Antithrombin Reduction Corrected Thrombin Generation in Samples from Hemophilia A and B Patients with Inhibitors

Background: Severe hemophilia A and B patients with inhibitors experience serious musculoskeletal hemorrhage as well as high risk of limb and life threatening bleeds. However, lack of effect of FVIII or FIX substitution therapy and short functional half-life of by-passing agents, leave these patients with very limited bleed preventive treatment options. ALN-AT3 (Alnylam Pharmaceuticals, Cambridge, MA, USA), a subcutaneously administered investigational RNAi therapeutic targeting reduction of antithrombin for potential treatment of hemophilia is currently in phase 1 clinical development in hemophilia A and B patients without inhibitors. Initial data from that ongoing study in 12 patients suggest an AT KD dependent correction of thrombin generation. This study aims to assess changes in peak thrombin generation in samples from patients with severe hemophilia A and B with inhibitors following in vitro reduction of antithrombin.

Materials and methods: Citrated plasma samples were obtained from patients with severe hemophilia A and B with high responding inhibitors. Samples were spiked in vitro with isotype specific control IgG or a monoclonal antibody (Haemtech Inc, Essex Junction, VT, USA) targeting antithrombin knockdown of 50% and 90%. Dynamic formation of thrombin was measured by calibrated automated thrombin generation using 1pM tissue factor PPP reagent and 4μM phospholipid (Thrombinoscope, Maastricht, The Nederlands). The primary effect measure was peak thrombin (nM). Data were tested by a 1-way ANOVA and p<0.05 was considered statistically significant.

Results: A total of 12 inhibitor hemophilia samples were investigated; 9 hemophilia A and 3 hemophilia B. All the control samples demonstrated a profound defect in thrombin generation with a median peak thrombin of 19.9 nM (range 6.7 - 42.4). Patients with severe hemophilia A and inhibitors had a median peak thrombin generation of 19.7 nM (range 6.7 - 42.4), whereas patients with severe hemophilia B and inhibitors had a median peak thrombin generation of 19.2nM (range 19.4 - 38.1). An AT reduction dependent improvement in peak thrombin generation was observed in all 12 tested plasma samples (Figure 1). In the first 12 subjects, peak thrombin generation was increased up to 363% from a mean of 22nM (control) to 39 nM (50% AT reduction) and 80nM (90% AT reduction) (p<0.05); levels comparable to thrombin generation observed in healthy male volunteers and in hemophilia patients treated with ALN-AT3.

Conclusions: These in vitro data suggest that reduction of AT is a promising approach for restoring hemostatic balance and correcting thrombin generation in hemophilia patients with inhibitors. Furthermore, the present laboratory data compare well with clinical data generated with ALN-AT3 administered to patients with hemophilia A or B. Thus, both laboratory and emerging clinical data suggest that targeting antithrombin could be a promising approach for restoring hemostatic balance in hemophilia. The potential for low volume subcutaneous administration, infrequent dosing, and applicability to persons with hemophilia who have inhibitors, make ALN-AT3 a particularly encouraging investigational therapy.

Figure 1.

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Figure 1.

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