Development of Fluorescent 4-[4-(3H-Spiro[isobenzofuran-1,4′-piperidin]-1′-yl)butyl]indolyl Derivatives as High-Affinity Probes to Enable the Study of σ Receptors via Fluorescence-Based Techniques

[Image: see text] Sigma (σ) receptor subtypes, σ(1) and σ(2), are targets of wide pharmaceutical interest. The σ(2) receptor holds promise for the development of diagnostics and therapeutics against cancer and Alzheimer’s disease. Nevertheless, little is known about the mechanisms activated by the σ(2) receptor. To contribute to the exploitation of its therapeutic potential, we developed novel specific fluorescent ligands. Indole derivatives bearing the N-butyl-3H-spiro[isobenzofuran-1,4′-piperidine] portion were functionalized with fluorescent tags. Nanomolar-affinity fluorescent σ ligands, spanning from green to red to near-infrared emission, were obtained. Compounds 19 (σ pan affinity) and 29 (σ(2) selective), which displayed the best compromise between pharmacodynamic and photophysical properties, were investigated in flow cytometry, confocal, and live cell microscopy, demonstrating their specificity for the σ(2) receptor. To the best of our knowledge, these are the first red-emitting fluorescent σ(2) ligands, validated as powerful tools for the study of σ(2) receptors via fluorescence-based techniques.