Positron emission tomography with 5-hydroxytryptophan in neuroendocrine tumors

Purpose: Carcinoid tumors, especially those of mid gut origin, produce serotonin via the precursors tryptophan and 5-hydroxytryptophan (5-HTP). We have evaluated the usefulness of positron emission tomography (PET) with carbon-11-labeled 5-HTP in the diagnosis and treatment follow-up evaluation of patients with neuroendocrine tumors. Patients and Methods: PET using C-11-labeled 5-HTP was compared with computed tomography (CT) in 18 patients (14 midgut, one foregut, one hindgut carcinoid, and two endocrine pancreatic tumors [EPT]). In addition, 10 of 18 patients were monitored with PET examinations during treatment. Results: All 18 patients, including two with normal urinary 5-hydroxyindole acetic acid (U-5-HIAA), had increased uptake of C-11-labeled 5-HTP in tumorous tissue as compared with normal tissue. Liver metastases, as well as lymph node, pleural, and skeletal metastases, showed enhanced 5-HTP uptake and PET could detect more lesions than CT in 10 patients and equal numbers in the others. Tumor visibility was better for PET than for CT due to the high and selective uptake of 5-HTP with a high tumor to-background ratio. Binding studies indicated an irreversible trapping of 5-HTP in the tumors. Linear regression analyses showed a clear correlation (r =.907) between changes in U-5-HIAA and changes in the transport rate constant for 5-HTP during treatment. Conclusion: PET with C-11-labeled 5-HTP demonstrated high uptake in neuroendocrine gastrointestinal tumors and thereby allowed improved visualization compared with CT. The in vivo data on regional tumor metabolism, as expressed in C-11-5-HTP uptake and transport rate, provided additional information over conventional radiologic techniques. The close correlation between the changes in C-11-5-HTP transport rare and U-HIAA during medical treatment indicates the potential of C-11-5-HTP-PET as a means to monitor therapy. (C) 1998 by American Society of Clinical Oncology.