System A amino acid transport is increased in transformed and malignant cells. The amino acid 4-cis[(18)F]fluoro-l-proline (cis-[(18)F]FPro) has been shown to be a substrate of the System A amino acid carrier. In this pilot study, we investigated the diagnostic potential of cis-[(18)F]FPro in patients with various tumors in comparison with [(18)F]fluorodeoxyglucose-positron emission tomography (FDG-PET).
Methods: Eight patients (seven females, one male, age range 43-77 years) with large primary, recurrent or metastatic tumors of different histologies were included in this study. One patient had a recurrent non-Hodgkin lymphoma; two patients, metastatic colon or rectal cancer; one, a metastatic endometrial cancer; one, a multiple myeloma; one, an Ewing sarcoma; one, a metastatic breast cancer and one, a gastrointestinal stromal tumor. PET scans of the trunk were acquired at 1 h after intravenous injection of 400 MBq cis-[(18)F]FPro and compared to PET scans with [(18)F]FDG.
Results: None of the tumors or metastatic lesions in this series of patients demonstrated relevant uptake of cis-[(18)F]FPro. In contrast, all tumors with exception of the multiple myeloma showed an intensive uptake of [(18)F]FDG. The mean standardized uptake value of cis-[(18)F]FPro in the tumor or metastases was significantly lower than that of [(18)F]FDG uptake (1.7+/-0.6 vs. 5.7+/-3.0; n=8; P<.01).
Conclusion: Although other System A-specific tracers have shown relevant tumor uptake, cis-[(18)F]FPro fails to detect most types of human tumors. Based on these results, we cannot recommend a further evaluation of this tracer as a tumor-seeking agent.