FDG PET/CT imaging one week after a single dose of pembrolizumab can predict response in patients with advanced melanoma

Immunologic response of melanoma to anti-PD-1 therapy can occur rapidly with T-cell responses detectable in one week. Given that activated immune cells are FDG avid, en early FDG PET/CT obtained approximately one week after starting pembrolizumab could be used to visualize a metabolic flare (MF), with increased tumor FDG activity due to infiltration by activated immune cells, or a metabolic response (MR), due to tumor cell death. A prospective study at the University of Pennsylvania (Philadelphia) included 19 patients receiving pembrolizumab for advanced melanoma. FDG PET/CT imagaing was performed at baseline and one week after starting pembrolizumab. MF or MR was identified in 6 of 11 responders (55%) and 0 of 8 nonresponders (0%), with ORR of 100% in the MF-MR group and 38% in the stable metabolism (SM) group. Median progression-free survival was not reached after 38 months follow-up in the MF-MR group and was 2.8 months in the SM group (p=0,017). Overall survival rate at 3 years was 83% in the MF-MR group and 62% in the SM group. The authors conclude that early FDG PET/CT identified metabolic changes in melanoma metastases that were predictive of response to pembrolizumab and significantly correlated with PFS.