Potent Antitumor Activity of AP24534, An Orally Active Inhibitor of Bcr-Abl, Flt3 and Other Kinases, in Both in Vitro and in Vivo Models of Acute Myeloid Leukemia (AML)

AP24534 is a potent, orally active inhibitor of Bcr-Abl and its mutants, including T315I, inhibiting kinase activity with IC50s of 0.3–2 nM. Potent cellular and in vivo activity of the compound has been demonstrated in models of chronic myeloid leukemia (CML). AP24534 also potently inhibits a discrete subset of other kinases, including Flt3 (IC50 13 nM), c-Kit (13 nM) and members of the FGF receptor family (2–18 nM), suggesting the potential for activity against other hematologic disorders characterized by activation of these proteins, such as acute myeloid leukemia (AML).

Methods: In this study, we examined the effects of AP24534 on AML cell lines characterized by expression of various activated kinase targets, including the internal tandem duplication (ITD) variant of Flt3, FGFR1 and c-Kit. Effects on cell viability in vitro were determined using an MTS assay, and correlated with biochemical assessment of target inhibition by Western blot analysis. In vivo activity was determined by daily oral administration of AP24534 for 4 weeks in a subcutaneous tumor model using Flt3-ITD-expressing cell line MV4-11.

Results: AP24534 potently inhibited the viability of AML cell lines expressing Flt3-ITD (MV4-11 cells), an activated FGFR1 fusion (KG-1 cells) or an activating c-Kit mutant N822K (Kasumi-1 cells) with IC50s of 0.7, 2.5 and 2.4 nM, respectively. In MV4-11, KG-1 and Kasumi-1 cells western blot analysis demonstrated that AP24534 inhibited the phosphorylation of the putative targets with IC50s of approximately 1, 10 and 18 nM, respectively. Furthermore, potent cellular activity (<10 nM) against all 3 activated kinases was a unique characteristic of AP24534 compared with other multi-targeted kinase inhibitors tested, including sunitinib and sorafenib. In vivo activity of AP24534 was examined in an MV4-11 mouse xenograft model. Statistically significant inhibition of tumor growth was demonstrated with once-daily oral doses as low as 1 mg/kg, and partial tumor regression with doses of 2.5 mg/kg. Doses of 5 or 10 mg/kg led to complete and durable tumor regression with no palpable tumors detected during a 4 week follow-up period. A single 10 mg/kg dose of AP24534 was sufficient to block phosphorylation of STAT5, a major downstream target of Flt-3. These potencies and responses observed in AML cell lines are comparable to the observed effects of AP24534 in analogous in vitro and xenograft studies using the Bcr-Abl-driven CML cell line K562.

Conclusions: These results indicate that AP24534 has the potential to be an effective treatment for AML, including the approximately one-third of AML cases characterized by the Flt3- ITD mutation that is correlated with a poor prognosis. The compound was particularly potent on the Flt3-driven cell line MV4-11. Inhibition by AP24534 of non-Flt3-dependent AML cell lines, such as those driven by c-Kit or FGF receptors, indicates the potential for activity across diverse AML subtypes and other c-Kit or FGF receptor-driven malignancies, such as multiple myeloma. Together with previous data showing potent activity in CML models, these results suggest a broad potential for AP24534 in hematologic malignancies. Based on these observations, a phase 1 clinical trial is now underway to evaluate AP24534 in patients with a range of hematologic malignancies, including AML and CML.