Abstract 360: Tumor specific imaging by L-5-F-18-ethoxy-Tryptophan mediated by LAT1 and IDO1

Purpose Radiolabeled tryptophan is capable of visualizing the amino acid metabolism of tumor cells. We investigated the indirectly labeled derivative L-5-F-18-ethoxy-Trp for its potential to differentiate tumors from inflammation by Positron-Emission-Tomography (PET) and for its possible intracellular trapping after catabolism by indoleamine-2,3-dioxygenase 1 (IDO1). Methods Human NCI-H460 lung cancer and PC-3 prostate cancer cells were used for in vitro studies. Cellular influx and efflux of L-5-F-18-ethoxy-Trp at 37°C was measured by a gamma-counter and via HPLC. Blocking studies were carried out under similar conditions to examine the transporter family being relevant for Trp uptake. The effect of IDO1 was investigated after stimulation of cells with 100 IU interferon-gamma (IFN-γ). Human plasma stability of 5-F-18-ethoxy-Trp was measured by radio-HPLC. The imaging potential was examined in tumor-bearing rodents and inflammation models. NCI-460 tumors in rats and PC-3 tumors in mice were subcutaneously inoculated. A sterile inflammation was induced by Complete Freund's Adjuvant (CFA) and an infection inflammation was induced by E.coli injection into rats. PET/CT imaging after 1 hour was conducted by an Inveon microPET camera. Results High cellular uptake of L-5-F-18-ethoxy-Trp was found in NCI-H460 and PC-3 cells with 10% of the applied radioactive dose per 100,000 cells and 30% per 100,000 cells, respectively. Uptake could be blocked by an excess of L-Trp, L-Leu and bicycloheptan-carboxylic acid indicating transport by the heterodimeric amino acid transporters of the SLC7 family with predominant transport by LAT-1. Both cellular uptake and retention were increased after stimulation of IDO1 activity by IFN-γ. HPLC of the efflux supernatant showed strict IDO1 dependence of the intracellular Trp catabolism. The tracer was rather instable in human plasma in vitro and degraded already after 15 min incubation. Nevertheless, PET imaging revealed good uptake of L-5-F-18-ethoxy-Trp in NCI-H460 and PC-3 tumors in rodents with low background. No uptake was found in both inflammation models in rats. Conclusion Tryptophan derivatives are promising tracers for cancer imaging utilizing enhanced metabolism by IDO1 which has been shown to be overexpressed in tumors to suppress the immune response of their microenvironment. Differentiation between malignant lesions and inflammation seems to be possible which would give this PET tracer an advantage over the currently use