Chronic Lymphocytic Leukaemia Is Associated with a Broad Platelet-Function Defect Which Is Exacerbated By Ibrutinib and Acalabrutinib
Authors: McGregor Scott R , Gilmore Grace , Gardiner Elizabeth E. , Hicks Sarah , Rakesh Shilpa , Tiao Jim , Baker Ross 
Published: October 2019

Abstract
INTRODUCTION
Bleeding from thrombocytopathy is a common complication of advanced chronic lymphocytic leukaemia (CLL). In addition to disease-related thrombocytopenia, the presence of the CLL clone and/or therapeutic interventions may further impair platelet function. In particular, the BTK inhibitors ibrutinib and acalabrutinib are known to inhibit platelet glycoprotein VI (GPVI)-mediated platelet aggregation. We compared platelet function and markers of GPVI activation between untreated CLL patients, ibrutinib-treated CLL patients and healthy controls, and studied the in vitro effects of ibrutinib and acalabrutinib on clinically utilised platelet function assays to assess their impact on GPVI-mediated as well as non-GPVI-mediated platelet activation pathways.
METHODS
Blood samples from 17 healthy volunteers and 8 untreated CLL patients were spiked with vehicle or comparable plasma concentrations of ibrutinib (0.3µM, 1.0µM) and acalabrutinib (1.8µM, 6.0µM) attainable during the treatment of CLL. Additional samples were obtained from 5 CLL patients undergoing ibrutinib treatment. Platelet function was evaluated using whole blood multiple electrode aggregometry (MEA - Multiplate®) and light transmission aggregometry (LTA - AggRAM®) in response to varying concentrations of aggregation-inducing reagents (collagen, CRP-XL, ADP, TRAP, ristocetin, arachidonic acid, and adrenaline). Shear-induced platelet adhesion was assessed using PFA-100®. Soluble GPVI plasma levels were assessed by ELISA.
RESULTS
In the absence of treatment, CLL patients exhibited significant platelet defects on whole-blood platelet function analyses in response to various agonists including ADP, ristocetin, TRAP and collagen (MEA) and prolongation of PFA-100® collagen/epinephrine closure time. This impairment was not replicated in assays using platelet-rich plasma (LTA). Ibrutinib-treated CLL patients demonstrated an additive impairment of platelet function, especially in regards to collagen-mediated activation by MEA or PFA-100®. There was no significant difference in soluble GPVI levels between normal, untreated or ibrutinib treated CLL patients.
Addition of clinically-attainable concentrations of ibrutinib and acalabrutinib in vitro produced similar concentration-dependent inhibition of platelet function in healthy controls, with inhibition of aggregation evident in response to various agonists including collagen, CRP-XL, ristocetin and ADP but not arachidonic acid or TRAP. Ibrutinib also impaired aggregation in response to epinephrine, and caused selective prolongation of the PFA-100® collagen/epinephrine closure time, an effect not observed with acalabrutinib. MEA appears more sensitive and reproducible than LTA to describe the various inhibitory effects on platelet aggregation. Similar concentration-dependent inhibition of platelet function was observed by adding ibrutinib and acalabrutinib in vitro to blood samples from untreated CLL patients.
CONCLUSIONS
CLL is associated with a broad platelet function defect, which can be exacerbated by BTK inhibitors. Acalabrutinib induces a platelet function defect similar but less potent to that observed with ibrutinib, with the exception of shear-induced platelet adhesion (PFA-100®) which was only abnormal with ibrutinib. Routine platelet function assays are capable of quantifying BTK inhibitor-induced platelet dysfunction in CLL patients, with the most sensitive and reproducible measure being collagen-induced aggregation by MEA. There was no evidence for BTK-dependent platelet GPVI cleavage. Whole-blood platelet function assays may have utility in managing CLL patients presenting with bleeding or requiring urgent surgery during therapy with BTK inhibitors.

Disclosures
McGregor: Pfizer: Other: Conference travel support; Bristol-Myers Squibb: Other: Conference travel support; Gilead Sciences: Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria. Baker:CSL Behring: Research Funding; Biogen Idec: Membership on an entity's Board of Directors or advisory committees, Research Funding; Astellas: Research Funding; Boehringer Ingelheim: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bayer: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Research Funding; Pfizer: Membership on an entity's Board of Directors or advisory committees, Research Funding; Daiichi Sankyo: Research Funding; Alexion Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Other: Conference travel support, Research Funding; Shire: Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees, Other: Conference travel support; Roche: Other: Conference travel support; Novo Nordisk: Other: Conference travel support.