Elucidation of Mild Bleeding Disorders Reported Under Ibrutinib (Imbruvica(R)) Therapy: Implications for Optimal Clinical Management

Introduction Ibrutinib is the first-in-class covalent inhibitor of Bruton's Tyrosine Kinase (BTK), now approved for the therapy of mantle cell lymphoma (MCL) and chronic lymphocytic leukemia (CLL). Mild bleeding disorders (grade 1-2) have been reported in 44-60% of patients across clinical trials, with <5% grade 3 hemorrhages after trauma. After vascular injury, platelets adhere onto von Willebrand factor (vWF, through GPIb-IX-V complex) and collagen (through a2b1 and GPVI receptor), and activate phospholipase Cg2 (PLCg2) through BTK phosphorylation. In this study, we sought to examine phosphorylation pathways and platelet functions in vitro and ex vivo from ibrutinib-treated patients. Patients and Methods Within the compassionate access program of ibrutinib in France (started Feb 2014), we investigated whether ibrutinib could impact on platelet functions in vitro and ex vivo, as measured at day 0 and day 15-30 by: aggregometry using various agonists, measurement of intra-cellular levels of phosphorylation of BTK and PLCg2 phosphorylations, monitoring adhesion onto vWF matrix under high shear rate. We next assess how in vitro tests could help identify bleeding risk in a larger cohort of patients from three institutions. Results First, we demonstrated that in healthy donors' platelets, ibrutinib inhibits collagen and collagen related peptide (CRP) -induced platelet aggregation in a dose-dependent manner (mean EC50=250nM, a dose achievable in patients). This effect was paralleled by the inhibition of PLCg2 phosphorylation on the Btk-dependent phosphorylation site Tyr753, and of the auto-phosphorylation Tyr223 site of BTK itself, suggesting a specific targeting by ibrutinib. Of note, adhesion on vWF under high shear rate was dramatically decreased. In parallel, in 7/14 patients had bleeding symptoms (5/7 with grade 1-2 bleedings) and they all presented a strong inhibition of platelet aggregation in response to collagen and a significant decrease in adhesion onto vWF. Thus, the easy-to-use collagen-induced platelet aggregation test in platelet rich plasma could help physicians to decide when to perform surgical procedures without haemostasis concerns. Moreover, we show that addition of 50% untreated platelets is sufficient to efficiently reverse the effects of ibrutinib, and that platelet functions recover following treatment interruption as physiological platelet renewal occurs, supporting the in vitro data. On the other hand, patients who received aspirin (n=6) had no cases of severe bleeding and no significant impact on collagen/CRP-induced platelet aggregation. Because aspirin+P2Y12 inhibitors (such as clopidogrel, Plavix®) is widely used in the elderly population, ibrutinib therapy should be given very cautiously to these patients (who receive then three major pathway platelet activation pathway inhibitor), as recommended for vitamin K antagonists drugs. Aggregometry tests may provide important information to physicians to predict the bleeding risk as observed in our cohort of >30 patients (as of June 2014, recruitment still ongoing). Two last points should be emphasized when considering bleeding risk of ibrutinib: (i) from our study, some patients had no anti-platelet detectable effect ex vivo under ibrutinib therapy, the mechanism of which still remains unclear, and (ii) in patients with mild bleedings, platelet functions recovery and cessation of symptoms occured in virtually all patients (except those on aspirin therapy) after 3-6 months despite ongoing lymphoma responses, suggesting a potential adaptative process in platelets. Summary and Conclusion We identified that ibrutinib affects collagen and Von Willebrand Factor-mediated platelet activation in vitro and ex vivo. The mild bleeding diathesis observed in a subgroup of ibrutinib-treated patients correlates with defects in collagen-induced platelet aggregation and platelet adhesion on von Willebrand Factor at high shear rate. Based on in vitro analyses and in vivo platelet turnover, 2-3 days ibrutinib cessation appears to be enough for effective aggregation response recovery, and reintroduction of the drug should be rapid to avoid disease recurrence. Our study also suggests that platelet transfusion at a dose sufficient to get 50 % of fresh platelets may correct haemostasis in emergency, provided it was given after elimination of ibrutinib from blood (4-6h). Disclosures Tam: Pharmacyclics and Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees.