Reduced Efficacy and mechanisms of drug resistance of FLT3 Tyrosine Kinase Inhibitors

Several targeted FLT3 TKIs, such as midostaurin, sorafenib, quizartinib, and crenolanib are undergoing phase 1, 2, and 3 of preclinical trials and show encouraging results when coupled with chemotherapy. cite my group partners However, resistance to this targeted molecular therapy is proving to be a major hindrance in both drug development and cancer research. Although the previously discussed TKIs have not been approved for clinical use, several studies highlight the development of drug resistant mechanisms. Pharmodynamic modeling has identified two types of FLT3 mutations conferring resistance: tyrosine kinase domain 1 (TKD1) mutations involving the ATP …show more content…
An early pharmakinetic study showed cytochrome P450 4AF, a hepatic oxidative enzyme, metabolizing midostaurin into it’s two metabolites: CGP62221 and CGP52421. This lead to a reduction in plasma concentrations of the of the drug, and ultimately, a loss in it’s function.2,3

Midostuarin’s acquisition of specific point mutations within the first tyrosine kinase domain (TKD1) of FLT3-ITD that confers drug resistance, and further contributes to it’s reduced efficacy. Heidel et al4 first described a conferred resistance to midostaurin in a patient via the mutation of Asn-676. Using pharmodynamic modeling, Cools et al5. confirmed this in vitro while further identifying Gly-697, Phe-691, Ala-627 as three additional novel amino acid residues whose mutations would confer an unfavorable binding conditions within the ATP binding cleft TKD1. Their modeling shows Phe-691 is in contact with a phenyl ring of midostaurin, and a mutation to a smaller amino acid removes the favorable hydrophobic interactions. Gly-697 is in contact with another phenyl ring of the drug, and a mutation to a larger residue creates a steric clash with the inhibitor, reducing midostaurin’s binding affinity. Additionally, a mutation of Ala-627 to a larger amino acid destabilizes the conformation of the P loop, a loop …show more content…
Unlike midostaurin, however, mutations in TKD2 are of primarily responsible for drug resistance. Using pharmadynamic modeling, Smith6 et al was the first to show two key resistance conferring mutations in the TKD2: residues D835 and Y842. Smith’s modeling offers compelling evidence that both D835 and Y842 stabilize the TKD2 domain via hydrogen bonding with neighboring residues. Thus, Smith shows that a mutation in either one of these residues leads to a reduction in binding affinity for Quizartinib. These findings are further confirmed by the fact that eight out of the eight AML patients with acquired resistance to Quizartinib also show mutations at both the Asp-835 and Y-842 positions. In addition to drug resistance, a missense mutation of the Asp-835 appears to result in the constitutive activation of the FLT3 receptor, resulting in a worse prognosis for patients.7 A more potent mutation of F691 residue, dubbed the gatekeeper residue, has frequently also been associated with quizartanib resistance. While the molecular undeprinnings of gatekeeper mutation mediated resistance are misunderstood, Smith et al8 reports that Quizartinib binding relies on essential interactions with the Phe-691 and highly conserved Asp-Phe-Gly motif within the TKD2. Thus a mutation causes resistance to Quizartinib by disrupting the