PS979 PI3K INHIBITION COMBINED WITH STANDARD CHEMOTHERAPY EFFECTIVELY INHIBITS KIT MUTANT AML

Background: AML patients currently demonstrate a dismal overall survival rate (5y OS 25%) in response to standard chemotherapy regimens. The c‐KIT‐encoded receptor protein tyrosine kinase for stem cell factor (KIT) is recurrently mutated in AML (5%). KIT mutations frequently cooccur in AML patients with inv(16)/t(16/16) (15%) and t(8;21) (38%) driver aberrations where they confer inferior prognosis. KIT activation, promoted by the stem cell factor, is essential for survival, migration, and proliferation of normal hematopoietic stem and progenitor cells (HSPCs). KIT mutations in AML, like the KIT(D816Y) in exon 17, confer conformational changes leading to constitutive activation of PI3K and its downstream signalling pathways, which ultimately promotes cell growth, survival and chemotherapy resistance. Aims: In the present we applied the inv(16)/KIT(D816Y) mouse AML model to investigate whether therapeutic targeting of PI3K in combination with standard chemotherapy would inhibit growth and survival of AML synergistically. Methods: Gene Expression Profiling of sorted normal BM progenitor cells and inv(16)/KIT(D816Y) AML cells as well as Western blot analyses demonstrated high PI3K pathway activity in AML cells (i.e. PI3K‐beta and PI3K‐delta) implicating that the inv(16)/KIT(D816Y) AML model would allow to explore the therapeutic potential of PI3K inhibition alone and in combination with standard chemotherapy in a preclinical AML trial program. Results: Colony forming‐cell (CFC) assays were applied to assess the inhibitory effect of a range of PI3K inhibitors and standard chemotherapeutics on clonogenic growth of inv(16)/KIT(D816Y) AML cells vs wild‐type BM cells. Among all tested drugs the pan‐PI3K inhibitor BKM120 (IC50 360 nM), Doxorubicin (IC50 5 nM) and Ara‐C (Cytarabine) (IC50 28 nM) were selected and subjected to subsequent drug combination tests, which demonstrated high synergy scores for BKM120+Doxorubicin (Delta Score = 21) and for BKM120+Ara‐C (Delta Score = 6). We subsequently conducted in vivo pre‐clinical AML trials using varying doses of BKM120 alone and in combination with varying doses of a Doxorubicin/Ara‐C 3+5 chemotherapy regimen. Treatment with BKM120 per oral gavage demonstrated in vivo inhibition of the PI3K pathway, decreased AML burden in bone marrow (BM), spleen, and peripheral blood (PB), and reduced splenomegaly in a dose dependent manner (BKM120: 50 mg/kg/d and 30 mg/kg/d). As expected, the Doxorubicin/Ara‐C 3+5 chemotherapy regimen also inhibited in vivo AML growth in a dose dependent manner (DA‐1: 0,75 mg/kg/d1–3 + 25 mg/kg/d1–5, DA‐2: 1,5 mg/kg/d1–3 + 50 mg/kg/d1–5). Combinatorial treatment with BKM120 and Doxorubin/Ara‐C, increased survival of mice with inv(16)/KIT(D816Y) AML significantly (i.e. 30–42d) compared to untreated mice (17–18d, p>0,001) and mice subjected to single treatment with either BKM120 (22–28d, p = 0,004) or chemotherapy (21–22d, p>0,001). Summary/Conclusion: The present study implicates that targeted inhibition of the KIT/PI3K signaling axis enhances the therapeutic efficiency of standard chemotherapy in KIT mutant AML. Significantly, our findings provide a rationale for development of precision medicine strategies that target cancer‐specific vulnerabilities such as oncogenic signaling in individual AML patients in order to enhance their sensitivity toward conventional chemotherapy, leading to improved clinical outcome.