Production of fluorine-18 labeled (3-N-methyl)benperidol for PET investigation of cerebral dopaminergic receptor binding

Photoredox Organocatalysis for the Enantioselective Synthesis of 1,7-Dicarbonyl Compounds

We describe an asymmetric organocatalytic method to synthesize 1,7-dicarbonyl compounds containing a β-stereocenter. The chemistry relies on the formation of γ-keto radicals, generated upon oxidative ring opening of cyclobutanols mastered by an organic photoredox catalyst. These nonstabilized primary radicals are stereoselectively intercepted by an iminium ion intermediate, formed upon activation of aliphatic and aromatic enals by a chiral secondary amine catalyst. This organocatalytic photoredox method served to prepare scaffolds found in natural products and drug molecules.

Radioligands for the dopamine receptor subtypes

The actions of the predominant neurotransmitter in the brain, dopamine, are mediated by the postsynaptic dopamine receptors. The five dopamine receptor subtypes and their regulation have been associated with a large variety of psychiatric diseases. Therefore, positron emission tomography (PET) imaging studies using suitable and selective (18) F-labeled and (11) C-labeled dopamine receptor radioligands could provide valuable knowledge on the impact of receptor density on the pathogenesis and evolvement of neuropsychiatric and neurological diseases. This special issue subchapter provides a summary of the most important (18) F-labeled and (11) C-labeled radioligands for PET imaging of the dopamine receptor subtypes, their radiochemistry, and characteristics from in vitro and in vivo applications, considering not only the already established PET ligands but also the recently published preclinical work.

A comparison of D2 receptor specific binding in obese and normal-weight individuals using PET with (N-[(11)C]methyl)benperidol

Previous PET imaging studies have demonstrated mixed findings regarding dopamine D2/D3 receptor availability in obese relative to nonobese humans. Nonspecific D2/D3 radioligands do not allow for separate estimation of D2 receptor (D2R) and D3 receptor (D3R) subtypes of the D2 receptor family, which may play different roles in behavior and are distributed differently throughout the brain. These radioligands are also displaceable by endogenous dopamine, confounding interpretation of differences in receptor availability with differing levels of dopamine release. The present study used PET imaging with the D2R-selective radioligand (N-[(11)C] methyl)benperidol ([(11)C]NMB), which is nondisplaceable by endogenous dopamine, to estimate D2R specific binding (BPND) and its relationship to body mass index (BMI) and age in 15 normal-weight (mean BMI = 22.6 kg/m(2)) and 15 obese (mean BMI = 40.3 kg/m(2)) men and women. Subjects with illnesses or taking medications that interfere with dopamine signaling were excluded. Striatal D2R BPND was calculated using the Logan graphical method with cerebellum as a reference region. D2R BPND estimates were higher in putamen and caudate relative to nucleus accumbens, but did not differ between normal-weight and obese groups. BMI values did not correlate with D2R BPND . Age was negatively correlated with putamen D2R BPND in both groups. These results suggest that altered D2R specific binding is not involved in the pathogenesis of obesity per se and underscore the need for additional studies evaluating the relationship between D3R, dopamine reuptake, or endogenous dopamine release and human obesity.

Characterization of extrastriatal D2 in vivo specific binding of [¹⁸F](N-methyl)benperidol using PET

PET imaging studies of the role of the dopamine D2 receptor family in movement and neuropsychiatric disorders are limited by the use of radioligands that have near-equal affinities for D2 and D3 receptor subtypes and are susceptible to competition with endogenous dopamine. By contrast, the radioligand [¹⁸F]N-methylbenperidol ([¹⁸F]NMB) has high selectivity and affinity for the D2 receptor subtype (D2R) and is not sensitive to endogenous dopamine. Although [¹⁸F]NMB has high binding levels in striatum, its utility for measuring D2R in extrastriatal regions is unknown. A composite MR-PET image was constructed across 14 healthy adult participants representing average NMB uptake 60 to 120 min after [¹⁸F]NMB injection. Regional peak radioactivity was identified using a peak-finding algorithm. FreeSurfer and manual tracing identified a priori regions of interest (ROI) on each individual's MR image and tissue activity curves were extracted from coregistered PET images. [¹⁸F]NMB binding potentials (BP(ND) s) were calculated using the Logan graphical method with cerebellum as reference region. In eight unique participants, extrastriatal BP(ND) estimates were compared between Logan graphical methods and a three-compartment kinetic tracer model. Radioactivity and BP(ND) levels were highest in striatum, lower in extrastriatal subcortical regions, and lowest in cortical regions relative to cerebellum. Age negatively correlated with striatal BP(ND) s. BP(ND) estimates for extrastriatal ROIs were highly correlated across kinetic and graphical methods. Our findings indicate that PET with [¹⁸F]NMB measures specific binding in extrastriatal regions, making it a viable radioligand to study extrastriatal D2R levels in healthy and diseased states.

Decreased striatal dopamine receptor binding in primary focal dystonia: a D2 or D3 defect?

Dystonia is an involuntary movement disorder characterized by repetitive patterned or sustained muscle contractions causing twisting or abnormal postures. Several lines of evidence suggest that abnormalities of dopaminergic pathways contribute to the pathophysiology of dystonia. In particular, dysfunction of D2-like receptors that mediate function of the indirect pathway in the basal ganglia may play a key role. We have demonstrated with positron emission tomography that patients with primary focal cranial or hand dystonia have reduced putamenal specific binding of [(18)F]spiperone, a nonselective D2-like radioligand with nearly equal affinity for serotonergic 5-HT(2A) sites. We then repeated the study with [(18)F]N-methyl-benperidol (NMB), a more selective D2-like receptor radioligand with minimal affinity for 5-HT(2A). Surprisingly, there was no decrease in NMB binding in the putamen of subjects with dystonia. Our findings excluded reductions of putamenal uptake greater than 20% with 95% confidence intervals. The analysis of the in vitro selectivity of NMB and spiperone demonstrated that NMB was highly selective for D2 receptors relative to D3 receptors (200-fold difference in affinity), whereas spiperone has similar affinity for all three of the D2-like receptor subtypes. These findings when coupled with other literature suggest that a defect in D3, rather than D2, receptor expression may be associated with primary focal dystonia.

Radiosynthesis of (N-[11C]methyl)benperidol for PET investigation of D2 receptor binding

A procedure for the reliable production of the novel PET D2 receptor ligand (N-[11C]methyl)benperidol ([11C]NMB) is described. The method is based on N-methylation of benperidol using [11C]methyl iodide from the PETtrace MeI MicroLab, with product purification via normal-phase HPLC. Batch yields of 2.5±0.9 GBq (68±25 mCi) of [11C]NMB are routinely (n=16) obtained, with a radiochemical purity >98% and an average specific activity of over 40.7 TBq (1100 Ci)/mmol. The overall radiosynthesis, purification, and pre-release quality control testing is accomplished within 50 minutes. The procedure described herein produces a radiopharmaceutical product that is suitable for D2 receptor studies using PET or microPET imaging.

Validation of the reference tissue model for estimation of dopaminergic D2-like receptor binding with [18F](N-methyl)benperidol in humans

Positron emission tomography measurements of dopaminergic D2-like receptors may provide important insights into disorders such as Parkinson's disease, schizophrenia, dystonia and Tourette's syndrome. The positron emission tomography (PET) radioligand [18F](N-methyl)benperidol ([18F]NMB) has high affinity and selectivity for D2-like receptors and is not displaced by endogenous dopamine. The goal of this study is to evaluate the use of a graphical method utilizing a reference tissue region for [18F]-NMB PET analysis by comparisons to an explicit three-compartment tracer kinetic model and graphical method that use arterial blood measurements. We estimated binding potential (BP) in the caudate and putamen using all three methods in 16 humans and found that the three-compartment tracer kinetic method provided the highest BP estimates while the graphical method using a reference region yielded the lowest estimates (P<.0001 by repeated-measures ANOVA). However, the three methods yielded highly correlated BP estimates for the two regions of interest. We conclude that the graphical method using a reference region still provides a useful estimate of BP comparable to methods using arterial blood sampling, especially since the reference region method is less invasive and computationally more straightforward, thereby simplifying these measurements.

Bilateral increase in striatal dopamine D2 receptor density in the 6-hydroxydopamine-lesioned rat: a serial in vivo investigation with small animal PET

Unilateral destruction of the substantia nigra by local application of 6-hydroxydopamine (6-OHDA) serves as an animal model for Parkinson's disease. In this study, the changes in neostriatal dopamine D(2) receptor density were investigated with a small animal positron emission tomograph (PET) before and after 6-OHDA lesion. PET scans were performed in 14 rats after injection of the D(2) receptor radioligand [(18)F] N-methylbenperidol. After the first scan (day 0), nigrostriatal pathways were lesioned by unilateral injections of 6-OHDA. Further PET scans were performed on days 2 and 14 post-lesion. For both striata, B(max) values were determined from saturation binding curves with non-linear regression analysis. In the striatum ipsilateral to the lesion, B(max) initially amounted to 19.3+/-1. 9 fmol/mg (mean+/-SD) and increased to 19.7+/-2.2 and 29.9+/-5.7 fmol/mg on days 2 and 14 post-lesion, respectively. Contralateral B(max) values increased from 19.2+/-2 fmol/mg prior to the lesion to 21.2+/-2.9 and 28.6+/-5.7 fmol/mg on days 2 and 14, respectively. On day 14, the ipsilateral saturation binding curve differed from the ipsilateral pre-lesion curve (P=0.04; F test). When the contralateral pre-lesion saturation binding curve was compared with the contralateral post-lesion curve on day 14, a P value of 0.08 was obtained. This first serial in vivo imaging study of 6-OHDA-lesioned rats showed a time-dependent increase in striatal D(2) receptor density on both sides, the increase being more pronounced ipsilateral to the lesion. This result implies that compensatory mechanisms in the intact hemisphere contribute to regenerative processes following nigrostriatal dopaminergic denervation. Overall, our findings show the feasibility of repetitive in vivo studies of striatal receptor density with a small animal tomograph. Moreover, the applied in vivo saturation binding technique provides a versatile method for the quantification of time-dependent changes in the concentration of receptor binding sites.

Influence of motor activity on striatal dopamine release: A study using iodobenzamide and SPECT

Pharmacologically induced dopamine release can influence the postsynaptic receptor binding of dopaminergic radioligands. This effect has recently been described using in vivo imaging methods and has been attributed to competition of radiotracers with the endogenous ligand. The present study examines the effect of a motor activation task on dopamine release and the consequences of this release on the binding of the selective D(2) receptor ligand (123)I-iodobenzamide (IBZM) to striatal dopamine D(2) receptors. Eight subjects were asked to write a text beginning immediately before IBZM injection and continuing for 30 min thereafter. Eighteen other subjects remained in a supine resting state during this period and served as a control group. All subjects were right handed. We hypothesized that the writing task would lead to an increase of dopamine release. The increased competition of the endogenous ligand with IBZM should lead to a decreased postsynaptic IBZM binding in the experimental group. Images were acquired and reconstructed identically and anatomically normalized to a computerized brain atlas. Regions of interest were drawn covering the striatum and three different reference regions. Ratios of striatal-to-reference-tissue radioactivity accumulation were calculated as semi-quantitative estimates of D(2) receptor binding potential. This decreased bilaterally, although right-sided significantly more than left, regardless of the choice of reference region. These data show that writing with the right hand compared to a supine resting state leads to a decrease of striatal IBZM accumulation. According to our primary hypothesis this reflects dopamine release.

Radiation dosimetry of [18F] (N-methyl)benperidol as determined by whole-body PET imaging of primates

Radiation absorbed doses due to IV administration of [18F](N-methyl) benperidol ([18F]NMB) were estimated by whole-body PET imaging of nonhuman primates. Time-activity curves were obtained for nine compartments (striatum, eyes, heart, lungs, liver, gallbladder, intestines, kidneys, bladder) by using dynamic PET scans of three different baboons given the radiotracer. These time-activity curves were used to calculate the residence times of radioactivity in these tissues. Human absorbed dose estimates were calculated using the updated MIRDOSE 3 S values and assuming the same biodistribution. Based on an average of three studies, the critical organs were the lower large intestine, gallbladder, and liver, receiving doses of 585, 281, and 210 mrad/mCi, respectively. The brain received a dose of 13 mrad/mCi; other organs received doses between 32-77 mrad/mCi. These results indicate that up to 8.5 mCi of [18F]NMB can be safely administered to human subjects for PET studies of D2 receptor binding.

18F-desmethoxyfallypride: a fluorine-18 labeled radiotracer with properties similar to carbon-11 raclopride for PET imaging studies of dopamine D2 receptors

We have developed (S)-N-[(1-allyl-2-pyrrolidinyl)methyl]-5-(3-18F-fluoropropyl)-2- methoxybenzamide (18F-desmethoxyfallypride) as a fluorine-18 radiotracer with properties analogous to that of 11C-raclopride. In vitro experiments in rat brain homogenates showed an association rate constant of 2.16 x 10(8) M(-1)min(-1) and a dissociation rate constant of 0.073 min(-1). High striatal uptake (up to 0.08% injected dose/cc) of 18F-desmethoxyfallypride in rhesus monkeys was observed in PET experiments. The radiotracer cleared from the striata with a dissociation rate of 1.80 x 10(-2) min(-1). Striatum to cerebellum ratios peaked at 3.0 in 30 min after which they decreased steadily. Intravenously administered haloperidol displaced specifically bound 18F-desmethoxyfallypride with a koff of 0.058 min(-1). Synaptic dopamine released by the treatment of the monkeys with d-amphetamine increased the dissociation rate of 18F-desmethoxyfallypride to 0.83 min(-1) thus reducing specifically bound 18F-desmethoxyfallypride by 56% over a period of 42 mins compared to a reduction of only 20% in controls during this time period. The sensitivity of 18F-desmethoxyfallypride towards competition with dopamine should make this radiotracer useful in PET studies to evaluate in vivo pharmacological effects of various agents that alter levels of endogenous dopamine.

Specific, reversible binding of [18F]benperidol to baboon D2 receptors: PET evaluation of an improved 18F-labeled ligand

[18F]Benperidol ([18F]BP), a positron-emitting analogue of the dopaminergic D2 antagonist benperidol, was evaluated as a radiopharmaceutical for use with positron emission tomography (PET). PET imaging of baboons after i.v. injection of [18F]BP indicated that the radiofluorinated ligand rapidly localized in vivo within dopaminergic receptor-rich cerebral tissues, and that selective disposition was retained for over 2 h. Pretreatment of an animal with unlabeled receptor-specific antagonists prior to injection of [18F]BP confirmed that the radioligand bound specifically to central D2 receptors in vivo, and not to S2 or D1 receptors. [18F]BP bound to D2 receptors in a reversible manner; unlabeled eticlopride displaced D2 receptor-bound [18F]BP in vivo. The radioligand was metabolized in the periphery to polar metabolites which are not expected to cross the blood-brain barrier. [18F]BP has advantages over other tracers as a radiopharmaceutical for PET study of central D2 receptor activity, and can be applied for noninvasive evaluation of the interaction of unlabeled drugs with central D2 receptor sites.