High performance liquid chromatography of carbon-11 labeled thymidine and its major catabolites for clinical PET studies

The identity and pharmacology of the rapidly formed radiolabeled metabolites formed following i.v. injection of C-11 thymidine (TdR) labeled in the 5-methyl-position are being studied in animal models and patients with cancer. A high performance liquid chromatographic (HPLC) procedure has been developed for the separation of these metabolites including thymine, dihydrothymine, beta-ureidoisobutyric acid, and beta-aminoisobutyric acid from plasma and tissue extracts. Information obtained regarding the pharmacokinetics of the metabolites are being used to generate mathematical models for C-11 TdR incorporation rates into DNA.

Myocardial perfusion with [11C]methyl triphenyl phosphonium: measurements of the extraction fraction and myocardial uptake

The present study describes extraction fraction and uptake measurements of the [11C]methyl triphenyl phosphonium (11C-MTP), a promising positron emission tomography (PET) agent for cardiac imaging. PET imaging was performed in mongrel dogs. Under physiological flow conditions 11C-MTP uptake reached a maximum within the first 10 minutes after injection and remained constant during the entire observation period of 80 minutes. Over the same time period, the heart/blood ratio was 46-106:1, and the heart/lung ratio 14:1. Following permanent occlusion of the left anterior descending coronary artery, 11C-MTP uptake in the normally perfused myocardium also reached a maximum at 10 minutes after injection, whereas in the infarcted area there was no significant accumulation of 11C-MTP. For a time period of 80 minutes the noninfarcted/infarcted myocardium ratio was 12:1. Extraction was measured in anesthetized dogs with a double isotope method using 99mTc-HSA as the reference tracer. The extraction fraction was 91% at a flow of 69 mL/min/100g. As flow increased to five-fold (342 mL/min/100g) following administration of adenosine, extraction fell to 61%. Following coronary artery occlusion, the 11C-MTP content in the myocardium was highly correlated (r = 0.93, p < 0.01; y = 10.46 + 0.92x) with the microsphere determined regional myocardial blood flow.

Multimodality correlative study of canine brain tumors. Proton magnetic resonance spectroscopy, positron emission tomography, and histology

RATIONALE AND OBJECTIVES

Structural/functional relationships in an induced canine brain tumor were studied using proton-magnetic resonance spectroscopy (1H-MRS), positron emission tomography (PET), and histology.

METHODS

Proton-MRS and PET data of implanted canine brain tumors were correlated with quantitative analysis of the tissue composition within the MRS and PET regions of interest (ROIs). Linear regression analysis was employed to correlate the 1H-MRS and PET data with the percent tumor and the percent total lesion (comprising tumor plus associated pathology ie, edema, cysts, hemorrhage, inflammation) within the ROI.

RESULTS

Using 1H-MRS, N-acetyl aspartate concentrations were indirectly correlated with the amount of tumor (P = .058), as well as the amount of tumor plus associated pathology (P = .032) within the ROI. Total creatine concentrations were indirectly correlated with the amount of tumor and the amount of tumor plus associated pathology within the ROI (P < .05). Lactate concentrations were directly correlated with the amount of tumor (P = .053) and the amount of tumor plus associated pathology (P = .058) within the ROI. Using PET, Oxygen metabolic rates were indirectly correlated with the amount of tumor and with the amount of tumor plus associated pathology within the ROI (P < .05). Glucose metabolic rates were directly correlated with both the amount of tumor and with the amount of tumor plus associated pathology at P < .05. Proton-MRS measured concentrations of choline and PET measured values for blood flow, and oxygen extraction showed correlations with the amount of tumor and with the amount of tumor plus associated pathology at P > or = .08.

CONCLUSIONS

The PET and MRS data were complementary with respect to suggesting anaerobic glucose metabolism for the tumor. Unlike other tumors, no increase in choline was noted in the canine tumor.

Comparison of [11C]diprenorphine and [11C]carfentanil in vivo binding to opiate receptors in man using a dual detector system

A simple dual detector coincidence system was used to measure the binding of [11C]carfentanil and [11C]diprenorphine to opiate receptors in normal volunteers before and after the administration of naloxone. Total radioactivity without naloxone and the ratio of total/non-specific radioactivity was 2 times greater for [11C]diprenorphine than [11C]carfentanil. The dose of naloxone required to maximally block specific [11C]diprenorphine binding was 10 times that for [11C]carfentanil, indicating that [11C]diprenorphine labels opiate receptor subtypes in addition to mu opiate receptors.

Buprenorphine reduces cerebral glucose metabolism in polydrug abusers

Buprenorphine is a mixed opioid agonist-antagonist, which acts as a partial mu agonist and a kappa antagonist. The present study evaluated the acute effects of buprenorphine on cerebral glucose metabolism (CMRglc) in six human substance abusers using a double-blind, placebo-controlled, counterbalanced, crossover design. Each subject participated in two positron emission tomographic (PET) studies, 1 week apart, following the injection of buprenorphine (1 mg, intramuscularly) and placebo. Buprenorphine significantly reduced CMRglc and the regional cerebral metabolic rate for glucose (rCMRglc) by up to 32% in all but three of 22 bilateral and in 4 midline regions (p < .05). No region showed an increase in rCMRglc. Buprenorphine also produced miosis, respiratory depression, and subjective ratings of euphoria and sedation in comparison to placebo (p < .05). These observations extend previous findings of reduced CMRglc following acute treatment with morphine and other nonopioid euphorigenic drugs.

Development of PET/SPECT ligands for the serotonin transporter

There is a great need for PET and SPECT ligands with high affinity and selectivity for the serotonin uptake site. These imaging agents would be useful in screening human populations at risk (e.g., individuals exposed to neurotoxic amphetamines such as MDMA and fenfluramine). Moreover, these radioligands would allow the study of serotonergic function in the normal living human brain, and they also would be useful in the examination of altered serotonergic neurotransmission in diseases such as depression and obsessive-compulsive and other neuropsychiatric disorders. Over the past several years, a number of radioligands have been studied in several laboratories for their in vivo binding to 5-HT uptake sites. Although [3H]paroxetine showed promising binding characteristics, conversion of authentic paroxetine into a PET or SPECT tracer turned out to be difficult and has not been achieved yet. Analogs of paroxetine displayed considerable loss of binding affinity and were, therefore, not useful for imaging purposes. For [11C]fluoxetine, [11C]citalopram, and cis-[11C]DDPI, target-to-nontarget (hypothalamus-to-cerebellar) ratios remained less than 2.0:1 over a 90-min period after injection. The most promising PET agents identified today are [11C]RTI-55 and [11C]McN-5652-X. [11C]RTI-55 labels both 5-HT and DA uptake sites. [11C]McN-5652-X is highly selective for 5-HT uptake sites, and its distribution is consistent with the neuroanatomical distribution of the 5-HT uptake site. Because [11C]McN-5652-Z is a racemic mixture of two stereoisomers, of which the (+) isomer (McN-5652-X) binds to the 5-HT uptake site in vivo and the (-) isomer (McN-5652-W) does not, the possibility exists that regional-specific binding can be determined by subtracting nonspecific binding of the (-) isomer from total radioactivity counts obtained with the (+) isomer. [11C]McN-5652-X is the best PET radioligand for the 5-HT uptake site described thus far. This tracer warrants further testing in nonhuman primates. Efforts are underway to obtain an investigational new drug application for use of the tracer in humans. Promising candidates as SPECT imaging agents for the 5-HT uptake site are [123I]RTI-55 and [123I]-iodo-6-nitroquipazine. Both agents are under intense investigation in different laboratories in the United States.

(+)-[C-11]-cis-N-benzyl-normetazocine: a selective ligand for sigma receptors in vivo

The in vivo biodistribution profile of the novel sigma (sigma) receptor ligand (+)-[C-11]-cis-N-benzyl-normetazocine ([C-11]-(+)-NBnNM) in mouse brain was examined. This radioligand displayed high brain uptake and a distribution consistent with the density of sigma receptors. Brain radioactivity levels peaked at 15 min postinjection and were largely maintained (ca. 80% of maximal values) up to 90 min postinjection. Pretreatment with several different sigma ligands (haloperidol, (+)-pentazocine, DuP 734, ifenprodil) effectively inhibited [C-11]-(+)-NBnNM binding in a dose-dependent manner in all brain regions. [C-11]-(+)-NBnNM binding sites were shown to be saturable with unlabeled (+)-NBnNM (ED50 = 0.02 mg/kg) and enantioselectively inhibited by the optical isomers of pentazocine. A blocking dose of the dopamine D2 antagonist spiperone (1 mg/kg) did not significantly inhibit [C-11]-(+)-NBnNM binding. Pretreatment with the phencyclidine (PCP) blocker 1-[1-(2-thienyl)cyclohexyl] piperidine (TCP) did not significantly alter total brain tissue radioactivity. Thus, [C-11]-(+)-NBnNM binds with high specificity and selectivity to sigma receptors in vivo and offers excellent potential to study sigma receptors in living human brain via positron emission tomography.

Dopamine D2 receptor density estimates in schizophrenia: a positron emission tomography study with 11C-N-methylspiperone

Positron emission tomography (PET) with 11C-N-methylspiperone as the radioligand was carried out in 25 chronic schizophrenic patients to determine dopamine D2 receptor density estimates in the corpus striatum. The sample included 18 neuroleptic-naive and 7 neuroleptic-free patients. Dopamine D2 receptor density estimates (Bmax) were obtained using a two-scan/four-compartment model. The Bmax estimates for the entire group (33.39 +/- 3.43 pmole/g) were significantly elevated when compared with estimates for the control group (Bmax = 15.63 +/- 2.38). The Bmax values for the entire group of schizophrenic patients showed a significant decline as a function of age. The Bmax values were significantly related to duration of illness (y = 13.2 + 10.3795x - 0.7931x2; r = 0.48). Thirteen patients and seven control subjects were added to our original publication sample (Wong et al., 1986c). The patients' Bmax values, when adjusted for age and sex effects, were significantly different compared with those of control subjects. Clinical data from the entire group were compared with published data from other research groups that have estimated dopamine D2 receptor density using different radioligands and different methods of data analysis. Comparisons of the clinical characteristics of the published studies show significant differences in patient populations, suggesting that discrepancies among published studies may reflect, in part, heterogeneity among groups of schizophrenic patients. The D2 receptor abnormality described in this study may be a late manifestation of disease, and the implications of this observation are discussed.

Assessment of brain muscarinic acetylcholinergic receptors in living mice using a simple probe, [125I]-4-iododexetimide and [125I]-4-iodolevetimide

This study describes assessment of brain muscarinic acetylcholinergic receptors in living mice using a single-crystal radiation detection system, the high-affinity antagonist [125I]-4-iododexetimide, and the inactive enantiomer [125I]-4-iodolevetimide. Kinetics of radioligand binding, as well as perturbation by atropine displacement, can be determined using this simple probe technique.

In vivo imaging of baboon and human dopamine transporters by positron emission tomography using [11C]WIN 35,428

[11C]WIN 35,428 was evaluated as a specific in vivo radioligand for the dopamine transporter site by PET scanning in nonhuman primates and humans. In studies with a baboon (Papio anubis), [11C]WIN 35,428 accumulated in brain regions containing dopamine transporters, i.e., the striata. This accumulation was partially blocked by prior administration of (-)cocaine (4 mg/kg, i.v.). Placement of a unilateral lesion of dopamine-containing nerve terminals with MPTP resulted in a unilateral reduction in [11C]WIN 35,428 accumulation in the striatum on the side of the lesion. Imaging of D2 dopamine receptors with [11C]NMSP in the same MPTP-treated animals showed much less reduction in the postsynaptic D2 dopamine receptors as compared to the much larger reduction in the dopamine transporters labeled with [11C]WIN 35,428. A total of ten normal human volunteers (five males and five females) with ages ranging from 19 to 81 years were studied. The caudate/cerebellar and putamen/cerebellar ratios ranged from 4.4 to 5.7 90 min after injection of the tracer. Preliminary kinetic modeling with arterial plasma sampling resulted in an average binding potential (k3/k4) of 4.98 in the caudate nucleus and 5.13 in putamen. To demonstrate in vivo blockade with dopamine reuptake inhibitors, two subjects received prior oral doses of 6 mg mazindol. Subject 5 had significant reductions of 29% in the caudate/cerebellar ratio at 90 min, 35% in the putamen/cerebellar ratio at 90 min, 45% in the caudate k3/k4 ratio from 6.7 to 3.7, and 46% in the putamen k3/k4 from 4.7 to 2.5. Subject 8 had significant reductions of 20% in both the caudate/cerebellar ratio and the putamen/cerebellar ratio at 90 min. During the human PET studies, a number of neuropsychological tests and physiological measurements were performed. No significant changes were found after administration of the [11C]WIN 35,428 alone. Taken together, these data indicate that [11C]WIN 35,428 is a promising radioligand for future studies of neuropsychiatric disorders that involve the dopamine transporter site.

PET-FDG test-retest reliability during a visual discrimination task in schizophrenia

OBJECTIVE

Reliability of assessment is important in any kind of neuropsychiatric study, but is particularly pivotal in schizophrenia research where symptom instability is common.

MATERIALS AND METHODS

Two fluorodeoxyglucose PET scans, 1 week apart, were carried out in schizophrenic patients while they were performing a simple visual discrimination task. Subject and scan conditions were held constant. Regional metabolic rates of glucose utilization were calculated as absolute and scaled; they were compared using correlational statistics.

RESULTS

Average differences between scans were 5-9% for the parietal and occipital cortices, but 1-3% for other cortical areas, differences comparable with reported variances in normal controls.

CONCLUSION

These results suggest that test-retest variance in metabolic imaging in schizophrenia is relatively low and that task performance increases metabolic stability in brain areas unrelated to task performance.

Positron emission tomographic imaging of the dopamine transporter with 11C-WIN 35,428 reveals marked declines in mild Parkinson's disease

Parkinson's Disease (PD) is characterized by a selective loss of nigrostriatal dopaminergic neurons that results in a marked reduction of dopaminergic nerve terminals in the striatum. Recently, 11C-WIN 35,428, a cocaine analogue that specifically labels the dopamine transporter, was developed and can be used to label dopaminergic nerve terminals in vivo by positron emission tomography. In healthy control subjects, binding of 11C-WIN 35,428 is highest in the striatum. In addition, 2 symmetrical focal areas of low binding were observed in the midbrain. The cerebellum functioned as an appropriate region for nonspecific binding. The binding of 11C-WIN 35,428 in patients with PD (Hoehn-Yahr II) was compared with that in healthy control subjects by using the (region-cerebellum)/cerebellum ratio for data acquired 34 to 82 minutes after injection. In control subjects, this ratio varied, at approximately 2, in the striatum. In patients with PD, binding in the posterior putamen was reduced by 78%, whereas the anterior putamen and the caudate nucleus showed a reduction of 59 and 39%, respectively. The reduction in 11C-WIN 35,428 binding was highest in the midbrain (84%). The high specific/nonspecific binding ratio and the pronounced reduction in binding in mild PD may permit detection of even earlier stages of PD or presymptomatic individuals with dopaminergic cell loss.

Decreased hippocampal muscarinic cholinergic receptor binding measured by 123I-iododexetimide and single-photon emission computed tomography in epilepsy

Regional binding of 123I-iododexetimide, a muscarinic acetylcholine receptor antagonist, was measured in vivo in the temporal lobes of 4 patients with complex partial seizures using single-photon emission computed tomography. In the anterior hippocampus ipsilateral to the electrical focus, 123I-iododexetimide binding was decreased by 40 +/- 9% (mean +/- SD, p < 0.01) compared with the contralateral hippocampus; 123I-iododexetimide binding in other temporal lobe regions was symmetrical. The data indicate a regionally specific change of muscarinic acetylcholine receptor in anterior hippocampus in complex partial seizures of temporal lobe origin.

Measurement of benzodiazepine receptor number and affinity in humans using tracer kinetic modeling, positron emission tomography, and [11C]flumazenil

Kinetic methods were used to obtain regional estimates of benzodiazepine receptor concentration (Bmax) and equilibrium dissociation constant (Kd) from high and low specific activity (SA) [11C]flumazenil ([11C] Ro 15-1788) positron emission tomography studies of five normal volunteers. The high and low SA data were simultaneously fit to linear and nonlinear three-compartment models, respectively. An additional inhibition study (pretreatment with 0.15 mg/kg of flumazenil) was performed on one of the volunteers, which resulted in an average gray matter K1/k2 estimate of 0.68 +/- 0.08 ml/ml (linear three-compartment model, nine brain regions). The free fraction of flumazenil in plasma (f1) was determined for each study (high SA f1: 0.50 +/- 0.03; low SA f1: 0.48 +/- 0.05). The free fraction in brain (f2) was calculated using the inhibition K1/k2 ratio and each volunteer's mean f1 value (f2 across volunteers = 0.72 +/- 0.03 ml/ml). Three methods (Methods I-III) were examined. Method I determined five kinetic parameters simultaneously [K1, k2, k3 (= konf2Bmax), k4, and konf2/SA] with no priori constraints. An average kon value of 0.030 +/- 0.003 nM-1 min-1 was estimated for receptor-rich regions using Method I. In Methods II and III, the konf2/SA parameter was specifically constrained using the Method I value of kon and the volunteer's values of f2 and low SA (Ci/mumol). Four parameters were determined simultaneously using Method II. In Method III, K1/k2 was fixed to the inhibition value and only three parameters were estimated. Method I provided the most variable results and convergence problems for regions with low receptor binding. Method II provided results that were less variable but very similar to the Method I results, without convergence problems. However, the K1/k2 ratios obtained by Method II ranged from 1.07 in the occipital cortex to 0.61 in the thalamus. Fixing the K1/k2 ratio in Method III provided a method that was physiologically consistent with the fixed value of f2 and resulted in parameters with considerably lower variability. The average Bmax values obtained using Method III were 100 +/- 25 nM in the occipital cortex, 64 +/- 18 nM in the cerebellum, and 38 +/- 5.5 nM in the thalamus; the average Kd was 8.9 +/- 1.0 nM (five brain regions).

A PET radiotracer for studying serotonin uptake sites: carbon-11-McN-5652Z

A radioligand for imaging central serotonin (5-hydroxytryptamine; 5-HT) uptake sites by positron emission tomography (PET) has yet to be developed. Such a tracer would be useful for the study of normal and altered serotonergic neurotransmission as well as for the detection of serotonergic neurotoxicity. This paper describes the labeling of the highly potent serotonin (5-HT) uptake blocker, McN-5652-Z (trans-1,2,3,5,6,10 beta-hexahydro-6-[4-(methylthio)phenyl]pyrrolo-[2,1-a]-isoquinoline; racemic mixture), with 11C and the evaluation of this radiotracer in rodents with respect to its in vivo binding characteristics. In mouse brain, 11C-McN-5652-Z accumulated rapidly in regions with high densities of 5-HT uptake sites. The ratio between hypothalamus and cerebellum was 1.5:1 at 15 min and increased with time to 4.6:1 at 90 min after injection. The distribution of 11C-McN-5652 in rat brain at 60 min correlated well with regional concentrations of 5-HT uptake sites (r = 0.86). The specificity and selectivity of 11C-McN-5652 binding to the 5-HT transporter were tested by preinjecting blocking doses of known 5-HT, dopamine and norepinephrine uptake inhibitors, and a 5-HT2 receptor blocker before injection of the radiotracer. Preinjection of increasing doses of unlabeled McN-5652-Z inhibited 11C-McN-5652-Z binding in a dose-dependent fashion. These results suggest that the in vivo binding of the radiotracer was specific, selective for 5-HT uptake sites, saturable and that 11C-McN-5652-Z holds promise as a radiotracer for PET imaging of 5-HT uptake sites in the mammalian brain.

A preliminary study of cortical S2 serotonin receptors and cognitive performance following stroke

Cortical serotonin receptor binding was measured with positron-emission tomography (PET) in uninjured regions of cortex in 26 stroke patients. Cognitive function was assessed with the Mini-Mental State Exam (MMSE) and a neuropsychological test battery. Left frontal cortex serotonin binding was correlated positively with MMSE total score (r = 0.50, P = 0.01) and with the MMSE concentration, writing, and copying tasks (r = 0.42, 0.56, 0.53, respectively; P < 0.05). Tests of orientation and repetition of difficult phrases were significantly correlated with serotonin binding (r = 0.53 and 0.52, respectively; P < 0.05). These findings suggest that cognitive performance after stroke may be influenced by alterations in the serotonergic system.

[11C](+)McN5652 as a radiotracer for imaging serotonin uptake sites with PET

The in vivo behavior of the stereoisomers of [11C]McN5652, a highly potent serotonin (5-HT) uptake blocker, was determined to evaluate their utility as radiotracers for imaging 5-HT uptake sites by positron emission tomography (PET). After intravenous injection into mice, [11C](+)McN5652 showed markedly higher uptake and longer retention in regions with high density of 5-HT uptake sites than the [11C]-labeled racemic mixture, while [11C](-)McN5652 washed out rapidly. With the [11C](+)-enantiomer, the ratio between hypothalamus and cerebellum reached 6 at 90 minutes. The binding of [11C](+)McN5652 was inhibited by 45-73% by pre-injection of 5 mg/kg of paroxetine, a selective 5-HT uptake blocker, in all regions examined except cerebellum where no significant effect of the drug was observed. [11C](-)McN5652 showed no specific binding in any of the regions. The [11C]-labeled cis isomer, [11C]McN5655, revealed surprisingly low brain penetration and showed no significantly higher uptake in regions of interest than cerebellum. These results suggest that [11C](+)McN5652 is a promising candidate as a PET radiotracer for studying 5-HT uptake sites in vivo.

In vivo studies of [125I]iodobenzamide and [11C]iodobenzamide: a ligand suitable for positron emission tomography and single photon emission tomography imaging of cerebral D2 dopamine receptors

Iodobenzamide (IMB) labeled with either [11C] or [125I] was studied in mice and baboons. Pharmacological studies demonstrated an in vivo binding profile compatible with D2 dopamine receptors. Mouse biodistribution studies with both [11C]IMB and [125I]IMB showed a similar brain distribution of radioactivity. Mouse [125I]IMB studies with amphetamine and reserpine pretreatment suggested that IMB may be less susceptible to endogenous dopamine competition for D2 receptor binding in vivo as compared to raclopride. Preliminary baboon studies showed haloperidol competition for IMB binding sites.

Dopamine transporter imaging with novel, selective cocaine analogs

RTI-121 and RTI-122 are 3 beta-substituted phenyltropane analogs of cocaine that have high, selective binding affinity for dopamine transporters. [123I]RTI-121 and [123I]RTI-122 bind to dopamine transporters in vivo after intravenous administration and permit imaging of the transporters.

Effects of vasopressin on blood-brain transfer of methionine in dogs

We used a simplified probe detection system for positron-emitting radionuclides in order to measure blood-brain barrier transport of amino acids in anesthetized dogs. Plasma and brain time-activity curves were recorded after intravenous bolus injection of L-[11C]methionine before and after administration of 1 microgram of vasopressin. Three-compartment models with three or four transfer coefficients were used to derive the kinetics of L-[11C]methionine uptake in brain. The blood-brain clearance of the tracer (K1) was 0.075 ml ml-1 min-1 before and 0.041 ml ml-1 min-1 after injection of vasopressin. The partition volume and the initial distribution (plasma) volume of methionine were unchanged and within the expected limits. The net accumulation rate of methionine (K), estimated by both the four-parameter (kinetic) and three-parameter (graphic) approaches, decreased after vasopressin injection in all six studies.

Paralimbic frontal lobe hypometabolism in depression associated with Huntington's disease

We measured regional cerebral glucose metabolism using 2-[18F]-fluoro-2-deoxy-D-glucose and positron emission tomography in depressed and nondepressed patients with early Huntington's disease (HD), compared with appropriately matched controls. Caudate, putamen, and cingulate metabolism was significantly lower in patients with HD than in control subjects, independent of mood state. Orbital frontal-inferior prefrontal cortex hypometabolism, however, differentiated depressed patients from both nondepressed patients and normal controls. These findings implicate selective dysfunction of the paralimbic regions of the frontal lobes in the mood disorder of HD. The metabolic pattern is similar to that in depression associated with Parkinson's disease, suggesting that the integrity of pathways linking paralimbic frontal cortex and the basal ganglia may be integral to the normal regulation of mood.

Imaging muscarinic cholinergic receptors in human brain in vivo with Spect, [123I]4-iododexetimide, and [123I]4-iodolevetimide

A method to image muscarinic acetylcholine receptors (muscarinic receptors) noninvasively in human brain in vivo was developed using [123I]4-iododexetimide ([123I]IDex), [123I]4-iodolevetimide ([123I]ILev), and single photon emission computed tomography (SPECT). [123I]IDex is a high-affinity muscarinic receptor antagonist. [123I]ILev is its pharmacologically inactive enantiomer and measures nonspecific binding of [123I]IDex in vitro. Regional brain activity after tracer injection was measured in four young normal volunteers for 24 h. Regional [123I]IDex and [123I]ILev activities were correlated early after injection, but not after 1.5 h. [123I]IDex activity increased over 7-12 h in neocortex, neostriatum, and thalamus, but decreased immediately after the injection peak in cerebellum. [123I]IDex activity was highest in neostriatum, followed in rank order by neocortex, thalamus, and cerebellum. [123I]IDex activity correlated with muscarinic receptor concentrations in matching brain regions. In contrast, [123I]ILev activity decreased immediately after the injection peak in all brain regions and did not correspond to muscarinic receptor concentrations. [123I]IDex activity in neocortex and neostriatum during equilibrium was six to seven times higher than [123I]ILev activity. The data demonstrate that [123I]IDex binds specifically to muscarinic receptors in vivo, whereas [123I]ILev represents the nonspecific part of [123I]IDex binding. Subtraction of [123I]ILev from [123I]IDex images on a pixel-by-pixel basis therefore reflects specific [123I]IDex binding to muscarinic receptors. Owing to its high specific binding, [123I]IDex has the potential to measure small changes in muscarinic receptor characteristics in vivo with SPECT. The use of stereoisomerism directly to measure nonspecific binding of [123I]IDex in vivo may reduce complexity in modeling approaches to muscarinic acetylcholine receptors in human brain.

Factor analysis of regional cerebral glucose metabolic rates in healthy men

Cerebral glucose utilization measured with fluorine-18-fluoro-2-deoxy-D-glucose is characterized by considerable variability both among different persons and for the same person examined on different occasions. The goal of this study was to explore whether some regions of the brain were more variable than others with respect to glucose utilization and whether there was a pattern in their covariance. The global and regional cerebral utilization of glucose was measured in 12 healthy young volunteers on 3 or 4 occasions. In all, 24 regions were examined. The interrelation of the glucose utilization rates of the brain regions was investigated by factor analysis of the metabolic rates. Some 70% of the total variance was attributable to only 1 factor, while 80% of the total variance could be attributed to 2 factors. Regions making up the first factor were the frontal and temporal cortex, cingulate gyrus, caudate nucleus, thalamus and putamen. These regions are functionally related to the limbic system. Regions of the second factor were the parietal cortex, occipital cortex and cerebellum, regions more clearly related to sensory and motor functions. The 2-factor pattern was highly reproducible, being found with different algorithms for factor extraction and rotation. Under resting conditions, the variance of cerebral metabolism seems to be primarily related to regions which are closely involved with the limbic system. Cortical regions involved primarily in motor and sensory functions have less influence on the variance.

Synthesis and biodistribution of a new radiotracer for in vivo labeling of serotonin uptake sites by PET, cis-N,N-[11C]dimethyl-3-(2',4'-dichlorophenyl)-indanamine (cis-[11C]DDPI)

A new PET radiotracer for in vivo labeling of serotonin (5-HT) uptake sites, cis-N,N-[11C]dimethyl-3-(2',4'-dichlorophenyl)-indanamine, cis-[11C]DDPI, was synthesized and its biological behavior was studied. The radiosynthesis of cis-[11C]DDPI was performed by N-methylation of cis-N-methyl-3-(2',4'-dichlorophenyl)-indanamine with [11C]iodomethane. The average radiochemical yield was approx. 8%, with an average specific activity of 600 mCi/mumol. Following intravenous administration, cis-[11C]DDPI accumulated in mouse brain regions rich in 5-HT uptake sites, such as olfactory tubercles, hypothalamus and frontal cortex. Following pre-injection of 1 mg/kg of paroxetine, a high affinity 5-HT uptake blocker, the binding of cis-[11C]DDPI in the olfactory tubercles, hypothalamus and frontal cortex was decreased by 23, 25 and 16%; this corresponds to 73, 82 and 59% of the specific binding in these regions. These results suggest that the accumulation of cis-[11C]DDPI in the tissues rich in 5-HT sites is a result of specific binding of cis-[11C]DDPI to 5-HT uptake sites. Due to the relatively high non-specific uptake and slow clearance of this compound from non-specific binding sites, the ratio between specific and non-specific binding increased slowly with time, reaching 1.5:1 at 60 min after injection.