title: Differential Metabolism and Pharmacokinetics of L-[1-(11)C]-Methionine and 2-[(18)F] Fluoro-2-deoxy-D-glucose (FDG) in Androgen Independent Prostate Cancer.
abstract: Metabolic imaging with positron emission tomography (PET) for the staging and monitoring of treatment response has important implications in clinical oncology. The choice of radiotracer is likely to be critically important. The objective of our study was to compare the pharmacokinetics of C-11-methionine with FDG in a group of androgen independent patients with metastatic prostate cancer, to determine the differential metabolism of the two tracers, and to determine the optimal time of imaging after injection in treated and untreated patients. A total of 29 dynamic scans (19 pretreatment and 10 posttreatment) were performed in 10 patients with progressive or new lesions on bone scans (index lesions). A total of 13 index lesions were identified in baseline scans. Patients were infused with 370 MBq C-11-methionine on the couch and 32 dynamic images acquired over 60 minutes. After at least 5 half-lives of C-11, patients were then dynamically imaged (15 frames) for 45 minutes with FDG. Index lesions demonstrated both C-11-methionine (13/13) and FDG uptake (12/13). The plateau of methionine uptake in tumor was reached by 10 minutes, and thereafter remained constant. FDG tumor uptake was slower and for some patients continued to rise beyond 45 minutes. The clearance of blood activity for C-11-methionine was more rapid than FDG and the plateau was 10 and 45 minutes respectively. In 5 patients scanned after therapy, 4 responded to treatment, which was reflected by a corresponding decrease in C-11-methionine and FDG tumor uptake. No change was observed in the relative shape of the uptake curves however, between the C-11-methionine and the FDG uptake, either in the 4 who responded to treatment or for one patient who did not respond. The SUV of C-11-methionine was significantly higher than for FDG (P <.008). Both C-11-methionine and FDG are taken up in index lesions in patients with progressive prostate cancer. The advantages of C-11-methionine over FDG are the higher tumor to blood ratio, the more rapid tumor uptake allowing earlier imaging, and a flatter plateau rendering lesion activity on whole body images more uniform and less susceptible to gradual change than FDG. This indicates the feasibility of whole body PET imaging with decay corrected C-11-methionine. Additional studies are planned to define optimal imaging times after different therapies in comparison to FDG and bone scans.