Dopamine D 2 receptor quantification in extrastriatal brain regions using [ 123 I]epidepride with bolus/infusion

The iodinated benzamide epidepride, which shows a picomolar affinity binding to dopamine D 2 receptors, has been designed for in vivo studies using SPECT. The aim of the present study was to apply a steady‐state condition by the bolus/infusion approach with [ 123 I]epidepride for the quantification of striatal and extrastriatal dopamine D 2 receptors in humans. In this way the distribution volume of the tracer can be determined from a single SPECT image and one blood sample. Based on bolus experiments, an algorithm using conventional convolution arguments for prediction of the outcome of a bolus/infusion (B/I) experiment was applied. It was predicted that a B/I protocol with infusion of one‐third of the initial bolus per hour would be appropriate. Steady‐state conditions were attained in extrastriatal regions within 3–4 h but the infusion continued up to 7 h in order to minimize the significance of individual differences in plasma clearance and binding parameters. A steady‐state condition, however, could not be attained in striatal brain regions using a B/I protocol of 20 h, even after 11 h. Under near steady‐state conditions a striatal:cerebellar ratio of 23 was demonstrated. Epidepride has a unique signal‐to‐noise ratio compared to [ 123 I]IBZM but present difficulties for steady‐state measurements of striatal regions. The bolus/infusion approach is particularly feasible for quantification of the binding potential in extrastriatal regions. Synapse 36:322–329, 2000. © 2000 Wiley‐Liss, Inc.