Species Differences in Blood-Brain Barrier Transport of Three Positron Emission Tomography Radioligands with Emphasis on P-Glycoprotein Transport Stina Syva ¨ nen, ¨ Orjan Lindhe, Mikael Palner, Birgitte R. Kornum, Obaidur Rahman, Bengt Långstro ¨ m, Gitte M. Knudsen, and Margareta Hammarlund-Udenaes Department of Pharmaceutical Biosciences, Uppsala University, Uppsala, Sweden (S.S., M.H.-U.); Uppsala Imanet, GE Healthcare, Uppsala, Sweden (S.S., O ¨ .L.); Uppsala Applied Science Lab, GEMS PET Systems, GE Healthcare, Uppsala, Sweden (O.R., B.L.); Neurobiology Research Unit, Department of Neurology, Copenhagen University Hospital, Copenhagen, Denmark (M.P., B.R.K., G.M.K.); and Department of Biochemistry and Organic Chemistry, Uppsala University, Uppsala, Sweden (B.L.) Received October 7, 2008; accepted November 24, 2008 ABSTRACT: Species differences occur in the brain concentrations of drugs, but the reasons for these differences are not yet apparent. This study was designed to compare brain uptake of three radiolabeled P-glycoprotein (P-gp) substrates across species using positron emission tomography. Brain concentrations and brain-to-plasma ratios were compared; [ 11 C]verapamil in rats, guinea pigs, and monkeys; [ 11 C](S)-(2-methoxy-5-(5-trifluoromethyltetrazol-1-yl)- phenylmethylamino)-2(S)-phenylpiperidine (GR205171) in rats, guinea pigs, monkeys, and humans; and [ 18 F]altanserin in rats, minipigs, and humans. The fraction of the unbound radioligand in plasma was studied along with its metabolism. The effect of P-gp inhibition was investigated by administering cyclosporin A (CsA). Pronounced species differences were found in the brain and brain- to-plasma concentrations of [ 11 C]verapamil, [ 11 C]GR205171, and [ 18 F]altanserin with higher brain distribution in humans, monkeys, and minipigs than in rats and guinea pigs. For example, the brain- to-plasma ratio of [ 11 C]GR205171 was almost 9-fold higher in hu- mans compared with rats. The species differences were still present after P-gp inhibition, although the increase in brain con- centrations after P-gp inhibition was somewhat greater in rats than in the other species. Differences in plasma protein binding and metabolism did not explain the species-related differences. The findings are important for interpretation of brain drug delivery when extrapolating preclinical data to humans. Compounds found to be P-gp substrates in rodents are likely to also be substrates in higher species, but sufficient blood-brain barrier permeability may be retained in humans to allow the compound to act at intracere- bral targets. Species differences in receptor and enzyme systems are well known, but very little is known about species differences in the transport of drug molecules across the blood-brain barrier. Transport across this barrier occurs by passive diffusion either alone or in combination with active influx or efflux. P-glycoprotein (P-gp; ABCB1; MDR1) is an ATP-dependent efflux transporter that carries a wide range of drug molecules at the blood-brain barrier as well as in other organs, such as the liver, kidney, and gastrointestinal tract. It has a profound influence on brain concentrations of its substrates. Because of its noninvasiveness, positron emission tomography (PET) is one of the few methods that allows for direct in vivo comparisons between preclinical species and humans regarding drug interactions with P-gp. [ 11 C]Verapamil is the most frequently used PET radioligand for P-gp studies (Hendrikse et al., 1999; Bart et al., 2003; Sasongko et al., 2005; Lee et al., 2006; Syva ¨nen et al., 2006). Other examples include [ 11 C]carazolol, [ 18 F]fluorocarazolol, [ 11 C]carvedilol, [ 11 C]GR218231, [ 11 C]loperamide, and [ 11 C]desmethyl loperamide (Doze et al., 2000; Bart et al., 2005; de Vries et al., 2005; Lazarova et al., 2008; Zoghbi et al., 2008). Some radioligands shown to be P-gp substrates in rodents are nonetheless taken up by the brain in primates, indicating that there may be species differences in P-gp function. For example, Liow et al. (2007) have shown that the 5HT 1A receptor antagonist [ 11 C]RWAY is a P-gp substrate in the rat and mouse and displays a low brain uptake in these This work was supported by GE Healthcare and EC-FP6-project DiMI [Grant LSHB-CT-2005-512146]. Article, publication date, and citation information can be found at http://dmd.aspetjournals.org. doi:10.1124/dmd.108.024745. ABBREVIATIONS: P-gp, P-glycoprotein; MDR, multidrug resistance; PET, positron emission tomography; 5HT, 5-hydroxytryptamine (serotonin); RWAY, 2,3,4,5, 6,7-hexahydro-1{4-[1[4-(2-methoxyphenyl)-piperazinyl]]-2-phenylbutyry}-1H-azepine; [ 18 F]MPPF, 4-(2'-methoxyphenyl)-1-[2'-(N- 2-pyridinyl)-p-[18F]fluorobenzamido]ethylpiperazine; GR205171, 2 (S)-(2-methoxy-5-(5-trifluoromethyltetrazol-1-yl)-phenylmethylamino)-2(S)- phenylpiperidine; NK, neurokinin; GF-120918, N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy- 9-oxo-4-acridine carboxamide; CsA, cyclosporin A; ROI, region of interest; f p , plasma protein unbound fraction; K p , brain-to-plasma concentration ratio; K p,increase , increase in K p after cyclosporin A administration; R buffer , radioactivity concentration in buffer; R plasma , radioactivity concentration in plasma; SUV, standardized uptake value; SUV increase , increase in standardized uptake value after cyclosporin A administration; PSC833, valspodar. 0090-9556/09/3703-635–643$20.00 DRUG METABOLISM AND DISPOSITION Vol. 37, No. 3 Copyright © 2009 by The American Society for Pharmacology and Experimental Therapeutics 24745/3435052 DMD 37:635–643, 2009 Printed in U.S.A. 635 at ASPET Journals on October 30, 2016 dmd.aspetjournals.org Downloaded from