Vemurafenib Plus Cobimetinib in Metastatic Melanoma

Molecular targeted therapy with BRAF inhibitor vemurafenib is now currently used as first line treatment for patients with unresectable stage IIIc or metastatic melanoma harboring the BRAFV600 mutation, which is present in about 50% of melanoma patients. Despite the improvement in Progression Free Survival (PFS) en Overall Survival (OS) compared to dacabarzine, acquired resistance that develops in virtually all patients treated with vemurafenib is a great concern. Combining a BRAF inhibitor with a MEK inhibitor that targets the MAPK pathway further downstream, however, may overcome acquired resistance to BRAF inhibition and recent studies in which both MEK inhibitors and BRAF inhibitors are combined as monotherapy seem promising. In a phase IB trial preliminary efficacy of vemurafenib with cobimetinib (GDC-0973), a highly selective inhibitor of MEK1 seems encouraging with an initial response rate of 85% and currently a phase III study of vemurafenib versus vemurafenib plus cobimetinib in BRAFV600 mutation positive patients with advanced stage melanoma is underway. It is expected that in the near future combined BRAF and MEK inhibition will be standard of care for patients with BRAFV600 mutated metastatic melanoma. Diagnostic CT cannot assess reduction in tumor size within days after the initiation of therapy and anatomic size does not provide information about the development of therapy response or resistance at a molecular level. It has been clearly demonstrated that alterations in metabolism occur earlier than anatomical size reduction after the initiation of therapy. Molecular imaging with PET visualizes metabolic activity in tumors and is a sensitive method to detect alterations in cell metabolism, even shortly after the start of therapy. 18F-Fluorodeoxyglucose (18F-FDG) is used to visualize glucose metabolism, whereas 18F-Fluoro-3'-deoxy-3'L-fluorothymidine (18F-FLT) is used to visualize proliferation. In preclinical mouse models 18F-FLT appears to predict response or resistance to therapy better than 18F-FDG. However, so far only 18F-FDG PET has been used to monitor response to vemurafenib in some BRAFV600 mutated metastatic melanoma patients, showing a rapid decline of 18F-FDG within 2 weeks following treatment. Preclinical studies and the observation that melanoma is a highly proliferative malignancy in most patients suggest that 18F-FLT might be the radiopharmaceutical of first choice in this setting. By detecting these metabolic alterations, responders might be distinguished from non-responders at an earlier phase compared with anatomical imaging with CT. This way, unnecessary expensive treatment of combined BRAF/MEK-inhibitor therapy and its side effects can be prevented in patients who will not benefit from this therapy. Furthermore, the level of decline in metabolic activity in the first two weeks after the initiation of therapy might predict progression free survival (PFS) as preliminary results in literature suggest. This is a single arm explorative phase II clinical trial in 90 subjects with advanced stage melanoma harbouring a BRAFV600 mutation. PET imaging and molecular diagnostics are combined in order to monitor response to treatment with vemurafenib plus cobimetinib, examine development of resistance and correlate changes in metabolic/proliferative activity with extend of target inhibition.