Tyrosine Kinase Inhibitors (TKIs) Targeting Syk and BTK Signaling Differentially Affect PI3K Signalosome Organization and Platelet Function

Small molecule tyrosine kinase inhibitors (TKIs) serve increasingly important roles in the treatment of hematologic malignancies and also have shown efficacy in a range of immunological and inflammatory conditions. However, many successful TKI therapies are associated with problematic effects - particularly on platelets - as bleeding complications have been reported for many TKIs, both on and off clinical trials. Bleeding risk is especially apparent for patients treated with ibrutinib and other irreversible inhibitors of Bruton's tyrosine kinase (BTK), several of which now effectively treat multiple B cell malignancies. Intriguingly, other therapeutic TKIs targeting mechanistic partners of BTK are not associated with platelet-related complications. For instance, inhibitors against the BTK-activating kinase Syk - both with selective (i.e., entospletinib) as well as promiscuous (i.e., fostamatinib) target profiles - are not associated with clinically increased bleeding, despite some reported antiplatelet effects.

To gain insight into the multifaceted effects of Syk and BTK inhibitors on platelet function, we analyzed the effects of a panel of eight different clinically relevant Syk- and BTK-directed TKIs (i.e., fostamatinib/R406, entospletinib, ibrutinib, acalabrutinib, AVL-292 and others) on essential platelet responses. We first assessed the effects of TKI treatment on Syk and BTK-mediated signaling events downstream of the platelet immunoreceptor tyrosine-based activation motif (ITAM) receptor GPVI. All Syk and BTK TKIs tested inhibited the phosphorylation of phospholipase C (PLCγ2) Y1217, protein kinase Cδ (PKCδ) Y311 and Akt T308, as well as Akt substrate phosphorylation following platelet stimulation with the GPVI receptor agonist, crosslinked collagen-related peptide (CRP-XL). Independent of TKI target profile, Syk and BTK TKIs differentially inhibited the phosphorylation of DAPP1 Y132, phosphoinositide 3-kinase (PI3K) p85α Y458, Akt S473 and PKC substrates. Syk and BTK TKIs also inhibited platelet adhesion to immobilized CRP-XL in a manner matching the effects of each TKI on ITAM-mediated signaling events. However, platelet signaling and adhesion phenotypes did not match the effects of these same TKIs on GPVI-triggered dense granule secretion, which was inhibited by all TKIs tested with the exception of R406. In contrast, all Syk and BTK inhibitors tested inhibited platelet spreading on immobilized fibrinogen.

Based on observations above, we hypothesized that TKIs that irreversibly bind to BTK may disrupt critical molecular interactions around BTK within the PI3K signalosome essential to the proper orchestration of platelet activation programs. To test the effects of Syk and BTK inhibitors on the organization of PI3K signaling in activating platelets, platelets were pretreated with TKIs prior to incubation on immobilized fibrinogen and processing for immunofluorescence microscopy. We found that under control conditions, PI3K p85α regulatory subunit localized to phospholipid PI(3,4)P2-rich regions in adherent platelets associated with active PKC signaling. While Syk and PI3K inhibitors abrogated platelet spreading on fibrinogen, PI3K p85α staining remained strong around undeveloped, nascent adhesions in platelets treated with Syk and PI3K inhibitors. In contrast, PI3K p85α staining was diffuse, diminished or absent in platelets adherent to fibrinogen in the presence of all irreversible BTK inhibitors tested.

Our results show that clinically effective kinase inhibitors that target Syk-BTK-PI3K signaling systems have varying effects on platelet function that may offer insights to adverse effects of therapeutic TKIs in different contexts. Moreover, our findings suggest that in addition inhibiting protein kinase activities, the discrepant effects of some TKIs may be attributed to, in part, altered protein relations around BTK, PLCγ2 and other components of the PI3K signalosome in activating platelets. Ongoing studies aim to specify protein:protein interactions affected by reversible as well as irreversible TKIs; to examine patient samples for evidence of PI3K mislocalization - especially in cases of BTK inhibitor-associated bleeding; and, to determine if PI3K signalosome disorganization has roles in other physiological, therapeutic and toxic effects of TKIs.