Abstract 1812: Adaptive and acquired resistance to GNAQ/11 inhibition in uveal melanoma

Abstract Uveal Melanoma (UM) is a highly lethal cancer with 50% of patients developing lethal metastatic disease, primarily the liver, with currently no effective treatment options. UM is driven by mutations constitutively activating the Gαq pathway, about 90% affecting the heterotrimeric G-protein alpha subunits GNAQ or GNA11 and less commonly (<10%) CYSLTR2 (a GNAQ/11 coupled receptor) or PLCB4 (a direct effector of Gαq). Directly targeting GNAQ/11 therefore could be of therapeutic relevance for the majority of UMs. Here we found that YM-254890, a cyclic depsipeptide, inhibited downstream signaling induced by GNAQQ209L and GNA11Q209L, but not GNA14Q205L, GNA15Q212L and GNASQ227L in 293T cells, confirming that it is a GNAQ/11-specific inhibitor. We systematically examined its activity against other recurring mutations in the Gαq pathway (GNAQ: Q209P, G48V, T175R, R183Q, F228L; GNA11:E191G, R183C, E234K; CYSLTR2: L129Q; PLCB4: D630Y). In 293T cells GNAQQ209P, GNAQR183Q, CYSLTR2L129Q and PLCB4D630Y activated downstream signaling whereas other Gαq mutants did not. YM-254890 inhibited the downstream signaling induced by GNAQQ209P, GNAQR183Q and CYSLTR2L129Q whereas it remained expectedly ineffective against PLCB4D630Y. Furthermore, we found that YM-254890 selectively inhibited Gαq signaling and cell proliferation in a panel of UM cell lines with GNAQ/11 mutations but had no effect on Gαq wild type melanoma cells. In vivo, YM-254890 slowed the growth of liver metastases in a xenograft model of GNAQ mutant UM cells but did not induce tumor shrinkage. Analysis of the tumor lysates revealed that inhibition of Gαq pathway signaling was incomplete. RNAseq analysis of 10 UM cell lines with various GNAQ/11 mutations exposed to YM-254890 revealed that GNAQ/11 inhibition led to significant upregulation of endothelin ET(B) receptor (EDNRB) compared to control treatment. EDNRB is a GNAQ/11 coupled receptor, and western blot confirmed that EDNRB protein level was significantly upregulated when mutant GNAQ/11 was inhibited with either YM-254890 or siRNA. The critical role of endothelin signaling in mediating adaptive resistance to Gαq inhibition was confirmed by demonstrating (1) expression of the EDNRB ligand endothelin 1 in liver metastases of the xenograft model; (2) pharmacological or genetic inhibition of EDNRB was able to break resistance. We also found that the therapeutic efficacy of YM-254890 can be subverted by acquiring additional mutations in the pathway. Upon chronic growth suppression of a GNA11Q209L mutant UM cell line a resistant clone emerged that was demonstrated to harbor a novel GNA11F75Y mutation, affecting the YM-254890 binding site. Concordantly, an engineered GNA11 with the two mutations in cis was resistant to the compound. Our data suggest that targeting mutant GNAQ/11 is promising but will require combinatorial targeting of EDNR signaling and possibly other pathways to reach maximal clinical efficacy. Citation Format: Jiafang Ma, Boris C. Bastian, Xu Chen. Adaptive and acquired resistance to GNAQ/11 inhibition in uveal melanoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1812.