In a recent hemophilia clinical trial, patients' TFPI plasma levels substantially increased after intravenous or subcutaneous administration of TFPI-inhibitory aptamer BAX 499. To better understand the consequences of elevated plasma full-length (fl)-TFPI and the potency of BAX 499, the TFPI-inhibitory activity of a wide concentration range of BAX 499 was tested at levels up to 10 nM fl-TFPI in two model systems and in two global hemostatic coagulation assays.
Factor Xa (FXa) inhibition assay measured the influence of BAX 499 on the interaction between TFPI and FXa. BAX 499 at 1000 nM efficiently inhibited fl-TFPI within the physiological concentration (< 0.5 nM). However, with increasing TFPI, inhibition of TFPI by BAX499 became considerably less efficient, reflected by increasing EC50 and decreasing maximum inhibition. At substantially elevated TFPI concentrations (10 nM) EC50 increased ∼20-fold, with TFPI retaining ∼70% of its FXa inhibitory activity. Similar results were observed in a more complex reaction system (TF-FVIIa-catalyzed FX activation). These results suggest that BAX 499 is a partial inhibitor of TFPI, efficiently neutralizing TFPI at low concentrations and less efficiently inhibiting TFPI at high TFPI levels.
In thrombin generation assays with FVIII-inhibited plasma, a substantial excess of BAX 499 was required to compensate the anticoagulant activity of increasing fl-TFPI. Based on peak thrombin values, fl-TFPI levels that could be neutralized by a given BAX 499 concentration were determined. For example, a ∼50-fold excess of BAX 499 neutralized 0.2 nM TFPI added to FVIII-inhibited plasma, whereas a 140-fold excess was required for neutralization of 7.3 nM fl-TFPI added. An equivalent ROTEM experiment performed in FVIII-inhibited whole blood confirmed these findings.
In summary, the data showed that even a high concentration of BAX 499 (1 μM) was not able to counteract the strong inhibitory effect of 10 nM fl-TFPI on FXa, on TF-FVIIa-catalyzed FX activation, on thrombin generation in plasma and clot formation in whole blood, due to the partially inhibitory properties of BAX 499. The current study offers an explanation for increased bleeding tendency in hemophilia patients associated with the ≥25-fold elevated fl-TFPI plasma levels after BAX 499 administration. BAX 499 does not appear to be able to compensate the anticoagulant activity of increased TFPI levels caused by partial inhibition in patients treated with BAX 499.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.