Novel Single Agents with Combined Anti-Coagulant and Anti-Platelet Activities: Potential Treatment Option for the Management of HIT.

A recently introduced series of antithrombotic agents brings the novel characteristic of dual anti-coagulant and anti-platelet actions in one molecule. These low molecular weight, synthetic, serine protease inhibitors, depending on structural modifications, have variable ratios of both anti-thrombin and anti-platelet activities. Studies have shown that these agents produce stronger antithrombotic actions relative to single targeted therapeutic agents (O. Iqbal #P0521 and D. Hoppensteadt #OR335, ISTH meeting Sydney, Australia August 2005). Heparin-induced thrombocytopenia (HIT) is an adverse effect of heparin in which both thrombin generation and platelet activation augment hypercoagulable and inflammatory states leading to a high probability of developing severe thrombosis. Current guidelines for patients who have HIT recommend the use of a direct thrombin inhibitor (DTI) to prevent or treat associated thrombosis. Clinical trial data, as well as practice outcomes, show that DTI treatment alone is not sufficient to overcome the pathology and resultant thrombosis in all HIT patients. Thus, more optimal treatment options are needed. A focus of treatment on inhibiting both platelet and coagulation activation is logical based on the pathophysiology of HIT. This study was undertaken to determine if the novel CanAm series of agents may have a role in the management of patients with HIT. Eight agents (MC45301, MC45308, CA207, CA216, CA234, CA247, CA250, CA254) with varying ratios of anticoagulant/anti-platelet activities were studied using the ¹⁴C-serotonin release assay (SRA) and flow cytometry for the detection of platelet P-selectin expression and platelet microparticle generation. The DTI argatroban, the FXa inhibitor fondaparinux, and the platelet GPIIb/IIIa receptor antagonist eptifibatide were included for comparison. Both cross-reactivity to HIT antibodies and amelioration of HIT antibody-induced platelet activation were assessed. In the absence of heparin, at 1-100 μg/ml none of the CanAm agents caused platelet activation in the presence of serum from patients with HIT (n=12) ruling out cross-reactivity with HIT antibodies. None of the comparator drugs showed cross reactivity with HIT antibodies. In the presence of heparin (0.1 U/ml) and serum from patients with HIT (n=12), the CanAm agents were able to inhibit all platelet activation responses at concentrations of 10–100 μg/ml. In comparison, the DTI and FXa inhibitor were not able to inhibit the HIT antibody/heparin induced platelet activation with any HIT serum. The GPIIb/IIIa inhibitor, however, showed a concentration-dependent inhibition of the platelet activities with complete blockade at 1 μg/ml, suggesting the importance of platelet activation inhibition for the treatment of HIT. Thus, compared to mono-therapeutic agents such as DTIs and fondaparinux, the dual-acting CanAm agents not only lack cross reactivity with HIT antibodies, but they have the added ability to block the antibody-induced platelet activation that occurs during an acute episode of HIT. A dual-acting anticoagulant/anti-platelet drug may, therefore, be of more value than single targeted anti-thrombin drugs for the management of HIT and associated thrombosis.