In vivo absolute quantification of striatal and extrastriatal D(2/3) receptors with [(123)I]epidepride SPECT

BACKGROUND: [(123)I]epidepride is a high-affinity radiotracer used in single-photon emission computed tomography (SPECT) imaging of the D(2/3) receptors. It binds with high affinity to striatal and extrastriatal receptors. Nevertheless, its slow kinetics in the striatum impedes quantification in this region. Thus, an approach that would allow a simultaneous quantification of both striatal and extrastriatal D(2/3) receptors would be of interest for preclinical and clinical SPECT neuroimaging. We describe a partial saturation protocol that allows us to produce an in vivo Scatchard plot and thus estimate B(avail) and appK(d) separately in both striatal and extrastriatal regions, through a single dynamic SPECT session. To validate this approach, a multi-injection protocol is used for the full kinetic modeling of [(123)I]epidepride using a two-tissue compartment, 5-parameter model (2T-5k). METHODS: Eighteen male rats were used. Binding parameters were estimated using the multi-injection protocol. Various simulations were performed to estimate the optimal conditions for the partial saturation protocol, which was applied at the region and voxel level. The results of the partial saturation study were compared to those obtained with the 2T-5k model. To illustrate the interest of the partial saturation approach, we performed a preliminary study of the effect of a chronic, subcutaneous administration of haloperidol (1 mg/kg/day), a D(2) receptor antagonist, on the B(avail) of [(123)I]epidepride in the rat striatum. RESULTS: A series of simulations demonstrated that a mass of 3 ug/kg of unlabeled epidepride allows the formation of an in vivo Scatchard plot. The partial saturation study led to robust estimations of B(avail) in all brain regions that highly correlated (r = 0.99) with the corresponding values from the multi-injection study. A chronic haloperidol treatment resulted in a 17.9% increase in the B(avail) values in the left Caudate Putamen nucleus (CP) (p = 0.07) and a 13.8% increase in the right CP (p = 0.12). CONCLUSION: A partial saturation method allowed the robust quantification of D(2/3) receptors in striatal and extrastriatal D(2/3) receptors with a single-scan approach. This approach may be applied in the mapping of the D(2/3) receptor in translational biological studies and potentially, in clinical SPECT imaging.