The effect of a P-glycoprotein inhibitor on rat brain uptake and binding of [18F]Altanserin: A micro-PET study

Flourine-18-labeled Altanserin {3-(2-[4-(4-fluorobenzoyl)piperidin-1-yl]- ethyl) -1,2-dihydro-2-thioxo- quinazolione} is a widely used 5HT2A receptor-selective positron emission tomography (PET) tracer (Ki 0.13 nM) in humans. Ex vivo studies in the rat show, however, that altanserin has a limited brain uptake and the altanserin binding potential (BP2) is highly variable with a low reproducibility. Therefore, we hypothesized that altanserin ? in parallel to what has been reported for the radiotracers [11C]Carazolol, [11C]Verapamil and [18F]MPPF ? is a substrate for P-glycoprotein (P-gp), an efflux transporter in the blood-brain barrier. We investigated rat brain uptake and specific binding of [18F]Altanserin before and after inhibition of P-gp with Cyclosporin A (CsA). Small animal PET was used to estimate brain percent standard uptake value (%SUV) of radioactivity in control and CsA treated rats. [18F]Altanserin (1-10 MBq) was injected i.v. and arterial blood samples were obtained from a femoral cathether during the scan. To assess the specific binding before and after CsA treatment, a displacement study with i.v.injection of the potent 5HT2(A/C) antagonist Ketanserin was performed. The area under the arterial input curves (AUC) was computed to compare radioligand levels in plasma and blood. Previous studies in rats have shown only minimal brain and plasma metabolism of [18F]Altanserin and for that reason, no metabolite analyses were carried out. Radiotracer in vivo distribution was compared to in vitro autoradiography images, from 20 μm rat brains sections. Specific binding, in terms of BP2 [(ROI-Cerebellum)/Cerebellum], was determined on the basis of the mean %SUV after 60 minutes and the outcome was compared to ex vivo dissection 60 min post injection of [18F]Altanserin. When normalized with the corresponding AUC's, brain images of CsA treated rats had a 52 % higher total brain uptake of [18F]Altanserin, and a 6.46 fold increase in frontal cortex BP2 compared to rats injected with [18F]Altanserin alone. As demonstrated in the figure, a substantially higher brain uptake of [18F]Altanserin was seen in the CsA treated (B) compared to controls (A: without CsA, C: with CsA and Ketanserin). The distribution of activity in the brain was found to be similar to the distribution found by in vitro 5HT2A receptor autoradiography. CsA treated rats had lower radioactivity in blood and plasma than controls, Ketanserin treated rats had lower radioactivity levels in plasma but not in blood, probably because of the Ketanserin-induced displacement of [18F]Altanserin from platelet 5-HT2 receptors. It is concluded that, CsA pronouncedly affects [18F]Altanserin uptake and binding in the rat brain. By contrast, [18F]Altanserin shows a high uptake and binding in the human brain. When new PET-tracers are evaluated, these species differences should be kept in mind.