Prothrombin Complex Concentrates (PCCs) Have Limited Effect on TF-Initiated Thrombin Generation in FXa Inhibitor-Anticoagulated Plasma: In Vitro Comparison between Direct Reversal By Andexanet Alfa and "Work Around" By PCCs
Authors: Lu Genmin , Lin Joyce P. , Bui Khanh , Curnutte John T. , Conley Pamela B. 
Published: February 2020

Introduction: Direct oral FXa inhibitors (DOACs, e.g., apixaban [Apix] and rivaroxaban [Riva]) exert their anticoagulant activity by specifically targeting FXa with subnanomolar affinities. Reversal of DOAC-induced anticoagulation can be achieved by immediate and effective sequestration of the inhibitors to relieve inhibition of FXa activity and restore normal thrombin generation (TG). Andexanet alfa (AnXa) is a modified human FXa protein approved in the US and EU for patients treated with Apix or Riva, when reversal of anticoagulation is needed due to life-threatening bleeding. Factor replacement therapies (e.g., 4 factor-PCCs [4F-PCCs]) have demonstrated clinical benefit for vitamin K antagonist-anticoagulated patients by replacing factors II, VII, IX, and X rendered deficient by the anticoagulant. In contrast, the effectiveness and clinical benefit of 4F-PCCs are unclear for reversing DOACs because PCCs do not affect anti-FXa activity. In the present study, we evaluated the effect of individual coagulation factors (FIX, FX, FII), as well as 4F-PCCs on TF-initiated TG (TF-TG) in human platelet poor plasma (PPP), with or without Apix or Riva, and compared it with AnXa under the same conditions.
Methods: TF-TG was measured using a calibrated automated thrombogram (CAT, 5 pM TF, Diagnostica Stago). Purified plasma proteins (FIX, FX, FII), PPP, and a commercially available 4F-PCC (Kcentra/Beriplex) were used. 4F-PCC was used with or without pretreatment with the heparinase cup (Haemonetics Corporation) at room temperature for 30 minutes. TF-TG was measured in PPP spiked with purified plasma proteins, 4F-PCC (0-1.0 IU/mL), 4F-PCC + Riva or Apix (0-250 ng/mL), or PPP with AnXa (0-4 μM) + Riva or Apix (0-2000 ng/mL). For comparison, reversal of warfarin anticoagulation by 4F-PCC was performed under similar conditions using individual plasma from warfarin-treated patients (INR = 1.5-6.9) spiked with 4F-PCC (0-1.0 IU/mL). Five CAT parameters were collected, and the endogenous thrombin potential (ETP) and Peak thrombin (Peak) were used for the comparisons.
Results: In normal PPP, addition of FX (up to 1.0 IU/mL) had minimal effect on TF-TG parameters, whereas FIX (1.0 IU/mL) mainly increased Peak (~60%). In contrast, FII (1.0 IU/mL) increased both the ETP (~2x) and Peak (~50%). These observations are consistent with the relative plasma concentration and affinity of these coagulation factors as the substrate for the respective enzyme complexes, demonstrating the predominant role of prothrombinase activity in the TF-TG. Addition of 4F-PCC (1.0 IU/mL, equivalent to the therapeutic dose of 50 IU/kg) to PPP caused similar increases of ETP (~2.4x) and Peak (~40%). However, individual plasma protein (FIX, FX, FII) or 4F-PCC up to 1.0 IU/mL was unable to restore normal ETP or Peak in the presence of Apix or Riva (75-250 ng/mL). ETP and Peak increased only when the inhibitor concentrations were sufficiently low (&lt;75 ng/mL), leading to less inhibition of FXa and increased FIIa formation, likely due to an increased FII level from 4F-PCC addition. Similar results were obtained with or without preincubation of 4F-PCC with heparinase prior to spiking in plasma. In contrast, AnXa (0-4.0 μM) dose-dependently reversed Apix- and Riva- (up to 2000 ng/mL) induced inhibition of TF-TG. In the same TF-TG assay, 4F-PCC dose-dependently reversed warfarin anticoagulation in warfarin-treated patients' plasma spiked with 4F-PCC.
Conclusions: Specific reversal agents are effective for restoration of TF-TG by either direct sequestration of the FXa inhibitor and restoration of FXa activity (in the case of AnXa with FXa inhibitors) or by replacement of inactive factors due to warfarin treatment (in the case of 4F-PCCs). Addition of coagulation factors or use of 4F-PCCs had limited effect on restoration of normal TG in the presence of a FXa inhibitor. These data demonstrate that AnXa can normalize TF-TG over a wide range of FXa inhibitor concentrations (0-2000 ng/mL), in particular over the range observed in bleeding patients in the ANNEXA-4 study, where 77% of bleeding patients had inhibitor levels ≥75 ng/mL at baseline. In contrast, 4F-PCCs were only able to restore normal TF-TG at low inhibitor levels (&lt;75 ng/mL). These in vitro data are consistent with ANNEXA-4, where AnXa demonstrated &gt;90% immediate reversal of anti-FXa activity and &gt;80% hemostatic efficacy with a wide range of FXa inhibitor levels and multiple bleed types.

Disclosures
Lu: Portola Pharmaceuticals: Employment, Equity Ownership. Lin:Portola Pharmaceuticals, Inc: Employment, Equity Ownership. Bui:Portola Pharmaceuticals, Inc.: Employment, Equity Ownership. Curnutte:Portola Pharmaceuticals: Employment, Equity Ownership. Conley:Portola Pharmaceuticals: Employment, Equity Ownership.