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An effective and safe anti-platelet drug is central to the management of patients with acute coronary syndrome (ACS). Glycoprotein VI (GPVI) is currently regarded as a potential target for novel anti-platelet agents due to its collagen-binding potential. Development of anti-thrombotics is associated with testing in animals. We have previously successfully evaluated anti-platelet drugs in the Cape Chacma baboon (Papio ursinus). However, various anti-GPVI agents did not have an effect on baboons when evaluated in our arterial thrombosis model. To evaluate the suitability of baboons for GPVI studies, we performed collagen-induced platelet aggregation, GPVI quantification and DNA sequencing. Baboon platelets needed double the amount of collagen compared to human platelets to illicit proper aggregation. GPVI quantification was unsuccessful due to non-binding of monoclonal antibodies. Sequencing of the GPVI gene revealed 36 SNPs leading to 14 amino acid changes, as well as a 9 bp deletion causing a 3 amino acid deletion. Several of the amino acid changes were within the ligand binding region of GPVI, causing limited binding of humanized anti-GPVI antibodies to the baboon platelets. Therefore, the baboon was deemed not suitable to evaluate human targeted anti-GPVI agents.

To prevent thrombosis in high-risk acute coronary syndrome patients undergoing percutaneous coronary intervention for re-vascularisation, concomitant administration of a glycoprotein IIb/IIIa inhibitor, such as abciximab, tirofiban or eptifibatide, is recommended. Abciximab and eptifibatide are mostly preferred over tirofiban, which is less effective in preventing ischaemic events. We compared the efficacy and bleeding potential of escalating doses of tirofiban and abciximab in non-human primates. The efficacy of tirofiban and abciximab in inhibiting cyclic flow reductions (CFRs) was tested in a high shear arterial thrombosis model. Bleeding was evaluated with the template bleeding time and an incision bleeding model. Abciximab completely inhibited arterial thrombosis after injection of its therapeutic bolus dose. With tirofiban, a dose three times higher than the recommended therapeutic dose caused weak inhibition characterised by a return of CFRs after re-injury. At nine times the recommended therapeutic dose, complete inhibition was observed, and the efficacy of tirofiban was comparable to abciximab at its therapeutic bolus dose. Blood loss was less than with abciximab at its effective dose. In this model, tirofiban compared favourably with abciximab, although only at a dose of 3-9 times the therapeutic dose, and caused less bleeding than abciximab.

The pathophysiology of thrombotic thrombocytopenic purpura (TTP) can be explained by the absence of active ADAMTS13, leading to ultra-large von Willebrand factor (UL-VWF) multimers spontaneously interacting with platelets. Preventing the formation of UL-VWF-platelet aggregates therefore is an attractive new treatment strategy. Here, we demonstrate that simultaneous administration of the inhibitory anti-VWF monoclonal antibody GBR600 and the inhibitory anti-ADAMTS13 antibody 3H9 to baboons (prevention group) precluded TTP onset as severe thrombocytopenia and hemolytic anemia were absent in these animals. In addition, partial VWF inhibition was not enough to prevent thrombocytopenia, demonstrating the specificity of this therapeutic strategy. GBR600 treatment of baboons during acute TTP (treatment group) resulted in a rapid recovery of severe thrombocytopenia similar to the platelet count increases observed in TTP patients treated by plasma exchange. Baboons in the control group only injected with 3H9 developed early stages of TTP as previously described. Hence, inhibiting VWF-GPIb interactions is an effective way to prevent and treat the early symptoms of acquired TTP in baboons.

ALX-0681 is a therapeutic Nanobody targeting the A1-domain of VWF. It inhibits the interaction between ultra-large VWF and platelet GpIb-IX-V, which plays a crucial role in the pathogenesis of thrombotic thrombocytopenic purpura (TTP). In the present study, we report the efficacy and safety profile of ALX-0681 in a baboon model of acquired TTP. In this model, acute episodes of TTP are induced by administration of an ADAMTS13-inhibiting mAb. ALX-0681 completely prevented the rapid onset of severe thrombocytopenia and schistocytic hemolytic anemia. After induction of TTP, platelet counts also rapidly recovered on administration of ALX-0681. This effect was corroborated by the full neutralization of VWF activity. The schistocytic hemolytic anemia was also halted and partially reversed by ALX-0681 treatment. Brain CT scans and post mortem analysis did not reveal any sign of bleeding, suggesting that complete neutralization of VWF by ALX-0681 under conditions of thrombocytopenia was not linked with an excessive bleeding risk. The results obtained in this study demonstrate that ALX-0681 can successfully treat and prevent the most important hallmarks of acquired TTP without evidence of a severe bleeding risk. Therefore, ALX-0681 offers an attractive new therapeutic option for acquired TTP in the clinical setting.