The First Human Application of an F-18-Labeled Tryptophan Analog for PET Imaging of Cancer

Preclinical studies showed the tryptophan analog PET radiotracer 1-(2-¹⁸F-fluoroethyl)-L-tryptophan (¹⁸F-FETrp) to accumulate in various tumors, including gliomas, and being metabolized via the immunosuppressive kynurenine pathway. In this first-in-human study, we tested the use ¹⁸F-FETrp-PET in patients with neuroendocrine and brain tumors.

We applied dynamic brain imaging in patients with gliomas (n = 2) and multi-pass 3D whole-body PET scans in patients with neuroendocrine tumors (n =4). Semiquantitative analysis of organ and tumor tracer uptake was performed using standardized uptake values (SUVs). In addition, organ dosimetry was performed based on extracted time-activity curves and the OLINDA software.

Neuroendocrine tumors showed an early peak (10-min post-injection) followed by washout. Both gliomas showed prolonged ¹⁸F-FETrp accumulation plateauing around 40 min and showing heterogeneous uptake including non-enhancing tumor regions. Biodistribution showed moderate liver uptake and fast clearance of radioactivity into the urinary bladder; the estimated effective doses were similar to other ¹⁸F-labeled radioligands.

The study provides proof-of-principle data for the safety and potential clinical value of ¹⁸F-FETrp-PET for molecular imaging of human gliomas.