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Compensation for cranial spill-in into the cerebellum improves quantitation of striatal dopamine D2/3 receptors in rats with prolonged [18F]-DMFP infusions

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Abstract

The condition of steady-state receptor binding in positron emission tomography (PET) studies is best obtained through the use of a bolus plus steady-infusion paradigm. This is a particularly important consideration in the context of in vivo competition studies, where a pharmacological challenge can be administered during the interval of steady-state ligand binding, as in the case of [11C]-raclopride studies with amphetamine challenge. However, the short half-life of 11C imposes limits on the practical duration of constant infusions. Therefore, we chose to test [18F]-DMFP as a tracer for dopamine D2/3 receptors in rat striatum in the paradigm. Using a conventional bolus injection, the [18F]-DMFP BPND was 3.8 in striatum of anesthetized rats. When followed by a constant infusion, we obtained quasi-stable BPND estimates of 4.5 within an interval of 45 min. During infusions lasting up to 4 h, BPND declined progressively. This seemed due to the progressive spill-in of radioactivity from the cranium to the cerebellum reference region, despite optimized iterative reconstruction of the images. Therefore, we propose a new concept of compensation for this spill-in effect using pharmacokinetic considerations, without requiring high-resolution anatomical images. Challenge with amphetamine (1 and 4 mg/kg) evoked an ∼25% reduction in BPND. There was no clear evidence of dose-dependence in the striatal-binding changes, despite the considerably greater physiological effect, as documented by ECG. Thus, the general applicability of the bolus plus infusion method with [18F]-DMFP for small animal studies is impeded by the substantial labeling of the cranium. The cranial uptake was linear, indicating first-order kinetics for the enzymatic defluorination of the tracer. Based on this phenomenon, we developed an analytic method compensating for the effects of progressive cranial labeling on the estimation of specific binding in striatum. Synapse, 2012. © 2012 Wiley Periodicals, Inc.

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