Abstract 2704: RP6530, a dual PI3K δ/γ inhibitor, potentiates ruxolitinib activity in the JAK2-V617F mutant erythroleukemia cell lines
Authors: Vakkalanka Swaroop , Nyayapathy Seeta , Viswanadha Srikant 
Published: August 2015

Abstract
Background: Myelofibrosis (MF) represents a life-threatening neoplasm that manifests particularly in the elderly population and is characterized by bone marrow fibrosis and extramedullary hematopoeisis. While ruxolitinib, a JAK1/2 inhibitor, has recently been approved by the USFDA for its disease modifying potential in MF patients, it is still not considered as a curative option. Targeting another kinase such as PI3K, downstream of JAK, could therefore be a more efficient way of treating myelofibrotic neoplasms. RP6530 is a novel, potent, and selective PI3K δ/γ inhibitor that demonstrated high potency against PI3Kδ (IC50 = 25 nM) and γ (IC50 = 33 nM) enzymes with selectivity over α (&gt;300-fold) and β (&gt;100-fold) isoforms. The objective of this study was to evaluate the effect of a combination of ruxolitinib and RP6530 in the JAK2-V617F mutant Human Erythroleukemia (HEL) cell line.
Methods: Passive resistance was conferred by incubating HEL cells with increasing concentrations of ruxolitinib over an 8-10-week period. Endogenous JAK2, PI3Kδ, PI3Kδ, and pAKT were estimated by Western Blotting. RP6530, ruxolitinib, and the combination of RP6530 + Ruxolitinib were tested for their effect on viability and apoptosis. Cell viability was assessed by a MTT assay. Induction of apoptosis was analyzed by Annexin V/PI staining.
Results: Resistance to ruxolitinib was confirmed by a right-ward shift in EC50 of ruxolitinib in a HEL cell proliferation assay (0.82 μM Vs. 12.2 μM). Endogeous pAKT expression was 3.7-fold higher in HEL-RR compared to HEL-RS cells indicating activation of the AKT signaling pathway. While single-agent activity of RP6530 was modest (33-46% inhibition @ 10 μM) in both HEL-RS and HEL-RR cells, addition of 10 μM RP6530 to ruxolitinib was synergistic resulting in a near-complete inhibition of proliferation (&gt;90% for HEL-RS and &gt;70% for HEL-RR). While the order of addition did not affect the potency of RP6530, addition of 5 μM RP6530, 4 h prior to the addition of ruxolitinib resulted in a significant reduction in EC50 of ruxolitinib (5.8 μM) in HEL-RR cells. On lines with cell proliferation data, incubation of 10 μM RP6530 with ruxolitinib for 72 h increased the percent of apoptotic cells (55% in HEL-RS and 37% in HEL-RR) compared to either agent alone (16-27% in HEL-RS and 17-21% in HEL-RR).
Conclusions: Ruxolitinib resistance in the V617F JAK-2 mutant HEL cells is accompanied by an increase in pAKT expression. Inhibition of pAKT via the addition of RP6530, a dual PI3K δ/γ inhibitor, resulted in a reversal of ruxolitinib resistance. Complementary activity was also observed in HEL-RS cells indicating that a combination of ruxolitinib and RP6530 could have a positive bearing on the clinical outcome in MF patients.
Citation Format: Swaroop Vakkalanka, Seeta Nyayapathy, Srikant Viswanadha. RP6530, a dual PI3K δ/γ inhibitor, potentiates ruxolitinib activity in the JAK2-V617F mutant erythroleukemia cell lines. [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 2704. doi:10.1158/1538-7445.AM2015-2704