Abstract 2420: The dual PI3K δ/γ inhibitor RP6530 in combination with Brentuximab Vedotin (SGN-35) synergistically induces cell death via inhibition of tubulin polymerization in Hodgkin lymphoma cell lines
Authors: Locatelli Silvia L. , Careddu Giuseppa , Viswanadha Srikant , Vakkalanka Swaroop , Castagna Luca , Santoro Armando , Carlo-Stella Carmelo 
Published: August 2015

Abstract
Introduction: The phosphatidylinositol 3-kinase (PI3K) pathway is consistently activated in relapsed/refractory Hodgkin lymphoma (HL). Expression of the δ and γ isoforms of PI3K is restricted to cells of the hematopoietic system, suggesting that RP6530, a novel dual PI3K δ/γ Inhibitor, might represent a promising approach in the treatment of lymphomas. The CD30-directed antibody-drug conjugate, Brentuximab Vedotin (BV), has recently been reported to induce a high overall response rate in relapsed/refractory HL, but is associated with limited response duration. Combination therapies aimed at enhancing the anti-tumor activity of BV and eventually reducing its side effects may have significant clinical impact in the treatment of relapsed/refractory HL. Our study aimed at investigating the activity and mechanism(s) of action of RP6530 in combination with BV in preclinical HL models.
Methods and Results: RP6530 was active against a panel of HL cell lines (L-540, KM-H2, L-428). Using a WST assay, increasing concentrations of RP6530 (1.25 - 20 μM) resulted in a dose-dependent inhibition of cell growth (range, 20% to 40%). BV, at clinically achievable concentrations induced a significant growth inhibition (range, 30% to 40%) in all HL cells. Remarkably, when RP6530 was combined with BV at minimal, or mildly inhibitory concentrations, a highly synergistic inhibition in all HL cells was observed. Following a 48 hour exposure, RP6530 (5 μM) and BV (10 ng/ml) synergistically inhibited the mean (±SEM) growth of HL cells (RP6530: 9 ± 3%; BV: 12 ± 3%; RP6530/BV: 26 ± 3%). While single agents induced only 20% HL cell death, combination of RP6530 and BV caused remarkable induction of apoptosis (90%), with Combination Index (CI) as low as 0.003. To assess potential effects of RP6530/BV on microtubule dynamics, we conducted a tubulin polymerization assay. Highly purified α/β-tubulin and GTP were incubated with RP6530 and/or BV. While single agent activity of RP6530 or BV was modest, RP6530/BV significantly inhibited tubulin polymerization in a time-dependent manner (P &lt; 0.0001), suggesting that RP6530/BV disrupts microtubules in cells by abrogating microtubule assembly. Finally, in vivo experiments were conducted to investigate the antitumor activity of the two drugs. Compared to mock controls, treatment of NOD/SCID mice bearing KM-H2 tumor nodules with RP6530 (150 mg/kg/BID/3 weeks) or BV (0.5 mg/kg/q4×4d) resulted in a mean tumor growth inhibition of 90% and 40%, respectively. No mice experienced any apparent treatment-related toxicity. Effect of RP6530/BV in the xenograft model is currently being evaluated and results will be presented.
Conclusions: The combination of RP6530 and BV exerts a marked in vitro synergistic antitumor activity against HL cells, thereby representing a promising therapeutic strategy in the treatment of relapsed/refractory HL.
Citation Format: Silvia L. Locatelli, Giuseppa Careddu, Srikant Viswanadha, Swaroop Vakkalanka, Luca Castagna, Armando Santoro, Carmelo Carlo-Stella. The dual PI3K δ/γ inhibitor RP6530 in combination with Brentuximab Vedotin (SGN-35) synergistically induces cell death via inhibition of tubulin polymerization in Hodgkin lymphoma 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 2420. doi:10.1158/1538-7445.AM2015-2420