Omacetaxine mepesuccinate and 4′-DMHHT do not inhibit major CYP enzymes in vitro at concentrations that can be expected clinically. The potential for omacetaxine mepesuccinate or 4′-DMHHT to induce CYP enzymes has not been determined.

Transporter Systems

Omacetaxine mepesuccinate is a P-glycoprotein (P-gp) substrate in vitro. Omacetaxine mepesuccinate and 4′-DMHHT do not inhibit P-gp mediated efflux of loperamide in vitro at concentrations that can be expected clinically.