A Novel HCK and BTK Dual Inhibitor Kin-8194 Shows Superior Activity over Ibrutinib and Overcomes BTK^C481S Mediated Ibrutinib Resistance in Vitro and In Vivo in MYD88 Mutated B-Cell Lymphomas

Activating mutations in MYD88 promote malignant cell growth and survival through multiple pathways that include BTK and HCK. HCK is transcriptionally upregulated and activated by mutated MYD88 and in turn activates BTK itself, as well as ERK and AKT. Ibrutinib is a covalent inhibitor that binds to BTK^Cys481 and shows activity in MYD88 mutated B-cell malignancies, including WM, MZL, ABC DLBCL, and PCNSL. Resistance to ibrutinib on the basis of BTK^Cys481 as well as downstream mutations is increasingly being recognized. We therefore sought to develop potent and selective inhibitors that target HCK. We developed a non-covalent dual inhibitor, KIN-8194, of HCK and BTK following a screen >220 clinical and preclinical kinase inhibitors, and lead optimization following synthesis >400 analogs. Dual HCK and BTK inhibition was confirmed by both KINOMEscan® and live-cell target engagement studies (KiNativ™ profiling and live cell ATP-biotin competition assay). KIN-8194 showed robust suppression of HCK (IC₅₀<0.495nM) and BTK (IC₅₀=0.915nM) in these studies, and blocked pHCK and pBTK in both wild-type and mutated BTK^Cys481 WM and ABC DLBCL cells, and primary WM cells. Importantly, KIN-8194 showed selective killing of MYD88 mutated WM and ABC DLBCL cells and overcame resistance to ibrutinib in WM and ABC DLBCL cells engineered to express mutated BTK^Cys481. KIN-8194 showed excellent microsomal stability across multiple species including human (T_1/2=49.5 minutes). Pharmacokinetic studies in mice showed excellent bioavailability (F=55%), serum half-life (T_1/2=15.1 hours) and drug clearance (CL=17.4mL/min/kg) amenable to once daily oral dosing. The compounds exhibited an excellent in vitro safety profiling including no relevant inhibition observed against a panel of 100 other receptor targets, including hERG, AMES was negative up to 50 µM, and Cyp inhibition studies showed acceptable inhibition up to 10 µM. Pharmacodynamic studies following oral administration showed that KIN-8194 blocked both pHCK and pBTK in wild-type and mutated BTK^Cys481 expressing TMD8 ABC DLBCL cells engrafted in NOD SCID mice. Continuous dosing up to 100 mg/kg was well tolerated in these mice. KIN-8194 treated NOD SCID mice xenografted with either wild-type (A) or mutated (B) BTK^Cys481 TMD cells (N=8/cohort) showed superior tumor growth suppression and survival over vehicle control or ibrutinib treated mice at 50 mg/kg. Among wild-type BTK^Cys481 TMD8 xenografted mice treated for 6 weeks, elimination of tumor was observed in half the mice with no subsequent growth following 12 additional weeks of observation, consistent with a cure. We therefore describe a novel, highly potent non-covalent dual HCK and BTK inhibitor that is well tolerated in mice, shows selective killing of MYD88 mutated WM and ABC DLBCL cells, and can overcome mutated BTK^Cys481 related ibrutinib resistance.

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