Background and purpose: BTK mediates B-cell receptor signaling and was validated as a target by the BTK inhibitor ibrutinib in several B-cell malignancies, including mantle cell lymphoma and chronic lymphocytic leukemia (CLL). However, patients may have or acquire resistance. Resistance mechanisms include mutation of the cysteine in the BTK active site that ibrutinib requires for covalent binding (C481). We identified and characterized SNS-062, a potent, noncovalent BTK inhibitor with activity towards BTK harboring resistance mutations that also inhibits ITK and may provide enhanced anti-tumor immune responses. SNS-062 shows restricted kinase selectivity and nonclinical pharmacology, pharmacokinetics (PK) and toxicology profiles distinct from ibrutinib and merits clinical investigation. Methods and Results: In in vitro kinase binding assays, SNS-062 bound to 9 kinases with Kd < 25 nM, including Tec kinase family members BTK (0.3 nM) and ITK (2.2 nM), but did not bind EGFR. Lack of EGFR inhibition by SNS-062 may offer safety advantages over ibrutinib, which has been associated with diarrhea and rash potentially related to its off-target effects on EGFR. Cellular inhibition of BTK was demonstrated by measuring inhibition of BTK auto-phosphorylation. SNS-062 inhibited pBTK in human whole blood with an average IC50 of 50 nM. A PK/ PD (pBTK) relationship was demonstrated in mice, with an in vivo IC50 for pBTK inhibition of 47 nM. Prolonged SNS-062-mediated in vivo inhibition of pBTK correlated with potent efficacy in a T cell-independent (type II) BTK dependent B cell mediated antibody response mouse model (ED50 11 mg/kg); efficacy was also observed in a type II collagen- induced arthritis (CIA) rat model (ED50 for summed knee and ankle histopathology 0.8 and 3.9 mg/kg). To assess the activity of SNS-062 and ibrutinib against BTK with acquired resistance mutations, inhibition of wild type (WT) and mutant C481S BTK was evaluated in direct kinase assays. SNS-062 inhibited WT and C481S BTK with similar IC50s (pBTK IC50s: WT BTK 2.9 nM, C481S BTK 4.4 nM) while ibrutinib potency was reduced 40-fold (pBTK IC50s: WT BTK 0.58 nM, C481S BTK IC50 25.7 nM). Similarly, ibrutinib showed a 100-fold loss of potency in C481S BTK expressing 293 cells (pBTK IC50s: WT BTK 0.016 μM, C481S BTK 1.7 μM) while SNS-062 activity was unaffected (pBTK IC50s: WT BTK 0.57 μM, C481S BTK 0.80 μM). SNS-062 had good oral bioavailability in rat and dog (%F ≥ 40%) and a terminal half- life of 5 to 6 hours. 28 day repeat-dose toxicology studies in rat and dog showed that SNS-062 was well tolerated with continuous drug levels and at exposures (AUC∞) much greater than those achieved for ibrutinib. These results suggest that SNS-062 plasma concentrations may be achieved that provide prolonged inhibition of both WT and mutant BTK. Conclusions: SNS-062 is a noncovalent BTK inhibitor that also inhibits ITK but not EGFR. SNS-062 activity against both WT and mutant C481S BTK was similar, while ibrutinib activity was decreased 40 to 100-fold. SNS-062 has a nonclinical PK/safety profile that will likely provide SNS-062 plasma concentrations for sustained inhibition of both WT and mutant BTK. These results support clinical development of SNS-062 for B cell malignancies, including CLL with acquired resistance to ibrutinib.
Citation Format: Minke E. Binnerts, Kevin L. Otipoby, Brian T. Hopkins, Tonika Bohnert, Stig Hansen, Gene Jamieson, Pamela A. Howland, Eric H. Bjerkholt, Deborah A. Thomas, Judith A. Fox, Adam R. Craig. SNS-062 is a potent noncovalent BTK inhibitor with comparable activity against wild type BTK and BTK with an acquired resistance mutation. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C186.