Synthesis and preclinical evaluation of [ 11 C]D617, a metabolite of (R)-[ 11 C]verapamil Joost Verbeek a, ⁎ , Stina Syvänen b , Robert C. Schuit a , Jonas Eriksson a , Elizabeth C. de Lange b , Albert D. Windhorst a , Gert Luurtsema a,1 , Adriaan A. Lammertsma a a Department of Nuclear Medicine & PET Research, VU University Medical Center, Amsterdam, the Netherlands b Division of Pharmacology, LACDR, Leiden University, Leiden, the Netherlands Received 5 September 2011; received in revised form 30 September 2011; accepted 22 October 2011 Abstract Objectives: (R)-[ 11 C]verapamil is widely used as a positron emission tomography (PET) tracer to evaluate P-glycoprotein (P-gp) functionality at the blood–brain barrier in man. A disadvantage of (R)-[ 11 C]verapamil is the fact that its main metabolite, [ 11 C]D617, also enters the brain. For quantitative analysis of (R)-[ 11 C]verapamil data, it has been assumed that the cerebral kinetics of (R)-[ 11 C]verapamil and [ 11 C]D617 are the same. The aim of the present study was to investigate whether the cerebral kinetics of (R)-[ 11 C]verapamil and [ 11 C] D617 are indeed similar and, if so, whether [ 11 C]D617 itself could serve as an alternative PET tracer for P-gp. Methods: [ 11 C]D617 was synthesized and its ex vivo biodistribution was investigated in male rats at four time points following intravenous administration of [ 11 C]D617 (50 MBq) without (n=4) or with (n=4) pretreatment with the P-gp inhibitor tariquidar (15 mg·kg -1 , intraperitoneally). Brain distribution was further assessed using consecutive PET scans (n=8) before and after pretreatment with tariquidar (15 mg·kg -1 , intravenously), as well as metabolite analysis (n=4). Results: The precursor for the radiosynthesis of [ 11 C]D617, 5-amino-2-(3,4-dimethoxy-phenyl)-2-isopropyl-pentanitrile (desmethyl D617), was synthesized in 41% overall yield. [ 11 C]D617 was synthesized in 58%–77% decay-corrected yield with a radiochemical purity of ≥99%. The homogeneously distributed cerebral volume of distribution (V T ) of [ 11 C]D617 was 1.1, and this increased 2.4-fold after tariquidar pretreatment. Conclusion: V T of [ 11 C]D617 was comparable to that of (R)-[ 11 C]verapamil, but its increase after tariquidar pretreatment was substantially lower. Hence, (R)-[ 11 C]verapamil and [ 11 C]D617 do not show similar brain kinetics after inhibition of P-gp with tariquidar. © 2012 Elsevier Inc. All rights reserved. Keywords: [ 11 C]verapamil; [ 11 C]D617; P-glycoprotein; PET; Pgp; Metabolite 1. Introduction The blood–brain barrier (BBB) is a tightly connected cell layer which protects the brain from harmful substances and regulates the transfer of various compounds between brain and blood. The tight junctions between endothelial cells of the BBB force drugs to penetrate through rather than between these cells. This can be achieved by passive diffusion, active influx or active efflux. Passive diffusion is promoted by (a) high lipophilicity, (b) ideally an octanol-water partition coefficient (log P) roughly between 0.9 and 2.5 [1], (c) low molecular weight (b500 Da), (d) small cross-sectional area (b80Å 2 ) and (e) lack of charged atoms [2]. Passive transport does not require energy, and molecules are moved along the concentration gradient. Active influx and efflux are processes that require energy such as adenosine triphosphate (ATP), which can therefore transport molecules against a concen- tration gradient. P-glycoprotein (P-gp) is an ATP-dependent 170–180-kDa transmembrane glycoprotein present at the luminal side of the BBB, lowering the brain concentration of its substrates. This phenomenon may be associated with drug resistance in several diseases, such as temporal lobe epilepsy Available online at www.sciencedirect.com Nuclear Medicine and Biology 39 (2012) 530 – 539 www.elsevier.com/locate/nucmedbio ⁎ Corresponding author. Nuclear Medicine & PET research, location: radionuclide centre, VU University Medical Center, P.O. box 7057, 1007 MB Amsterdam, the Netherlands. Tel.: +31 0 20 4445989. E-mail address: jverbeek@rnc.vu.nl (J. Verbeek). 1 Present address: Department of Nuclear Medicine & Molecular Imaging, University Medical Center Groningen, University of Groningen, Groningen, the Netherlands. 0969-8051/$ – see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.nucmedbio.2011.10.017