Abstract 1689: Synergistic effect of JQ1 and rapamycin for treatment of human osteosarcoma

Abstract

Bromodomain and extra terminal domain (BET) proteins are important epigenetic regulators facillitating the transcription of genes in chromatin areas linked to acetylated histones. JQ1, a BET protein inhibitor, has antiproliferative activity againts many hematological and solid cancers, mainly through inhibition of c-myc and upregulation of p21. In this research, we investigated the use of JQ1 for human osteosarcoma (OS) treatment, due to the frequent copy number gain and overexpression of c-myc in OS cells. JQ1 significantly inhibited the proliferation and survival of seven OS cells (U2OS, G292, MG-63, HT-161, MNNG/HOS, SAOS-2, SJSA), at a relatively higher dose than other cancers (median IC50 = 7.35 μM). JQ1 induced G1 cell cycle arrest and premature senescence of OS cells, but produced little apoptosis. Interestingly, c-myc protein levels in JQ1-treated cells remained unchanged, whereas the upregulation of p21 protein was still observable, as noted by Western blots. Although effective in vitro, JQ1 failed to reduce the size of the MNNG/HOS xenografts in immunocompromised mice in vivo. To overcome the resistance of OS cells to JQ1 treatment, we tested JQ1 in combination with rapamycin. Rapamycin inhibits the assembly of mTOR complexes which is one of the important growth signaling pathways in many cancers including OS. JQ1 and rapamycin synergistically inhibited the growth and survival of OS cells in vitro. The drug combination nearly abrogated expression of c-myc protein, enhanced levels of p21, and induced apoptosis in OS cells in vitro. After 24 days of treatment in vivo, JQ1 (50mg/kg body weight) or rapamycin (1.5 mg/kg body weight) alone showed little effect on the size of human OS xenografts in nude mice. However, combination of both drugs significantly inhibited tumor growth by 85% and 73% (p<0.05 for both) compared to either JQ1 or rapamycin alone, respectively. In conclusion, JQ1 inhibited proliferation and survival of OS cells in vitro but failed to reduce growth of OS xenografts in immunodeficient mice. When combined with rapamycin, JQ1 synergistically inhibited proliferation of OS cells both in vitro and in vivo. Citation Format: Dhong Hyun Tony Lee, Jun Qi, James Bradner, Jonathan Said, Ngan Doan, Charles Forscher, H Phillip Koeffler. Synergistic effect of JQ1 and rapamycin for treatment of human osteosarcoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1689. doi:10.1158/1538-7445.AM2014-1689