Mapping of human cerebral sigma1 receptors using positron emission tomography and [11C]SA4503

The objective of this study was to establish the kinetic analysis for mapping sigma1 receptors (σ1Rs) in the human brain by positron emission tomography (PET) with [11C]SA4503. The σ1Rs are considered to be involved in various neurological and psychiatric diseases. [11C]SA4503 is a recently developed radioligand with high and selective affinity for σ1Rs, and we have first applied it to clinical studies. Nine healthy male subjects each underwent a dynamic 90-min PET scan after injection of [11C]SA4503. In addition to the baseline measurement, three of the nine subjects underwent a second [11C]SA4503-PET after partial blockade of σ1Rs by oral administration of haloperidol, a sigma receptor antagonist. Full kinetic analysis using two times nonlinear estimations was applied for fitting a two-tissue three-compartment model to determine the binding potential (BP) and total distribution volume (tDV) of [11C]SA4503. Graphical analysis with a Logan plot was also applied for estimations of tDV. The regional distribution patterns of BP and tDV in 11 regions were compatible with those of previously reported σ1Rs in vitro. The reduced binding sites of σ1Rs by haloperidol were appropriately evaluated. The tDVs derived from the two methods matched each other well. The Logan plot offered images of the tDV, which reflected σ1R densities, and the tDV in the images decreased after haloperidol loading. Moreover, comparison of BPs calculated with and without metabolite correction for plasma input function indicated that the metabolite correction could be omitted. We concluded that this method enables the quantitative analysis of σ1Rs in the human brain.