460. PHNO in vivo affinity for D2 and D3 receptors: a PET study in rhesus monkeys
J.D. Beaver1, J.D. Gallezot2, N. Nabulsi2, D. Weinzimmer2, T. Singhal2, M. Slifstein3, R.N. Gunn1, M. Laruelle4, Y.-S. Ding2, Y. Huang2, R.E. Carson2 and E.A. Rabiner1
1GSK, Clinical Imaging Center, London, UK; 2Yale University PET Center, New Haven, Connecticut, USA; 3Columbia University, New York, New York, USA; 4GSK, Schizophrenia & Cognition DPU, Harlow, UK
Objectives: We previously used SB-277011 (a D3 selective antagonist) and [11C]PHNO PET in non-human primates to determine the proportion of [11C]PHNO BPND attributable to D3 and D2 components. Additional data from the same subjects using SB-277011 with [11C]raclopride and [11C]fallypride were also used to estimate the relative regional density of D3 and D2 receptors. These data implied that in vivo [11C]PHNO has a ∼20-fold higher affinity for D3 relative to D2;1,2 however, this method of estimating the regional density of D3 and D2 receptors assumes that [11C]raclopride and [11C]fallypride are non-selective for D2 over D3. Here we used an independent method, that does not rely on these assumptions, to test directly the affinity of [11C]PHNO for D3 and D2 receptors.
Methods: Two rhesus monkeys were scanned four times each on a Focus 220 PET scanner. 54±26 MBq of [11C]PHNO was injected using a bolus-plus-infusion protocol (Kbol = 50 mins) over 120 mins. Infusion rates varied from 0.032 to 2.1 μg/kg/h. Images were reconstructed with FORE/FBP. The concentration of non-displaceable [11C]PHNO was assumed to be uniform in the brain at equilibrium (t⩾90 mins), and was estimated in the cerebellum. Scatchard plots were drawn for selected regions of interest and analyzed using 1- (1S) and 2-site (2S) models, from which Kd/fND and Bmax parameters were estimated. For the 2S model, the two Kd/fND values were assumed to be identical across brain regions.
Results: Using a 1S model, Kd/fND estimates were highest in the caudate (CN, 14 and 5 nmol/L for subjects 1 and 2, respectively) and putamen (PU, 9 and 7 nmol/L for subjects 1 and 2, respectively), intermediate in the pallidum (GP, 3 nmol/L for both subjects) and lowest in the substantia nigra (SN, 0.3 and 0.5 nmol/L for subjects 1 and 2, respectively). Using a 2S model, the D3 Kd/fND for subjects 1 and 2 was 0.2 and 0.8 nmol/L, respectively, whereas the D2 Kd/fND was 13 and 6 nmol/L, respectively. The D3 BPND comprised ∼65% of the total BPND in GP and >95% in SN. The D2 BPND comprised >80% of the total BPND in the CN and PU.
Conclusions: This study provides the first direct evaluation of PHNO in vivo affinity. Our findings demonstrate a ∼20-fold selectivity of [11C]PHNO for D3 over D2 receptors, confirming our previous findings established using SB-277011 with [11C]PHNO, [11C]raclopride and [11C]fallypride.1,2 The D3 BPND proportion of the total BPND observed in the present study is also consistent with the previous data acquired using SB-277011 (i.e., 95% in SN, 72% in GP, 24% in CN and 10% in PU). Understanding PHNO affinity at the D3 and D2 sites allows the estimation of regional D3 receptor distribution in vivo.
579. Effect of risperidone on high-affinity state of dopamine D2 receptor; a pet study with [C-11]MNPA
F. Kodaka1,2, H. Ito1, H. Takano1, H. Takahashi1, R. Arakawa1, M. Miyoshi1, M. Okumura1, T. Otsuka1, K. Nakayama2 and T. Suhara1
1Department of Molecular Neuroimaging, Clinical Neuroimaging Section, National Institute of Radiological Sciences, Chiba; 2Department of Psychiatry, Jikei University School of Medicine, Tokyo, Japan
Objectives: Early in vitro studies have revealed that dopamine D2 receptors (D2 receptors) have two interconvertible affinity states for endogenous dopamine, referred to as high- and low-affinity state.1 [C-11]-(R)-2-CH3O-N-n-propylnorapomorphine ([C-11]MNPA), a newly developed D2 receptor agonist ligand, is more sensitive to displacement by endogenous dopamine than [C-11]raclopride in the primate brain.2 For this reason, [C-11]MNPA represents a promising radioligand for positron emission tomography (PET) imaging of the high-affinity state of the dopamine D2 receptor. Risperidone, which is well known as a serotonin-dopamine antagonist and a commonly prescribed antipsychotic to alleviate positive symptoms of schizophrenia, acts as an antagonist against dopamine D2 receptors. While the occupancy of dopamine D2 receptors by risperidone has been measured by PET with conventional D2 receptor antagonist ligands such as [C-11]raclopride and [C-11]FLB457, little is known about its pharmacological behavior against the high-affinity state of D2 receptor because those conventional D2 receptor antagonist ligands are sensitive to both the high- and low-affinity state of D2 receptor. Here, we measured D2 receptor occupancy of [C-11]MNPA and [C-11]raclopride after oral administration of risperidone to evaluate its pharmacological action against the high-affinity state of D2 receptor.
Methods: PET studies were performed on eleven healthy men (21 to 39 years) under resting condition and oral administration of a single dose of risperidone (0.5 to 2.0 mg) on separate days. In each condition, PET scans using [C-11]raclopride and [C-11]MNPA were performed sequentially. For each PET study, the binding potentials (BPs) in the striatum were calculated by reference tissue model method with use of the cerebellum as reference region. The occupancy of dopamine D2 receptors was then calculated from the BP values of resting and drug challenge conditions. Relations between dopamine D2 receptor occupancy and the administered dose of risperidone were analyzed for each radioligand.
Results: The occupancies of dopamine D2 receptors in [C-11]raclopride and [C-11]MNPA studies ranged from 24% to 70% and from 22% to 66% in the striatum, respectively. The occupancy of [C-11]raclopride was positively correlated with that of [C-11]MNPA (r = 0.72, P = 0.012). The relation between dopamine D2 receptor occupancy and the dose of risperidone (dose-occupancy curve) with [C-11]raclopride and [C-11]MNPA was positive and logarithmically fitted (r = 0.85 for [C-11]raclopride, r = 0.69 for [C-11]MNPA). ED50 values calculated from the dose-occupancy curves with [C-11]raclopride and [C-11]MNPA were 0.98 and 1.03 mg, respectively.
Conclusions: The positive correlation of occupancies between both [C-11]raclopride and [C-11]MNPA studies and similar ED50 values of both studies indicate that risperidone blocks both high- and low-affinity state of dopamine D2 receptors in a similar dose-dependent manner.
768. Dose occupancy relationship for a dopamine D1 receptor agonist using [11C] NNC 112 in non-human primates
M. Slifstein1,2, R. Nardi3, J. Javitch1,4, T. Cooper5, J. Lieberman1,4 and A. Abi-Dargham1,2
1Psychiatry, Columbia University; 2Division of Translational Imaging, New York State Psychiatric Institute, New York, New York, USA; 3Sentia Therapeutics Inc., NA; 4Psychiatry, New York State Psychiatric Institute, New York, USA; 5Analytical Psychopharmacology, Nathan Kline Institute, Orangeburg, New York, USA
Objectives: DAR100A is the active enantiomer of DAR100 and is a potent D1 agonist under development to treat cognitive impairment and negative symptoms in schizophrenia. Our aim here was to demonstrate a dose-occupancy relationship for DAR100A using PET imaging in baboons with [11C]NNC112. Knowledge of the dose occupancy relationship would inform the choice of doses to be investigated in humans.
Methods: 2 adult male baboons were scanned at baseline and following 5 doses of DAR100A (total scans: 10 = 3*2+2*2) on the HR+ scanner. Scans were 90 mins. DAR100A was given as a constant infusion starting 10 mins prior to scanning and lasting through the entire scan. Arterial blood samples were taken to measure concentration of DAR100A in plasma. PET data were analyzed by SRTM with cerebellum as reference region. Plasma data were analyzed by HPLC. Brain regions were cortical (frontal, parietal, temporal and occipital) and striatum. Occupancy was computed as the fractional decrease in BPND (ΔBP) following DAR100A.
Results: A clear relationship was observed between plasma concentration of DAR100A (both the integral and initial value at the start of the scan) and ΔBP. Average occupancy across cortical regions was lower than in striatum at each plasma level of DAR100A (Figure 1, left). Fits to a concentration-occupancy relationship showed maximal occupancy of 53% in striatum and 22% in cortex (Figure 1, right) when maximal occupancy and EC50 were treated as free parameters, or 51% and 26% when EC50 was constrained to the same value in both regions (constrained EC50 of DAR100A concentration at start of scan = 85 ng/mL), consistent with literature showing approximately 25% of the specific binding signal of [11C]NNC112 in baboon cortex is due to 5HT2A binding,1 whereas striatal binding is due entirely to D1-like receptors.
D1 Occupancy by DAR100A.
Conclusions: DAR100A reduces D1 binding of [11C]NNC112 in a dose dependent manner. Cortical ΔBP was lower than striatal ΔBP consistent with the known D1:5HT2A binding profile of [11C]NNC112. Estimated maximal occupancy was 53%, but this pharmacological ceiling effect will need confirmation with additional studies at higher doses than those described here. Previous failure to show vulnerability of dopamine D1 PET tracers to endogenous dopamine release has suggested that these tracers may not be sensitive to agonist competition. This is the first demonstration of significant occupancy by a full D1 agonist in vivo.
180. Norepinephrine transporter occupancy by antidepressant in human brain measured using positron emission tomography with (S,S)-[18F]FMENER-D2
M. Sekine1,2, I. Hiroshi1, R. Arakawa1,2, M. Okumura1,2, T. Sasaki1, H. Takahashi1, H. Takano1, Y. Okubo2 and T. Suhara1
1Molecular Neuroimaging Group, Molecular Imaging Center, National Institute of Radiological Sciences, Chiba; 2Department of Neuropsychiatry, Nippon Medical School, Tokyo, Japan
Objectives: Norepinephrine transporter (NET) is one of the main targets of antidepressants. Although serotonin transporter occupancy was reported to be over 80% at clinical doses of serotonin reuptake inhibitors (SSRIs) during the treatment of depression,1 NET occupancy by antidepressants in human brain has not been reported because of a lack of suitable radioligands for NET. (S,S)-[18F]FMeNER-D2 was recently developed as a radioligand for the measurement of NET binding with positron emission tomography (PET).2,3 In this study, we investigated the degree of NET occupancy by different doses of an antidepressant, nortriptyline, using PET and (S,S)-[18F]FMeNER-D2.
Methods: Six healthy males (age 22 to 39 yrs; mean±s.d., 30.5±6.3 yrs) participated in this study. PET scans with (S,S)-[18F]FMeNER-D2 were performed before and after oral administration of a single dose of nortriptyline on separate days. The nortriptyline doses were 10, 25 and 75 mg in two subjects each. After a bolus i.v. injection of 188.9±4.2 MBq of (S,S)-[18F]FMeNER-D2, scanning was performed for 0 to 90 mins (1 min × 10, 2 mins × 15, 5 mins × 10), followed by scanning for 120 to 180 mins (10 mins × 6). Regions of interests were drawn on the thalamus and caudate in each PET image. The thalamus was used as target region, and the caudate as reference region. Areas under the curve (AUCs) of radioactivity in the target and reference regions were calculated for 120 to 180 mins. The ratio of the thalamus-to-caudate AUCs minus 1 was used as the binding potential (BPND) for NET. NET occupancy was defined as the percentage reduction of BPND. Venous blood samples were obtained to measure the concentrations of nortriptyline just before injection of the tracer and at 180 mins after. This study was approved by the Ethics and Radiation Safety Committee of the National Institute of Radiological Sciences, Chiba, Japan. After complete description of this study, written informed consent was obtained from all subjects.
Results: The mean NET occupancies by nortriptyline doses were 16.4% at 10 mg, 33.2% at 25 mg, and 41.1% at 75 mg, respectively. The mean plasma concentration of nortriptyline was 0 ng/mL at 10 mg, 23.7 ng/mL at 25 mg, and 50.5 ng/mL at 75 mg. Estimated ED50 (50% effective dose) was 76.8 mg of administration dose and 59.8 ng/mL of plasma concentration.
Conclusions: NET occupancy by nortriptyline corresponding to the administration dose and plasma concentration was observed. To determine the optimal administration dose of nortriptyline for patients with depression, the relation between therapeutic effects and NET occupancy needs to be investigated in further studies.
