Abstract
The MEK inhibitors cobimetinib and trametinib are used in combination with BRAF inhibitors to treat metastatic melanoma but increase rates of hemorrhage relative to BRAF inhibitors alone. Platelets express several members of the MAPK signalling cascade including MEK1 and MEK2 and ERK1 and ERK2 but their role in platelet function and haemostasis is ambiguous as previous reports have been contradictory. It is therefore unclear if MEK inhibitors might be causing platelet dysfunction and contributing to increased hemorrhage. In the present study we performed pharmacological characterisation of cobimetinib and trametinib in vitro to investigate potential for MEK inhibitors to cause platelet dysfunction.We report that whilst both cobimetinib and trametinib are potent inhibitors of platelet MEK activity, treatment with trametinib did not alter platelet function. Treatment with cobimetinib results in inhibition of platelet aggregation, integrin activation, alpha-granule secretion and adhesion but only at suprapharmacological concentrations. We identified that the inhibitory effects of high concentrations of cobimetinib are associated with off-target inhibition on Akt and PKC. Neither inhibitor caused any alteration in thrombus formation on collagen under flow conditions in vitro.Our findings demonstrate that platelets are able to function normally when MEK activity is fully inhibited, indicating MEK activity is dispensable for normal platelet function. We conclude that the MEK inhibitors cobimetinib and trametinib do not induce platelet dysfunction and are therefore unlikely to contribute to increased incidence of bleeding reported during MEK inhibitor therapy.
Highlights
Protein kinases mediate many critical signalling events in platelet activation and enable them to fulfil their role in primary haemostasis
Further investigation of the effect of the MEK inhibitors on the profile of platelet aggregation using real-time traditional light transmission aggregometry identified no inhibition of the kinetics or extent of platelet aggregation to either 1 μg/ml CRP-XL or 1 μM TRAP-6 by either cobimetinib or trametinib even at the highest concentrations tested (10 μM)
We found that accumulation of platelets on collagen coated flow chambers, normalised to maximum vehicle-treated response to compensate for differences between donors, occurred at a similar rate following treatment with 10 μM cobimetinib or trametinib compared to vehicle-treated controls (Figure 3a)
Summary
Protein kinases mediate many critical signalling events in platelet activation and enable them to fulfil their role in primary haemostasis. Dasatinib (Src), ibrutinib (Btk) and imatinib (Bcr-Abl) all cause platelet dysfunction via inhibition of platelet kinases, while drugs like idelalisib (PI3K, P110δ) inhibit platelet kinases without increasing bleeding risk. This necessitates strategies to manage bleeding risk, such as contraindication with antiplatelet medication or cessation of therapy prior to surgical procedures. Cobimetinib binds to phosphorylated MEK (with a 100x greater potency for MEK1 over MEK2) whilst trametinib binds to non-phosphorylated MEK1 and 2 with similar potencies, though both inhibit downstream ERK signalling Both cobimetinib and trametinib are used to inhibit BRAFV600E/K-mediated MEK1 activation, phospho-MEK1 activity, cellular phosphorylation of ERK, and cellular proliferation. ERK2 activation is thought to be mediated by MEK1/2 and seems to be dependent on protein kinase C (PKC) [10]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
