Synthesis and In Vitro Evaluation of Novel Nortropane Derivatives as Potential Radiotracers for Muscarinic M2 Receptors

Disturbances of the cerebral cholinergic neurotransmitter system are present in neurodegenerative disorders. SPECT or PET imaging, using radiotracers that selectively target muscarinic receptor subtypes, may be of value for in vivo evaluation of such conditions. 6β-acetoxynortropane, a potent muscarinic M2 receptor agonist, has previously demonstrated nanomolar affinity and high selectivity for this receptor. Based on this compound we synthesized four nortropane derivatives that are potentially suitable for SPECT imaging of the M2 receptor. 6β-acetoxynortropane and the novel derivatives were tested in vitro for affinity to the muscarinic M1−3 receptors. The original 6β-acetoxynortropane displayed high affinity (Ki = 70–90 nM) to M2 receptors and showed good selectivity ratios to the M1 (65-fold ratio) and the M3 (70-fold ratio) receptors. All new derivatives showed reduced affinity to the M2 subtype and loss of subtype selectivity. It is therefore concluded that the newly synthesized derivatives are not suitable for human SPECT imaging of M2 receptors

Novel molecular imaging ligands targeting matrix metalloproteinases 2 and 9 for imaging of unstable atherosclerotic plaques

Molecular imaging of matrix metalloproteinases (MMPs) may allow detection of atherosclerotic lesions vulnerable to rupture. In this study, we develop a novel radiolabelled compound that can target gelatinase MMP subtypes (MMP2/9) with high selectivity and inhibitory potency. Inhibitory potencies of several halogenated analogues of MMP subtype-selective inhibitors (N-benzenesulfonyliminodiacetyl monohydroxamates and N-halophenoxy-benzenesulfonyl iminodiacetyl monohydroxamates) were in the nanomolar range for MMP2/9. The analogue with highest inhibitory potency and selectivity was radiolabelled with [I-123], resulting in moderate radiochemical yield, and high radiochemical purity. Biodistribution studies in mice, revealed stabilization in blood 1 hour after intravenous bolus injection. Intravenous infusion of the radioligand and subsequent autoradiography of excised aortas showed tracer uptake in atheroprone mice. Distribution of the radioligand showed co-localization with MMP2/9 immunohistochemical staining. In conclusion, we have developed a novel selective radiolabeled MMP2/9 inhibitor, suitable for single photon emission computed tomography (SPECT) imaging that effectively targets atherosclerotic lesions in mice

Acute administration of haloperidol does not influence 123I-FP-CIT binding to the dopamine transporter

A recent (123)I-FP-CIT ((123)-I-N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)nortropane) SPECT study on rats suggested that a single 1 mg/kg dose of the antipsychotic haloperidol induces enough dopamine release to compete with (123)I-FP-CIT for binding to the dopamine transporter. Taking into account the far-reaching consequences of this proposition, we were interested in testing whether we could reproduce this finding using storage phosphor imaging. Twenty rats were pretreated with saline or haloperidol (1 mg/kg of body weight) and then injected with (123)I-FP-CIT. Two hours after (123)I-FP-CIT injection, the rats were sacrificed and binding in the striatum, nucleus accumbens, and cerebellum (nonspecific binding) was measured. In contrast to the earlier SPECT finding, acute administration of haloperidol did not induce a significant change in (123)I-FP-CIT binding ratios in the striatum and nucleus accumbens. Changes in synaptic dopamine due to acute haloperidol administration were not detectable with (123)I-FP-CI

The effects of ecstasy (MDMA) on brain serotonin transporters are dependent on age-of-first exposure in recreational users and animals.

RATIONALE AND OBJECTIVE: Little is known on the effects of ecstasy (MDMA, a potent 5-HT-releaser and neurotoxin) exposure on brain development in teenagers. The objective of this study was to investigate whether in humans, like previous observations made in animals, the effects of MDMA on the 5-HT system are dependent on age-of-first exposure. METHODS: 5-HT transporter (SERT) densities in the frontal cortex and midbrain were assessed with [(123)I]β-CIT single photon emission computed tomography in 33 users of ecstasy. Subjects were stratified for early-exposed users (age-at-first exposure 14-18 years; developing brain), and late-exposed users (age-at-first exposure 18-36 years; mature brain). In parallel, we investigated the effects of age experimentally with MDMA in early-exposed (adolescent) rats and late-exposed (adult) rats using the same radioligand. RESULTS: On average, five years after first exposure, we found a strong inverse relationship, wherein age-at-first exposure predicted 79% of the midbrain SERT variability in early (developing brain) exposed ecstasy users, whereas this was only 0.3% in late (mature brain) exposed users (p=0.007). No such effect was observed in the frontal cortex. In rats, a significant age-BY-treatment effect (p<0.01) was observed as well, however only in the frontal cortex. CONCLUSIONS: These age-related effects most likely reflect differences in the maturational stage of the 5-HT projection fields at age-at-first exposure and enhanced outgrowth of the 5-HT system due to 5-HT's neurotrophic effects. Ultimately, our findings stress the need for more knowledge on the effects of pharmacotherapies that alter brain 5-HT levels in the pediatric population

The role of SPECT imaging of the dopaminergic system in translational research on Parkinson's disease

Imaging of the dopaminergic system with single photon emission computed tomography (SPECT), and particularly of the dopamine transporter (DAT) located in the striatum, is a well accepted tool in clinical practice to confirm or exclude loss of nigrostriatal dopamine (DA) neurons in patients suspected to suffer from Parkinson's disease (PD). SPECT techniques were developed successfully to image neurotransmitter systems, including the presynaptic DAT and postsynaptic dopamine D2/3 receptors, in rat and mouse models of PD. Here we review the results of preclinical SPECT studies of the dopaminergic system in rat and mouse models of PD. Initially, SPECT studies in animal models of PD were performed to validate that micro-SPECT is able to accurately assess parts of the dopaminergic system in small animals in-vivo. However, more recently, micro-SPECT DAT is increasingly used as a research tool to support the interpretation of human DAT SPECT studies in PD, including clinical trials examining the effects of potential neuroprotective drugs. Translational research with SPECT is an interesting development which may further increase our understanding of the pathophysiology and treatment of P

Varenicline increases striatal dopamine D(2/3) receptor binding in rats

Increasing dopamine D(2/3) receptor availability is postulated to be a treatment for drug addiction. Varenicline, an alpha4beta2-nicotinic partial agonist, is effective for nicotine dependence. We hypothesize that varenicline increases dopamine D(2/3) receptor availability. Twenty male drug-naïve rats were randomized to varenicline (2 mg/kg) or placebo for 14 days, and then injected with the dopamine D(2/3) radiotracer 123I-IBZM. We found significantly higher striatum-to-cerebellum binding ratios in both dorsal and ventral striatum for the varenicline group compared with placebo. Varenicline increases dopamine D(2/3) receptor availability in drug-naïve rats. Therefore, varenicline may be an effective treatment for addictions other than smokin

Free-Choice and No-Choice High-Fat Diets Affect Striatal Dopamine D-2/3 Receptor Availability, Caloric Intake, and Adiposity

Different types of high-fat (HF) diets are used to study diet-induced obesity (DIO) in rodents and this has led to different phenotypes. This study assesses whether different HF diets differentially affect striatal dopamine D-2/3 receptor (DRD2/3) availability, as decreased striatal DRD2/3 availability has been implicated in obesity in relation to reward deficiency for food. Thirty rats were randomized to either a free-choice HF diet (HF-choice), a premixed HF diet (HF-no-choice), or a standard chow diet for 28 days. Striatal DRD2/3 was measured using 123I-IBZM storage phosphor imaging at day 29. DRD2/3 availability was significantly decreased in the dorsal striatum in the HF-choice rats compared to chow rats, but not in HF-no-choice rats. Additionally, caloric intake of the HF-choice rats was significantly higher than that of HF-no-choice rats and serum leptin and percentage abdominal fat store weight of total body weight were significantly higher in the HF-choice rats compared to chow rats. These preliminary results suggest that the choice element in HF diets, which is possibly related to the motivational aspects of eating, leads to overconsumption and to a distinct state of obesity. These results are relevant for future studies on DIO when considering choice of diet typ

Clinical evaluation of [123I]FP-CIT SPECT scans on the novel brain-dedicated InSPira HD SPECT system: a head-to-head comparison

The InSPira HD system, a novel brain-dedicated SPECT scanner, allows for imaging with a high spatial resolution. Here, we tested whether this scanner can be used to image the dopamine transporter adequately. Therefore, striatal phantom and patient data acquired on the InSPira were compared head-to-head with the well-validated brain-dedicated NeuroFocus system. A striatal phantom filled with [123I] and 14 subjects (after [123I]FP-CIT injection) were scanned on both systems. [123I]FP-CIT SPECT scans were visually assessed. Striatal binding ratios were calculated automatically using the software package BRASS. Striatal phantom and patient data showed strong correlations with respect to striatal ratios (R = 0.99 and R = 0.92; p < 0.05 and p < 0.01, respectively). Slightly higher ratios were found for the NeuroFocus patient data, probably due to differences in system performance. Visual assessment of [123I]FP-CIT scans showed agreement between systems in 13 of the 14 cases. We conclude that [123I]FP-CIT SPECT imaging can be performed adequately on the new InSPira system

Hepatobiliary function assessed by 99mTc-mebrofenin cholescintigraphy in the evaluation of severity of steatosis in a rat model

PURPOSE: This study evaluated the utility of non-invasive assessment of hepatobiliary function by 99mTc-mebrofenin cholescintigraphy in a rat model of diet-induced steatosis. METHODS: Male Wistar rats (250-300 g) were fed a standard methionine- and choline-deficient (MCD) diet for up to 5 weeks, thereby inducing hepatic fat accumulation, progressive inflammation and fibrogenesis corresponding with clinical steatosis. 99mTc-mebrofenin pinhole scintigraphy was used to evaluate the hepatocyte mebrofenin uptake rate, the time of maximum hepatic uptake (T(peak)) and the time required for peak activity to decrease by 50% (T(1/2peak)). Scintigraphic parameters were correlated with biochemical and serological parameters and with liver histopathology. RESULTS: MCD diet induced mild steatosis after 1 week and severe steatosis with prominent inflammation after 5 weeks. T(peak), T(1/2peak) prolonged and the uptake rate decreased significantly, while the severity of steatosis increased (p <0.05). There was a strong, significant correlation between the severity of steatosis (histopathology, hepatic triglyceride content) and the 99mTc-mebrofenin uptake rate (r2=0.83, p <0.0001 and r2=0.82, p <0.0001, respectively). In addition, the uptake rate correlated significantly with the increased inflammation (plasma and hepatic TNF-alpha, r2=0.72, p <0.0001 and r2=0.52, p=0.001, respectively). The correlation of the uptake rate with hepatocellular damage was weak (AST and ALT, r2=0.29 and 0.32, respectively), but correlation with synthetic function was strong (prothrombin time, r2=0.70, p <0.001). CONCLUSION: Hepatobiliary function assessed by 99mTc-mebrofenin scintigraphy correlates with the extent and progression of steatosis. These results suggest a potential role for mebrofenin scintigraphy as a non-invasive functional follow-up method for disease progression in steatotic patient