The Safety and Efficacy of Full Versus Reduced Dose Betrixaban in the Acute Medically Ill VTE (Venous Thromboembolism) Prevention with Extended Duration Betrixaban (APEX) Trial

Background: The exposure of all the currently licenced DOACs is increased in renal impairment and by certain drug interactions. The Acute Medically Ill VTE (Venous Thromboembolism) Prevention with Extended Duration Betrixaban (APEX) trial assessed the safety and efficacy of extended-duration thromboprophylaxis using betrixaban versus standard dosing of enoxaparin among acutely ill medical patients. The full 80 mg dose of betrixaban was halved to 40 mg among subjects with severe renal insufficiency (calculated creatinine clearance <30ml/min), or receiving a concomitant strong P-glycoprotein (P-gp) inhibitor.

Objectives and Methods: The goal of this analysis is to assess the efficacy and safety of full (80 mg) and reduced dose (40 mg) betrixaban relative to enoxaparin in the APEX trial. Subjects were stratified into the two dose groups prior to randomization according to the pre-specified dosing criteria. The primary efficacy outcome was the composite of asymptomatic proximal and symptomatic venous thromboembolism (proximal or distal deep-vein thrombosis, symptomatic nonfatal pulmonary embolism, or death from venous thromboembolism). The principal safety outcome was major bleeding.

Results: The median concentration of betrixaban among subjects administered the 80 mg dose was higher than that of the 40 mg dose (19 ng/ml vs 11 ng/ml, p<0.0001). In the primary analysis Cohort 1 (the elevated D-dimer +ve patients), the primary efficacy outcome was significantly reduced among subjects treated with 80 mg of extended dose betrixaban vs enoxaparin [6.3% (95/1516) vs 8.4% (130/1549), RRR = 0.26 (0.04-0.42), p=0.023]. A similar reduction was observed in the entire modified Intention to Treat (mITT) population [4.87% (122/2506) vs 7.06% (181/2562), RRR = 0.30 (0.13 - 0.44), p=0.001]. In contrast, among subjects receiving 40 mg of betrixaban there was no significant difference in the primary outcomes compared with enoxaparin across Cohorts 1, 2, and 3. There was no excess of major bleeding associated with administration of either the full 80 mg dose or the reduced 40 mg dose of betrixaban as compared with enoxaparin.

Conclusion: For extended duration prophylaxis against VTE in hospitalized medically ill patients, the full 80 mg dose of extended duration betrixaban achieves higher serum concentrations than the 40 mg dose and is associated with improved efficacy across all cohorts relative to standard dose enoxaparin, without an excess risk of major bleeding. Based upon the approximate halving of plasma concentrations in this analysis and the lack of improvement in clinical outcomes, the reduced 40 mg dose may have been excessively downwardly adjusted. Conversely, the 80 mg dose was efficacious in all cohorts, including cohort 1. The inclusion of the 40 mg dose in the primary analysis may explain at least in part the marginal statistical result in the original report.