Visualization and Tissue Distribution of α1L-Adrenoceptor in Human Prostate by the Fluorescently Labeled Ligand Alexa-488-Silodosin.

Purpose: Although α_1L-adrenoceptor is recognized as a target of α₁ antagonist therapy for benign prostatic hyperplasia, the most common techniques, such as immunohistochemistry and in situ hybridization, are not applicable to examine α_1L-AR vs α_1A-AR tissue distribution because α_1L-AR is now considered another phenotype sharing the α_1A-AR gene and protein molecule. We labeled the α_1A and α_1L-adrenoceptor selective antagonist silodosin (Kissei Pharmaceutical, Matsumoto, Japan) with the fluorophore Alexa Fluor® 488 (Alexa-488-silodosin) to visualize α_1L-AR expression. Materials and Methods: Radioligand binding and functional bioassay experiments were done to assess α₁-AR expression in Chinese hamster ovary cells and human prostate tissues. Confocal imaging was subsequently performed. Results: Although Alexa-488-silodosin had about 10 times lower affinity for all α₁-AR subtypes than silodosin in binding and functional studies, it had high selectivity to α_1A and α_1L-ARs. Confocal imaging revealed clear localization of fluorescence on the membrane of Chinese hamster ovary cells expressing α_1A-AR but not α_1B-and α_1D-ARs, and in the muscle layer of the human prostate. The fluorescent signal in Chinese hamster ovary cells disappeared in the presence of 3 nM prazosin but fluorescence was observed in the human prostate even in the presence of 100 nM prazosin. Conclusions: Alexa-488-silodosin is a powerful fluorescent probe with high selectivity to α_1A and α_1L-ARs. Thus, Alexa-488-silodosin successfully visualizes the site of α_1L-ARs in the muscle layer of the human prostate without losing its distinct pharmacological profile. [Copyright &y& Elsevier]