In a pre-clinical study comparing racemic 4-, 6-, and 7-18F-FEHTrp [66] (Figure 6), 6-18F-FEHTrp was found to have the highest tumor-to-reference ratio (2.6 ± 0.2) via the LAT transport system in a small cell lung cancer xenograft model, but there was no evidence of tracer metabolism. This evidence concerns the gene LAT and small cell lung carcinoma.