Adaptive and acquired responses of mutant braf melanomas to RAF/MEK inhibitors
Since the discovery of BRAF mutations in over 50% of melanomas, various small molecule inhibitors against mutant BRAF have been developed for the treatment of metastatic melanoma. Early clinical data from these inhibitors showed significant efficacy with high response rates. However, these benefits were short-lived as the majority of patients relapsed while on treatment. Our work suggests mutant BRAF melanomas overcome BRAF inhibition in a stepwise process involving both acquired and adaptive mechanisms of resistance. Acquired mechanisms of resistance have been studied in great detail since the arrival of mutant BRAF inhibitors. Here we show that the acquisition of an NRAS Q61K mutation working in concert with mutant BRAF is sufficient to overcome BRAF inhibition in mutant BRAF melanomas. Our group and others have shown that NRAS Q61K can promote the formation of BRAF/CRAF heterodimers in mutant BRAF melanomas leading to an increase in ERK signaling capable of compensating for BRAF inhibition. We also show that this mutation requires the RAS scaffolding protein SHOC2 in order to mediate resistance. Loss of SHOC2 in mutant BRAF cells harboring an NRAS Q61K mutation prevented the compensatory ERK reactivation, and rendered the cells susceptible to BRAF inhibition in vitro. Targeting the pathway downstream of SHOC2 and BRAF/CRAF heterodimers should prevent the compensatory ERK reactivation seen in NRAS Q61K resistant cells, however, recent BRAF/MEK combination therapies have failed to prevent relapse in the clinic. As a result, we investigated the adaptive mechanisms which precede acquired mutations in mutant BRAF melanomas. Adaptive mechanisms are acute cellular responses resulting from ERK inhibition that promote the survival of cells in the presence of mutant BRAF inhibitors. These mechanisms afford the cells time to acquire permanent secondary mutations and allow the cells to become fully resistant to treatment. Thus adaptive mechanisms can be exploited to reduce the occurrence of acquired mutations and, therefore, relapse in patients. Data from our lab has shown that the EGFR tyrosine kinase family member ERBB3 is adaptively upregulated following ERK inhibition in a high percentage of mutant BRAF melanomas. We used RPPA analysis to show that ERRB3 primarily signals to the PI3K/AKT pathway in order to promote the survival of mutant BRAF melanoma cells in the presence of BRAF inhibition. Targeting ERBB3 using the monoclonal antibody huHER3-8, provided by Immunogen, in combination with BRAF inhibition enhanced cell death and reduced proliferation in vitro. The combination of BRAF inhibition and huHER3-8 in vivo resulted in a significant reduction of tumor growth compared to BRAF inhibition alone. Additionally, a high percentage of mice treated with the mutant BRAF inhibitor, PLX4720, and huHER3-8 saw a complete tumor response which persisted following removal of treatment. The findings presented here suggest targeting ERBB3 in combination with mutant BRAF can improve the efficacy of mutant BRAF inhibitors and help to promote long-term durable responses in the clinic.
Kugel, Curtis H., "Adaptive and acquired responses of mutant braf melanomas to RAF/MEK inhibitors" (2015). ETD Collection for Thomas Jefferson University. AAI3705079.