The role of species-dependent metabolism in the regional brain retention of 18F-labeled muscarinic acetylcholine receptor ligands.

We have developed PET radioligands for the muscarinic acetylcholine receptor designed to be sensitive to endogenous acetylcholine changes. These radioligands were based on the piperidyl and pyrrolidyl benzilate scaffold and include (R)-N-(2-[18F]fluoroethyl)-3-piperidyl benzilate (1b), (R)-N-(2-[18F]fluoroethyl)-3-pyrrolidyl benzilate (2b), and N-(2-[18F]fluoroethyl)-4-piperidyl benzilate (3b). In the mouse, intravenous injection of 2b produced a heterogeneous receptor-mediated regional retention of radioactivity, whereas in the rat a homogeneous brain distribution was observed. Analyses of blood and brain extracts showed a radiolabeled metabolite for 2b which was formed to a much greater extent in mice than rats. This metabolite may have a higher receptor binding affinity than authentic 2b, and thus be responsible for the apparent receptor-mediated binding in the mouse brain. Our findings emphasize the importance of metabolite analysis in multiple species when developing novel radiopharmaceuticals for in vivo use.

Acetylcholinesterase inhibition increases in vivo N-(2-[18F]fluoroethyl)-4-piperidyl benzilate binding to muscarinic acetylcholine receptors.

Although the inhibition of acetylcholinesterase remains the primary treatment of Alzheimer's disease, little is known of the results of increased acetylcholine levels on muscarinic receptor occupancy or function. Using N-(2-[18F]fluoroethyl)-4-piperidyl benzilate ([18F]FEPB), a moderate affinity (Ki = 1.7 nmol/L) nonsubtype-selective muscarinic receptor antagonist, the authors examined the sensitivity of equilibrium in vivo radioligand binding in rat brain with changes in endogenous acetylcholine levels produced by treatments with acetylcholinesterase inhibitors. Phenserine administration 30 minutes before resulted in a dose-dependent into muscarinic cholinergic receptors, reaching a maximum increase of 90% in the striatum at a dose of 5 mg/kg intraperitoneally. Constant infusion of physostigmine at a dosage of 250 microg/kg/min produced an identical increase in radioligand binding. This agonist-induced increase of in vivo mAChR radioligand binding offers a new method for monitoring of the efficacy of acetylcholinesterase inhibitors or other drugs to enhance acetylcholine actions at the muscarinic receptors.

Radiolabeled cholinesterase substrates: in vitro methods for determining structure-activity relationships and identification of a positron emission tomography radiopharmaceutical for in vivo measurement of butyrylcholinesterase activity.

There is currently great interest in developing radiolabeled substrates for acetylcholinesterase and butyrylcholinesterase that would be useful in the in vivo imaging of patients with Alzheimer's disease. Using a simple in vitro spectrophotometric assay for determination of enzymatic cleavage rates, the structure-activity relationship for a short series of 1-methyl-4-piperidinyl esters was investigated. Relative enzymatic hydrolysis rates for the well-characterized 1-methyl-4-piperidinyl acetate, propionate, and i-butyrate esters were in agreement with literature values. The 4 and 5 carbon esters of 1-methyl-4-piperidinol were specific for butyrylcholinesterase and cleaved in the rank order n-valerate > n-butyrate >> 2-methylbutyrate, iso-valerate. These spectrophotometric results were also in agreement with in vitro hydrolysis rates in mouse blood and with in vivo regional retention of radioactivity in mouse brain of 11C-labeled analogs. Brain uptake and apparent enzymatic rate constants for 1-[11C]methyl-4-piperidinyl n-butyrate and n-valerate were calculated from in vivo measurements in M. nemistrina using positron emission tomography. Based on higher brain uptake of radioactivity and superior pharmacokinetics, 1-[11C]methyl-4-piperidinyl n-butyrate was identified as a new radiopharmaceutical for the in vivo measurement of butyrylcholinesterase activity.

Synthesis, (18)F-labeling, and biological evaluation of piperidyl and pyrrolidyl benzilates as in vivo ligands for muscarinic acetylcholine receptors.

A series of 31 compounds based on the piperidyl or pyrrolidyl benzilate scaffold were prepared from methyl benzilate and 4-piperidinol, (R)-(+)-3-piperidinol, or (R)-(+)-3-pyrrolidinol. Amine substituents included alkyl and aralkyl groups. In vitro K(i) values ranged from 0.05 nM to >100 nM. (R)-N-(2-Fluoroethyl)-3-piperidyl benzilate (3-FEPB, 22, K(i) = 12.1 nM) and N-(2-fluoroethyl)-4-piperidyl benzilate (4-FEPB, 8, K(i) = 1. 83 nM) were selected for radiolabeling with fluorine-18. Using alkylation with 2-[(18)F]fluoroethyl triflate, 3-[(18)F]FEPB (42) and 4-[(18)F]FEPB (43) were produced in 7-9% radiochemical yield and >97% radiochemical purity. For in vivo studies, retention was moderate in mouse brain for 42; however, blocking with scopolamine showed that uptake was not muscarinic cholinergic receptor-mediated. Conversely, 43 exhibited high, receptor-mediated retention in mouse brain, with significant clearance after 1 h. These results suggest that 43 could have applications as an in vivo probe for measuring endogenous acetylcholine levels.

Synthesis and in vivo evaluation of (E)-N-[(11)C]Methyl-4- (3-pyridinyl)-3-butene-1-amine ([(11)C]metanicotine) as a nicotinic receptor radioligand.

(E)-N-[(11)C]Methyl-4-(3-pyridinyl)-3-butene-1-amine ([(11)C]metanicotine), a high affinity (K(i) = 16 nM) CNS-selective nicotinic agonist, was prepared by the [(11)C]alkylation of the desmethyl precursor with [(11)C]methyl trifluoromethanesulfonate. In vivo distribution studies in mice demonstrated good blood brain permeability but essentially uniform regional brain distribution and no evidence of specific binding to nicotinic cholinergic receptors. Identical results were obtained in an imaging study performed in a monkey brain. Therefore, despite literature reports supporting the use of metanicotine as a cognition enhancing nicotinic agonist, (E)-N-[(11)C]methyl-4-(3-pyridinyl)-3-butene-1-amine does not appear to be a suitable candidate for in vivo imaging studies of nicotinic acetylcholine receptors in the mammalian brain.

Simplified methods for in vivo measurement of acetylcholinesterase activity in rodent brain.

Simplified methods for in vivo studies of acetylcholinesterase (AChE) activity in rodent brain were evaluated using N-[11C]methylpiperidinyl propionate ([11C]PMP) as an enzyme substrate. Regional mouse brain distributions were determined at 1 min (representing initial brain uptake) and 30 min (representing trapped product) after intravenous [11C]PMP administration. Single time point tissue concentrations (percent injected dose/gram at 30 min), tissue concentration ratios (striatum/cerebellum and striatum/cortex ratios at 30 min), and regional tissue retention fractions (defined as percent injected dose 30 min/percent injected dose 1 min) were evaluated as measures of AChE enzymatic activity in mouse brain. Studies were carried out in control animals and after dosing with phenserine, a selective centrally active AChE inhibitor; neostigmine, a peripheral cholinesterase inhibitor; and a combination of the two drugs. In control and phenserine-treated animals, absolute tissue concentrations and regional retention fractions provide good measures of dose-dependent inhibition of brain AChE; tissue concentration ratios, however, provide erroneous conclusions. Peripheral inhibition of cholinesterases, which changes the blood pharmacokinetics of the radiotracer, diminishes the sensitivity of all measures to detect changes in central inhibition of the enzyme. We conclude that certain simple measures of AChE hydrolysis rates for [11C]PMP are suitable for studies where alterations of the peripheral blood metabolism of the tracer are kept to a minimum.

Synthesis and evaluation of 6-[11C]methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2- benzisoxazole as an in vivo radioligand for acetylcholinesterase.

6-Methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2-benzisoxazole is a high affinity (K(i) = 8.2 nM) reversible inhibitor of acetylcholinesterase (AChE). The carbon-11 labeled form was prepared in high (>97%) radiochemical purity and with specific activities of 37+/-20 GBq/micromol at end of synthesis, by the alkylation of the desmethyl precursor with [11C]methyl trifluoromethanesulfonate in N,N-dimethyl-formamide at room temperature. In vivo studies in mice demonstrated good blood brain permeability but essentially uniform regional brain distribution. Thus, despite in vitro and in vivo activity as an AChE inhibitor, 6-[11C]methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2-benzis oxa zole does not appear to be a good candidate for in vivo imaging studies of AChE in the mammalian brain.

Heterogeneous cardiac sympathetic denervation and decreased myocardial nerve growth factor in streptozotocin-induced diabetic rats: implications for cardiac sympathetic dysinnervation complicating diabetes.

Heterogeneous myocardial sympathetic denervation complicating diabetes has been invoked as a factor contributing to sudden unexplained cardiac death. In subjects with diabetic autonomic neuropathy (DAN), distal left ventricular (LV) denervation contrasts with preservation of islands of proximal innervation, which exhibit impaired vascular responsiveness. The aims of this study were to determine whether this heterogeneous pattern of myocardial sympathetic denervation occurs in a rat model of diabetes and to explore a potential association with regional fluctuations in myocardial nerve growth factor (NGF) protein. Myocardial sympathetic denervation was characterized scintigraphically using the sympathetic neurotransmitter analog C-11 hydroxyephedrine ([11C]HED) and compared with regional changes in myocardial NGF protein abundance and norepinephrine content after 6 and 9 months in nondiabetic (ND) and streptozotocin-induced diabetic (STZ-D) rats. In ND rats, no difference in [11C]HED retention or norepinephrine content was detected in the proximal versus distal myocardium. After 6 months, compared with ND rats, myocardial [11C]HED retention had declined in the proximal segments of STZ-D rats by only 9% (NS) compared with a 33% decrease in the distal myocardium (P < 0.05). Myocardial norepinephrine content was similar in both ND and STZ-D rats. At 6 months, LV myocardial NGF protein content in STZ-D rats decreased by 52% in the proximal myocardial segments (P < 0.01 vs. ND rats) and by 82% distally (P < 0.01 vs. ND rats, P < 0.05 vs. proximal segments). By 9 months, [11C]HED retention had declined in both the proximal and distal myocardial segments of the STZ-D rats by 42% (P < 0.01 vs. ND rats), and LV norepinephrine content and NGF protein were decreased in parallel. Therefore, 6 months of STZ-induced diabetes results in heterogeneous cardiac sympathetic denervation in the rat, with maximal denervation occurring distally, and is associated with a proximal-to-distal gradient of LV NGF protein depletion. It is tempting to speculate that regional fluctuations of NGF protein in the diabetic myocardium contribute to heterogeneous cardiac sympathetic denervation complicating diabetes.

Intensity and duration of intensive case management services.

OBJECTIVE: Two policy issues related to intensive case management programs were examined: limiting caseload size to ensure that services are intensive and providing intensive services to the same clients in perpetuity. METHOD: The Denver Acuity Scale, which indicates need for services on a 5-point scale, was used for determining the service intensity needed by consumers and for increasing case managers' efficiency when caseload size varied. The acuity ratings of individuals in the Denver intensive case management program were examined to evaluate the effects of service duration on decompensation. Each consumer was rated at every service contact. The percentage of individuals readmitted to the program after they graduated was calculated, and the trajectories of 87 individuals who continued to be served by the program after they attained the highest rating of functioning were examined. RESULTS: Of the 112 individuals who graduated from the program in the 29-month study period, four (4 percent) were readmitted. More than half of the 87 consumers who achieved the highest functioning level did not deviate from that level for the remainder of the study period. Nearly a fifth showed some deterioration shortly after achieving that level but then improved. Slightly more than a fourth continued to deteriorate, but many never reached the lowest levels of functioning. CONCLUSIONS: To increase efficiency and ensure appropriate service levels, service intensity should be based on individual consumers' functioning levels. Most consumers are unlikely to need intensive case management in perpetuity. Providing more intensive services than needed or providing services longer than needed is inefficient and may even impede consumer recovery.

[18F]fluoroethoxy-benzovesamicol, a PET radiotracer for the vesicular acetylcholine transporter and cholinergic synapses.

Loss of cholinergic transmission in the cortex and hippocampus is a characteristic feature of Alzheimer's disease, and visualization of functional cholinergic synapses in the brain with PET could be a useful method for studying this degenerative condition in living humans. We investigated [18F]fluoroethoxybenzovesamicol, (-)-[18F] FEOBV,(-)-(2R,3R)-trans-2-hydroxy-3-(4-phenylpiperidino)-5-(2-[18F ]fluoroethoxy)-1,2,3,4-tetralin, a high affinity positron emitting ligand for the vesicular acetylcholine transporter, as a potential in vivo cholinergic synapse mapping agent. Rodent biodistribution, dosimetry, stereospecificity of biological effects, pharmacologic blocking studies, in vivo rodent brain autoradiography and metabolites were examined. (-)-[18F]FEOBV brain uptake following intravenous injection was robust, with 2.65% dose/brain in mice at 5 min, and the regional localization matched the known distributions of presynaptic cholinergic markers at later times. Both the cholinergic localization and curare-like effects of FEOBV were associated with the "(-)"-enantiomer exclusively. (-)-[18F]FEOBV regional brain distribution in rodents was changed little by pretreatment with haloperidol, (+)-3-PPP, or E-2020, indicating FEOBV, unlike other vesamicol analogs, did not interact in vivo with dopamine or sigma receptor systems. Autoradiography of rat brain 3 h following i.v. injection of (-)-[18F]FEOBV showed high localization in brain areas rich in presynaptic cholinergic elements. Metabolic defluorination in rodents was modest, and analysis of brain tissue following tracer administration found FEOBV as the only extractable radioactive species. (-)-[18F]FEOBV dosimetry calculated from rat data estimate 10 mCi doses can be given to humans. These studies show FEOBV maps cholinergic areas with high specificity in vivo, and may provide a noninvasive means to safely and accurately gauge the functional integrity of cholinergic synapses in man using PET.

Computer-assisted creation of psychiatric advance directives.

Results are reported from a project that involved design, production, and feasibility testing of the use of a multimedia computer program to: 1) educate MH consumers about psychiatric advance directives (ADs), and 2) assist them in creating ADs. The feasibility results, based on a random sample of 60 adults meeting criteria for having a serious and persistent mental illness, were positive. Sixty-five percent of the sample were able to complete their ADs. Educational level, presence of a learning disability, and minority-group membership, but not diagnosis, were related to completion rates. The results dispel several popular myths about psychiatric ADs.

Patterns of services and consumer outcome in an intensive case management program.

This study examined the patterns of services provided to individuals with serious and persistent mental illness during their first year in an intensive case management program. Services in 10 content areas were examined, and patterns for more versus less "successful" individuals were compared. Differences emerged for services focusing on family and housing, suggesting that the need for community support services influences the need for continued intensive case management. Linear reductions in rehabilitation services suggest that such services may indeed be effective early in the treatment process. Finally, differences among case managers in service patterns for 5 of the 10 content areas suggest that case managers play an important role in determining the course of treatment.

Simplifying the dosimetry of carbon-11-labeled radiopharmaceuticals.

UNLABELLED: A two time-point sacrifice method is proposed as an alternative to conventional multiple time-point sacrifice methods to determine the organ cumulated activity of 11C-labeled radiopharmaceuticals. METHODS: Rat biodistribution data for 10 11C-labeled radiopharmaceuticals were analyzed to determine organ cumulated activity. Data were obtained at four sacrifice intervals 2-5 min, 10-15 min, 30-45 min and 1-1.5 hr postinjection. The organ absorbed dose per unit administered radioactivity (mGy/MBq) was calculated using all four data points and combinations of limited data. The objective was to determine if a limited sampling technique would provide sufficient accuracy in estimating absorbed dose. RESULTS: Residence times calculated using two time-points acquired during the first half-life of 11C were either equivalent or positively biased compared to using all sacrifice times. Overall, 87% of the residence times assessed were conservative compared to the multipoint method. For bladder organs, a consistent negative bias was observed with the reduced sacrifice method. CONCLUSION: Analysis of animal biodistributions using a reduced sacrifice protocol provides results in good agreement with and generally conservative to results using all sacrifice intervals. Correction factors are required for the urinary bladder and gallbladder when using the simplified technique due to bias. The bladder was often the limiting organ in determining human administered activity.

Vesamicol receptor mapping of brain cholinergic neurons with radioiodine-labeled positional isomers of benzovesamicol.

UNLABELLED: Alzheimer's disease is characterized by progressive cerebral cholinergic neuronal degeneration. Radiotracer analogs of benzovesamicol, which bind with high affinity to the vesamicol receptor located on the uptake transporter of acetylcholine storage vesicles, may provide an in vivo marker of cholinergic neuronal integrity. Five positional isomers of racemic iodobenzovesamicol (4'-, 5-, 6-, 7-, and 8-IBVM) were synthesized, exchange-labeled with iodine-125, and evaluated as possible in vivo markers for central cholinergic neurons. Only two isomers, 5-IBVM (5) and 6-IBVM (10), gave distribution patterns in mouse brain consistent with cholinergic innervation: striatum >> hippocampus > or = cortex > hypothalamus >> cerebellum. The 24-h tissue-to-cerebellum concentration ratios for 5-IBVM (5) were 3-4-fold higher for striatum, cortex, and hippocampus than the respective ratios for 6-IBVM (10). Neither 8-IBVM (16) nor 4'-IBVM (17) exhibited selective retention in any of the brain regions examined. In the heart, only 5-IBVM (5) exhibited an atria-to-ventricles concentration ratio consistent with high peripheral cholinergic neuronal selectivity. The 7-IBVM (14) isomer exhibited an anomalous brain distribution pattern, marked by high and prolonged retention in the five brain regions, most notably the cerebellum. This isomer was screened for binding in a series of 26 different biological assays; 7-IBVM (14) exhibited affinity only for the delta-receptor with an IC50 of approximately 30 nM. Drug-blocking studies suggested that brain retention of 7-IBVM (14) reflects high-affinity binding to both vesamicol and delta-receptors. Competitive binding studies using rat cortical homogenates gave IC50 values for binding to the vesamicol receptor of 2.5 nM for 5-IBVM (5), 4.8 nM for 6-IBVM (10), and 3.5 nM for 7-IBVM (14). Ex vivo autoradiography of rat brain after injection of (-)-5-[125I]IBVM ((-)-[125I]5) clearly delineated small cholinergic-rich areas such as basolateral amygdala, interpeduncular nucleus, and facial nuclei. Except for cortex, regional brain levels of (-)-5-[123I]IBVM ((-)-[123I]5) at 4 h exhibited a linear correlation (r2 = 0.99) with endogenous levels of choline acetyltransferase. CONCLUSION: Vesamicol receptor mapping of cholinergic nerve terminals in murine brain can be achieved with 5-IBVM (5) and less robustly with 6-IBVM (10), whereas the brain localization of 7-IBVM (14) reflects high-affinity binding to both vesamicol and delta-receptors.

Effects of dopaminergic drug treatments on in vivo radioligand binding to brain vesicular monoamine transporters.

The effects of various dopaminergic drug treatments on the in vivo regional brain distribution of high-affinity radioligands ([11C]dihydrotetrabenazine and [11C]methoxytetrabenazine) for the rat brain vesicular monoamine transporter (VMAT2) were determined. Acute treatments with reserpine (2 mg/kg i.p.), tetrabenazine (10 mg/kg i.v.) or related benzoisoquinolines significantly reduced radiotracer binding in vivo. In contrast, radiotracer distributions remained unchanged after treatments with other dopaminergic drugs, whether given by single injection (haloperidol, 1 mg/kg i.p., pargyline 80 mg/kg), repeatedly (pargyline, 80 mg/kg s.c., 14 days), or by continuous infusion (deprenyl, 10 mg/kg/day, 5 days; L-DOPA methyl ester 100 mg/kg/day, 5 days). Repeated injections of tetrabenazine (5 mg/kg i.p., twice daily, 3 days) did not alter in vivo radioligand binding measured after allowing drug washout from the brain. These studies support the proposal that in vivo PET imaging of VMAT2 radioligands in patients with extrapyramidal movement disorders will not be affected by concurrent use of L-DOPA or deprenyl.

In vivo studies of acetylcholinesterase activity using a labeled substrate, N-[11C]methylpiperdin-4-yl propionate ([11C]PMP).

Two esters, N-[11C]methylpiperidyl acetate ([11C]AMP) and N-[11C]methylpiperidyl propionate ([11C]PMP), were synthesized in no-carrier-added forms and evaluated as in vivo substrates for brain acetylcholinesterase (AChE). After peripheral injection in mice, each ester showed rapid penetration into the brain and a regional retention of radioactivity (striatum > cortex, hippocampus > cerebellum) reflecting known levels of AChE activity in the brain. Regional brain distributions after [11C]PMP administration showed better discrimination between regions of high, intermediate, and low AChE activities. Chromatographic analysis of blood and brain tissue extracts showed rapid and nearly complete hydrolysis of [11C]PMP within 10 min after injection. For both [11C]AMP and [11C]PMP, retention of radioactivity in all regions was reduced by pretreatment with diisopropylfluorophosphate (DFP), a specific irreversible AChE inhibitor. DFP treatment also significantly increased the proportions of unhydrolyzed ester in both blood and brain. Radioactivity localization in brain after peripheral injection was thus dependent on AChE-catalyzed hydrolysis to the hydrophilic product N-[11C]methylpiperidinol. PET imaging of [11C]AMP or [11C]PMP distributions in monkey brain showed clear accumulation of radioactivity in areas of highest AChE activity (striatum, cortex). These esters are thus in vivo substrates for brain AChE, with potential applications as in vivo imaging agents of enzyme action in the human brain. [11C]PMP, the ester with a slower rate of hydrolysis, appears to be the better candidate radiotracer for further development.

Synthesis and evaluation of [123I]-iodo-PK11195 for mapping peripheral-type benzodiazepine receptors (omega 3) in heart.

An iodinated analog of PK11195, 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)isoquinoline-3-carboxamide , a specific antagonist of the peripheral-type benzodiazepine receptor (omega 3), has been synthesized in three steps with an overall chemical yield of 40%. Both [123I]- and [125I]-Iodo-PK11195 have been synthesized by solid-state isotopic exchange in > 60% isolated radiochemical yield and specific activity of 233-348 mCi/mmol. Tissue distribution studies in rats indicate a high uptake of radioactivity in adrenal glands, heart, lung and kidneys, which was blocked 63-87% by preadministration of cold PK11195. Single photon emission computer tomography (SPECT) imaging of the canine heart has been accomplished with [123I]PK11195. These results suggest that [123I]PK11195 has potential as a SPECT radiotracer for studying the omega 3 receptor in humans.

In vitro and in vivo studies of benzisoquinoline ligands for the brain synaptic vesicle monoamine transporter.

Tetrabenazine is a high-affinity inhibitor of the vesicular monoamine transporter in mammalian brain. As part of a program to develop in vivo imaging agents for these transporters in human brain, a series of 2-alkylated dihydrotetrabenazine ligands was synthesized and evaluated in vitro and in vivo for binding to the brain vesicular monoamine transporter. Additions of organometallic reagents to tetrabenazine produced 2-methyl, 2-ethyl, 2-n-propyl, 2-isopropyl, and 2-isobutyl derivatives of dihydrotetrabenazine. The stereochemistry and conformation of the addition products were thoroughly verified by two-dimensional NMR techniques. All of these alkyl derivatives displayed in vitro affinity for the vesicular monoamine transporter binding site in rat brain using competitive assays with the radioligand [3H]methoxytetrabenazine. Except for the isopropyl derivative, all compounds when tested at 10 mg/kg iv showed an ability to inhibit in vivo accumulation of the radioligand [11C]methoxytetrabenazine in the mouse brain striatum. Derivatives with small alkyl groups (methyl, ethyl) were more effective than those with large groups (propyl, isobutyl). These studies suggest that large groups in the 2-position of the benzisoquinoline structure will significantly diminish both in vitro and in vivo binding of these compounds to the vesicular monoamine transporter.

Mutant mouse strains as models for in vivo radiotracer evaluations: [11C]methoxytetrabenazine ([11C]MTBZ) in tottering mice.

The regional brain distribution of [11C]methoxytetrabenazine ([11C]MTBZ), a high affinity radioligand for the vesicular monoamine transporter, has been examined in normal and tottering mice, which are neurological mutants. The tottering mice show significantly higher radiotracer accumulation (150-195% of controls) in all brain regions examined (striatum, cortex, hippocampus, hypothalamus, cerebellum and thalamus). The increases in [11C]MTBZ binding correlate (r = 0.91) with the reported increases in norepinephrine concentrations in these regions, and are consistent with the noradrenergic hyperinnervation characteristic of the tottering mutant mouse. These studies demonstrate that a mutant mouse model of increased innervation can be used to evaluate the sensitivity of an in vivo radiotracer measure, in this case [11C]MTBZ binding to vesicular transporters, to an increase in the numbers of binding sites in specific regions of the brain.

Synthesis of carbon-11-, fluorine-18-, and iodine-125-labeled GABAA-gated chloride ion channel blockers: substituted 5-tert-butyl-2-phenyl-1,3-dithianes and -dithiane oxides.

A series of substituted 5-tert-butyl-2-phenyl-1,3-dithianes and 5-tert-butyl-2-phenyl-1,1,3,3-tetraoxo-1,3-dithianes was synthesized as ligands for the GABAA receptor complex-associated neuronal chloride ion channels. The in vitro binding affinities of these compounds for the GABA-gated chloride ion channel were determined by their ability to compete with [3H]TBOB for binding to rat brain slices. Of the eight compounds tested, trans-5-tert-butyl-2-(4-cyanophenyl)-2-methyl-1,1,3,3-tetraoxo+ ++-1,3-dithiane, 9b, trans-5-tert-butyl-2-(4-fluorophenyl)-1,1,3,3-tetraoxo-1,3-dithian e, 10, and trans-5-tert-butyl-2-(4-iodophenyl)-2-methyl-1,1,3,3-tetraoxo-1,3- dithiane, 11, showed moderately high binding affinities (Ki = 41, 180, and 105 nM, respectively). Four radioligand candidates from this series, 5-tert-butyl-2-(4-cyanophenyl)-2-[11C]methyl-1,3-dithiane, [11C]6, 5-tert-butyl-2-(4-[18F]fluorophenyl)-1,3-dithiane, [18F]7, 5-tert-butyl-2-(4-[18F]-fluorophenyl)-1,1,3,3-tetraoxo-1,3- dithiane, [18F]10, and 5-tert-butyl-2-(4-[125I]iodophenyl)-2-methyl-1,1,3,3- tetraoxo-1,3-dithiane, [125I]11, have been successfully prepared for evaluation as in vivo imaging agents useful for positron emission tomography and single photon emission computed tomography. Preliminary in vivo studies indicate significant uptake into mouse brain for [18F]7, [18F]10, and [125I]11.