Increased glucose metabolism of inflammatory tissues is the main source of false positive FDG-PET findings in oncology. For this reason, we have tested whether the selectivity of sigma receptor ligands for tumor versus inflammation is higher than that of FDG and FLT. The sigma receptor is strongly over-expressed in a variety of tumors, making it an attractive target for oncology-PET. It has been demonstrated that the receptor density of the sigma-2 subtype shows a high correlation with cellular proliferation rate 1 . Therefore sigma receptor mapping may result in more selective tumor imaging. To test this hypothesis, we compared the biodistribution of
Wistar rats, which were tumor-bearing (C6 rat glioma in the right shoulder) and also had a sterile inflammation in the left calf muscle (induced by injection of 0.1 ml turpentine). The data were compared with those previously obtained with FDG and another cell proliferation marker FLT 2 . Twenty-four hours after turpentine injection, the rats received an intravenous bolus (10-20 MBq) of either of the two sigma receptor ligands (n=5). The rats were sacrificed after 60 min biodistribution. Tumor/muscle ratios of [ 18 F]FE-SA5845 and [ 11 C]SA4503 were 3.6 Β± 1.4 and 4.9 Β± 1.6, respectively. These numbers are comparable to a previously reported study 3 . Tumor/muscle ratios of FDG at 2 h post injection (13.2 Β± 3.0) were higher, but those of FLT (3.8 Β± 1.3) were similar 2 . The sigma receptor ligands did not accumulate in the inflamed tissue. Inflammation/muscle ratios for [ 18 F]FE-SA5845 and [ 11 C]SA4503 were 0.9 Β± 0.2 and 1.3 Β± 0.4, respectively. For comparison, FDG accumulated in the inflamed muscle, with 4.8 Β± 1.2 times higher uptake in the affected thigh than in the contralateral healthy thigh, in contrast to FLT for which this ratio was not significantly different from unity (1.3 Β± 0.4) 2 . From these biodistribution data can be concluded that the two sigma receptor ligands have comparable selectivity for tumor versus inflammation as FLT in our animal model, whereas FDG showed both higher uptake in tumor and inflammatory tissue. Further evaluation of the sigma receptor ligands in cancer patients will show whether sigma mapping can be of clinical relevance.