Christopher J. Bench

A database of [11C]WAY-100635 binding to 5-HT1A receptors in normal male volunteers: Normative data and relationship to methodological, demographic, physiological, and behavioral variables

PET studies of [(11)C]WAY-100635 binding are proving to be a useful tool to evaluate 5-HT(1A) receptor function in vivo in humans. We describe the pattern of [(11)C]WAY-100635 binding in 61 healthy male brains and examine its variability. For all PET scans, binding potential (BP) values for [(11)C]WAY-100635 in different regions were calculated using a simplified reference tissue model, with the cerebellum as reference region. Specifically we describe (1) region of interest and SPM databases of PET [(11)C]WAY-100635 binding, including test-retest variability; (2) the sensitivity of [(11)C]WAY-100635 binding to manipulations of endogenous 5-HT; and (3) correlations between [(11)C]WAY-100635 binding and radiochemical, demographic, physiological, and behavioral variables. The regional distribution of [(11)C]WAY-100635 binding in healthy human brain was similar to that reported in vitro. The test-retest variability was approximately 12% (range 9-16%) and was similar for all methods of regional sampling. The binding of [(11)C]WAY-100635 was insensitive to changes in brain 5-HT induced by tryptophan infusion and depletion. Although BP values varied greatly across subjects (range 2.9-6.8), there were no significant correlations of regional and global BP with common radiochemical, demographic, physiological, and personality variables. Specifically, in contrast with two recent small studies, we found no decline of [(11)C]WAY-100635 binding with age in our large cohort over the age range of 24 to 53 years. Assessment of 5-HT(1A) receptors in vivo using PET and [(11)C]WAY-100635 gives reliable measures of 5-HT(1A) binding. The large between-subject variability observed could not be explained by common methodological, physiological, or behavioral factors and hence the biological basis of this variability remains to be clarified.

Characterisation of the Appearance of Radioactive Metabolites in Monkey and Human Plasma from the 5-HT1A Receptor Radioligand, [carbonyl-11C]WAY-100635—Explanation of High Signal Contrast in PET and an Aid to Biomathematical Modelling

N-(2-(4-(2-Methoxy-phenyl)-1-piperazin-1-yl)ethyl)-N-(2-pyridyl)++ +cyclohexanecarboxamide (WAY-100635), labelled in its amido carbonyl group with 11C (t1/2 = 20.4 min), is a promising radioligand for the study of brain 5-HT1A receptors with positron emission tomography (PET). Thus, in PET experiments in six cynomolgus monkeys and seven healthy male volunteers, [carbonyl-11C]WAY-100635 was taken up avidly by brain. Radioactivity was retained in regions rich in 5-HT1A receptors, such as occipital cortex, temporal cortex and raphe nuclei, but cleared rapidly from cerebellum, a region almost devoid of 5-HT1A receptors. [Carbonyl-11C]WAY-100635 provides about 3- and 10-fold higher signal contrast (receptor-specific to nonspecific binding) than [O-methyl-11C]WAY-100635 in receptor-rich areas of monkey and human brain, respectively. To elucidate the effect of label position on radioligand behaviour and to aid in the future biomathematical interpretation of the kinetics of regional cerebral radioactivity uptake in terms of receptor-binding parameters, HPLC was used to measure [carbonyl-11C]WAY-100635 and its radioactive metabolites in plasma at various times after intravenous injection. Radioactivity cleared rapidly from monkey and human plasma. Parent radioligand represented 19% of the radioactivity in monkey plasma at 47 min and 8% of the radioactivity in human plasma at 40 min. [Carbonyl-11C]desmethyl-WAY-100635 was below detectable limits in monkey plasma and at most a very minor radioactive metabolite in human plasma. [11C]Cyclohexanecarboxylic acid was identified as a significant radioactive metabolite. In human plasma this maximally represented 21% of the radioactivity at 10 min after radioligand injection. All other major radioactive metabolites in monkey and human plasma were even more polar. No-carrier-added [carbonyl-11C]cyclohexanecarboxylic acid was prepared in the laboratory and after intravenous administration into cynomolgus monkey was shown with PET to give only a low uptake of radioactivity into brain tissue. The acid rapidly gave rise to several radioactive metabolites of higher polarity in plasma. The observed lack of any significant metabolism of [carbonyl-11C]WAY-100635 to highly lipophilic or pharmacologically potent radioactive compounds is consistent with its high signal contrast in primate brain.

beta-blocker binding to human 5-HT(1A) receptors in vivo and in vitro: implications for antidepressant therapy.

A novel strategy for improving the treatment of depressive illness is augmentation of antidepressants with a 5-HT11A autoreceptor antagonist. However, trials using the 5-HT11A/β-blocker pindolol are proving inconsistent. We report how positron emission tomography (PET) and in vitro autoradiography can inform trials of antidepressant augmentation. We show that in healthy volunteers, in vivo, pindolol (n = 10) and penbutolol (n = 4), but not tertatolol (n = 4) occupy the human 5-HT1A receptors, at clinical doses. Pindolol, as well as the β-blockers penbutolol and tertatolol, has high affinity for human 5-HT1A receptors in post-mortem brain slices (n = 4). Pindolol shows preference for 5-HT1A autoreceptors versus the post-synaptic receptors both in vitro and in vivo. Our data reveal that pindolol doses used in antidepressant trials so far are suboptimal for significant occupancy at the 5-HT1A autoreceptor. Penbutolol or higher doses of pindolol are candidates for testing as antidepressant augmenting regimes in future clinical trials.

Evidence for endogenous opioid release in the amygdala during positive emotion.

Endogenous opioid release has been linked to relief from aversive emotional memories, thereby promoting a euphoric state and subsequent interactions towards social stimuli resulting in the formation of social preferences. However, this theory remains controversial. Using positron emission tomography and [(11)C]diprenorphine (DPN) in healthy volunteers, we found significantly reduced DPN binding to opioid receptor in the hippocampus during positive mood induction compared to neutral mood. Furthermore, the magnitude of positive mood change correlated negatively with DPN binding in the amygdala bilaterally. Our finding of reduced DPN binding is consistent with increased release of endogenous opioids, providing direct evidence that localised release of endogenous opioids is involved in the regulation of positive emotion in humans.

A Positron Emission Tomography Study of the 5-Ht1A Receptor in Schizophrenia and during Clozapine Treatment

Several post-mortem studies have identified increases of 5-HT1A receptor density in frontal cortical areas in schizophrenic patients, and one has found increases in the cerebellar vermis. Clozapine has moderate affinity at the 5-HT1A receptor, and this may be of therapeutic importance. This positron emission tomography (PET) study attempted to replicate the post-mortem findings in vivo and sought an occupancy effect of clozapine at the 5-HT1A receptor. We recruited healthy controls, and patients with schizophrenia who were divided into those receiving clozapine and those receiving neuroleptics lacking 5-HT1A receptoraffinity. Each volunteer received a PET scan, using the 5-HT1A receptor radioligand [carbonyl-11C]WAY-100635, and a magnetic resonanceimaging scan. The cerebellar vermis was examined by comparing time-activity data between groups. For other brain regions (the raphe and subdivisions of the cerebral cortex), binding potential images weregenerated to reflect receptor density, then analysed using ‘region of interest’ and voxel-by-voxel methods. No significant changes of 5-HT1A receptor density were found in schizophrenic patients compared tocontrols. Two other PET studies, containing drug naïve rather thanmedicated schizophrenic patients, have also reported no increase in 5-HT1A receptor density in the frontal cortex. The results obtained in vivo bring into question the importance of the receptor in thepathophysiology of the illness. Clozapine did not occupy the 5-HT1A receptor at clinical doses. This is consistent with recent related PET results: 5-HT1A agonists do not appear to measurably block the binding of antagonist radiotracers in man at doses that are pharmacologically active but which are limited by tolerability.

Neuroimaging of mood disorders

In the past year there has been a continued accumulation of data showing regional functional and structural (in older patients) abnormalities in depression.The desired links between altered structure, function and neurochemistry remain elusive but a refined dissection of brain-behaviour relationships is emerging and radiotracers in development will allow a more precise description of the neurochemistry of the depressed state.

PET measurement of the influence of corticosteroids on serotonin-1A receptor number

BACKGROUNDnThe hypothalamic-pituitary-adrenal (HPA) axis and serotonergic system interact functionally. The modulatory effect of corticosteroids on 5-HT(1A) receptor number and function has been repeatedly demonstrated in preclinical studies suggesting that raised corticosteroid levels decrease 5-HT(1A) receptor number and function in the hippocampus.nnnMETHODSnWe used positron emission tomography (PET) to quantify the number of 5-HT(1A) receptors in two studies, the first in normal subjects given a single dose of hydrocortisone using a random-order, double-blind, placebo-controlled design and second in patients treated long-term with corticosteroids.nnnRESULTSnWe did not find that exposure to elevated levels of corticosteroids in either the short or long term alters 5-HT(1A) receptor binding in the hippocampus or other brain regions examined.nnnCONCLUSIONSnThis study does not support the hypothesis that corticosteroids exert a major inhibitory regulatory control over the 5-HT(1A) receptor binding in the human brain.

Preclinical Development of a Radioligand for the Study of Central 5-HT1A Receptors with PET — [11C]Way-100635

N-(2-(1-(4-(2-Methoxyphenyl)-1 -piperazinyl)ethyl))-N-(2-pyridyl)cyclohexanecarboxamide (WAY-100635) has properties that favour its development as the first radioligand for studies of 5-HT1A receptors in vivo with positron emission tomography (PET). These include high potency, high selectivity, pure antagonist action and amenability to labelling with the positron-emitter, carbon-11 (t 1/2 = 20.4 min). Here the status of our preclinical development of [11 C] WAY-100635 as a PET radioligand is summarised. On the basis of studies in rats and human volunteers, it is concluded that [11C]WAY-100635 is a promising radioligand for the study of central 5-HT 1A receptors with potential for application to the study of neuropsychiatric disorders and to the human pharmacology of psychoactive drugs.