Treatment and Resistance to Paradox-Breaker PLX8394 in cutaneous melanoma

FDA-approved BRAF inhibitors produce high response rates and improve overall survival in patients with BRAF V600E/K mutant melanoma, but are linked to pathologies associated with paradoxical ERK1/2 activation in wild-type BRAF cells. To overcome this limitation, a next-generation paradox breaking RAF inhibitor (PLX8394) has been designed. Here we show that by using a quantitative reporter assay, PLX8394 rapidly suppressed ERK1/2 reporter activity and growth of mutant BRAF melanoma xenografts. Ex vivo treatment of xenografts and use of a patient-derived explant system (PDeX) revealed that PLX8394 suppressed ERK1/2 signaling and elicited apoptosis more effectively than the FDA-approved BRAF inhibitor, vemurafenib. Furthermore, PLX8394 was efficacious against vemurafenib-resistant BRAF splice-variant expressing tumors and reduced splice-variant homodimerization. Importantly, PLX8394 did not induce paradoxical activation of ERK1/2 in wild-type BRAF cell lines or PDeX. Continued in vivo dosing of xenografts with PLX8394 led to the development of acquired resistance via ERK1/2 reactivation through heterogeneous mechanisms; however, resistant cells were found to have differential sensitivity to ERK1/2 inhibitor. These findings highlight the efficacy of a paradox-breaking selective BRAF inhibitor and the use of PDeX system to test efficacy of therapeutic agents. RNAs were obtained from parental and PBRT resistant cell lines established from in vivo tumors PBRT15 and PBRT16 were treated with 500 nM PLX8394