Bleeding complications and platelet dysfunction in patients with lymphoproliferative disease receiving Bruton tyrosine kinase inhibitors

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Ibrutinib (IBR) is an irreversible inhibitor of Bruton tyrosine kinase (Btk)

that is approved for the treatment of chronic lymphocytic leukemia (CLL) and indolent non-Hodgkin lymphomas (NHLs). Mild to moderate bleedings attributed to platelet inhibition by IBR have been reported in 44-60% of patients across clinical trials and there is great interest toward development of diagnostic and therapeutic strategies to manage these side effects1.

Btk is expressed in platelets and it is involved in a common platelet activation pathway downstream of different receptors, namely the collagen receptor glycoprotein (GP) VI2, the CLEC-2 receptor3 and the receptor for vonWillebrand factor (VWF), the GPIb-IX-V complex4. The GPVI-mediated platelet activation has been the most investigated IBR target, and several studies have consistently shown that IBR causes inhibition of the collagen-mediated platelet aggregation, which correlates with the occurrence of clinical bleedings5-7. The VWF-GPIb mediated pathway has been investigated in a few studies with conflicting results. One study reported that ristocetin-induced platelet aggregation (RIPA), which is dependent on vWF binding to GPIb, was inhibited in CLL patients treated with IBR, and the authors suggested the use of RIPA for predicting and monitoring the bleeding tendency in these patients8. Another study did not confirm these results, showing no short term or long term inhibition of RIPA in IBR treated CLL patients 9. These two studies made use of different techniques to evaluate platelet function, i.e. impedence aggregometry versus light transmission aggregometry, which could contribute to the different results. The CLEC-2-dependent platelet functions are difficult to explore, and they have not been investigated in IBR treated patients.

So far, the severe inhibition of collagen induced platelet aggregation seems to be correlated with the bleedings and it is an easy-to-use test to measure the platelet dysfunction caused by IBR, helping physicians to decide when to perform surgical procedures without haemostasis problems after IBR suspension.

However, Btk inhibition alone cannot explain the bleedings associated with IBR because patients with X-linked agammaglobulinemia, who have congenital Btk deficiency, do not exhibit increased risk of bleeding10. In fact, studies performed using genetically modified mice demonstrated redundancy between Btk and the other kinase Tec in platelet signalling because ablation of both kinases was required to abolish collagen induced platelet activation11. Hence, collagen-dependent aggregation is also dependent on Tec, and it is the off-target inhibition of Tec that is believed to contribute to IBR-induced platelet dysfunction. Interestingly, Btk and Tec kinases themselves become active upon phosphorylation of tyrosine residues by other kinases important for platelet GPVI signalling, i.e. Src family kinases (SFKs) that are targeted by dasatinib, whose administration in chronic myeloid leukemia is also associated with increased bleeding ris 12.


Acalabrutinib is a second-generation Btk inhibitor that was developed with improved specificity for Btk over Tec and SFKs.

No major bleeding events were reported during a phase 2 trial of acalabrutinib for treatment of CLL13.

A recent paper demonstrated differential activity of IBR and acalabrutinib on platelet function in patients with CLL receiving these drugs14. Acalabrutinib and IBR both inhibit GPVI-mediated platelet aggregation, which are mediated by Btk and Tec family kinase activity. However, IBR and not acalabrutinib inhibited Src family kinases, which mediate thrombus formation on collagen. Accordingly, thrombus formation on collagen under conditions of shear stress was reduced and unstable in the presence of IBR, whereas it was not affected by acalabrutinib. Thus, loss of SFK function and inhibition of stable thrombus formation is likely to contribute to bleeding risk in IBR treated patients. Lack of bleeding events in patients treated with acalabrutinib might be explained by only partial inhibition of SFK, which does not affect formation of normal thrombi on collagen.

In the same manuscript the authors tried to reverse the effect of drugs by adding ex vivo von Willebrand factor and factor VIII to blood from IBR and acalabrutinib treated patients. Indeed, addition of von Willebrand factor and FVIII significantly improved the thrombus formation in both IBR and acalabrutinib treated patients’ samples, as in healthy donors and Btk-inhibitor naïve CLL patients as well. Thus, a therapeutic approach based on the increase of circulating VWF levels, such as administration of desmopressin, might be considered in lymphoproliferative patients treated with Btk inhibitors in case of bleedings or urgent need for surgery, but this approach needs confirmation of efficacy in vivo.

It has to be considered, however, that in untreated CLL patients levels of vWF are higher than in control subjects, most probably because systemic inflammation and/or vascular damage occur in the presence of high lymphocyte counts7,9. Although IBR significantly reduces plasma VWF levels after 15/30 days of therapy through an unknown mechanism, increased thrombotic risk has to be evaluated when administering desmopressin causing further elevation of vWF levels in elderly patients, such as most of CLL patients, who usually have several comorbidities.

In conclusions, Btk inhibitors with higher selectivity for Btk over Tec and SFKs display a safer profile with regard to bleeding side effects in patients with lymphoproliferative disorders. Acalabrutinib seems a better option for the management of CLL patients with higher bleeding risk because of low platelet count, personal history of bleeding or concomitant antithrombotic therapy.



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