Abstract LB-121: ACK in BRAF inhibitor resistance

Abstract

Abstract BRAF inhibition by the small-molecule drug vemurafenib or dabrafenib achieves enormous success in the treatment of melanoma. However, melanoma patients lose their responsiveness after 6 months of treatment. Activation of receptor tyrosine kinase (RTK) was found in vemurafenib resistant melanomas and considered as a critical factor contributing to vemurafenib resistance. We hypothesize that the quick relapse after BRAFi therapy is at least partly due to the off targets of these drugs. Using trans-cyclooctene(TCO) moiety conjugated vemurafenib to treat melanoma cells in culture, we were able to confirm that ACK is a bona-fide intracellular target of vemurafenib. ACK, a protein kinase that regulates turnover of EGFR and other RTKs, was then identified to be downregulated in melanoma cells after short term vemurafenib treatment. Furthermore, ACK protein was reduced in multiple vemurafenib resistant melanoma cells. To prove ACK downregulation plays a role in vemurafenib resistance development, we knocked down ACK by shRNA. We found that ACK reduction delays EGFR degradation speed and increases EGFR protein expression. Since EGFR overexpression has been demonstrated as one important driver of vemurafenib resistance, we expected ACK reduction by shRNA would lead to BRAFi resistance, which was then confirmed by viability assay. During the development of BRAFi resistance, MITF is lost, which is accompanied by overexpression of growth factors and cytokines. We proved that higher resistance to vemurafenib can be induced by combination of ACK reduction and addition of EGFR ligands and cytokines. Our data suggest that EGFR activation in vemurafenib resistant melanoma is mediated by a combination of ACK and MITF downregulation, which lead to RTK and growth factor/cytokine upregulation. ACK binding and downregulation by BRAFi might be the first step toward resistance and a loophole inherent in these compounds themselves. Thus, optimizing BRAFi to ensure more selectivity should be a critical step further to improve the long-term benefits of this class of drugs. Note: This abstract was not presented at the meeting. Citation Format: Zhenyu Ji, Hensin Tsao, Mikula Hannes, Jenny Njauw, Raj Kumar. ACK in BRAF inhibitor resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-121. doi:10.1158/1538-7445.AM2017-LB-121