Predictive value of early 18F-FDG PET/CT studies for treatment response evaluation to ipilimumab in metastatic melanoma: preliminary results of an ongoing study

Abstract

Ipilimumab is a newly approved immunotherapeutic agent that has been shown to provide a survival benefit in patients with metastatic melanoma. ¹⁸F-FDG PET/CT has demonstrated very satisfying results in detecting melanoma metastases in general. Using ¹⁸F-FDG PET/CT we monitored patients with metastatic melanoma undergoing ipilimumab therapy during the course of treatment. The aim of our study was to evaluate the role of ¹⁸F-FDG PET/CT performed after two cycles of ipilimumab in predicting the final response to therapy. In 22 patients suffering from unresectable metastatic melanoma, scheduled for ipilimumab treatment PET/CT scanning was performed before the start of treatment (baseline scan), after two cycles of treatment (early response) and at the end of treatment after four cycles (late response). Evaluation of the patient response to treatment on PET was based on the European Organization for Research and Treatment of Cancer 1999 criteria. Progression-free survival (PFS) and overall survival (OS) data are presented. After the end of treatment, 15 patients were characterized as having progressive metabolic disease (PMD) and five as having stable metabolic disease (SMD), and two patients showed a partial metabolic response (PMR). Early PET/CT performed after two ipilimumab cycles predicted treatment response in 13 of the 15 PMD patients, in five of the five SMD patients and in neither of the two PMR patients. Both patients with PMR showed pseudoprogression after the second cycle and were therefore wrongly classified. According to the patients’ clinical outcome, patients with late PMD had a median PFS of 3.6 months (mean 5.6 months), while patients with late SMD had a median PFS of 9.8 months (mean 9.0 months). In comparison, patients with early PMD had a median PFS of 2.7 months (mean 5.5 months) and patients with early SMD had a median PFS of 6.3 months (mean 7.5 months). The difference in PFS between the two groups was statistically significant for both early and late responses (log-rank p < 0.001). Median OS among patients with late PMD was 9.1 months (mean 11.2 months) and among those with late SMD 9.8 months (mean 10.7 months). The difference in OS between the two groups was statistically significant (log-rank p = 0.013). The median OS among patients with early PMD was 8.8 months (mean 12.0 months) and among those with early SMD 9.8 months (mean 10.0 months). The difference in OS between the two groups was statistically significant (log-rank p < 0.001). ¹⁸F-FDG PET/CT after two cycles of ipilimumab is highly predictive of the final treatment outcome in patients with PMD and SMD.