Design, Synthesis and in Vitro Biological Evaluation of Reference Compounds of 123I and 99Tcm Labeled Indole Radiotracers for σ2 Receptor Imaging

Novel indole radiotracers for 123I and 99Tcm labeling were designed. The corresponding reference compounds (Indole-I and Indole-MAMA-Re) and the precursor for 99Tcm labelling were synthesized. The compounds were characterized by IR, NMR, and MS analyses. Competition binding assays in vitro show that the Ki values of Indole-I for σ1 and σ2 receptors are (0.574 ± 0.355) μmol/L and (0.162 ± 0.030 2) μmol/L, respectively. The Ki values of Indole-MAMA-Re for σ1 and σ2 receptors are (3.75 ± 2.22) μmol/L and (7.83 ± 4.87) μmol/L, respectively. Furthermore, 99Tcm-Indole-MAMA was successfully prepared. The radio-chemical purity (RCP) of 99Tcm-Indole-MAMA after purification was higher than 90% by HPLC analysis. The compounds reported in this paper may be used as lead compounds for further structural modification to develop indole SPECT tumor imaging agents.