Crenolanib Is A Highly Potent, Selective, FLT3 TKI with Activity Against D835 Mutations

Abstract 1341

Approximately 23% of acute myeloid leukemia (AML) patients have internal tandem duplications (ITD) of the type III receptor tyrosine kinase FLT3. FLT3/ITD mutations correlate with poor survival rates in AML patients. This has made FLT3 a promising target for treating AML, leading to the development of a series of small molecule FLT3 tyrosine kinase inhibitors (TKIs). Quizartinib is the most potent FLT3 TKI developed to date, and has achieved a high response rate as a monotherapy in relapsed/refractory FLT3/ITD AML patients. However, during clinical trials of quizartinib, multiple patients developed point mutations of aspartate 835 (D835) which conferred resistance to treatment with FLT3 TKIs. FLT3/D835 mutations can also occur at disease presentation with or without the presence of ITD mutations, though the prognostic significance in these cases is less well-defined. Therefore, a FLT3 TKI which also has activity against FLT3/D835 point mutations has significant potential to be clinically beneficial for a number of patients. Crenolanib is a next generation FLT3 TKI which is nearly as potent as quizartinib (IC50 in human plasma of 35 nM versus 18 nM for quizartinib). Furthermore, in clinical trials of quizartinib, some subjects experienced myelosuppression, which may have been associated with inhibition of the related type III receptor tyrosine kinase, c-Kit. Crenolanib is 33-fold less active against c-Kit as compared to FLT3. The functional significance of this finding was tested with colony formation assays of normal donor bone marrow (n=3). The abundance of erythroid colonies with 200 nM of drug compared to control was 23% and 75% when treated with quizartinib and crenolanib, respectively. These data suggest that clinically, crenolanib may not have the significant myelosuppressive effects as observed for quizartinib in clinical trials. Crenolanib was studied in phase II trials as a PDGFR inhibitor in glioma and GIST patients. Pharmacokinetic data from these trails show that patients are able to achieve drug levels well above the IC90 for FLT3 in plasma. Plasma samples were obtained from these patients and utilized for plasma inhibitory activity (PIA) assays. The results of these assays suggest that adequate FLT3 inhibitory concentrations of this drug can easily be achieved with safe and tolerable dosing schedules. The activity of crenolanib was tested in series of primary samples with a variety of FLT3 mutations. One patient sample containing a D835V mutation was treated with 20 nM of sorafenib, quizartinib, and crenolanib and Western blotted for pFLT3. Only crenolanib showed any activity against this sample, with an IC50 of 2 nM in culture medium. In another case, blasts were obtained from a patient treated with quizartinib who then relapsed. In a comparison of sorafenib, quizartinib, and crenolanib, only crenolanib had a significant cytotoxic effect. These data strongly suggest that crenolanib is an important potential new TKI for treating FLT3/D835 mutated AML. Accrual into the trial is ongoing (NCT01522469).