Mohammad Sib Ansari

Midbrain Dopamine Receptor Availability Is Inversely Associated with Novelty-Seeking Traits in Humans

Novelty-seeking personality traits are a major risk factor for the development of drug abuse and other unsafe behaviors. Rodent models of temperament indicate that high novelty responding is associated with decreased inhibitory autoreceptor control of midbrain dopamine neurons. It has been speculated that individual differences in dopamine functioning also underlie the personality trait of novelty seeking in humans. However, differences in the dopamine system of rodents and humans, as well as the methods for assessing novelty responding/seeking across species leave unclear to what extent the animal models inform our understanding of human personality. In the present study we examined the correlation between novelty-seeking traits in humans and D2-like (D2/D3) receptor availability in the substantia nigra/ventral tegmental area. Based on the rodent literature we predicted that novelty seeking would be characterized by lowered levels of D2-like (auto)receptor availability in the midbrain. Thirty-four healthy adults (18 men, 16 women) completed the Tridimensional Personality Questionnaire-Novelty-Seeking Scale and PET scanning with the D2/D3 ligand [18F]fallypride. Novelty-Seeking personality traits were inversely associated with D2-like receptor availability in the ventral midbrain, an effect that remained significant after controlling for age. We speculate that the lower midbrain (auto)receptor availability seen in high novelty seekers leads to accentuated dopaminergic responses to novelty and other conditions that induce dopamine release.

Amphetamine-Induced Displacement of ( 18 F) Fallypride in Striatum and Extrastriatal Regions in Humans

This study examined D-amphetamine (D-AMPH)-induced displacements of [18F] fallypride in striatal and extrastriatal regions and the correlations of these displacements with cognition, affect, and sensation-seeking behavior. In all, 14 normal subjects, six females and eight males (ages 21–32, mean age 25.9 years), underwent positron emission tomography (PET) with [18F]fallypride before and 3 h after a 0.43 mg/kg oral dose of D-AMPH. Levels of dopamine (DA) D2 receptor density were calculated with the reference region method of Lammerstma. Percent displacements in striatal and extrastriatal regions were calculated for the caudate, putamen, ventral striatum, medial thalamus, amygdala, substantia nigra, and temporal cortex. Correlations of changes in cognition, affect, and sensation seeking with parametric images of D-AMPH-induced DA release were computed. Significant displacements were seen in the caudate, putamen, ventral striatum substantia nigra, and temporal cortex with a trend level change in the amygdala. Greatest displacements were seen in striatal subdivisions—5.6% in caudate, 11.2% in putamen, 7.2% in ventral striatum, and 6.6% in substantia nigra. Lesser decrements were seen in amygdala—4.4%, temporal cortex—3.7%, and thalamus—2.8%. Significant clusters of correlations of regional DA release with cognition and sensation-seeking behavior were observed. The current study demonstrates that [18F]fallypride PET studies using oral D-AMPH (0.43 mg/kg) can be used to study D-AMPH-induced DA release in the striatal and extrastriatal regions in humans, and their relationship with cognition and sensation-seeking behavior.

Relationship of Dopamine Type 2 Receptor Binding Potential With Fasting Neuroendocrine Hormones and Insulin Sensitivity in Human Obesity

OBJECTIVE Midbrain dopamine (DA) neurons, which are involved with reward and motivation, are modulated by hormones that regulate food intake (insulin, leptin, and acyl ghrelin [AG]). We hypothesized that these hormones are associated with deficits in DA signaling in obesity. RESEARCH DESIGN AND METHODS We assessed the relationships between fasting levels of insulin and leptin, and AG, BMI, and insulin sensitivity index (SI) with the availability of central DA type 2 receptor (D2R). We measured D2R availability using positron emission tomography and [18F]fallypride (radioligand that competes with endogenous DA) in lean (n = 8) and obese (n = 14) females. Fasting hormones were collected prior to scanning and SI was determined by modified oral glucose tolerance test. RESULTS Parametric image analyses revealed associations between each metabolic measure and D2R. The most extensive findings were negative associations of AG with clusters involving the striatum and inferior temporal cortices. Regional regression analyses also found extensive negative relationships between AG and D2R in the caudate, putamen, ventral striatum (VS), amygdala, and temporal lobes. SI was negatively associated with D2R in the VS, while insulin was not. In the caudate, BMI and leptin were positively associated with D2R availability. The direction of associations of leptin and AG with D2R availability are consistent with their opposite effects on DA levels (decreasing and increasing, respectively). After adjusting for BMI, AG maintained a significant relationship in the VS. We hypothesize that the increased D2R availability in obese subjects reflects relatively reduced DA levels competing with the radioligand. CONCLUSIONS Our findings provide evidence for an association between the neuroendocrine hormones and DA brain signaling in obese females.

Occupancy of striatal and extrastriatal dopamine D2/D3 receptors by olanzapine and haloperidol.

There have been conflicting reports as to whether olanzapine produces lower occupancy of striatal dopamine D2/D3 receptor than typical antipsychotic drugs and preferential occupancy of extrastriatal dopamine D2/D3 receptors. We performed [18F] fallypride PET studies in six schizophrenic subjects treated with olanzapine and six schizophrenic subjects treated with haloperidol to examine the occupancy of striatal and extrastriatal dopamine receptors by these antipsychotic drugs. [18F] setoperone PET studies were performed in seven olanzapine-treated subjects to determine 5-HT2A receptor occupancy. Occupancy of dopamine D2/D3 receptors by olanzapine was not significantly different from that seen with haloperidol in the putamen, ventral striatum, medial thalamus, amygdala, or temporal cortex, that is, 67.5–78.2% occupancy; olanzapine produced no preferential occupancy of dopamine D2/D3 receptors in the ventral striatum, medial thalamus, amygdala, or temporal cortex. There was, however, significantly lower occupancy of substantia nigra/VTA dopamine D2/D3 receptors in olanzapine-treated compared to haloperidol-treated subjects, that is, 40.2 vs 59.3% (p=0.0014, corrected for multiple comparisons); in olanzapine-treated subjects, the substantia nigra/VTA was the only region with significantly lower dopamine D2/D3 receptor occupancy than the putamen, that is, 40.2 vs 69.2% (p<0.001, corrected for multiple comparison). Occupancy of 5-HT2A receptors was 85–93% in the olanzapine- treated subjects. The results of this study demonstrated that olanzapine does not produce preferential occupancy of extrastriatal dopamine D2/D3 receptors but does spare substantia nigra/VTA receptors. Sparing of substantia nigra/VTA dopamine D2/D3 receptor occupancy may contribute to the low incidence of extrapyramidal side effects in olanzapine-treated patients.

Estimation of baseline dopamine D2 receptor occupancy in striatum and extrastriatal regions in humans with positron emission tomography with [18F] fallypride.

BACKGROUND This study examined whether positron emission tomography (PET) studies with [18F] fallypride performed before and after alpha-methyl-para-tyrosine (AMPT) administration can be used to estimate baseline dopamine (DA) D2 receptor occupancy in striatal and extrastriatal regions. METHODS Six normal subjects underwent PET with [18 F] fallypride before and after administration of AMPT. The DA D2 receptor binding potentials (bp) were calculated with the reference region method. Percent changes in bp in striatal and extrastriatal regions were calculated with both region-of-interest analysis and on a voxel by voxel basis with parametric images of DA D2 receptor levels. RESULTS The results of the current study indicate that AMPT treatment significantly increased the bp in the caudate, putamen, ventral striatum, and substantia nigra. A trend level increase was seen in the medial thalamus. CONCLUSIONS This study demonstrates that PET with [18F] fallypride can be used to estimate baseline DA D2 receptor occupancy in striatal and extrastriatal regions.

Fluorinated Cyclooxygenase-2 Inhibitors as Agents in PET Imaging of Inflammation and Cancer

COX-2 is a major contributor to the inflammatory response and cancer progression so it is an important target for prevention and therapy. COX-2 is absent or expressed at low levels in most epithelial cells but is found at high levels in inflammatory lesions, and many premalignant and malignant tumors. Thus, it is an attractive target for molecular imaging. We report a series of novel fluorinated imaging agents, derived from indomethacin or celecoxib that selectively inhibit COX-2. The most promising lead, compound 7, was a fluorinated derivative of celecoxib. Kinetic analysis revealed that this fluorinated compound is a slow, tight-binding inhibitor of COX-2 and exhibits minimal inhibitory activity against COX-1. Efficient incorporation of 18F into compound 7 by radiochemical synthesis and intravenous injection provided sufficient signal for in vivo positron emission tomography (PET) imaging. Selective uptake of 18F-7 was observed in inflamed rat paws compared with the noninflamed contralateral paws and uptake was blocked by pretreatment with the COX-2 inhibitor, celecoxib. Uptake of 18F-7 was not observed when inflammation was induced in COX-2–null mice. In nude mice bearing both a COX-2–expressing human tumor xenograft (1483) and a COX-2–negative xenograft (HCT116), 18F-7 selectively accumulated in the COX-2–expressing tumor. Accumulation was blocked by pretreatment of the animals with celecoxib. The in vitro and in vivo properties of compound 7 suggest it will be a useful probe for early detection of cancer and for evaluation of the COX-2 status of premalignant and malignant tumors. Cancer Prev Res; 4(10); 1536–45. ©2011 AACR.

Quantitative, Preclinical PET of Translocator Protein Expression in Glioma Using 18F-N-Fluoroacetyl-N-(2,5-Dimethoxybenzyl)-2-Phenoxyaniline

Translocator protein (TSPO), also referred to as peripheral benzodiazepine receptor (PBR), is a crucial 18-kDa outer mitochondrial membrane protein involved in numerous cellular functions, including the regulation of cholesterol metabolism, steroidogenesis, and apoptosis. Elevated expression of TSPO in oncology correlates with disease progression and poor survival, suggesting that molecular probes capable of assaying TSPO levels may have potential as cancer imaging biomarkers. In preclinical PET studies, we characterized a high-affinity aryloxyanilide-based TSPO imaging ligand, 18F-N-fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline (18F-PBR06), as a candidate probe for the quantitative assessment of TSPO expression in glioma. Methods: Glioma-bearing rats were imaged with 18F-PBR06 in a small-animal PET system. Dynamic images were acquired simultaneously on injection of 18F-PBR06 (70–100 MBq/0.2 mL). Over the course of scanning, arterial blood was collected to derive the input function, with high-performance liquid chromatography radiometabolite analysis performed on selected samples for arterial input function correction. Compartmental modeling of the PET data was performed using the corrected arterial input function. Specific tumor cell binding of PBR06 was evaluated by radioligand displacement of 3H-PK 11195 with PBR06 in vitro and by displacement of 18F-PBR06 with excess PBR06 in vivo. Immediately after imaging, tumor tissue and adjacent healthy brain were harvested for assay of TSPO protein levels by Western blotting and immunohistochemistry. Results: 18F-PBR06 was found to preferentially accumulate in tumors, with modest uptake in the contralateral brain, facilitating excellent contrast between tumor and adjacent tissue. Infusion with PBR06 (10 mg/kg) displaced 18F-PBR06 binding by approximately 75%. The accumulation of 18F-PBR06 in tumor tissues and adjacent brain agreed with the ex vivo assay of TSPO protein levels by Western blotting and quantitative immunohistochemistry. Conclusion: These preclinical studies illustrate that 18F-PBR06 is a promising tracer for visualization of TSPO-expressing tumors. Importantly, the close correlation between 18F-PBR06 uptake and TSPO expression in tumors and normal tissues, coupled with the high degree of displaceable binding from both tumors and the normal brain, represents a significant improvement over other TSPO imaging ligands previously evaluated in glioma. These data suggest the potential of 18F-PBR06 to elucidate the role of TSPO in oncology, as well as its potential development as a cancer imaging biomarker.

Changes in dopamine release and dopamine D2/3 receptor levels with the development of mild obesity

Dopamine (DA) neurotransmission is an important modulator of both homeostatic food consumption mediated by the hypothalamus, and reward-related/ hedonic food consumption mediated by the brain reward circuit (Lutter and Nestler, 2009; Schwartz et al., 2000; Wise, 2008). Although there are monogenetic mutations that affect the regulation of body weight, the majority of cases of human obesity appear to result from the interaction of polygenetic predispositions to obesity with an environment in which highly palatable foods are readily available leading to excessive consumption of such foods (Walley et al. 2009). There have been two major hypotheses regarding the role of DA neurotransmission in the development of obesity. The first postulates a rewarddeficient state leading to compensatory overeating to remedy an anhedonic state; decreased brain reward circuit DA neurotransmission is hypothesized to be a significant factor in this reward-deficient state (Blum et al., 2000; Geiger et al., 2008). The second hypothesis postulates that increased behavioral salience of food produces excessive intake of highly palatable foods leading to obesity; this increased behavioral salience is believed to be mediated by increased ventral striatal DA release (Berridge et al., 2009). Although studies of obese animals and obesity prone animals have consistently shown decreased striatal DA release and DA D2/3 receptor levels (DA D2r), to date there have been no published data in humans on changes in DA release in the evolution of obesity and varying results on changes in DA D2r with the development of obesity (Eisenstein et al., 2013; Geiger et al., 2008; Haltia et al., 2007; Johnson and Kenny, 2010; Wang et al., 2001). The purpose of this study was to evaluate the changes in DA release and DA D2r with the development of mild obesity. Baseline [F]fallypride PET studies were performed in 33 healthy subjects (M5 18, F515, mean age5 25.8, range 18–35; mean BMI5 24.8, range 19–35) as previously described to determine regional DA D2r (Kessler, 2009). To evaluate regional DA release, 16 of the 33 subjects having the baseline [F]fallypride positron emission tomography (PET) study (8M, 8F; mean age5 24.3, age range5 21–32; mean BMI5 25.2, body mass index (BMI) range of 19–35) had a second [F]fallypride PET study 3 h following oral administration of 0.43 mg/kg d-amphetamine.(Riccardi et al., 2006). Regions believed to be involved in the development of obesity including ventral striatum, ventral midbrain/substantia nigra, amygdala, caudate, and putamen were specifically examined. High-resolution T1-weighted MRI scans and PET scans of the brain were performed and coregistered to each other. Regions of interest were delineated on the coregistered MRI, and regional estimates of DA D2r (BPND’s) and amphetamine-induced DA release were calculated as previously described (Kessler, 2009; Riccardi et al., 2006). Correlations of regional DA D2r and DA release with BMI were performed using a Pearson product moment correlation covaried for age. Correlations of DA D2r with BMI covaried for age demonstrated small negative correlations in the regions examined (Table 1). Correlations in the right

Multimodal image coregistration and inducible selective cell ablation to evaluate imaging ligands

We combined multimodal imaging (bioluminescence, X-ray computed tomography, and PET), tomographic reconstruction of bioluminescent sources, and two unique, complementary models to evaluate three previously synthesized PET radiotracers thought to target pancreatic beta cells. The three radiotracers {[18F]fluoropropyl-(+)-dihydrotetrabenazine ([18F]FP-DTBZ), [18F](+)-2-oxiranyl-3-isobutyl-9-(3-fluoropropoxy)-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinoline (18F-AV-266), and (2S,3R,11bR)-9-(3-fluoropropoxy)-2-(hydroxymethyl)-3-isobutyl-10-methoxy-2,3,4,6,7,11b-hexahydro-1H-pyrido[2,1-a]isoquinolin-2-ol (18F-AV-300)} bind vesicular monoamine transporter 2. Tomographic reconstruction of the bioluminescent signal in mice expressing luciferase only in pancreatic beta cells was used to delineate the pancreas and was coregistered with PET and X-ray computed tomography images. This strategy enabled unambiguous identification of the pancreas on PET images, permitting accurate quantification of the pancreatic PET signal. We show here that, after conditional, specific, and rapid mouse beta-cell ablation, beta-cell loss was detected by bioluminescence imaging but not by PET imaging, given that the pancreatic signal provided by three PET radiotracers was not altered. To determine whether these ligands bound human beta cells in vivo, we imaged mice transplanted with luciferase-expressing human islets. The human islets were imaged by bioluminescence but not with the PET ligands, indicating that these vesicular monoamine transporter 2-directed ligands did not specifically bind beta cells. These data demonstrate the utility of coregistered multimodal imaging as a platform for evaluation and validation of candidate ligands for imaging islets.

Relationship between in vivo receptor occupancy and efficacy of metabotropic glutamate receptor subtype 5 allosteric modulators with different in vitro binding profiles.

Allosteric modulators of the metabotropic glutamate receptor subtype 5 (mGlu5) have exciting potential as therapeutic agents for multiple brain disorders. Translational studies with mGlu5 modulators have relied on mGlu5 allosteric site positron emission tomography (PET) radioligands to assess receptor occupancy in the brain. However, recent structural and modeling studies suggest that closely related mGlu5 allosteric modulators can bind to overlapping but not identical sites, which could complicate interpretation of in vivo occupancy data, even when PET ligands and drug leads are developed from the same chemical scaffold. We now report that systemic administration of the novel mGlu5 positive allosteric modulator VU0092273 displaced the structurally related mGlu5 PET ligand, [18F]FPEB, with measures of in vivo occupancy that closely aligned with its in vivo efficacy. In contrast, a close analog of VU0092273 and [18F]FPEB, VU0360172, provided robust efficacy in rodent models in the absence of detectable occupancy. Furthermore, a structurally unrelated mGlu5 negative allosteric modulator, VU0409106, displayed measures of in vivo occupancy that correlated well with behavioral effects, despite the fact that VU0409106 is structurally unrelated to [18F]FPEB. Interestingly, all three compounds inhibit radioligand binding to the prototypical MPEP/FPEB allosteric site in vitro. However, VU0092273 and VU0409106 bind to this site in a fully competitive manner, whereas the interaction of VU0360172 is noncompetitive. Thus, while close structural similarity between PET ligands and drug leads does not circumvent issues associated with differential binding to a given target, detailed molecular pharmacology analysis accurately predicts utility of ligand pairs for in vivo occupancy studies.