Quantification of Serotonin 5-HT 1A Receptors in Monkey Brain With [ 11 C](R)-()-RWAY FUMIHIKO YASUNO, 1 * SAMI S. ZOGHBI, 1 JULIE A. McCARRON, 1 JINSOO HONG, 1 MASANORI ICHISE, 1 AMIRA K. BROWN, 1 ROBERT L. GLADDING, 1 JOHN D. BACHER, 2 VICTOR W. PIKE, 1 AND ROBERT B. INNIS 1 1 Molecular Imaging Branch, National Institute of Mental Health, Bethesda, Maryland 2 Division of Veterinary Resources, Office of Research Services, National Institutes of Health, Bethesda, Maryland KEY WORDS serotonin 5-HT 1A ;[ 11 C](R)-()-RWAY; kinetic analysis; P-glycoprotein; PET ABSTRACT [ 11 C](R)-()-RWAY ([ 11 C]2, 3, 4, 5, 6, 7-hexahydro-1{4-[1[4-(2-methoxy- phenyl)-piperazinyl]]-2-phenylbutyry}-1H-azepine) is a new radioligand for imaging brain 5-HT 1A receptors with positron emission tomography. In [ 11 C](R)-()-RWAY, the direction of the amide bond is expected to reduce metabolism by hydrolysis while allowing easy 11 C-labeling at the methoxy position. The purposes of this study were to evaluate different tracer kinetic models in nonhuman primates to quantify 5-HT 1A receptors with [ 11 C](R)-()-RWAY and to test for the possible action of P-glycoprotein (P-gp), one of the known efflux pumps at the blood–brain barrier. The brain uptake of radioactivity from [ 11 C](R)-()-RWAY into 5-HT 1A receptor-rich brain regions was sev- eralfold greater than for its antipode ([ 11 C](S)-(+)-RWAY) and could be displaced by re- ceptor saturating doses of the selective 5-HT 1A antagonist, WAY-100635. Pretreatment with tariquidar, a potent inhibitor of P-gp, increased brain uptake of [ 11 C](R)-()- RWAY about 1.5-fold and the plasma free fraction about 1.8-fold. Thus, the effect of tariquidar on brain uptake may have been caused by displacement of the radioligand binding to plasma proteins. Mathematical modeling showed that the estimated values of regional binding potential were correlated strongly between two-tissue compartment model and multilinear reference tissue model, and thus, supported the use of the cere- bellum as a reference region. Synapse 60:510–520, 2006. Published 2006 Wiley-Liss, Inc. { INTRODUCTION The first successful positron emission tomography (PET) radioligand for the human 5-HT 1A receptor was WAY-100635 labeled with 11 C in the O-methoxy posi- tion (Pike et al., 1995) (Fig. 1A). Unfortunately, [O-methyl- 11 C]WAY-100635 is rapidly metabolized in human and nonhuman primates with cleavage of the amide bond. The resulting radiometabolite, [O-methyl- 11 C]WAY-100634, enters brain in a significant (Osman et al., 1998). Because of this metabolic path, WAY- 100635 was labeled in the carbonyl position (McCarron et al., 1996), since amide cleavage of [carbonyl- 11 C] WAY-100635 generates the polar metabolite, [car- boxyl- 11 C]cyclohexanecarboxylic acid, that does not easily cross the blood–brain barrier (Pike et al., 1996) (Fig. 1B). Two disadvantages of [carbonyl- 11 C]WAY-100635 are: (1) labeling in the carbonyl position is more difficult than >O-methylation and (2) nonspecific uptake in cere- bellum is low. Reference tissue analysis is strongly pre- ferred to standard compartmental modeling, because it does not require arterial line placement or radiometabo- lite analysis. This relatively simple method calculates the ratio of brain uptake in receptor-rich to receptor-free regions, and assumes that all activity derives from the parent tracer and not radiometabolite(s). In the case of [carbonyl- 11 C]WAY-100635, the denominator (cerebellar activity) is so low that the ratio is vulnerable to mea- surement error and to even small amounts of radiome- tabolite(s). We developed [O-methyl- 11 C](R)-()-RWAY Published 2006 WILEY-LISS, INC. { This article is a US government work and, as such, is in the public domain in the United States of America. Contract grant sponsor: NIMH; Contract grant number: Z01-MH002795-04; Contract grant sponsor: Japan Society for the Promotion of Science. *Correspondence to: Fumihiko Yasuno, MD, PhD, Molecular Imaging Branch, National Institute of Mental Health, Building 1, Room B3-10, One Center Drive, Bethesda, MD 20892-0135, USA.. E-mail: yasunof@mail.nih.gov Received 25 May 2006; Accepted 18 July 2006 DOI 10.1002/syn.20327 Published online in Wiley InterScience (www.interscience.wiley.com). SYNAPSE 60:510–520 (2006)