TTT-3002 Is a Novel FLT3 Tyrosine Kinase Inhibitor That Has the Potential to Overcome Some of the Limitations of Current FLT3 Inhibitors in Treatment of Acute Myeloid Leukemia

Abstract 866

Acute myeloid leukemia (AML) is a hematologic malignancy characterized by increased myeloproliferation and a block in differentiation of hematopoietic stem/progenitor cells, leading to infiltration of immature blasts in the bone marrow and peripheral blood. FMS-like tyrosine kinase-3 (FLT3) is a receptor tyrosine kinase expressed in hematopoietic stem/progenitor cells. It can activate cell growth/proliferation pathways via STAT5, AKT/PI3K, and RAS/MAPK signaling. Greater than 35% of AML patients harbor a constitutively activating mutation in the FLT3 gene, and the most common type, an internal tandem duplication (ITD), confers poor prognosis. These ITD mutations typically occur in the juxtamembrane domain and consist of variable length sequence repeats. In addition, activating mutations in the kinase domain are observed in 7–10% of patients. Thus, FLT3 is a validated target for the treatment of AML. Towards this end there have been a number of clinical trials testing the clinical efficacy of FLT3 tyrosine kinase inhibitors (TKIs) in FLT3 mutant AML, either alone or combined with chemotherapy. While some patients have responded, there have been many others who fail to demonstrate significant improvement. Additionally, many patients who initially respond to FLT3 TKI have developed resistance. Inadequate achievement of FLT3 inhibition has been one of the factors limiting efficacy in these trials. This appears most often to be due to a combination of insufficiently potent drugs, decreased efficacy against some FLT3 activating mutations, high plasma protein binding, and the selection for resistance-conferring point mutations within the FLT3/ITD gene. For all of these reasons, there is the need to develop additional FLT3 TKIs able to overcome some of these limitations.

We report for the first time on TTT-3002, a kinase inhibitor that has activity against FLT3 and may overcome some of the limitations that current FLT3 TKIs have demonstrated in clinical trials. TTT-3002 is one of the most potent FLT3 inhibitors discovered to date with regard to both inhibition of FLT3 autophosphorylation and cell proliferation. Utilizing human FLT3/ITD mutant leukemia cell lines the half maximal inhibitory concentration (IC₅₀) for inhibiting FLT3 autophosphorylation was less than 250 pM. The IC₅₀ for TTT-3002 in inhibiting proliferation in these same FLT3/ITD expressing cells was 490–920 pM. TTT-3002 also showed potent activity when tested against a broad spectrum of known, non-ITD FLT3 activating mutations, including the most frequently occurring D835Y mutation. It also shows potent activity against a number of FLT3/ITD resistance mutations that have been selected for in patients from clinical trials of other FLT3 TKI or through in vitro drug screening.

Studies utilizing human plasma samples from healthy donors and AML patients determined that TTT-3002 is only moderately protein bound, resulting in high levels of free drug able to bind target, and thus maintain activity, in the presence of physiological concentrations of major human plasma proteins including alpha-1-acid glycoprotein and human serum albumin. Therefore, relatively low levels of total drug would need to be achieved in vivo to achieve an effective concentration of free drug in clinical trials. These encouraging findings have been validated both ex vivo and in vivo utilizing mouse models of FLT3-associated AML. Survival and tumor burden of mice in a number of FLT3/ITD transplant models is significantly improved by administration of TTT-3002 via oral dosing. Finally, we demonstrate that TTT-3002 is cytotoxic to leukemic blasts isolated from FLT3/ITD expressing AML patients, while displaying minimal toxicity against normal hematopoietic stem/progenitor cells from healthy bone marrow donors. Therefore, TTT-3002 has demonstrated preclinical potential as a promising new FLT3 TKI that may overcome some of the limitations of other TKI in the treatment of FLT3-mutant AML.