Donald Burns

Gender and age affect NK1 receptors in the human brain - a positron emission tomography study with [18F]SPA-RQ.

Substance P (SP) is a neurotransmitter and neuromodulator that mediates its effects in the brain predominantly via the neurokinin-1 receptors (NK1Rs). NK1Rs and SP have been shown clinically to be involved in nausea and emesis after chemotherapy (CINV) and have been implicated preclinically in a range of neuropsychiatric disorders but unlike CINV their blockade in these conditions does not have proven clinical value. We investigated whether age and gender affects NK1R binding potential (NK1R-BP; an index of receptor availability) in the living human brain using PET and [18F]SPA-RQ, a highly specific NK1R antagonist. Forty-five healthy volunteers (35 male and 10 female), aged between 19 and 55 years were studied. NK1R-BP was estimated using the simplified reference tissue model with cerebellum as a reference region. A regression analysis indicated that that a loss of NK1R is associated with normal ageing as shown by decreased NK1R-BP (average rate 7% per decade). Statistically significant negative associations between age and NK1R-BP were observed in temporal, parietal and frontal cortex, hippocampus and parahippocampal formation. In addition preliminary data were obtained suggesting possible gender differences in NK1R-BP in the cortex and putamen with females having a lower NK1R-BP. The exact physiological significance of these results remains to be elucidated but conceptually they could be involved in age-related CNS disorders or those with gender differences in prevalence.

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.

11C-MK-8278 PET as a Tool for Pharmacodynamic Brain Occupancy of Histamine 3 Receptor Inverse Agonists

The histamine 3 (H3) receptor is a presynaptic autoreceptor in the central nervous system that regulates the synthesis and release of histamine and modulates the release of other major neurotransmitters. H3 receptor inverse agonists (IAs) may be efficacious in the treatment of various central nervous system disorders, including excessive daytime sleepiness, attention deficit hyperactivity disorder, Alzheimer disease, ethanol addiction, and obesity. Methods: Using PET and a novel high-affinity and selective radioligand 11C-MK-8278, we studied the tracer biodistribution, quantification, and brain H3 receptor occupancy (RO) of MK-0249 and MK-3134, 2 potential IA drugs targeting cerebral H3 receptors, in 6 healthy male subjects (age, 19–40 y). The relationship among H3 IA dose, time on target, and peripheral pharmacokinetics was further investigated in 15 healthy male volunteers (age, 18–40 y) with up to 3 PET scans and 3 subjects per dose level. Results: The mean effective dose for 11C-MK-8278 was 5.4 ± 1.1 μSv/MBq. Human brain kinetics showed rapid high uptake and fast washout. Binding potential values can be assessed using the pons as a reference region, with a test–retest repeatability of 7%. Drug RO data showed low interindividual variability per dose (mean RO SD, 2.1%), and a targeted 90% RO can be reached for both IAs at clinically feasible doses. Conclusion: 11C-MK-8278 is a useful novel PET radioligand for determination of human cerebral H3 receptor binding and allows highly reproducible in vivo brain occupancy of H3-targeting drugs, hereby enabling the evaluation of novel compounds in early development to select doses and schedules.

Brain neurokinin-1 receptor availability in never-medicated patients with major depression - A pilot study.

BACKGROUND Neurotransmitter substance P (SP) and its preferred neurokinin-1 receptor (NK1R) have been implicated in the treatment of affective and addiction disorders. Despite promising preclinical data on antidepressant action, the clinical trials of NK1R antagonists in major depression have been disappointing. There are no direct in vivo imaging studies on NK1R characteristics in patients with a major depressive disorder (MDD). METHODS In this cross-sectional case-control study, we recruited nine never-medicated patients with moderate to severe MDD and nine matched healthy controls. NK1R availability (NK1R binding potential, BPND) was measured with in vivo 3-D positron emission tomography and a specific NK1 receptor tracer [18F]SPA-RQ. Clinical symptoms were assessed with the 17-item Hamilton Rating Scale for Depression (HAM-D17). RESULTS NK1R-BPND did not differ statistically significantly between patients with MDD and healthy controls. HAM-D17 total scores (range 21-32) correlated positively with NK1R-BPND in cortical and limbic areas. HAM-D17 subscale score for anxiety symptoms correlated positively with NK1R-BPND in specific brain areas implicated in fear and anxiety. LIMITATIONS Small sample size. Low variability in the clinical HAM-D subscale ratings may affect the observed correlations. CONCLUSIONS Our preliminary results do not support a different baseline expression of NK1Rs in a representative sample of never-medicated patients with MDD during a current moderate/severe depressive episode. The modulatory effect of NK1Rs on affective symptoms is in line with early positive results on antidepressant action of NK1 antagonists. However, the effect is likely to be too weak for treatment of MDD with NK1R antagonists alone in clinical practice.