Fluorescent Derivatives of σ Receptor Ligand 1-Cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (PB28) as a Tool for Uptake and Cellular Localization Studies in Pancreatic Tumor Cells

Fluorescent derivatives of σ2 high affinity ligand 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1- yl)propyl]piperazine 1 (PB28) were synthesized. NBD or dansyl fluorescent tags were connected through a 5- or 6-atom linker in two diverse positions of 1 structure. Good σ2 affinities were obtained when the fluorescent tag was linked to 5-methoxytetralin nucleus replacing the methyl function. NBD-bearing compound 16 displayed high σ2 affinity (Ki = 10.8 nM) and optimal fluorescent properties. Its uptake in pancreatic tumor cells was evaluated by flow cytometry, showing that it partially occurs through endocytosis. In proliferating cells, the uptake was higher supporting that σ2 receptors are markers of cell proliferation and that the higher the proliferation is, the stronger the antiproliferative effect of σ2 agonists is. Colocalization of 16 with subcellular organelles was studied by confocal microscopy: the greatest was in endoplasmic reticulum and lysosomes. Fluorescent σ2 ligands show their potential in clarifying the mechanisms of action of σ2 receptors.