Crenolanib is a selective type I pan-FLT3 inhibitor

Abstract

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.sorafenib | activation-loop mutations | D835 mutations P ioneering studies demonstrated that imatinib, the first small molecule tyrosine kinase inhibitor (TKI), binds to an inactive kinase conformation of the Abelson protein tyrosine kinase (ABL) (1). Compounds that bind to kinases in this manner have been termed "type II", whereas "type I" inhibitors bind to an active conformation. Type II inhibitors typically target the ATP binding region and an adjacent allosteric site available only in an inactive conformation. Because this region is less well conserved among kinases than the ATP binding region, interactions with this area allow for greater selectivity.Despite the clinical success of imatinib for the treatment of chronic myeloid leukemia (CML), durability of response is compromised by secondary kinase domain (KD) mutations in BCR-ABL (1), many of which destabilize the inactive conformation required for imatinib binding (1). Second-generation ABL inhibitors, dasatinib (type I) and nilotinib (type II), were subsequently developed to retain efficacy against most imatinib-resistant BCR-ABL KD mutants, and each are vulnerable to only approximately five resistance-conferring mutations (2, 3). In contrast, the third-gene...

How I treat refractory and early relapsed acute myeloid leukemia

“…It is important to keep in mind that these TKIs vary significantly in their specificity and their activity against resistance-conferring kinase domain mutations in FLT3. 76 This may translate to differences in their clinical efficacy. Thus, it is unlikely that the question of the efficacy of TKIs will be solved in the very near future.…”

Structural and biochemical studies of the PDGFRA kinase domain

“…It is interesting to note that unlike most RTK inhibitors, crenolanib is an inhibitor that preferentially binds to phosphorylated active kinases with the 'DFG in' conformation motif [25,26,27]. It is possible that it adopts a binding mode different from imatinib, and therefore remains sensitive.…”

Emerging strategies for high-risk and relapsed/refractory acute myeloid leukemia: Novel agents and approaches currently in clinical trials

“…PLX3397 has potent activity against the FLT3 F691L mutant [28]. Crenolanib has activity against FLT3-ITD mutants, FLT3/D835 point mutations and, at least in vitro, can overcome the resistance to quizartinib and sorafenib [29]. Another potential advantage of crenolanib is its reduced inhibition of c-Kit compared with quizartinib.…”