Abstract 2653: Preclinical activity of dual PI3K/mTOR inhibitor SPR965 in multiple myeloma
Authors: Larsen Jeremy T. , Ramakrishnan Vijay , Haug Jessica , Kimlinger Teresa , Sen Somdutta , Mahajan Dinesh , Dugar Sundeep , Rajkumar S. Vincent , Kumar Shaji K. 
Published: August 2015

Introduction: Relapsed and refractory multiple myeloma (MM) remains a significant clinical challenge and drugs with new mechanisms of action to overcome resistance are needed. Constitutive activation of the PI3K/Akt/mTOR pathway in MM promotes tumorigenesis through propagation of the cell cycle, protein synthesis, and inhibition of apoptosis. Anti-MM effects of rapalogs are limited due to feedback activation of the PI3K/Akt signaling pathway. Agents capable of targeting PI3K and mTORC1 or both mTORC1/C2 are able to partially overcome such resistance mechanisms. However, the mTORC2 mediated increase in pAkt (Ser473) after PI3K/mTORC1 inhibition and the activation of PI3K after mTORC1/C2 inhibition could still contribute to resistance to such agents. We sought to investigate the effects of SPR965 (synthesized and provided by Sphaera Pharma, Singapore under an MTA), an orally bioavailable novel small molecule with inhibition of class 1 PI3 kinase and mTORC1/C2 kinases on MM cell lines and patient cells.

Results: SPR965 induced cytotoxicity and inhibited proliferation in all MM cell lines examined with IC50 values between 25-500nM. To understand if SPR965 induced apoptotic cell death, annexin/PI staining followed by flow cytometric assays were performed, which showed a clear increase in cells undergoing apoptosis. Western blots showed increase in caspase-3, -9, and PARP cleavage confirming apoptotic induction by SPR965. Importantly, SPR965 caused potent increase in apoptosis in cytogenetically distinct MM patient cells. Next, we examined the mechanism of action of SPR965. SPR965 caused potent mTORC1 inhibition evidenced by decreased p4E-BP1, p-p70S6K, and pS6 at doses as low as 25nM. SPR965 was able to inhibit PI3K activity as shown by decreased pPDK1 and pBTK at slightly higher concentrations of 75nM. Phosphorylation of Akt S473, a TORC2 substrate, increased at initial low concentrations of SPR965, but was attenuated at concentrations of 100 nM and above, demonstrating dose-dependent inhibition of mTORC2. Such effects were also observed when we performed western blots on MM patient derived primary plasma cells. Increased levels of p27 (KIP1) were observed suggestive of G1 growth arrest, which was confirmed on cell cycle analysis. Since SPR965 caused an increase in pAkt (both T308 and S473) at doses up to 75nM, we examined if SPR965 was able to synergize with an Akt inhibitor MK2206 at doses lower than 75nM. Our results showed potent synergy suggesting that the pAkt up regulation contributes to partial resistance, which is inhibited by SPR965 at doses of 100nM and higher, doses that are still clinically achievable.

Conclusion: Our findings demonstrate SPR965 induces apoptosis of MM cells through the inhibition of PI3K and mTORC1/C2 activity. Further studies are underway to better characterize the mechanism of action of SPR965, all of which will be informative for the drug to be used as a single agent or in combination with other agents in relapsed MM.

Citation Format: Jeremy T. Larsen, Vijay Ramakrishnan, Jessica Haug, Teresa Kimlinger, Somdutta Sen, Dinesh Mahajan, Sundeep Dugar, S. Vincent Rajkumar, Shaji K. Kumar. Preclinical activity of dual PI3K/mTOR inhibitor SPR965 in multiple myeloma. [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 2653. doi:10.1158/1538-7445.AM2015-2653