Evaluation of [¹8F]MK-0911, a positron emission tomography (PET) tracer for opioid receptor-like 1 (ORL1), in rhesus monkey and human.

Antagonism of the central opioid receptor like-1 receptor (ORL1) has been implicated in cognition, and has been a focus of drug discovery efforts to ameliorate the cognitive deficits that remain during the stable treatment of schizophrenia with current antipsychotics. In order to facilitate dose selection for phase II clinical testing an ORL1-specific PET tracer was developed to determine drug plasma concentration versus occupancy relationships in order to ensure that the doses selected and the degree of target engagement were sufficient to ensure adequate proof of concept testing. MK-0911 is a selective, high affinity antagonist for the ORL1 receptor radiolabeled with high specific activity (18)F for positron emission tomography (PET) studies. Evaluation of [(18)F]MK-0911 in rhesus monkey PET studies showed a pattern of brain uptake which was consistent with the known distribution of ORL1. In vitro autoradiography with [(18)F]MK-0911 in rhesus monkey and human brain tissue slices showed a regional distribution that was consistent with in vivo imaging results in monkey. Pre-treatment of rhesus monkeys with high doses of structurally diverse ORL1 antagonists MK-0584, MK-0337, or MK-5757 achieved blockade of [(18)F]MK-0911 in all gray matter regions. Baseline PET studies with [(18)F]MK-0911 in healthy human subjects showed tracer distribution and kinetics similar to that observed in rhesus monkey. Quantification of [(18)F]MK-0911 uptake in repeat human baseline PET studies showed a test-retest variability in volume of distribution (V(T)) averaging 3% across brain regions. Humans dosed orally with MK-5757 showed reduced [(18)F]MK-0911 tracer concentration in brain proportional with MK-5757 dose and plasma level. [(18)F]MK-0911 was useful for determining MK-5757-induced receptor occupancy of ORL1 to guide MK-5757 dose-selection for clinical proof-of-concept studies. Additionally, [(18)F]MK-0911 may be a useful tool for studying the pharmacology of ORL1 in various human populations and disease states.

Biodistribution and radiation dosimetry of [18F]F-PEB in nonhuman primates.

INTRODUCTION: The metabotropic glutamate receptor subtype 5 (mGluR5) is distributed throughout the central nervous system (CNS), and has been suggested to be a potential target for several CNS disorders suchas Parkinson's disease, pain, anxiety, depression, schizophrenia, and addiction. We report here on the rhesus monkey biodistribution and radiation dosimetry of [18F]3-fluoro-5-[(pyridine-3-yl)ethynyl]benzonitrile, [18F]F-PEB, a mGluR5 positron emission tomography (PET) radiotracer. METHODS: Three male and two female rhesus monkeys were imaged using the Discovery ST PET/computed tomography scanner. A total of 25 whole body PET emissions were acquired over 3 h (23 emissions in one subject). Regions of interest were drawn in the brain, lungs, heart, liver, spleen, bladder, and testes. The absorbed radiation dose was calculated using OLINDA v1. RESULTS: At the end of the imaging session, 45% of the [18F]F-PEB activity had been excreted by the liver and into the gastrointestinal tract and 10% had been excreted into the urinary bladder. When extrapolating to the adult human, the largest absorbed radiation doses were located in the upper large intestine (males: 0.18 mGy/MBq, females: 0.20 mGy/MBq) and small intestine (males: 0.16 mGy/MBq, females: 0.19 mGy/MBq). Effective radiation dose was 0.033 mSv/MBq for males and 0.034 mSv/MBq for females, similar to many other [18F] ligands. CONCLUSION: The effective radiation dose of [18F]F-PEB obtained from rhesus is similar to many other clinically utilized [18F] ligands.

Discovery of N-{(1S,2S)-2-(3-cyanophenyl)- 3-[4-(2-[18F]fluoroethoxy)phenyl]-1-methylpropyl}- 2-methyl-2-[(5-methylpyridin-2-yl)oxy]propanamide, a cannabinoid-1 receptor positron emission tomography tracer suitable for clinical use.

The discovery of a structurally distinct cannabinoid-1 receptor (CB1R) positron emission tomography tracer is described. Starting from an acyclic amide CB1R inverse agonist (1) as the lead compound, an efficient route to introduce 18F to the molecule was developed. Further optimization focused on reducing the lipophilicity and increasing the CB1R affinity. These efforts led to the identification of [18F]-16 that exhibited good brain uptake and an excellent signal-to-noise ratio in rhesus monkeys.

In vitro characterization of [3H]MethoxyPyEP, an mGluR5 selective radioligand.

We have characterized the in vitro properties of 3-[3H]methoxy-5-(pyridin-2-ylethynyl)pyridine ([3H]MethoxyPyEP), an analogue of the mGluR(5) receptor subtype antagonist MPEP [2-methyl-6-(phenylethynyl)-pyridine], in rat tissue preparations using tissue homogenates and autoradiography. Binding of [3H]MethoxyPyEP to rat cortex, hippocampus, thalamus and cerebellum membrane preparations revealed saturable, high affinity binding (3.4 +/- 0.4 nM, n = 4 in rat cortex) to a single population of receptors in all regions studied except for cerebellum. Binding was found to be relatively insensitive to pH and insensitive to DTT. High concentrations of NEM both reduce receptor concentration and binding affinity for the radioligand. In time-course studies at room temperature k(on) and k(off) were determined as 2.9 x 10(7) M(-1) min(-1) and 0.11 min(-1) respectively. The rank order of affinities, as assessed by equilibrium competition studies, of a variety of ligands suggested binding of the radioligand selectively to mGluR5 (MPEP > trans-azetidine-2,4-dicarboxylic acid congruent with (S)-4-carboxyphenylglycine congruent with (+)MK801 congruent with CP-101,606 congruent with clozapine congruent with atropine congruent with ketanserin congruent with yohimbine congruent with benoxathian). Autoradiographic studies with [3H]MethoxyPyEP showed that binding was regioselective, with high density of binding in caudate and hippocampus, intermediate binding in thalamus and very low density in the cerebellum. These data show that [3H]MethoxyPyEP is a high affinity radioligand useful for the in vitro study of mGluR5 receptor distribution and pharmacologic properties in brain.

Potent, brain-penetrant, hydroisoindoline-based human neurokinin-1 receptor antagonists.

3-[(3aR,4R,5S,7aS)-5-{(1R)-1-[3,5-bis(trifluoromethyl)phenyl]ethoxy}-4-(4-fluorophenyl)octahydro-2H-isoindol-2-yl]cyclopent-2-en-1-one (17) is a high affinity, brain-penetrant, hydroisoindoline-based neurokinin-1 (NK(1)) receptor antagonist with a long central duration of action in preclinical species and a minimal drug-drug interaction profile. Positron emission tomography (PET) studies in rhesus showed that this compound provides 90% NK(1) receptor blockade in rhesus brain at a plasma level of 67 nM, which is about 10-fold more potent than aprepitant, an NK(1) antagonist marketed for the prevention of chemotherapy-induced and postoperative nausea and vomiting (CINV and PONV). The synthesis of this enantiomerically pure compound containing five stereocenters includes a Diels-Alder condensation, one chiral separation of the cyclohexanol intermediate, an ether formation using a trichloroacetimidate intermediate, and bis-alkylation to form the cyclic amine.

Myocardial protection with the non-selective endothelin receptor antagonist L-753,037 following acute coronary artery occlusion in the dog.

SUMMARY: Efficacy of a new, potent non-selective endothelin antagonist, l-753037, was examined in a model of canine coronary artery occlusion and reperfusion to assess whether blockade of both ETA and ETB receptors would enhance or reduce myocardial ischemic injury. Instrumented dogs were randomized to receive vehicle (n = 9) or l-753037 (0.1 microg/kg/min, n = 9) by intracoronary infusion 30 minutes before a 90-minute LCx coronary artery occlusion and through 4 hours of reperfusion. After 4 hours of reperfusion, plasma ET-1 levels rose significantly in both groups: 24 +/- 3 fmol/ml in vehicle animals (P < 0.01) versus 42 +/- 5 fmol/ml with l-753037 (P < 0.05). Treatment with l-753037 normalized total LCx flow and regional myocardial flow after 4 hours of reperfusion in all regions. LCx flow was reduced 16% from pre-occlusion baseline (P = 0.45) with treatment compared with 35% with vehicle (P < 0.01). Endocardial flow in the risk region returned to baseline values with l-753037 treatment but was reduced approximately 50% in vehicle animals. l-753037 treatment was associated with a 38% reduction in infarct size (24.1 +/- 3.9% AAR with l-753037 treatment versus 38.7 +/- 3.1% with vehicle, P < 0.01). Thus, a non-selective endothelin antagonist provides significant myocardial protection primarily by improving regional myocardial flow distribution following reperfusion and demonstrated no detrimental effects associated with blockade of the ETB receptor.