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J. Series et al.
We provide evidence that one factor contributing to predis- pose platelets to ibrutinib sensitivity in vitro is the drug efflux pump system. Indeed, inhibition of drug efflux pumps was sufficient to induce ibrutinib sensitivity in the LS group of healthy donors. This suggests that the actual dose of ibrutinib reaching intracellular targets is critical and will determine the extent of inhibition of Btk and Tec and possibly other undesired targets such as Src-kinases. These data are in line with those of a very recent study suggesting that the ibrutinib-mediated increase of bleeding is due to off-targets effects of the drug occurring because of unfavor- able pharmacodynamics.27 The authors propose that the bleeding side effect of ibrutinib may be avoided by reduc- tion of the dose. Our data suggest that an efficient platelet drug efflux pump system may limit the multifactorial antiplatelet effects of ibrutinib. This is important new infor- mation to take into consideration when interpreting the results of in vitro experiments with ibrutinib. Moreover, it could be interesting to analyze a potential link between polymorphisms of drug efflux pumps and the risk of bleed- ing in patients treated with ibrutinib. These data may also stimulate pharmacists to look for intake of P-glycoprotein inhibitors among co-medications in patients with pro- longed bleeding under ibrutinib. Since verapamil increases the plasma concentrations of amiodarone and likely the intra-platelet concentration of ibrutinib, cardiologists may favor the use of β-blockers when prescribing drugs to lower the heart rate. Evaluating the sensitivity of a patient’s platelets to ibrutinib before starting therapy could also help clinicians to establish a personalized therapeutic strategy.
Our standard in vitro aggregation tests indicated that acal- abrutinib had no effect on the maximal platelet aggregation response in the ibrutinib LS group of healthy donors. In the
ibrutinib HS group of healthy donors, acalabrutinib affected maximal platelet aggregation only in a few cases. However, acalabrutinib consistently delayed platelet aggregation in both groups. As expected, acalabrutinib had no effect on platelet aggregation induced by TRAP, U46619 or ADP. These data point to a better profile of acalabrutinib on platelets from healthy donors compared to that of ibrutinib.
Importantly, when acalabrutinib was tested on PRP from CLL patients, it had no effect on the maximal platelet aggre- gation response in the ibrutinib LS group, but significantly inhibited platelet aggregation in the ibrutinib HS group of patients. These data suggest that a switch from ibrutinib to acalabrutinib therapy may not be systematically appropri- ate to prevent bleeding in CLL patients.5 The results from clinical trials show that, although no grade 3 bleeding was observed in relapsed CLL patients treated with acalabruti- nib, low-grade hemorrhages occurred in a significant pro- portion of patients, comparable with those observed with ibrutinib.4,9,24
Ibrutinib has been shown to reduce the stability of platelet thrombus on collagen24 and firm platelet adhesion on the von Willebrand factor matrix.10 Consistent with the data reported by Bye et al.,24 we found here that acalabruti- nib had no effect on thrombus formation on collagen in the ibrutinib LS group of healthy donors. However, in the ibru- tinib HS group, while acalabrutinib did not affect platelet adhesion it was able to significantly reduce thrombus vol- ume. Again these data show a better profile of acalabruti- nib, although with some significant impact in the ibrutinib HS group.
The effects of ibrutinib and acalabrutinib on the Btk- mediated tyrosine phosphorylation of PLCγ2 on Tyr-753 and on the Src autophosphorylation site Tyr-418 (activated
AB
Figure 5. Effect of ibrutinib and acalabrutinib in association with anti-platelet drugs. (A, B) Platelet-rich plasma from healthy donors with (A) high sensitivity (HS) or (B) low sensitivity (LS) to ibrutinib was pre-treated or not with the indicated doses of acalabrutinib (ACP) or ibrutinib for 1 h at 37°C and anti-platelets drugs (10 mM indomethacin and/or 10 mM ARC69931MX) were added 10 min before stimulation with collagen 3.3 mg/mL. Platelet aggregation was assessed by turbidimetry dur- ing 10 min and results, expressed as percentage of maximal aggregation, are the mean ± standard error of mean of three to five independent experiments. *P<0.05, **P<0.01, ***P<0.001 according to one-way and two-way analyses of variance.
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haematologica | 2019; 104(11)