Thrombin Generation Test in Patients with Hemophilia and Inhibitors Treated with by-Passing Agents: Results From In Vivo Studies

The development of inhibitors is the most serious complication of modern hemophilia therapy. Patients with inhibitors are often treated with by-passing agents (namely, aPCC and rFVIIa) whose haemostatic efficacy is not always predictable even in the same subject. Indeed clinical response to by-passing therapies may vary between patients and the lack of a specific laboratory test aimed at monitoring their ability in triggering blood coagulation and at correlating clotting activation to clinical outcome renders the management of these drugs somehow empirical. The thrombin generation (TG) test is a global coagulation assay that may serve as a candidate in this setting. Recently, dose tailoring of by-passing agents was performed using in vitro spiking experiments in order to establish the most adequate doses to cover elective surgery in hemophilic patients with inhibitors. Methods: In this study TG capacity was evaluated for the first time in vivo by drawing plasma samples from hemophilic patients with inhibitors treated either with aPCC or rFVIIa in a non-bleeding state after a minimum wash-out period of 3 days from the last infusion. TG test was performed in platelet-rich (PRP) and platelet-poor (PPP) plasma with the addition of corn trypsin inhibitor (CTI, final concentration 18.3 mcg/ml). Blood was drawn at baseline, 30 minutes, 3 hours (if rFVIIa), 6 hours (if aPCC) and 24 hours after drug administration. Four parameters of the TG curve were evaluated: lagtime, endogenous thrombin potential (ETP), peak and time to peak. Patients were defined as responders to by-passing agents when able to control mild/moderate bleeding episodes by home treatment with those drugs. Results: Eight patients with hemophilia A and high-responding inhibitors (historical peak above 5 BU/ml) with a median age of 33 years (IQR: 20–38) received rFVIIa at a standard dose of 90–120 mcg/kg and aPCC at a dose of 80 IU/kg. In 6 patients the TG test was performed also after the administration of a rFVIIa dose of 270 mcg/kg. Four patients were responders to aPCC, one to rFVIIa, 2 to both drugs and one was non-responder to both. Median values of the TG curve observed at baseline and 30 minutes after drug administration are shown in the Table. Similar variations in the TG curve were observed after administration of either rFVIIa or aPCC or after the administration of rFVIIa at the 2 different dosages. By evaluating only the patients responders to aPCC (n=4), there was no difference in the TG curves obtained after the administration of aPCC or rFVIIa. Conclusions: Our preliminary results show that the in vivo administration of by-passing agents causes an increase of TG capacity in hemophilic patients with inhibitors. However, the extent of such increase does not seem to be related neither with the type nor with the dose of these drugs. The evaluation of TG curve in response to by-passing agents will be performed also in haemophilic patients with inhibitors who underwent or will undergo major surgical procedures.

No relevant conflicts of interest to declare.