3'-Deoxy-3'-18F-Fluorothymidine and 18F-Fluorodeoxyglucose positron emission tomography for the early prediction of response to Regorafenib in patients with metastatic colorectal cancer refractory to all standard therapies.

PURPOSE: The purpose of this study was to evaluate the value of 3'-deoxy-3'-18F-fluorothymidine (18F-FLT) and 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT) for early prediction of standard anatomic response and survival outcomes in patients with metastatic colorectal cancer (mCRC) receiving Regorafenib. METHODS: Sixty-eight patients with mCRC refractory to standard cytotoxic chemotherapy were enrolled and received Regorafenib (160 mg/day on days 1-21, following a 7-day break). Standard anatomical response was evaluated every 8 weeks. Both scans were performed before and on day 21 of Regorafenib. RESULTS: Of the 61 patients included in per-protocol analysis, complete response was not observed, but partial response was observed in 8.2% (n = 5), stable disease in 67.2% (n = 41), and progressive disease in 24.6% (n = 15). The objective response rate was 8.2% and disease control rate 75.4%. Five responders (8.2%) and 13 non-responders (21.3%) met the CT and 18F-FLT PET/CT criteria (maximum standardized uptake value decrease ≥ 10.6% for responders). Forty-three (70.5%) exhibited discordant responses on CT and 18F-FLT PET/CT (McNemar test, P < 0.001). At a median follow-up of 8.9 months, median progression-free survival (PFS) and median overall survival (OS) were 3.6 months (95% confidence interval [CI], 3.34-3.80 months) and 8.5 months (95% CI, 6.95-10.10 months), respectively. Comparison of PFS and OS according to 18F-FLT PET/CT response revealed slightly longer PFS (P = 0.015) in responders, but the correlation with OS was not significant. The PET Response Criteria in Solid Tumours (PERCIST) of 18F-FDG PET/CT revealed differences in PFS and OS between partial metabolic response (PMR) and non-PMR (P = 0.048 and P = 0.014, respectively), and between progressive metabolic disease (PMD) and non-PMD (P = 0.189 and P = 0.007, respectively). CONCLUSIONS: Survival outcome was significantly associated with PERCIST using 18F-FDG PET/CT but the change of 18F-FLT uptake was only slightly associated with PFS. 18F-FDG PET/CT can be used as imaging biomarker to predict clinical outcomes early in patients with mCRC receiving Regorafenib.