Cyclosporine, a P-glycoprotein modulator, increases [18F]MPPF uptake in rat brain and peripheral tissues: microPET and ex vivo studies.

PURPOSE: Pretreatment with cyclosporine, a P-glycoprotein (P-gp) modulator increases brain uptake of 4-(2'-methoxyphenyl)-1-[2'-(N-2"-pyridinyl)-p-[(18)F]fluorobenzamido]ethylpiperazine ([(18)F]MPPF) for binding to hydroxytryptamine(1A) (5-HT(1A)) receptors. Those increases were quantified in rat brain with in vivo microPET and ex vivo tissue studies. MATERIALS AND METHODS: Each Sprague-Dawley rat (n = 4) received a baseline [(18)F]MPPF microPET scan followed by second scan 2-3 weeks later that included cyclosporine pretreatment (50 mg/kg, i.p.). Maximum a posteriori reconstructed images and volumetric ROIs were used to generate dynamic radioactivity concentration measurements for hippocampus, striatum, and cerebellum, with simplified reference tissue method (SRTM) analysis. Western blots were used to semiquantify P-gp regional distribution in brain. RESULTS: MicroPET studies showed that hippocampus uptake of [(18)F]MPPF was increased after cyclosporine; ex vivo studies showed similar increases in hippocampus and frontal cortex at 30 min, and for heart and kidney at 2.5 and 5 min, without concomitant increases in [(18)F]MPPF plasma concentration. P-gp content in cerebellum was twofold higher than in hippocampus or frontal cortex. CONCLUSIONS: These studies confirm and extend prior ex vivo results (J. Passchier, et al., Eur J Pharmacol, 2000) that showed [(18)F]MPPF as a substrate for P-gp. Our microPET results showed that P-gp modulation of [(18)F]MPPF binding to 5-HT(1A) receptors can be imaged in rat hippocampus. The heterogeneous brain distribution of P-gp appeared to invalidate the use of cerebellum as a nonspecific reference region for SRTM modeling. Regional quantitation of P-gp may be necessary for accurate PET assessment of 5-HT(1A) receptor density when based on tracer uptake sensitive to P-gp modulation.

Radiochemical synthesis and tissue distribution of p-[18F]DMPPF, a new 5-HT1A ligand for PET, in rats.

Several studies have demonstrated the potential of p-[(18)F]MPPF as a radiopharmaceutical to study the 5-HT(1A) receptor family in animals and humans. A structural modification leading to a higher radioactive signal at an equipotent dose would greatly enhance this potential. With this goal, the desmethylated 4-(2'-methoxyphenyl)-1-[2'-[N-(2''-pyridinyl)-p-fluorobenzamido]ethyl]-piperazine (p-MPPF), identified as p-DMPPF, was synthesized, labeled with fluorine-18 and evaluated through ex vivo tissue distribution in rats. The new compounds p-DMPPF, p-DMPPNO(2), MEM-p-MPPF and MEM-p-MPPNO(2) were isolated and fully identified ((1)H and (13)C NMR, LC-MS). The final compound, p-[(18)F]DMPPF, was obtained ready for injection, with an overall radiochemical yield of 10% (EOB corrected) within 90 min and a specific activity of 62 GBq/mumol. Tissue distributions showed that the carbon-fluorine bond was stable in vivo and that this compound could cross the blood-brain barrier. For kidney, lung, heart, spleen, bone, testicle, liver and muscle, the percentage of injected dose per gram of tissue obtained with p-[(18)F]DMPPF was of the same order of magnitude as that of p-[(18)F]MPPF. The amount of radioactivity reaching the brain was much higher (approximately fivefold at 60 min) for p-[(18)F]DMPPF compared with p-[(18)F]MPPF, which was taken as reference. The distribution and specificity were in total agreement with the known localization of 5-HT(1A) receptors in rats. The radioactivity increase was more important for specific tissues (hippocampus and frontal cortex) than for cerebellum or striatum, leading to better contrast (hippocampus/cerebellum=5.8 at 60 min). The levels of metabolites found in plasma showed that p-[(18)F]DMPPF appears to be less metabolized than p-[(18)F]MPPF. p-[(18)F]DMPPF deserves further evaluation as a radiopharmaceutical candidate.