Crenolanib is a selective type I pan-FLT3 inhibitor

Abstract

Rapid evolution of drug resistance associated with secondary kinase domain (KD) mutations is the best characterized mechanism of acquired resistance to effective tyrosine kinase inhibitor (TKI) therapy. Medicinal chemistry efforts have largely been devoted toward synthesizing type II TKIs that, by targeting an inactive kinase conformation, are believed to afford greater selectivity than type I TKIs that bind an active kinase conformation. The only previously described TKI with the ability to successfully suppress all resistance-conferring KD mutants (i.e. “pan-kinase” inhibitor) is the type II multikinase TKI ponatinib. Here, we demonstrate that a type I TKI can be potent, selective, and invulnerable to resistance-conferring KD mutation as a mechanism of resistance. Efforts to develop potent, selective type I pan-kinase inhibitors are warranted.Tyrosine kinase inhibitors (TKIs) represent transformative therapies for several malignancies. Two critical features necessary for maximizing TKI tolerability and response duration are kinase selectivity and invulnerability to resistance-conferring kinase domain (KD) mutations in the intended target. No prior TKI has demonstrated both of these properties. Aiming to maximize selectivity, medicinal chemists have largely sought to create TKIs that bind to an inactive (type II) kinase conformation. Here we demonstrate that the investigational type I TKI crenolanib is a potent inhibitor of Fms tyrosine kinase-3 (FLT3) internal tandem duplication, a validated therapeutic target in human acute myeloid leukemia (AML), as well as all secondary KD mutants previously shown to confer resistance to the first highly active FLT3 TKI quizartinib. Moreover, crenolanib is highly selective for FLT3 relative to the closely related protein tyrosine kinase KIT, demonstrating that simultaneous FLT3/KIT inhibition, a prominent feature of other clinically active FLT3 TKIs, is not required for AML cell cytotoxicity in vitro and may contribute to undesirable toxicity in patients. A saturation mutagenesis screen of FLT3–internal tandem duplication failed to recover any resistant colonies in the presence of a crenolanib concentration well below what has been safely achieved in humans, suggesting that crenolanib has the potential to suppress KD mutation-mediated clinical resistance. Crenolanib represents the first TKI to exhibit both kinase selectivity and invulnerability to resistance-conferring KD mutations, which is unexpected of a type I inhibitor. Crenolanib has significant promise for achieving deep and durable responses in FLT3–mutant AML, and may have a profound impact upon future medicinal chemistry efforts in oncology.