title: Determination of in vivo Bmax and Kd for 11C-GR103545, an agonist PET tracer for κ-opioid receptors: a study in nonhuman primates.
abstract: The κ-opioid receptors (KOR) are involved in mood disorders and addictive conditions. In vivo imaging studies of this receptor in humans have not been reported because of the lack of a selective ligand. We used a recently developed selective KOR agonist tracer, (11)C-GR103545, and performed a study in rhesus monkeys to estimate the in vivo receptor concentration (Bmax) and dissociation equilibrium constant (Kd).
  
  
  METHODS
  Four rhesus monkeys underwent 12 scans with (11)C-GR103545 on a PET scanner under baseline and self-blocking conditions. The injected mass was 0.042 ± 0.014 μg/kg for the baseline scans and ranged from 0.16 to 0.3 μg/kg for the self-blocking scans. The radiotracer was administered in a bolus-plus-infusion protocol, and cerebellum was used as the reference region in kinetic analysis. Binding potential (BPND) values were computed as [(CROI/CREF) - 1], where CROI and CREF are the mean of the radioactivity concentrations from 90 to 120 min after tracer administration in a given region of interest (ROI) and in the cerebellum. In 6 scans, arterial input functions and free fraction in plasma (fp) were measured. In addition, a 2-tissue-compartment model was used to compute the volume of distribution in the cerebellum (VT_REF), which was then used to estimate the free-to-nondisplaceable concentration ratio (fND) as fp/VT_REF. A Scatchard plot was used to estimate Bmax, and Kd(ND) = Kd/fND, the Kd value with respect to the cerebellar concentration. Individual data were first analyzed separately and then pooled together. When Kd(ND) was allowed to vary among ROIs, results were variable; therefore, Kd(ND) was constrained to be constant across ROIs, whereas Bmax was allowed to be ROI-dependent and animal-dependent.
  
  
  RESULTS
  A global estimate of 1.72 nM was obtained for Kd(ND). Estimated Bmax ranged from 0.3 to 6.1 nM across ROIs and animals. The Kd estimate of 0.048 nM, obtained by correcting Kd(ND) by the factor fND, was in good agreement with the half maximal inhibitory concentration (IC50) of 0.018 nM determined from functional assays in rabbit vas deferens and inhibition constant (Ki) of 0.02 nM measured in radioligand competition assays using cloned human receptors. On the basis of these data, a suitable tracer dose of 0.02 μg/kg was selected for use in humans.
  
  
  CONCLUSIONS
  The use of a bolus-plus-infusion protocol with the KOR agonist tracer (11)C-GR103545 permitted the successful estimation of Bmax and Kd(ND) in vivo. On the basis of the estimated Kd value, a tracer dose of 1.4 μg (3.38 nmol) for an average body weight of 70 kg was chosen as the mass dose limit in human studies using this novel agonist radiotracer.
authors: PET Center, Department of Diagnostic Radiology, Yale University, New Haven, CT 06520, USA.
authors: Giampaolo
authors: G
authors: Tomasi
authors: Nabeel
authors: N
authors: Nabulsi
authors: Ming-Qiang
authors: MQ
authors: Zheng
authors: David
authors: D
authors: Weinzimmer
authors: Jim
authors: J
authors: Ropchan
authors: Laura
authors: L
authors: Blumberg
authors: Clive
authors: C
authors: Brown-Proctor
authors: Yu-Shin
authors: YS
authors: Ding
authors: Richard E
authors: RE
authors: Carson
authors: Yiyun
authors: Y
authors: Huang
mesh_terms: D000818:Animals; D002250:Carbon Radioisotopes; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D008253:Macaca mulatta; D008297:Male; D010879:Piperazines / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D049268:Positron-Emission Tomography; D011759:Pyrrolidines / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D011817:Rabbits; D011849:Radioactive Tracers; D011854:Radiochemistry; D017473:Receptors, Opioid, kappa / Q000819:agonists* / Q000378:metabolism*
chemical_list: D002250:Carbon Radioisotopes; D010879:Piperazines; D011759:Pyrrolidines; D011849:Radioactive Tracers; D017473:Receptors, Opioid, kappa; C071609:GR 89696