Placental transport of vitamin B12 in the pregnant rat.

Placental transport of vitamin B(12) was studied in the pregnant rat in two series of experiments. In the first series animals were given cyanocobalamin-(57)Co intravenously at various stages of gestation. High specific activity tracer was used and doses of B(12) were 1-2 ng per animal. The rats were killed from 15 min to 24 hr after injection and the fetuses, placentas, and serum were assayed for radioactivity. In the second series using uninjected animals, absolute amounts of vitamin B(12) in fetuses and placentas were measured at stages of gestation from day 12 through day 20. There was a progressive increase in B(12) transferred to the fetus during gestation. Although the quantity of vitamin B(12) transported per 24 hr was proportional to fetal weight, the amount transported per gram of placenta increased tenfold from day 10 through day 19. Uptake of tracer B(12) by placenta was initially rapid; however, no radioactivity appeared in the fetus until 2 hr after injection. The actual amount of B(12) in placenta increased throughout gestation, and the placental concentration of B(12) was greater than maternal plasma and fetal tissue concentrations at all times measured. These data suggest that the ability of placenta to transport B(12) increased throughout gestation, and that the rate-limiting step in the transport process was either the passage of B(12) from the maternal to the fetal side of placenta or the transfer from placenta into fetal plasma.

99mTc labeling of proteins: initial evaluation of a novel diaminedithiol bifunctional chelating agent.

The initial evaluation of a novel thiolactone bifunctional chelating agent, 1, for facile introduction of the diaminedithiol (DADT) ligand system onto proteins for subsequent labeling with 99mTc under mild conditions is presented. For human serum albumin and a monoclonal anti-colon carcinoma antibody (B72.3), time-dependent coupling was achieved at pH 7-9 at room temperature. The coupled proteins could be labeled in a direct fashion (Sn2+/99mTcO4-) and by exchange labeling (with preformed 99mTc-glucoheptonate) in good to excellent yields at pH 7. In contrast, the unmodified proteins could be labeled only by direct methods with maximum yields of 30%. The labeled, coupled proteins were purified by high performance liquid chromatography and found to be stable in vitro over a 20-h period of study with no evidence for the loss of label. This exceptional stability is due to the chelation of 99mTc by the DADT ligand since 66% (3.5 h) and 47% (20 h) of the label were lost from directly labeled unmodified human serum albumin and the unmodified antibody, respectively. Biodistribution studies in normal mice and rabbits confirmed the stability of the 99mTc-DADT-labeled proteins compared to the 99mTc-labeled unmodified proteins in vivo. Thus, the novel thiolactone 1 is a useful bifunctional chelating agent for introducing the DADT ligand system onto a variety of proteins for subsequent incorporation of technetium under mild conditions such that the resultant labeled products are stable in vitro and in vivo.

Positron emission tomography of 5-HT transporter sites in the baboon brain with [11C]McN5652.

[11C]McN5652 is a new radioligand specific for 5-hydroxytryptamine (5-HT; serotonin) transporters. In this study we used [11C]McN5652 to image the 5-HT transporter sites in baboon brain by positron emission tomography (PET). Dynamic PET studies were performed in three Papio anubis baboons. The animals were injected intravenously first with 11C-labeled (+)-McN5652([11C](+)McN5652), then with pharmacologically inactive enantiomer 11C-labeled (-)-McN5652 ([11C](-)McN5652); two animals received a third study with [11C](+)McN5652 after pretreatment with the specific 5-HT uptake site inhibitor fluoxetine (5 mg/kg). Initial uptake into the brain was similar for both [11C](+)McN5652 and [11C](-)McN5652. At later times (45-120 min after injection), only [11C](+)McN5652 showed a distribution characteristic for 5-HT uptake sites. In contrast, in studies with [11C](-)McN5652 and in those with [11C](+)McN5652 after 5-HT uptake site blockade with fluoxetine, 11C radioactivity concentrations were significantly lower and the distribution pattern was relatively even. The differences between [11C](+)-and (-)McN5652 were calculated for the time interval 95-125 min postinjection and used to estimate specific binding. Specific binding correlated well (r = 0.95, p < 0.001) with the known density of 5-HT uptake sites in human brain. These results indicate that [11C](+)McN5652 is suitable for PET imaging of 5-HT uptake sites in primate brain.

Kinetic analysis of [11C]McN5652: a serotonin transporter radioligand.

The impulse response function of a radioligand is the most fundamental way to describe its pharmacokinetics and to assess its tissue uptake and retention pattern. This study investigates the impulse response function of [11C](+)McN5652, a radioligand used for positron emission tomography (PET) imaging of the serotonin transporter (SERT) in the brain. Dynamic PET studies were performed in eight healthy volunteers injected with [11C](+)McN5652 and subsequently with its pharmacologically inactive enantiomer [11C](-)McN5652. The impulse response function was calculated by deconvolution analysis of regional time-activity curves, and its peak value (f(max)), its retention value at 75 minutes (fT), and its normalized retention (f(rel) = fT/f(max)) were obtained. Alternatively, compartmental models were applied to calculate the apparent total distribution volume (DV(T)) and its specific binding component (DV(S)). Both the noncompartmental (fT,f(rel)) and the compartmental parameters (DV) were investigated with and without correction for nonspecific binding by simple subtraction of the corresponding value obtained with [11C](-)McN5652. The impulse response function obtained by deconvolution analysis demonstrated high tracer extraction followed by a slow decline in the form of a monoexponential function. Statistical analysis revealed that the best compartmental model in terms of analysis of variance F and condition number of the parameter variance-covariance matrix was the one that was based on a single tissue compartment with parameters k1 and k2 and that also included the parameter of regional cerebral blood volume (BV). The parameter f(rel) demonstrated low between-subject variance (coefficient of variation [CV] = 19%), a midbrain to cerebellum ratio of 1.85, and high correlation with the known density of SERT (r = 0.787 where r is the coefficient of linear correlation between the parameter and the known density of SERT). After correction for nonspecific binding, f(rel) demonstrated further improvement in correlation (r = 0.814) and midbrain to cerebellum ratio (3.09). The variance of the distribution volumes was acceptable when the logarithmic transform lnDV was used instead of DV (17% for the three-parameter model), but correlation of this compartmental parameter was slightly less (r = 0.652 for the three-parameter model) than the correlation of the noncompartmental f(rel) with the known density of SERT, and the midbrain to cerebellum ratio was only 1.5 (uncorrected) and 1.8 (corrected). At the expense of increasing variance, the correlation was increased after correction for nonspecific binding using the inactive enantiomer (r = 0.694; CV = 22%). These results indicate that the kinetics of [11C](+)McN5652 can best be described by a one-tissue compartment model with three parameters (k1, k2, and BV), and that both the noncompartmental parameter f(rel) and the compartmental distribution volumes have the potential for quantitative estimation of the density of SERT. Further validation of the radioligand in experimental and clinical situations is warranted.

Repeated administration of MDMA causes transient down-regulation of serotonin 5-HT2 receptors.

The present study examined short- and long-term effects of MDMA (3,4-methylene-dioxymethamphetamine) on serotonin (5-HT2 and 5-HT1c) receptors in the brain of the rat. N1-Methyl-2-[125I]lysergic acid diethylamide ([125I]MIL) was used to label these receptors in vitro and in vivo. The usefulness of [125I]MIL for in vivo detection of changes in 5-HT2 receptors was confirmed in preliminary experiments in which rats were treated chronically with mianserin (5 mg/kg, once daily for 10 days). Decreases in specific in vivo binding of [125I]MIL, after treatment with mianserin were found to be of the same magnitude as those determined by others, using in vitro methods. The MDMA (8 doses; 5-20 mg/kg each) was administered to rats over a period of 4 days. At various times after administration of the last dose of MDMA, the binding of [125I]MIL was measured. Acutely, treatment with MDMA (20 mg/kg) reduced specific in vivo binding of [125I]MIL in all regions of brain studied. For example, in the frontal cortex, specific binding of [125I]MIL was decreased by 80% at 6 hr and by 62% at 24 hr, after cessation of treatment with MDMA. Twenty-one days after administration of MDMA however, the number of binding sites for [125I]MIL was back to control levels. Reductions in in vivo binding of [125I]MIL in frontal cortex were dependent on the dose of MDMA injected and were associated with decreases in the number of binding sites for [125I]MIL (Bmax values) in tissue homogenates of the same area. Autoradiographic studies of MDMA-treated rats confirmed the decreased density of 5-HT2 receptors and also suggested that the 5-HT1c receptor of the choroid plexus was not affected. These results indicate that repeated administration of MDMA caused transient down-regulation of 5-HT2 receptors in the brain of the rat. Further, they demonstrated that [125I]MIL is a suitable radioligand for labeling 5-HT2 receptors, both in vitro and in vivo. Once labeled with an appropriate radionuclide for SPECT (single photon emission computed tomography) or PET (positron emission tomography), MIL should prove useful for monitoring changes in the density of serotonin receptors in the living mammalian brain.

Radiosynthesis, cerebral distribution, and binding of [125I]-1-(p-iodophenyl)-3-(1-adamantyl)guanidine, a ligand for sigma binding sites.

An analog of 1,3-di-o-tolylguanidine (DTG), [125I]-labeled 1-(p-iodophenyl)-3-(1-adamantyl)guanidine (PIPAG), was synthesized as a potential ligand for cerebral sigma binding sites. Data from in vitro binding experiments and in vivo experiments on brain distribution suggested that PIPAG binds to sigma binding sites with high affinity (Kd in low nanomolar range) as determined by Scatchard analysis and relative potencies of sigma-specific drugs. Haloperidol had the highest potency to inhibit [125I]PIPAG binding. It was followed by DTG, BMY 14802, and (+)-N-allylnormetazocine. Compounds with high affinities for dopamine receptors (but low affinity for sigma binding sites), for opioid receptors, for nicotinic acetylcholine receptors, and for phencyclidine receptors were ineffective inhibitors of [125I]PIPAG binding.

Synthesis and evaluation of N-[11C]methylated analogues of epibatidine as tracers for positron emission tomographic studies of nicotinic acetylcholine receptors.

Four halogen-substituted analogues of N-methylepibatidine, a nicotinic acetylcholine receptor (nAChR) ligand, were synthesized. They were (+/-)-exo-N-methyl-2-(2-halogeno-5-pyridyl)-7-azabicyclo[2. 2.1]heptanes, where halogeno = F (1a), Cl (2a), Br (3a), I (4a). (+/-)-N-Ethylepibatidine (2b) also was synthesized. The compounds 1a, 2a, 3a, and 4a and their corresponding normethyl analogues 1, 2, 3, and 4 inhibited the in vitro binding of [3H]epibatidine to nAChRs to a similar degree, with affinities in the 27-50 pM range. The binding affinity of N-ethylepibatidine (2b), however, was substantially lower. The N-[11C]methyl derivatives of 1, 2, and 3 were synthesized from high-specific radioactivity [11C]methyl iodide using a high-temperature/high-pressure technique. The corresponding radiolabeled compounds [11C]1a, [11C]2a, and [11C]3a were administrated to mice intravenously. The pattern of regional distribution of the three tracers in the mouse brain following intravenous administration matched those of [3H]epibatidine, [3H]norchloroepibatidine, and (+/-)-exo-2-(2-[18F]fluoro-5-pyridyl)-7-azabicyclo[2.2.1]heptane ([18F]FPH), which are highly specific nAChR probes. The initial brain uptake of the 11C analogues and the acute toxicity of the corresponding authentic nonlabeled compounds appeared to be related to their lipophilicity.

Mechanism of gallium-67 accumulation in tumors.

Neoplasms are characterized by increased perfusion, increased permeability of their capillary beds to macromolecules, and a delay in new lymphatic vessel growth. These lead to the increased entry and residency time of macromolecules in the interstitial space of tumors. Multiple factors contribute to the localization of 67Ga in tumors. Adequate blood supply is essential; at areas with no blood supply such as the necrotic center of a large tumor, there is no 67Ga accumulation. Gallium-67, mainly in the form of transferrin-67Ga complex, is delivered to the tumor through capillaries with increased permeability. In tumors, some 67Ga is taken up by tumor cells; some may also be taken up by inflammatory cells when they are present. Gallium-67 binding proteins, such as lactoferrin or ferritin, may also contribute to the accumulation and retention of 67Ga in tumors; however, their roles are less clear. The intensity of these various factors determine their relative contribution and the degree of 67Ga accumulation in tumors.

Labeling of human platelets with [111In) 8-hydroxyquinoline.

We have evaluated the factors influencing the labeling of human platelets in the presence of autologous plasma. The labeling efficiency was found to be dependent on a) the time and temperature of incubation, b) the platelet concentration, c) the concentration of citrate ions (in ACD anticoagulant), and d) the concentration of 8-hydroxyquinoline in the suspending medium. Contrary to what was expected, unsaturated transferrin was found not to interfere with the transfer of In-111 from the [111In] 8-hydroxyquinoline complex to the platelets. Based on the findings of this study, a protocol was established by which human platelets can be labeled with In-111 in plasma with a labeling efficiency of 55.5 +/- 9.3 (mean +/- 1 s.d.) percent.

Enhancement of hepatic gallium-67 uptake by asialo-transferrin.

A significant fraction of intravenously injected 67Ga localizes in the liver. The mechanism of 67Ga uptake by the liver is not clear. Hepatocyte membranes contain galactose-specific receptors which selectively remove asialo-glycoproteins from the circulation by way of endocytosis. In this investigation, we examined whether endocytic uptake of asialo-transferrin would involve 67Ga transport into hepatocytes. We demonstrated that asialo-transferrin bound 67Ga as effectively as apotransferrin. Human asialo-transferrin markedly enhanced 67Ga uptake by isolated, perfused rat livers. Human asialo-orosomucoid, but not orosomucoid competitively inhibited hepatic uptake of the 67Ga asialo-transferrin complex, indicating that hepatic 67Ga uptake in the presence of asialo-transferrin occurred by way of galactose-specific receptors. Our results point to a novel pathway for metal ion transport into hepatocytes by way of galactose receptor mediated endocytosis.

Gallium-67 uptake by the liver: studies using isolated rat hepatocytes and perfused livers.

We studied the hepatic uptake of carrier-free Ga-67 using isolated rat hepatocytes and perfused livers. The results indicate that: (a) the liver can take up Ga-67 and secrete it into the bile, even in the absence of transferrin; (b) transferrin inhibits hepatic uptake of Ga-67 and its biliary excretion; and (c) iron deficiency markedly enhances hepatic uptake of Ga-67.

Gallium-67 uptake by hepatoma: studies in cell cultures, perfused livers, and intact rats.

In this investigation, the effect of transferrin on 67Ga uptake by rat hepatoma was studied at three levels: at the level of individual tumor cells in culture; at the level of isolated, perfused livers with implanted intrahepatic tumors; and in intact animals bearing intrahepatic tumors. This approach was possible using H-4-II-E hepatoma cells which grew into discrete tumors when implanted intrahepatically. Transferrin at low concentrations (0.05-0.5 mg/ml) stimulated, while at a higher concentration (1.0 mg/ml) it inhibited 67Ga uptake by tumor cells in culture. In contrast, in isolated, perfused livers with intrahepatic tumors, transferrin at concentration levels of 0.05 and 0.1 mg/ml had no effect, while at 0.25-1.0 mg/ml transferrin inhibited 67Ga uptake by intact tumors. Administration of transferrin which markedly enhanced the serum unsaturated iron binding capacity, had no effect on 67Ga accumulation in the intrahepatic tumors in vivo. These results indicate that, although transferrin at low concentration promotes the uptake of 67Ga by individual tumor cells in culture, it does not do so in intact tumors in isolated rat liver preparations or in tumor bearing rats. We conclude that the mechanism of 67Ga uptake by intact tumors is different from that of tumor cells growing in culture.

The source of gallium-67 in gastrointestinal contents: concise communication.

The sources of Ga-67 in gastrointestinal (GI) contents, and factors affecting its secretion were studied in rats. To prevent loss of fecal Ga-67, the anus was sutured before intravenous injection of Ga-67 citrate. Secretion of Ga-67 into the contents of the GI tract was rapid, 3, 6, and 9% of the injected dose were secreted at 1, 6, and 24 hr after injection, respectively. In contrast, Ga-67 concentration in the GI tissues remained relatively constant throughout this period. Analysis of Ga-67 contents of various parts of the GI trace revealed that small intestine is its major source, contributing 60% while the bile contributes 20%, large intestine 10%, GI contents. In contrast, the serum unbound iron-binding capacity (UIBC) played an important role in the Gl secretion of Ga-67 reducing the serum UIBC reduced the Ga-67 secretion into GI contents.

Studies on gallium accumulation in inflammatory lesions: I. Gallium uptake by human polymorphonuclear leukocytes.

The mechanism of ionic gallium-67 localization in inflammatory lesions was studied. Human polymorphonuclear leukocytes (PMN) had higher Ga-67 uptake than lymphocytes, whereas red blood cells had no affinity for Ga-67. Uptake by PMN showed temperature dependence, was independent of Ga-67 concentrations, and was not inhibited by metabolic inhibitors. However, its binding to PMN could be removed by trypsin but not by neuraminidase. These results are consistent with the hypothesis that the plasma membrane serves as a diffusion barrier and Ga-67 only binds to the surface of the PMN plasma membrane. When this membrane's permeability barrier was disrupted, as in heat-killed PMN, Ga-67 uptake increased markedly. Experimental abscesses were induced with E. coli or turpentine in rabbits. Twenty-four hours after i.v. injection, only 20% of Ga-67 in abscesses was in fractions containing intact PMN, cell debris or bacteria; the remainder was in a soluble, non-cellular fraction (2,500-g supernatant).

Comparison of toadfish-serum competitive binding and microbiologic assays of vitamin B12.

Toadfish serum (TFS) offers several advantages over other proteins as the binder in a competitive-binding assay for vitamin B12. It is unaffected by pH changes in the range 5.6-9.4 or by the addition of human serum albumin. Prolonged incubation with charcoal does not disrupt the TFS-cyanocobalamin bond, and the addition of albumin as a protein source in the standard tubes was proven unnecessary. The binding capacity of TFS does not increase significantly with increasing concentrations of cyanocobalamin as does the binding capacity of intrinsic factor, normal serum, or transcobalamin I. A single extract was prepared from each of 44 sera and measured for vitamin B12 content simultaneously by the TFS assay and the conventional microbiologic method using Lactobacillus leichmannii. The values obtained with TFS were in each instance higher than those obtained by the microbiologic assay (p less than 0.001).

Accumulation of indium-111-labeled neutrophils and gallium-67 citrate in rabbit abscesses.

A rabbit abscess model was developed to study the effect of abscess age on the accumulation of indium-111-labeled neutrophils ([111In]N) versus gallium-67 citrate (67Ga). Abscesses 1-2 hr, 6-8 hr, 24 hr, and 7 days old were induced by subcutaneous injection of autoclaved colon contents prior to i.v. administration of either [111In]N or 67Ga. Radioactivity in the abscesses was determined 48 hr postinjection. Accumulation of [111In]N was inversely proportional to abscess age. Seven-day-old abscesses were inconsistently seen on [111In]N scans. In contrast, 67Ga accumulation was not affected by abscess age and all abscesses could be identified on a scan 48 hr postinjection. Scans with [111In]N were clearly superior to 67Ga scans for demonstrating early abscesses. Since 67Ga accumulated to a greater extent than [111In]N in abscesses 7 days old, it may be a superior imaging agent for older abscesses.

In vitro and in vivo characterization of 4-[125I]iododexetimide binding to muscarinic cholinergic receptors in the rat heart.

4-[125I]iododexetimide binding to muscarinic cholinergic receptors (mAChR) was evaluated in the rat heart. 4-[125I]iododexetimide displayed high in vitro affinity (Kd = 14.0 nM) for rat myocardial mAChR. In vivo, there was high accumulation of 4-[125I]iododexetimide in the rat atrium and ventricle which could be blocked by approximately 60% by preinjection of atropine. In contrast, accumulation of the radiolabeled stereoisomer, 4-[125I]iodolevetimide, was 63% lower than 4-[125I]iodolevetimide and was not blocked by atropine. The blood clearance of 4-[125I]iododexetimide was rapid, providing heart-to-blood ratios of up to 14:1; however, heart-to-lung and heart-to-liver ratios were below unity. The data indicate that 4-[125I]iododexetimide binds potently to rat mAChR. However, since nonspecific binding is relatively high, it is not clear whether iododexetimide labeled with 123I will be useful in SPECT imaging studies of myocardial mAChR. Further studies in humans are indicated.

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.

Comparison of technetium-99m aminoalkyl diaminodithiol (DADT) analogs as potential brain blood flow imaging agents.

N-ethyl piperidinyl diaminodithiol (NEP-DADT), complexed with 99mTc has been developed as an agent for the measurement of brain blood flow using SPECT. Studies in patients have shown that 99mTc NEP-DADT enters rapidly into the brain, but also clears rapidly (t1/2 = 17 min). In this study nine new aminoalkyl DADT derivatives were synthesized, labeled with 99mTc and tested in mice with the aim of developing an agent with increased retention in the brain. In addition, relationships between chemical properties of the derivatives and their in vivo localization were investigated. The results were as follows: (a) the R-group and its isomeric configuration has a profound influence on the biodistribution; (b) 99mTc aminoalkyl DADT derivatives with apparent pKa values of greater than 6.9 show poor brain uptake (less than 0.40% dose at 5 min); (c) lengthening of the chain between the DADT moiety and the amino-R group from ethyl to hexyl generally increases the apparent pKa and consequently lowers brain uptake; (d) a correlation (r = 0.71) exists between initial brain uptake and the octanol-buffer partition coefficient; (e) 99mTc-4'-methyl NEP-DADT has the highest partition coefficient, relatively high uptake, and longest retention in the mouse brain. This complex has characteristics suited for brain blood flow measurements.

Imaging nicotinic acetylcholine receptors with fluorine-18-FPH, an epibatidine analog.

UNLABELLED: Nicotinic acetylcholine receptors (nAChRs) have been implicated in a variety of central processes, such as learning and memory and analgesia. These receptors also mediate the reinforcing properties of nicotine in tobacco products and are increased in postmortem samples of brains of smokers. On the other hand, brains of individuals who have died from dementia of the Alzheimer type show abnormally low densities of nAChRs. In this study, the distribution and kinetics of [(+/-)-exo-2-(2-[18F] fluoro-5-pyridyl)-7-azabicyclo[2.2.1]heptane (18F-FPH), a high-affinity nAChR agonist, was evaluated in a baboon using PET. METHODS: After intravenous injection of 5 mCi [185 MBq] 18F-FPH into a 25-kg anesthetized baboon, sequential quantitative tomographic data were acquired over a period of 150 min. Regions of interest were placed and time-activity curves were generated. Brain kinetics of the radiotracer were calculated, and the in vivo regional binding in the baboon brain was compared with the known in vitro regional distribution of nAChRs in the rat and human brain. RESULTS: Brain activity reached a plateau within 60 min after injection of the tracer, and the binding was reversible. Elimination of 18F-FPH was relatively rapid from the cerebellum (clearance t[1/2] = 3 hr), intermediate from the hypothalamus/midbrain (t[1/2] = 7 hr) and slow from the thalamus (t[1/2] = 16 hr). Radioactivity due to 18F-FPH at 130 min postinjection was highest in the thalamus and hypothalamus/midbrain, intermediate in the neocortex and hippocampus and lowest in the cerebellum. Subcutaneous injection of 1 mg/kg cytisine 45 min after injection of the radiotracer reduced brain activity at 130 min by 67%, 64%, 56% and 52% of control values in the thalamus, hypothalamus/midbrain, hippocampus and cerebellum, respectively. The regional binding of 18F-FPH at 130 min was highly correlated with the known densities of nAChR measured in vitro in human (r = 0.81) and rat brain (r = 0.90). CONCLUSION: These results demonstrate the feasibility of imaging nAChRs in vivo. Fluorine-18-FPH appears to be a suitable tracer to study nAChRs in the human brain.