Paradox breaker BRAF inhibitors have comparable potency and MAPK pathway reactivation to encorafenib in BRAF mutant colorectal cancer

// Oliver J. Pickles 1 , * , Aneta Drozd 2 , * , Louise Tee 2 , Andrew D. Beggs 2 and Gary W. Middleton 1 1 Institute of Immunology and Immunotherapy, University of Birmingham, Edgbaston, Birmingham, UK 2 Institute of Cancer and Genomic Sciences, University of Birmingham, Edgbaston, Birmingham, UK * These authors contributed equally to this work Correspondence to: Gary W. Middleton, email: G.Middleton@bham.ac.uk Keywords: colorectal cancer; BRAF; paradox breaker; encorafenib; PLX8394 Received: May 13, 2020 Accepted: June 30, 2020 Published: August 25, 2020 ABSTRACT The BEACON CRC trial demonstrated a survival advantage over chemotherapy for a combination of targeted agents comprising the potent BRAF inhibitor encorafenib together with cetuximab and binimetinib. Resistance to BRAF inhibition in CRC arises in part through the generation and activation of RAF dimers resulting in MEK-ERK pathway reactivation. Paradox breaker BRAF inhibitors, such as PLX8394, are designed to inhibit RAF dimer formation. We analyzed whether paradox breakers reduce pathway reactivation and so have enhanced potency compared with encorafenib in BRAF mutant CRC. The potency of encorafenib and PLX8394 was greater than vemurafenib and the degree of pathway reactivation somewhat less. However, dose response curves for encorafenib and PLX8394 were similar and there was no significant differences in degree of pathway reactivation. To our knowledge these data represent the first comparative data of encorafenib and paradox breaker inhibitors in BRAF mutant CRC. Whilst these results support further investigation of PLX8394, all three agents tested reactivated the pathway in melanoma cells, a disease in which monotherapy is effective. Strategies focused on restricting RAF dimerization fail to address the impact that specific context of BRAF mutation in CRC has on targeted therapy outcomes.