Paige Finley

In Vivo Detection of Short- and Long-Term MDMA Neurotoxicity—A Positron Emission Tomography Study in the Living Baboon Brain

The present study evaluated short‐ and long‐term effects of MDMA (3,4‐methylenedioxymethamphetamine) in the baboon brain using PET and [11C](+)McN 5652, a potent 5‐HT transporter ligand, as well as [11C]RTI‐55, a cocaine derivative which labels both 5‐HT and dopamine transporters. Following baseline PET scans with [11C](+)McN5652, [11C](−)McN5652 (the inactive enantiomer of the active enantiomer [11C](+)McN5652) and [11C]RTI‐55, a baboon was treated with MDMA (5 mg/kg, s.c., twice daily for four consecutive days). PET studies at 13, 19, and 40 days post‐MDMA revealed decreases in mean radioactivity levels in all brain regions when using [11C](+)McN 5652, but not with [11C](−)McN5652 or [11C]RTI‐55. Reductions in specific [11C](+)McN5652 binding (calculated as the difference in radioactivity concentrations between (+) and (−)[11C]McN5652) ranged from 44% in the pons to 89% in the occipital cortex. PET studies at 9 and 13 months showed regional differences in the apparent recovery of 5‐HT transporters, with increases in some brain regions (e.g., hypothalamus) and persistent decreases in others (e.g., neocortex). Data obtained from PET studies correlated well with regional 5‐HT axonal marker concentrations in the CNS measured after sacrifice of the animal. The results of these studies indicate that PET imaging of the living nonhuman primate brain with [11C](+)McN 5652 can detect changes in regional 5‐HT transporter density secondary to MDMA‐induced neurotoxicity. Using PET, it should also be feasible to use [11C](+)McN5652 to determine whether human MDMA users are also susceptible to MDMAs neurotoxic effects. Synapse 29:183–192, 1998.

2-[18F]fluoro-A-85380, an in vivo tracer for the nicotinic acetylcholine receptors

6-[18F]Fluoro-3-(2(S)-azetidinylmethoxy)pyridine (6-[18F]fluoro-A-85380 or 6-[18F]FA), a new tracer for positron emission tomography, was synthesized by no-carrier-added [18F] fluorination of 6-iodo-3-((1-tert-butoxycarbonyl-2(S)-azetidinyl)methoxy)pyridine followed by acidic deprotection. 6-[18F]FA followed the regional densities of brain nicotinic acetylcholine receptors (nAChRs) reported in the literature. Evidence of binding to nAChRs and high specificity of the binding in vivo was demonstrated by inhibition with nAChR selective ligands as well as with unlabeled 6-FA. A preliminary toxicology study of the 6-FA showed a relatively low biological effect.

Synthesis and biodistribution of [11C]A-836339, a new potential radioligand for PET imaging of cannabinoid type 2 receptors (CB2)

Recently, A-836339 [2,2,3,3-tetramethylcyclopropanecarboxylic acid [3-(2-methoxyethyl)-4,5-dimethyl-3H-thiazol-(2Z)-ylidene]amide] (1) was reported to be a selective CB2 agonist with high binding affinity. Here we describe the radiosynthesis of [11C]A-836339 ([11C]1) via its desmethyl precursor as a candidate radioligand for imaging CB2 receptors with positron-emission tomography (PET). Whole body and the regional brain distribution of [11C]1 in control CD1 mice demonstrated that this radioligand exhibits specific uptake in the CB2-rich spleen and little specific in vivo binding in the control mouse brain. However, [11C]1 shows specific cerebral uptake in the lipopolysaccharide (LPS)-induced mouse model of neuroinflammation and in the brain areas with Abeta amyloid plaque deposition in a mouse model of Alzheimers disease (APPswe/PS1dE9 mice). These data establish a proof of principle that CB2 receptors binding in the neuroinflammation and related disorders can be measured in vivo.

5-[I-125/123] Iodo-3 (2 (S)-azetidinylmethoxy) pyridine, a radioiodinated analog of A-85380 for in vivo studies of central nicotinic acetylcholine receptors

The in vivo biodistribution profile of the novel nicotinic acetylcholine receptor (nAChR) radioligand 5-[I-125/123]Iodo-3(2(S)-azetidinylmethoxy)pyridine, [I-125/123]-5-IA, in mouse brain was examined. This radiotracer displayed good brain penetration (3.1% of the injected dose (ID) in whole brain at 15 min post-radioligand injection). Radioligand distribution was consistent with the density of high affinity nAChRs with highest uptake observed in the nAChR-rich thalamus (14.9 %ID/g at 60 min), moderate uptake in cortex (8.5 %ID/g at 60 min), and lowest uptake in the cerebellum (2.4 %ID/g at 60 min). Pretreatment with several different nAChR agonists (A-85380, (-)-nicotine, cytisine) significantly inhibited [I-125]-5-IA binding in all brain regions studied (P < 0.01) demonstrating the high specificity of the radioligand for nAChRs. Blocking doses of the muscarinic antagonist scopolamine and the non-competitive nAChR channel blocker mecamylamine had no significant effect on radioactive uptake supporting the in vitro selectivity of [I-125]-5-IA for the nAChR component of the cholinergic system. [I-125]-5-IA binding sites were shown to be saturable with unlabeled 5-IA. With a relatively low acute toxicity (LD50 > 3 mg/kg via intravenous injection in mice) and high in vivo specificity and selectivity, 5-IA labeled with the imaging radionuclide I-123 may prove useful for single photon emission computed tomography (SPECT) studies of nAChRs in human subjects.

[125/123I]IPH: A radioiodinated analog of epibatidine for in vivo studies of nicotinic acetylcholine receptors

Tomographic imaging of central nicotinic acetylcholine receptors (nAChRs) via single photon emission computed tomography (SPECT) has been hampered by the lack of a radioligand with suitable in vivo binding characteristics. Therefore, a novel analog of epibatidine, (±)‐exo‐2‐(2‐iodo‐5‐pyridyl)‐7‐azabicyclo[2.2.1]heptane (IPH), labeled with [125I] or [123I] was evaluated as an in vivo marker of central nicotinic acetylcholine receptors (nAChRs). [125I]IPH showed substantial brain penetration (4.2% of the injected dose at 30 min) and a cerebral biodistribution in mice consistent with the in vivo labeling of nAChRs (% injected dose/gram of thalamus, superior colliculi ≫ cerebellum). [125I]IPH binding sites were shown to be saturable with unlabeled IPH (ED50 approximately 1 μg/kg). The uptake of [125I]IPH was blocked significantly by the nicotinic agonists, cytisine, lobeline, and (−)‐nicotine, but not by the noncompetitive nAChR antagonist, mecamylamine. Antagonists of muscarinic (scopolamine), serotonin (ketanserin), and opioid (naloxone) receptors had no significant effect on [125I]IPH binding. A preliminary SPECT imaging study with [123I]IPH in a baboon showed [123I]IPH to localize in nAChR‐rich areas of brain (thalamus > frontal cortex > cerebellum). [123I]IPH binding in baboon brain was also displaced (35–45% displacement) by a challenge dose of cytisine showing that a well‐characterized nicotinic agonist effectively competes for [123I]IPH binding sites. [123I]IPH seems well suited for imaging studies of nAChRs and, to our knowledge, is the first SPECT agent that has allowed for the visualization of nAChRs in primate brain. Synapse 26:392–399, 1997.

NICOTINE INDUCED UP-REGULATION OF NICOTINIC RECEPTORS IN CD-1 MICE DEMONSTRATED WITH AN IN VIVO RADIOTRACER : GENDER DIFFERENCES

Up‐regulation of brain nicotinic receptors (nAChRs) by chronic nicotine treatment has chiefly been demonstrated by in vitro binding assays. Here, we report that up‐regulation of nAChRs in CD‐1 mice can be detected using a specific in vivo radioligand for nAChRs, [125I]IPH. After 10 days of (−)‐nicotine administration, male and female mice demonstrated a significant elevation of [125I]IPH binding in all brain regions studied. [125I]lPH uptake also displayed significant gender differences with male animals showing a more pronounced increase in [125I]IPH accumulation. Synapse 29: 116–118, 1998.

Biodistribution of [18f] SR144385 and [18f] SR147963: selective radioligands for positron emission tomographic studies of brain cannabinoid receptors

ABSTRACT. [(18)F] SR144385 and [(18)F] SR147963 were synthesized in a multistep reaction in which fluorine-18 was introduced by nucleophilic halogen displacement on a bromo precursor. The fluorine-18-labeled intermediate was deprotected and coupled with the appropriate alkyl amine to give the final products. Both radioligands had appropriate regional brain distribution for cannabinoid receptors with a target to nontarget ratio of 1.7 for [(18)F] SR147963 and 2.5 for [(18)F] SR144385 at 60 and 90 min postinjection, respectively. The uptake of both tracers was blocked with a 1 mg/kg dose of SR141716A.

18F-Fluorobenzyl Triphenyl Phosphonium: A Noninvasive Sensor of Brown Adipose Tissue Thermogenesis

Recent studies have proposed activation of brown adipose tissue (BAT) thermogenesis as a new strategy to combat obesity. Currently, there is no effective noninvasive imaging agent to directly detect unstimulated BAT and quantify the core mechanism of mitochondrial thermogenesis. We investigated an approach to detect BAT depots and monitor thermogenesis using the mitochondria-targeting voltage sensor radiolabeled fluorobenzyltriphenyl phosphonium (FBnTP). Methods: 18F-FBnTP, 14C-FBnTP, 18F-FDG, and 99mTc-sestamibi uptake in BAT at room temperature (n = 8) and cold-treated (n = 8) Lewis rats was assayed. The effect of the cold condition on 18F-FBnTP retention in BAT was assessed in 8 treated and 16 control rats. The effect of the noradrenergic inhibitor propranolol on 14C-FBnTP response to cold stimulation was investigated in an additional 8 treated and 8 control mice. Results: At room temperature, 18F-FBnTP accumulated in BAT to an extent similar to that in the heart, second only to the kidney and twice as much as 99mTc-sestamibi. Prior exposure to cold (4°C) for 4 h resulted in an 82% decrease of 14C-FBnTP uptake and an 813% increase of 18F-FDG uptake in BAT. 99mTc-sestamibi uptake was not affected by cold. Administration of 18F-FBnTP at room temperature 60 min before 120 and 240 min of exposure to cold resulted in marked washout of the tracer from BAT. Propranolol significantly diminished the effect of cold on 14C-FBnTP and 18F-FDG uptake into BAT. Conclusion: The intense uptake of 18F-FBnTP into BAT at room temperature and the response to cold stimulation suggest the unique potential advantage of 18F-FBnTP not only in detecting unstimulated BAT at high contrast but also in quantifying the mitochondrial thermogenic activity. 18F-FBnTP PET may serve as a useful technique to assess BAT volume and function.

[11C]-methyl 4-[(3,4-dichlorophenyl)acetyl]-3-[(1-pyrrolidinyl)methyl]-1-piperazinecarboxylate ([11C]GR89696): synthesis and in vivo binding to kappa opiate receptors

GR89696, racemic methyl 4-[(3,4-dichlorophenyl)acetyl]-3-[(1-pyrrolidinyl) methyl]-1-piperazinecarboxylate, a kappa opioid receptor ligand, was labeled with [11C]methyl chloroformate. The radiochemical yield was 20% with an observed specific radioactivity of 75.5 GBq/micromol at end of synthesis (2,040 mCi/micromol). Five minutes after intravenous administration, 5.4% of the injected dose accumulated in mouse whole brain. Brain region to cerebellar ratios increased over time with ratios at 90 min of 7.8, 5.6, and 4.5 for the hypothalamus, olfactory tubercle, and striatum, respectively. The uptake of [11C]GR89696 correlated with known kappa opioid receptor densities and was inhibited by kappa opioid selective drugs.

Fenfluramine-induced loss of serotonin transporters in baboon brain visualized with PET.

The present study sought to determine whether or not Positron Emission Tomography (PET) with the newly developed positron emitting serotonin (5‐HT) transporter ligand, (+)[11C]McN‐5652, could be used to detect fenfluramine‐induced 5‐HT neurotoxicity in the brain of living primates (baboons). Six PET imaging studies were performed: three before treatment with fenfluramine (5 mg/kg, s.c., twice daily for 4 days) and three after (18, 45, and 81 days after treatment). The dose of fenfluramine used in this study (5 mg/kg) is known to produce 5‐HT neurotoxicity in primates, and to be approximately two times higher than a dose of fenfluramine reported to produce small and inconsistent weight loss in baboons (2 mg/kg). Following fenfluramine treatment, marked lasting reductions in regional brain specific binding of (+)[11C]McN‐5652 were found by means of PET. Findings with PET corresponded well with post‐mortem neurochemical findings indicative of serotonergic neurotoxicity (lasting depletions of regional brain 5‐HT, 5‐HIAA, and 5‐HT uptake sites). These results suggest that PET imaging with (+)[11C]McN‐5652 will be useful for evaluating the 5‐HT neurotoxic potential of fenfluramine and related drugs in living humans.