Abstract 3683: Crenolanib, a novel Type I, mutant-specific inhibitor of Class III receptor tyrosine kinases, preferentially binds to phosphorylated kinases

Introduction: Crenolanib (CP-868,596) is a benzimidazole tyrosine kinase inhibitor (TKI), known to potently inhibit the platelet derived growth factor receptors (PDGFR) alpha and beta, and the Fms-like tyrosine kinase 3 (FLT3). Crenolanib has been evaluated as a single agent (Lewis N, et al, J Clin Oncol 2009), and in combination with docetaxel ± axitinib (Michael M, et al, Br J Cancer 2010), and was found to be well tolerated, with transient elevation of liver enzymes at higher doses. We describe here the characterization of crenolanib as a unique chemotype, mutant-specific Type I TKI. Methods: The relative affinities of tyrosine kinase inhibitors for phosphorylated and non-phosphorylated ABL1 have been previously used to accurately distinguish type I from type II inhibitors (Wodicka LM, et al, Chem Biol 2010). The effect of ABL1 A-loop phosphorylation on crenolanib affinity was investigated in a competition binding assay (Karaman MW, et al, Nat Biotech 2008). The binding constants (Kds) of crenolanib for FLT3 (wild-type and five mutant isoforms) were also determined using the KINOMEscan KdELECT Assay (DiscoveRx, San Diego, CA), and compared with the Kds of 442 TKIs reported (Davis MI, et al, Nat Biotech 2011). The Kds of crenolanib for PDGFRα, PDGFRα, and c-KIT (wild-type and two mutant isoforms) are being determined. Results: The Kd of crenolanib for phosphorylated ABL1 and ABL(T315I) were 7- and 15-fold lower than the Kds for non-phosphorylated ABL1 and ABL(T315I) respectively. Based on this higher affinity for p-ABL1 relative to np-ABL1, crenolanib appears to be a Type I inhibitor. Furthermore, crenolanib has a 10-fold affinity shift between the non-autoinhibited and autoinhibited states of FLT3, which is in the range of affinity shifts reported for other Type I TKIs, in contrast to the 100- to 1000-fold affinity shift reported for Type II TKIs. Crenolanib also has the highest affinity for three constitutively activate isoforms of FLT3 (FLT3-ITD, FLT3-D835Y, FLT3-D835V) compared to 442 kinases reported (Davis MI, et al, Nat Biotech 2011). Crenolanib is a next-generation, mutant-specific TKI, currently in Phase II development in adult and pediatric gliomas, and in PDGFRAD842V-mutant GIST. The unique and specific kinase inhibition profile is hypothesized to translate to greater therapeutic effect, accompanied by fewer adverse events in the clinic. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the