p-Fluoro-hexahydro-sila-difenidol exhibits poor selectivity between M3 and M1 muscarinic receptors

We investigated the potential ability of p-fluoro-hexahydro-sila-difenidol (p-F-HHSiD) to discriminate between M1 and M3 muscarinic receptor subtypes using Chinese hamster ovary cells stably transfected with the genes encoding the two receptors. Both radioligand binding and functional assays were utilized for this purpose. In contrast to initial reports of a 14-fold selectivity of this antagonist for M3 versus M1 receptors, we have detected a qualitatively similar selectivity that was markedly smaller in magnitude.