Background: Bruton’s tyrosine kinase (BTK) plays an important role in platelet signalling downstream of GPVI receptor and has been proposed as a novel target to prevent thrombosis in high-risk patients. Current, clinically used BTK inhibitors (e.g. acalabrutinib, ibrutinib), however, have off target effects and are associated with an increased risk of bleeding.

Aims: Our study is focussed on the targeted chemical degradation of BTK in human platelets by using recently developed heterobifunctional small molecule degraders (PROteolysis TArgeting Chimeras, PROTACs) that utilise the proteasomal system to degrade BTK. We aim to explore the repurposing potential of PROTACs as novel anti-platelet therapy and to see whether these compounds can be used as pharmacological research tools to study platelet function.

Methods: Human platelet rich plasma (PRP) was treated with various concentrations of the PROTACs. Samples were then either lysed and subjected to immunoblotting, or were assessed for integrin a_IIbb₃ activation, P-selectin expression and Annexin V binding in response to the GPVI and PAR agonists CRP or a-thrombin, respectively. 

Results: The multi-tyrosine kinase degrader TL 12-186 and BTK degraders show potent BTK degradation, which can be overcome by addition of proteasomal inhibitors. GPVI-mediated integrin activation, P-selectin expression and PS exposure were significantly impaired, whereas PAR-mediated responses were unaffected. Tandem mass tagging confirmed the high specificity of BTK degradation.

Conclusions: Our data confirm the high susceptibility of human platelets to BTK degraders, therefore PROTACs can be successfully utilised in modulating and studying human platelet function.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/targeted-proteasomal-degradation-of-platelet-btk-and-tec-as-a-novel-approach-in-thrombosis-prevention/