The Cape Chacma baboon is not suitable for evaluating human targeted anti-GPVI agents

The effects of streptokinase in a Chacma baboon (Papio ursinus) model of acquired thrombotic thrombocytopenic purpura

TTP is a life-threatening disorder with limited pharmaceutical treatment options. Recently, the potential of streptokinase in the treatment of acquired TTP was demonstrated in humans in vitro, and in vivo in a mouse model. We aimed to determine the in vitro and in vivo effects of streptokinase in an established Papio ursinus model of acquired TTP. In vitro: VWF activities & multimer patterns and thromboelastograms were assessed with increasing concentrations of streptokinase. In vivo: After induction of TTP, escalating streptokinase doses (ranging from 50,000 to 900,000 IU) were administered, and the effects of streptokinase assessed on peripheral blood counts, fibrinolysis, VWF activities & multimer patterns and thromboelastograms. In an extension of the study, high-dose streptokinase (1,500,000-3,000,000 IU) was administered to another baboon. After spiking, fibrinolysis with loss of large VWF multimers was observed at [2200 IU/mL]-roughly equivalent to 1,500,000 IU. However, administration of escalating intravenous streptokinase doses had no in vivo effect on the TTP phenotype, and in vivo increases in plasmin activity were mild when compared with baseline, even at high doses. Minimal effect on VWF multimer patterns was observed but only at doses ≥ 1500,000 IU. Streptokinase is not effective in resolving TTP in a Papio ursinus model of TTP, possibly due to limited activation of the baboon fibrinolytic system. Modifications to this model, the use of alternative higher animal models, or alternative thrombolytics, should be considered to establish proof-of-concept.

Platelet function in baboons and humans - A comparative study of whole blood using impedance platelet aggregometry (Multiplate®)

BACKGROUND

Platelets play a pivotal role in coagulation, inflammation and wound healing. Suitable animal models that have the potential to mimic human platelet function are limited. The objective of the current study was to compare platelet aggregation response in the whole blood of baboons and humans using impedance aggregometry.

METHODS

Blood was drawn from 24 anesthetised male baboons and 25 healthy volunteers. The platelet aggregation response was determined by impedance aggregometry (Multiplate®). Platelets in the hirudinised whole blood samples were stimulated with four different activators: adenosine diphosphate (ADP), collagen (COL), thrombin receptor activating peptide-6 (TR1AP), and activation of PAR-4 thrombin receptor subtype (TR4AP) at standard concentrations. Higher than standard concentrations were tested in a subgroup of the animals.

RESULTS

The cell counts showed no differences between baboons and humans. The platelet aggregation response was significantly lower in baboons compared to humans when stimulated with the platelet agonists ADP (p<0.0001), COL (p=0.021) and TR4AP (p<0.0001). TR1AP did not stimulate platelet aggregation in the baboon blood. Doubling the concentration of ADP and of TR4AP significantly increased the AUC compared to the standard concentration. In contrast, increased COL levels did not further increase the AUC.

CONCLUSION

The current study revealed that testing the platelet function in baboon blood by impedance aggregometry is feasible with ADP, COL and TR4AP, but not with TR1AP. Compared to humans, the aggregation response is lower in baboons. Considering the limitations in accordance to these results, baboons might represent a potential species for further platelet research.

Comparison of common platelet receptors between the chacma baboon (Papio ursinus) and human for use in pre-clinical human-targeted anti-platelet studies

Anti-platelet agents play a central part in the treatment and prevention of acute thrombotic events. Discriminating animal models are needed for the development of novel agents. The chacma baboon has been extensively used as a model to evaluate anti-platelet agents. However, limited data exist to prove the translatability of this species to humans. We aimed to determine the suitability of the chacma baboon in preclinical human targeted GPIIb/IIIa, GPIbα and P2Y12 studies. Light-transmission platelet aggregometry (LTA), whole blood impedance aggregometry, receptor number quantification and genomic DNA sequencing were performed. Baboon ADP and arachidonic acid-induced LTA aggregation results differed significantly from human values, even at increased concentrations. LTA ristocetin-induced agglutination was comparable between species, but baboon platelets needed twice the concentration of ristocetin to elicit a similar response. Citrated baboon blood had significantly less aggregation than humans when evaluated with impedance aggregometry. However, hirudinised baboon whole blood gave similar aggregation as humans at the same agonist concentrations. GPIIb, GPIIIa and GPIbα numbers were significantly more on the baboon platelets. None of the amino acids deemed vital for receptor function, ligand binding or receptor inhibition, were radically different between the species. However, a conservative change in a calcium-binding region of GPIIb may render the baboon platelets more sensitive to calcium-binding agents. The chacma baboon may be used for the evaluation of human-targeted GPIIb/IIIa-, GPIbα- and P2Y12-inhibiting agents. However, the best anticoagulant, optimal agonist concentrations, increase in receptor number and sequence differences must be considered for any future studies.