Small molecules targeting the FLT3 kinase with an internal tandem duplication mutation (FLT3-ITD) and the Bruton's Tyrosine Kinase (BTK) have shown great potential to treat hematologic malignancies. However, acquired mutations have emerged in resistant/relapsed patients treated with the FLT3 inhibitor quizartinib (Smith et al., 2012) or the BTK inhibitor ibrutinib (Woyach et al., 2017), respectively. This study explored the potency and molecular mechanisms of CG'806, a pan-FLT3/pan-BTK inhibitor, in hematologic malignancies relative to other FLT3 or BTK inhibitors commercialized or in development.

In FLT3-ITD AML cells, CG'806 induced apoptosis through inhibition of FLT3 signaling (decreased phospho-FLT3, -STAT5 and -ERK) and promotion of G0/G1 cell cycle arrest determined by immunoblotting and flow cytometry, and CG'806 was approximately 10-fold more potent than quizartinib. Although FLT3-ITD is found in 25-30% of AML patients, most AML patients express wild type (WT) FLT3. CG'806 had an IC50 = 11 nM against FLT3 WT-transfected Ba/F3 cells and was superior to quizartinib, gilteritinib and crenolanib FLT3 inhibitors (1,956, 500 and 2,617 nM, respectively). In FLT3-WT AML cell lines, or Ba/F3 cells transfected with FLT3-WT, D835Y, ITD+D835Y, or ITD+F691L, CG'806 markedly decreased phosphorylation of BTK, aurora kinases (AURK) and H3S10, resulting in G2/M arrest or polyploidy, and apoptosis with less or no effect on FLT3-WT activity. In contrast, quizartinib did not affect BTK or AURK signaling, and CG'806 was >2,000-fold more effective than quizartinib on FLT3-WT AML cells in MTS based cell proliferation assays.

In B cell malignancies, BTK signaling plays a pivotal pathogenic role. CG'806 decreased BTK phosphorylation in all malignant B cell lines tested (n =10) and inhibited cell proliferation and colony formation 50-6,000 times more potently than ibrutinib, an effect which could not be explained by the exclusive inhibition of BTK signaling. Further analysis revealed CG'806 effectively inhibited AURK signaling and caused polyploidy and apoptosis in B cell lines, sequelae which were not induced by ibrutinib. BTK-C481S is the most common mutation induced by treatment with ibrutinib and is associated with ibrutinib resistance in the clinic. CG'806 equivalently inhibited BTK-WT and BTK-C481S in HEK293 transfected cells, whereas ibrutinib was much less potent against the BTK-C481S mutant.

The ability of CG'806 to target all WT and mutant forms of FLT3 and BTK, and to inhibit multiple signaling pathways, produces killing of diverse subtypes of hematologic malignancies driven by different genomic aberrations. Considering the efficacy of CG'806 in the absence of observed toxicity in murine AML models, CG'806 appears superior to other FLT3 or BTK inhibitors and is suitable for development in patients with either AML or B-cell malignancy.

Citation Format: Hongying Zhang, Andrea Local, Khalid Benbatoul, Peter Folger, Susan Sheng, Cheng-Yu Tsai, Stephen B. Howell, William G. Rice. CG'806, a first-in-class pan-FLT3/pan-BTK inhibitor, targets multiple pathways to kill diverse subtypes of acute myeloid leukemia and B-cell malignancy in vitro [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 794.