Direct factor Xa inhibitors have demonstrated compelling anticoagulant efficacy and/or safety profiles across multiple diverse patient populations. A specific antidote to reverse anticoagulation during episodes of serious uncontrolled bleeding or before urgent/emergent surgery is lacking. Andexanet alfa (proposed INN)(AnXa, PRT064445) is a modified, recombinant human fXa molecule that is catalytically inactive but retains high-affinity binding to direct fXa inhibitors. It thus acts as a decoy to reverse fXa inhibitor-mediated anticoagulation in preclinical and early clinical studies.


This ongoing Phase 2, double-blind, placebo-controlled study is examining the reversal by AnXa of the anticoagulant activity of rivaroxaban (riva), as well as the pharmacokinetics and safety in healthy subjects. Reversal of riva anticoagulation will be studied with up to 6 different dose cohorts/ regimens of AnXa or placebo in a 6:3 ratio (i.e., 9 subjects per cohort). Riva is administered at an oral dose of 20 mg qd for 6 days and AnXa administered intravenously on Day 6, 3 hours after the last riva dose – the approximate time of maximum riva concentration (mean ± SD: 0.64 ± 0.22 mM, n=18). Pharmacodynamic and safety data are collected through Day 48 with pharmacokinetic data through Day 10.


We report here available data from the first 2 AnXa dose cohorts (210 mg and 420 mg, n =18). Immediately after completion of the 210 mg and 420 mg doses, anti-fXa activity decreased dose-dependently by 20% and 53%, respectively, from the pre-AnXa level and returned to placebo levels by approximately 2 hours after treatment (Figure). In parallel, the plasma concentrations of unbound riva were decreased by 32% and 51%, respectively, relative to pre-AnXa values. In addition, riva-induced inhibition of thrombin generation and prolongation of both prothrombin time and activated clotting time were also rapidly partially reversed by AnXa in a dose-dependent manner. At 2 minutes after AnXa administration, the molar ratio of AnXa to total plasma riva was 0.8 for the 210 mg dose (1.2 µM/1.6 µM, respectively) and 1.2 for the 420 mg dose (2.6 µM/2.1 µM, respectively). AnXa infusion was not associated with increases in prothrombin fragments F1+2, thrombin-antithrombin, or D-dimer (all values were within normal ranges). As expected, tissue factor pathway inhibitor activity decreased due to its binding to AnXa. AnXa was well tolerated and there were no thrombotic events, serious, or severe adverse events. Adverse events occurring in 1 or more AnXa or placebo recipients included infusion-related reactions (n = 3, all mild in severity) and post-procedural hematoma, headache, or postural dizziness (n = 2 each).


Results from this ongoing clinical trial demonstrate that AnXa is able to dose-dependently partially reverse the anticoagulant effects of rivaroxaban, as assessed by pharmacodynamic markers, in healthy subjects. These data are consistent with previously reported results with apixaban in that AnXa sequesters rivaroxaban and apixaban in a similar stoichiometric manner. Additional data with higher doses of AnXa will also be presented. AnXa is well-tolerated and a potentially promising, universal antidote for fXa inhibitors.


Mark:Portola Pharmaceuticals: Consultancy. Off Label Use: The use of PRT064445 as an antidote for reversal of anticoagulation from direct and indirect fXa inhibitors is investigational. Vandana:Portola Pharmaceuticals: Consultancy. Michael:Portola Pharmaceuticals: Employment, Equity Ownership. Genmin:Portola Pharmaceuticals: Employment, Equity Ownership. Conley:Portola Pharmaceuticals: Employment, Equity Ownership. Stanley:Portola Pharmaceuticals: Employment, Equity Ownership. Castillo:Portola Pharmaceuticals: Employment, Equity Ownership. Hutchaleelaha:Portola Pharmaceuticals: Consultancy. Karbarz:Portola Pharmaceuticals: Employment. Lin:Portola Pharmaceuticals: Employment. Barron:Portola Pharmaceuticals: Employment. Russell:Portola Pharmaceuticals: Employment. Levy:Portola Pharmaceuticals: Employment. Connolly:Portola Pharmaceuticals: Consultancy. Curnutte:Portola Pharmaceuticals: Employment, Equity Ownership.

Author notes


Asterisk with author names denotes non-ASH members.