Xianghong Wang

Preclinical Evaluation of AMG 925, a FLT3/CDK4 Dual Kinase Inhibitor for Treating Acute Myeloid Leukemia

Acute myeloid leukemia (AML) remains a serious unmet medical need. Despite high remission rates with chemotherapy standard-of-care treatment, the disease eventually relapses in a major proportion of patients. Activating Fms-like tyrosine kinase 3 (FLT3) mutations are found in approximately 30% of patients with AML. Targeting FLT3 receptor tyrosine kinase has shown encouraging results in treating FLT3-mutated AML. Responses, however, are not sustained and acquired resistance has been a clinical challenge. Treatment options to overcome resistance are currently the focus of research. We report here the preclinical evaluation of AMG 925, a potent, selective, and bioavailable FLT3/cyclin-dependent kinase 4 (CDK4) dual kinase inhibitor. AMG 925 inhibited AML xenograft tumor growth by 96% to 99% without significant body weight loss. The antitumor activity of AMG 925 correlated with the inhibition of STAT5 and RB phosphorylation, the pharmacodynamic markers for inhibition of FLT3 and CDK4, respectively. In addition, AMG 925 was also found to inhibit FLT3 mutants (e.g., D835Y) that are resistant to the current FLT3 inhibitors (e.g., AC220 and sorafenib). CDK4 is a cyclin D–dependent kinase that plays an essential central role in regulating cell proliferation in response to external growth signals. A critical role of the CDK4–RB pathway in cancer development has been well established. CDK4-specific inhibitors are being developed for treating RB-positive cancer. AMG 925, which combines inhibition of two kinases essential for proliferation and survival of FLT3-mutated AML cells, may improve and prolong clinical responses. Mol Cancer Ther; 13(4); 880–9. ©2014 AACR.

AMG 925 Is a Dual FLT3/CDK4 Inhibitor with the Potential to Overcome FLT3 Inhibitor Resistance in Acute Myeloid Leukemia

Resistance to FLT3 inhibitors is a serious clinical issue in treating acute myelogenous leukemia (AML). AMG 925, a dual FLT3/CDK4 inhibitor, has been developed to overcome this resistance. It is hypothesized that the combined inhibition of FLT3 and CDK4 may reduce occurrence of the FLT3 resistance mutations, and thereby prolong clinical responses. To test this hypothesis, we attempted to isolate AML cell clones resistant to AMG 925 or to FLT3 inhibitors. After a selection of over 8 months with AMG 925, we could only isolate partially resistant clones. No new mutations in FLT3 were found, but a 2- to 3-fold increase in total FLT3 protein was detected and believed to contribute to the partial resistance. In contrast, selection with the FLT3 inhibitors sorafenib or AC220 (Quizartinib), led to a resistance and the appearance of a number of mutations in FLT3 kinase domains, including the known hot spot sites D835 and F691. However, when AC220 was combined with the CDK4 inhibitor PD0332991 (palbociclib) at 0.1 μmol/L or higher, no resistance mutations were obtained, indicating that the CDK4-inhibiting activity of AMG 925 contributed to the failure to develop drug resistance. AMG 925 was shown to potently inhibit the FLT3 inhibitor–resistant mutation D835Y/V. This feature of AMG 925 was also considered to contribute to the lack of resistance mutations to the compound. Together, our data suggest that AMG 925 has the potential to reduce resistance mutations in FLT3 and may prolong clinical responses. Mol Cancer Ther; 14(2); 375–83. ©2014 AACR.

Abstract 787: FLX925 (AMG 925) is a rationally designed FLT3, CDK4/6 inhibitor that retains potency against clinically relevant secondary resistance mutations in FLT3

Acquired secondary resistance mutations to clinically active kinase inhibitors remains a key obstacle between valid therapeutic hypotheses and meaningful patient benefit. In AML, evidence suggests that inhibition of FLT3 (particularly in FLT3-ITD mutated cancers) can be efficacious; however, relapse from complete remission is common and often rapid. As with other cancers driven by key oncogenic kinase mutations (e.g. BCR-ABL in CML), a primary mechanism of resistance is the acquisition of secondary resistance mutations in the oncogenic kinase themselves. Multiple strategies have been pursued to address such resistance, including the development of kinase inhibitors that either bind their respective targets differently or by targeting multiple important pathways simultaneously. Herein we describe a rationally conceived next generation FLT3 inhibitor, FLX925 (previously AMG 925), that was prospectively designed to address or avoid common resistance mechanism to earlier FLT3 inhibitors with its unique binding mode and potent activity against CDK4/CDK6. FLX925 is a potent and selective type 1 inhibitor of FLT3 that retains its cellular potency against clinically relevant secondary resistance mutations in FLT3 occurring with quizartinib or sorafenib treatment (FLX925 IC50: MOLM13ITD, 15 nM; MOLM13ITD/D835, 28 nM; MV4-11ITD, 16 nM; MV4-11ITD/D835, 19 nM; MV4-11ITD/N841, 16 nM; MV4-11ITD/F691, 73 nM). Indeed, while compounds currently in the clinic became more than 200-fold less potent against a number of mutants, FLX925 remained relatively equipotent (+/- 5-fold the parental cell line IC50) in these same resistant clones. This is in stark contrast to the striking cross-resistance observed with quizartinib in sorafenib resistant cells. Moreover, the few clones that grew out of a screen for resistance to FLX925 displayed a ‘persistence’ phenotype with modestly reduced sensitivity to FLX925 (∼5-fold IC50 shift) that was rapidly reversible. This persistence was associated with higher FLT3 protein levels and no detectable secondary mutations in FLT3. In addition to its suppression of FLT3 signaling, FLX925 potently inhibits CDK4/CDK6, central components of the cell cycle machinery. This unique profile may reduce the likelihood of emergent resistant clones and extends the therapeutic potential of FLX925 to other malignancies dependent on these pathways (e.g. MCL). Indeed, the addition of PD0332991 (a selective CDK4/6 inhibitor) to a relatively selective FLT3 inhibitor reduced the frequency of acquired resistance in a cell based screen, relative to a FLT3 inhibitor alone. These data suggest the unique profile of FLX925 makes it an ideal inhibitor for the treatment of cancers driven by FLT3 signaling, such as AML. A phase I clinical trial evaluating the safety, tolerability pharmacokinetics and pharmacodynamics effects of FLX925 in patients with AML is ongoing. Citation Format: Cong Li, Lingming Liang, Liqin Liu, Zhen Xia, Zhihong Li, Xianghong Wang, Lawrence McGee, Angus Sinclair, Sasha Kamb, Dineli Wickramasinghe, Sachie Marubayashi, Juan C. Jaen, Jordan S. Fridman, Kang Dai. FLX925 (AMG 925) is a rationally designed FLT3, CDK4/6 inhibitor that retains potency against clinically relevant secondary resistance mutations in FLT3. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 787. doi:10.1158/1538-7445.AM2015-787