Abstract 1697: CPI-169, a novel and potent EZH2 inhibitor, synergizes with CHOP <i>in vivo</i> and achieves complete regression in lymphoma xenograft models

Abstract Enhancer of Zeste Homolog 2 (EZH2) is the histone lysine methyltransferase (HKMT) component of the Polycomb Repressive Complex 2 (PRC2). In conjunction with other members of the complex, EZH2 represses gene expression through methylation of histone H3 on lysine 27 (H3K27). EZH2 overexpression is implicated in tumor progression and correlates with poor prognosis in several tumor types and enzymatic hyperactivity of EZH2 has been linked to aberrant repression of tumor suppressor genes in diverse cancers. Recently, direct inhibition of EZH2 HKMT activity by small molecules has been shown to be effective in inhibiting the proliferation of EZH2 mutant diffuse large B-cell lymphoma (DLBCL) cell lines and the growth of tumors in EZH2 mutant DLBCL xenografts. We have identified and optimized a series of small molecule EZH2 inhibitors that is structurally distinct from previously published chemotypes. CPI-169, a representative compound from that effort, inhibits the catalytic activity of PRC2 with an IC50 of &lt; 1nM, decreases cellular levels of H3K27me3 with an EC50 of 70 nM, and triggers cell cycle arrest and apoptosis in a variety of cell lines. Importantly, compound treatment triggers a sequence of downstream functional consequences of EZH2 inhibition whereby apoptosis is not induced before ten days of continuous target engagement. Administered subcutaneously at 200 mpk twice daily (BID), CPI-169 is well tolerated in mice with no observed toxic effect or body weight loss. In the present study we show that CPI-169 treatment led to tumor growth inhibition (TGI) of an EZH2 mutant KARPAS-422 DLBCL xenograft. The TGI is proportional to the dose administered and to the reduction of the pharmacodynamic marker H3K27me3. The highest dose, 200 mpk, BID led to complete tumor regression. Since CHOP (cyclophosphamide, doxorubicin, vincristine and prednisone) is the standard treatment of advanced DLBCL, we were interested in combining a suboptimal dose of CPI-169 (100 mpk, BID) with a single dose of CHOP in the KARPAS-422 model. After a week of combinatorial treatment the tumors rapidly regressed and became unpalpable. Four weeks after the last dose only a single mouse presented a palpable tumor. The immunohistochemical analysis of tumor samples revealed a strong correlation between the global decrease of H3K27me3, the decrease in the proliferation marker Ki-67 and the increase in cleaved-caspase 3 positive cells. In conclusion, we identified a strong synergistic anti-tumor activity between the standard of care CHOP and CPI-169, a distinct EZH2 inhibitor in an in vivo model of DLBCL. Citation Format: Vidya Balasubramanian, Priya Iyer, Shilpi Arora, Patrick Troyer, Emmanuel Normant. CPI-169, a novel and potent EZH2 inhibitor, synergizes with CHOP in vivo and achieves complete regression in lymphoma xenograft models. [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 1697. doi:10.1158/1538-7445.AM2014-1697