B. Gulyás

Fenfluramine‐induced serotonin release decreases [11C]AZ10419369 binding to 5‐HT1B‐receptors in the primate brain

The need for positron emission tomography (PET)‐radioligands that are sensitive to changes in endogenous serotonin (5‐HT) levels in brain is recognized in experimental and clinical psychiatric research. We recently developed the novel PET radioligand [11C]AZ10419369 that is highly selective for the 5‐HT1B receptor. In this PET‐study in three cynomolgus monkeys, we examined the sensitivity of [11C]AZ10419369 to altered endogenous 5‐HT levels. Fenfluramine‐induced 5‐HT release decreased radioligand binding in a dose‐dependent fashion with a regional average of 27% after 1 mg/kg and 50% after 5 mg/kg. This preliminary study supports that [11C]AZ10419369 is sensitive to endogenous 5‐HT levels in vivo and may serve as a tool to examine the pathophysiology and treatment of major psychiatric disorders. Synapse 64:573–577, 2010.

PET studies on the brain uptake and regional distribution of [11C]vinpocetine in human subjects

Gulyás B, Halldin C, Sandell J, Karlsson P, Sóvágó J, Kárpáti E, Kiss B, Vas Á, Cselényi Z, Farde L. PET studies on the brain uptake and regional distribution of [11C]vinpocetine in human subjects. Acta Neurol Scand 2002: 106: 325–332.

Regional increases in [11C]flumazenil binding after epilepsy surgery

Introduction – Animal experiments suggest that epileptic seizures alter the expression of mRNA for neuro‐receptors. PET measurements with [11 C]flumazenil show that patients with partial seizures have a reduced density of benzodiazepine (BZ) receptors in the epileptogenic regions (ER) and some of the target areas for seizure activity, the so called projection areas. Recent data suggest that the degree of BZ receptor reduction in ER is correlated to seizure frequency. We therefore hypothesized that seizure activity can alter the BZ receptor binding, and that some of these changes could normalize when the seizures disappeared. Methods – In 4 patients whose seizures were generated by mesial temporal lobe structures, BZ receptor density was measured with [11 C]flumazenil PET before, and 1 year after the epilepsy surgery and cessation of seizures. By use of a computerized anatomical brain atlas the same regions were analyzed in both PET scans, and the results related to data from 7 healthy controls. Results – Presurgical PET scans showed reductions in BZ receptor density in the epileptogenic region and some of its primary projection areas. Other cortical regions had normal values. Postsurgically, the calculated BZ receptor density normalized (29±17% increase) in several of the affected projection areas, whereas the values in other cortical regions remained unaltered. Conclusion – Regional reductions in BZ receptor density may be dynamic and related to seizures. The present preliminary observations encourage further studies on seizure‐related changes in regional receptor binding in humans.

Correlation between crossed cerebellar diaschisis and clinical neurological scales

Szilágyi G, Vas Á, Kerényi L, Nagy Z, Csiba L, Gulyás B. Correlation between crossed cerebellar diaschisis and clinical neurological scales. 
Acta Neurol Scand: 2012: 125: 373–381. 
© 2011 John Wiley & Sons A/S.

PET examination of [11C]5-methyl-6-nitroquipazine, a radioligand for visualization of the serotonin transporter

Radiohalogenated 5-halo-6-nitroquipazine analogues have been shown to be potential radioligands for visualization of the serotonin transporter (5-HTT) with PET and SPECT. In the present study a methylated analogue, 5-methyl-6-nitroquipazine (MNQP), was radiolabeled with carbon-11 in a two step reaction via a palladium catalyzed cross coupling reaction between N-t-BOC-protected 5-tributylstannyl-6-nitroquipazine and [(11)C]methyl iodide as key step. [(11)C]MNQP was examined in the cynomolgus monkey brain with positron emission tomography (PET) and the appearance of labeled metabolites in monkey plasma was measured with gradient HPLC. Radioactivity increased continuously in all brain regions during the 90 minutes acquisition time. Highest accumulation of radioactivity was observed in the thalamus and brainstem, regions with a known high density of 5-HTT. The calculated ratios between the thalamus and brainstem to the 5-HTT poor cerebellum were 1.5 and 1.3-1.4, respectively, 80 minutes after radioligand injection. Pretreatment with citalopram prior to the PET measurement markedly reduced the binding in the thalamus and the brainstem. At 15 and 30 minutes after injection of [(11)C]MNQP approximately 90% and 60%, respectively, of radioactivity in plasma represented unchanged radioligand. The slow kinetics and moderate ratios recorded however, may limit its use as a PET radioligand for quantitative studies of the serotonin transporter with PET.

The dopamine D1 receptor agonist (S)‐[11C]N‐methyl‐NNC 01‐0259 is not sensitive to changes in dopamine concentration—a positron emission tomography examination in the monkey brain

Dopamine D2 receptor positron emission tomography (PET) radioligands have proven useful for indirect assessment of the endogenous dopamine concentration in the living brain. On the contrary, dopamine D1 receptor antagonist radioligands have shown no sensitivity to changes in the dopamine concentration. A recent approach to enhance the sensitivity of radioligands to the dopamine concentration has been the development of dopamine D2 receptor agonist radioligands. The aim of this study was to evaluate the dopamine sensitivity of a dopamine D1 receptor agonist radioligand. For this purpose, we developed (S)‐[11C]N‐methyl‐NNC 01–0259 ((S)‐[11C]1) and characterized the receptor binding of (S)‐[11C]1 using in vitro receptor binding assays and in vivo PET measurements in monkeys. In vitro, both enantiomers of 1 were partial dopamine D1 receptor agonists, with (S)‐1 having a 10–50 times higher affinity than (R)‐1. PET studies in monkey confirmed the stereoselectivity of [11C]1 in vivo. In monkey, administration of the dopamine D1‐like receptor antagonist (R)‐(+)‐SCH 23390 decreased the striatal binding potential of (S)‐[11C]1 by 97%, but administration of the dopamine concentration enhancer d‐amphetamine did not affect (S)‐[11C]1 binding. We conclude that the agonist (S)‐[11C]1 provides specific binding to dopamine D1‐like receptors, possibly representing binding to the high‐affinity state of the receptors. The partial dopamine D1 receptor agonist radioligand has, however, no enhanced sensitivity to endogenous dopamine concentrations in comparison with antagonist radioligands. Synapse 67:586–595, 2013.

Dopamine D2/3 receptor occupancy of apomorphine in the non-human primate brain does not support the existence of two affinity states in vivo

Introduction: Binding studies in vitro have demonstrated that the dopamine D2 receptor, like other G-protein coupled receptors, may exist in two affinity states for agonists. The high affinity state is thought to represent the functional state of the receptor and it has been suggested that proportions might alter during disease. In vitro studies further indicate that agonists used in the treatment of Parkinsons disease induce measurable D2/3 receptor occupancy at clinically relevant concentrations, but only when occupancy is measured at the high affinity state. Recent development of new radioligands, such as [C]MNPA, has now made it possible to directly study agonist binding in vivo. The aim of the present PET study was to compare the inhibition by the exogenous dopamine agonist apomorphine of agonist and antagonist radioligand binding to D2 receptors in vivo.

A radiometabolite study of the serotonin transporter PET radioligand [11C]MADAM

INTRODUCTION (11)C]MADAM is a radioligand suitable for PET studies of the serotonin transporter (SERT). Metabolite analysis in human and non-human plasma samples using HPLC separation has shown that [(11)C]MADAM was rapidly metabolized. A possible metabolic pathway is the S-oxidation which could lead to SOMADAM and SO2MADAM. In vitro evaluation of these two potential metabolites has shown that SOMADAM exhibited a good affinity for SERT and a good selectivity for SERT over NET and DAT. METHODS Comparative PET imaging studies in non-human primate brain with [(11)C]MADAM and [(11)C]SOMADAM were carried out, and plasma samples were analyzed using reverse phase HPLC. We have explored the metabolism of [(11)C]MADAM in rat brain with a view to understand its possible interference for brain imaging with PET. RESULTS PET imaging studies in non-human primate brain using [(11)C]SOMADAM indicated that this tracer does not bind with high amounts to brain regions known to be rich in SERT. The fraction of [(11)C]SOMADAM in non-human primate plasma was approximately 5% at 4min and 1% at 15min after [(11)C]MADAM injection. HPLC analysis of brain sample after [(11)C]MADAM injection to rats demonstrated that [(11)C]SOMADAM was not detected in the brain. CONCLUSIONS (11)C]SOMADAM is not superior over [(11)C]MADAM as a SERT PET radioligand. Nevertheless, [(11)C]SOMADAM has been identified as a minor labeled metabolite of [(11)C]MADAM measured in monkey plasma. [(11)C]SOMADAM was not detected in rat brain.

Synthesis of 11C-labeled (S,S)-CFMME and (R)-OHDMI and their evaluation as candidate radioligands for imaging norepinephrine transporters with PET

Introduction: (R)-1-(10,11-Dihydro-dibenzo[b,f]azepin-5-yl)-3-methylamino-propan-2-ol ((R)-OHDMI) and (S,S)-1-cyclopentyl-2-(5-fluoro2-methoxy-phenyl)-1-morpholin-2-yl-ethanol ((S,S)-CFMME) were synthesized and found to be potent inhibitors of norepinephrine reuptake (inhibitory constants (K is) were 8.29 and 10.8 nM for (R)-OHDMI and (S,S)-CFMME, respectively, as compared with (S,S)-2-(a-(2methoxyphenoxy)benzyl)morpholine (S,S)-MeNER) which has a Ki of 7.93 nM). Here, we have explored the labeling of these ligands with carbon-11 for evaluation as candidate radioligands for imaging central norepinephrine transporters (NET).