Abstract
BackgroundThe BRAF inhibitor, vemurafenib, has recently been approved for the treatment of metastatic melanoma in patients harboring BRAFV600 mutations. Currently, dual BRAF and MEK inhibition are ongoing in clinical trials with the goal of overcoming the acquired resistance that has unfortunately developed in some vemurafenib patients. FDG-PET measures of metabolic activity are increasingly employed as a pharmacodynamic biomarker for guiding single-agent or combination therapies by gauging initial drug response and monitoring disease progression. However, since tumors are inherently heterogeneous, investigating the effects of BRAF and MEK inhibition on FDG uptake in a panel of different melanomas could help interpret imaging outcomes.Methods18 F-FDG uptake was measured in vitro in cells with wild-type and mutant (V600) BRAF, and in melanoma cells with an acquired resistance to vemurafenib. We treated the cells with vemurafenib alone or in combination with MEK inhibitor GDC-0973. PET imaging was used in mice to measure FDG uptake in A375 melanoma xenografts and in A375 R1, a vemurafenib-resistant derivative. Histological and biochemical studies of glucose transporters, the MAPK and glycolytic pathways were also undertaken.ResultsWe demonstrate that vemurafenib is equally effective at reducing FDG uptake in cell lines harboring either heterozygous or homozygous BRAFV600 but ineffective in cells with acquired resistance or having WT BRAF status. However, combination with GDC-0973 results in a highly significant increase of efficacy and inhibition of FDG uptake across all twenty lines. Drug-induced changes in FDG uptake were associated with altered levels of membrane GLUT-1, and cell lines harboring RAS mutations displayed enhanced FDG uptake upon exposure to vemurafenib. Interestingly, we found that vemurafenib treatment in mice bearing drug-resistant A375 xenografts also induced increased FDG tumor uptake, accompanied by increases in Hif-1α, Sp1 and Ksr protein levels. Vemurafenib and GDC-0973 combination efficacy was associated with decreased levels of hexokinase II, c-RAF, Ksr and p-MEK protein.ConclusionsWe have demonstrated that 18 F-FDG-PET imaging reflects vemurafenib and GDC-0973 action across a wide range of metastatic melanomas. A delayed post-treatment increase in tumor FDG uptake should be considered carefully as it may well be an indication of acquired drug resistance.Trial registrationClinicalTrials.gov NCT01271803
Highlights
The BRAF inhibitor, vemurafenib, has recently been approved for the treatment of metastatic melanoma in patients harboring BRAFV600 mutations
In vitro 18 F-FDG uptake demonstrates vemurafenib BRAFV600E selectivity as well as striking MEK potentiation F-FDG uptake was evaluated in a panel of melanoma cells, as well as the HCT 116 colorectal line, that had been treated with BRAF inhibitor vemurafenib alone, or in combination with MEK inhibitor GDC-0973 for a total of 3 days (Figure 1A)
Coadministration with GDC-0973 resulted in significant decreases in 18 F-FDG uptake across almost all cell lines, independent of mutation status; the combination treatment strikingly overcame the A375R1 and R3 cell lines lack of response to vemurafenib alone
Summary
The BRAF inhibitor, vemurafenib, has recently been approved for the treatment of metastatic melanoma in patients harboring BRAFV600 mutations. Methods: 18 F-FDG uptake was measured in vitro in cells with wild-type and mutant (V600) BRAF, and in melanoma cells with an acquired resistance to vemurafenib. PET imaging was used in mice to measure FDG uptake in A375 melanoma xenografts and in A375 R1, a vemurafenib-resistant derivative. This, is beginning to change as personalized medicines targeting the most commonly mutated oncogenes are being evaluated in clinical trials. Vemurafenib is such an example and is an orally available ATP competitive inhibitor of the kinase domain within the BRAF oncogene [3]. BRAF is mutated in more than 50% of all melanomas, with BRAFV600E being the predominant mutation, increasing the protein's kinase activity and, thereby, driving downstream cellular proliferation through the MAPK pathway [4]. BRAFV600K/R mutations have been reported to occur in melanoma but seem to be very rare and found mostly in other types of cancer [5]
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