The receptor tyrosine kinase fms-like tyrosine kinase 3 (FLT3), involved in regulating survival, proliferation, and differentiation of hematopoietic stem/progenitor cells, is expressed on acute myeloid leukemia (AML) cells in most patients. Mutations of FLT3 resulting in constitutive signaling are common in AML, including internal tandem duplication (ITD) in the juxtamembrane domain in 25% of patients and point mutations in the tyrosine kinase domain in 5%. Patients with AML with FLT3-ITD have a high relapse rate and short relapse-free and overall survival after chemotherapy and after transplant. A number of inhibitors of FLT3 signaling have been identified and are in clinical trials, both alone and with chemotherapy, with the goal of improving clinical outcomes in patients with AML with FLT3 mutations. While inhibitor monotherapy produces clinical responses, they are usually incomplete and transient, and resistance develops rapidly. Diverse combination therapies have been suggested to potentiate the efficacy of FLT3 inhibitors and to prevent development of resistance or overcome resistance. Combinations with epigenetic therapies, proteasome inhibitors, downstream kinase inhibitors, phosphatase activators, and other drugs that alter signaling are being explored. This review summarizes the current status of translational and clinical research on FLT3 inhibitors in AML, and discusses novel combination approaches. Mol Cancer Ther; 16(6); 991–1001. ©2017 AACR.

BACKGROUND: Acute myeloid leukemia (AML) is the common form of acute leukemia in adults, accounting for over 80% of all acute leukemias in individuals aged >18 years. Overall 5‐year survival remains poor in older AML patients; it is 65 years. In this study, the authors examined whether survival has improved for subsets of geriatric AML patients over 3 successive decades.METHODS: Surveillance, Epidemiology and End Results (SEER) data were used to determine trends in relative survival by age among 19,000 patients with AML over 3 successive decades (1977‐1986, 1987‐1996, and 1997‐2006). Relative survival rates (RRs) with 95% confidence intervals (CIs) were calculated as measures of survival.RESULTS: Overall, the RRs increased for each successive decade (1977‐1986, 1987‐1996, and 1997‐2006) in patients ages 65 to 74 years, with improvements in 12‐month survival from 20%, to 25%, to 30%, respectively. Findings were similar for 24‐month, 36‐month, 48‐month, and 60‐month survival. However, survival rates did not improve in patients aged ≥75 years. The oldest old patients (aged ≥85 years) had the lowest survival rates, with no apparent improvement.CONCLUSIONS: This analysis of a large data set demonstrated that, although overall survival remained unsatisfactory among older patients, it improved in the younger old (ages 65‐74 years). Survival of older old AML patients has not been favorably impacted by available AML therapies or supportive care, and intervention in this age group is best undertaken on a clinical trial. Cancer 2013;119:2720–2727. © 2013 American Cancer Society.

Purpose: Cancer and Leukemia Group B (CALGB) 9720 evaluated subcutaneous low-dose recombinant interleukin-2 (rIL-2) maintenance immunotherapy as a strategy for prolonging remission in older patients with acute myeloid leukemia (AML).Patients and Methods: AML patients age 60 years and older in first complete remission after induction and consolidation chemotherapy were randomly assigned to no further therapy or a 90-day regimen of 14-day cycles of low-dose rIL-2, aimed at expanding natural killer (NK) cells, followed by 3-day higher doses aimed at activating cytotoxicity of expanded NK cells to lyse residual AML cells. All randomly assigned patients were included in an intention-to-treat analysis.Results: A total of 163 (64%) of 254 patients who completed induction and consolidation chemotherapy on CALGB 9720 were randomly assigned to rIL-2 (n = 81) or no further therapy (n = 82); the most common reasons for lack of random assignment were patient refusal and relapse. Fifteen patients randomly assigned to rIL-2 never initiated it because of refusal, intercurrent medical problems, or relapse, and 24 patients initiated rIL-2 but stopped early because of toxicity or relapse. Grade 4 toxicities during rIL-2 therapy included thrombocytopenia (65%) and neutropenia (64%), and grade 3 toxicities included anemia (33%), infection (24%) and malaise/fatigue (14%). Forty-two patients (52%) randomly assigned to rIL-2 completed the full 90-day course. Patients in both arms had similar distributions of both disease-free (combined median = 6.1 months; P = .47) and overall survival (combined median = 14.7 months; P = .61) after random assignment. Moreover, the 42 patients who completed all planned therapy did not show prolongation of disease-free or overall survival.Conclusion: Low-dose rIL-2 maintenance immunotherapy is not a successful strategy in older AML patients.

Ponatinib is a novel tyrosine kinase inhibitor with potent activity against BCR-ABL with mutations, including T315I, and also against fms-like tyrosine kinase 3. We tested interactions between ponatinib at pharmacologically relevant concentrations of 50 to 200 nmol/L and the MDR-associated ATP-binding cassette (ABC) proteins ABCB1, ABCC1, and ABCG2. Ponatinib enhanced uptake of substrates of ABCG2 and ABCB1, but not ABCC1, in cells overexpressing these proteins, with a greater effect on ABCG2 than on ABCB1. Ponatinib potently inhibited [¹²⁵I]-IAAP binding to ABCG2 and ABCB1, indicating binding to their drug substrate sites, with IC₅₀ values of 0.04 and 0.63 μmol/L, respectively. Ponatinib stimulated ABCG2 ATPase activity in a concentration-dependent manner and stimulated ABCB1 ATPase activity at low concentrations, consistent with it being a substrate of both proteins at pharmacologically relevant concentrations. The ponatinib IC₅₀ values of BCR-ABL–expressing K562 cells transfected with ABCB1 and ABCG2 were approximately the same as and 2-fold higher than that of K562, respectively, consistent with ponatinib being a substrate of both proteins, but inhibiting its own transport, and resistance was also attenuated to a small degree by ponatinib-induced downregulation of ABCB1 and ABCG2 cell-surface expression on resistant K562 cells. Ponatinib at pharmacologically relevant concentrations produced synergistic cytotoxicity with ABCB1 and ABCG2 substrate chemotherapy drugs and enhanced apoptosis induced by these drugs, including daunorubicin, mitoxantrone, topotecan, and flavopiridol, in cells overexpressing these transport proteins. Combinations of ponatinib and chemotherapy drugs warrant further testing. Mol Cancer Ther; 11(9); 2033–44. ©2012 AACR.

The oncogenic serine/threonine kinase Pim-1 phosphorylates and activates the ATP-binding cassette transporter breast cancer resistance protein (ABCG2). The ABC transporter P-glycoprotein (Pgp; ABCB1) also contains a Pim-1 phosphorylation consensus sequence, and we hypothesized that Pim-1 also regulates Pgp. Pgp is exported from the endoplasmic reticulum (ER) as a 150-kDa species that is glycosylated to 170-kDa Pgp, translocates to the cell surface, and mediates drug efflux; alternatively, 150-kDa Pgp is cleaved to a 130-kDa proteolytic product by ER proteases or undergoes ubiquitination and proteasomal degradation. Pim-1 and Pgp interaction was studied in GST pull-down and phosphorylation in in vitro kinase assays. Pim-1 knockdown and inhibition effects on Pgp expression were studied by immunoblotting and flow cytometry and on Pgp stability by immunoblotting after cycloheximide treatment. Pim-1 directly interacted with and phosphorylated Pgp in intact cells and in vitro. Pim-1 knockdown or inhibition decreased cellular and cell surface 170-kDa Pgp, in association with both transient increase in 130-kDa Pgp and increased Pgp ubiquitination and proteasomal degradation. Pim-1 inhibition also decreased expression of 150-kDa Pgp in the presence of the glycosylation inhibitor 2-deoxy-d-glucose. Finally, Pim-1 inhibition sensitized Pgp-overexpressing cells to doxorubicin. Thus, Pim-1 regulates Pgp expression by protecting 150-kDa Pgp from proteolytic and proteasomal degradation and enabling Pgp glycosylation and cell surface translocation and thus Pgp-mediated drug efflux. Pim-1 inhibitors are entering clinical trials and may provide a novel approach to abrogating drug resistance.