Background: BRAFV600E mutations occur in ~10% of colorectal cancer (CRC), are associated with poor survival and have limited responses to BRAF/MEK inhibition with or without EGFR inhibition. There is an unmet need to understand the biology of poor prognostic BRAFMT CRC. Using gene expression data from BRAFMT CRC patient samples, we recently identified that dopamine receptor degradation and unfolded protein response are dominant pathways deregulated in the BRAFMT subgroup with the poorest outcome. The aim of this study was to investigate the role of the dopamine receptor 2 (DRD2) pathway as novel target in BRAFMT CRC cells. Methods: Small molecule DRD2 antagonists ONC-201 and ONC-206 (Oncoceutics Inc) and a panel of isogenic paired and non-isogenic BRAFMT and BRAFWT cells were used. MTT, Flow Cytometry, Western blotting and caspase- 8, 3/7 activity assays were used to measure cell survival/death. DR5 cell localization was performed using flow cytometry. A compound library including small molecules approved by the FDA was used. Results: BRAFMT CRC cells were highly sensitive to the DRD2 antagonists ONC-201 and ONC-206 with IC50 values between 1.9-4.5μM and 0.16 and 0.24μM respectively. Treatment with ONC-201 and ONC-206 resulted in marked increases in expression levels of the endoplasmic reticulum stress proteins ATF4, CHOP and the active (spliced) form of XBP1 (sXBP1) (indicators of activation of the PERK and IRE1α UPR branches), and this was associated with apoptosis induction as indicated by PARP cleavage, caspase-9 cleavage and increased caspase-3/7 activity in the BRAFMT VACO432 cell line but not in the WT VT1 clone. Importantly, no significant effect on proliferation or apoptosis was obtained in the normal colon CCD-18 fibroblast cell line following treatment with ONC-201 or ONC-206. Using a small molecule compound library, we found that the taxanes paclitaxel and docetaxel resulted in strong synergy and apoptosis when combined with ONC-201 or ONC-206, in particular in BRAFMT CRC cells. Mechanistically, we found that the apoptosis induced by combined ONC-201/paclitaxel treatment was dependent on caspase-8 activation and on up-regulation of the death receptor 5 (DR5). Conclusions: Taken together, we have identified a role for DRD2 signalling in the survival of BRAFMT CRC cells. Our data support the development of DRD2 antagonists, in particular in combination with taxanes, for the treatment of BRAFMT CRC tumours.

Citation Format: Arman Javadi, Nicholas Forsythe, Alaa Refaat, Jessica-Ann Weir, Hajrah Khawaja, David Waugh, Rohinton Tarapore, Joshua E. Allen, Patrick Johnston, Sandra Van Schaeybroeck. Targeting the dopamine receptor 2 in BRAF mutant colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3448.