Kinetic modeling of [99mTc]TRODAT-1: a dopamine transporter imaging agent.

UNLABELLED: [99mTc]Technetium[2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo [3.2.1] oct-2-yl]-methyl] (2-mercaptoethyl) amino] ethyl] amino] ethane-thiolato(3-)-N2,N2',S2,S2']oxo-[1R-(exo-exo)] ([99mTc] TRODAT-1) is a useful imaging agent for central nervous system dopamine transporters. The purpose of this study was to characterize the in vivo binding potential and kinetic rate constants of this agent in nonhuman primates. METHODS: A series of four SPECT scans were performed on each of two female baboons with a bolus injection of [99mTc]TRODAT-1 (717+/-78 MBq; 19.38+/-2.12 mCi). Dynamic images of the brain were acquired over 4 h using a triple-head camera equipped with fan-beam collimators. Arterial and venous blood were sampled frequently using a peristaltic pump throughout the duration of the study. Regions of interest were drawn on the corresponding MRI scan to which each functional image was coregistered. Using analytical solutions to the three-compartment model with the Levenberg-Marquardt minimization technique, each study was individually fitted to a kinetic parameter vector (method I). Additionally, within each subject, three corresponding intrasubject studies were fitted simultaneously to a single parameter vector by constraining the binding potential, distribution volume and dissociation rate constant to improve the identifiability of the parameter estimates (method II). RESULTS: The results clearly indicated that [99mTc] TRODAT-1 localized in the striatum with slower washout rate than other brain regions. A maximal target/nontarget ratio of 3.5 between striatum and cerebellum was obtained. SPECT image analysis of the striatum yielded unconstrained k3/k4 values of 3.4+/-1.4, 2.4+/-0.7, 3.0+/-1.5, and 4.0+/-10.3, with respective constrained (fixed k4) values of 2.9 +/- 0.4, 2.4 +/- 0.4, 1.7+/-0.4 and 1.8+/-0.4 in one baboon using method I. With method II, the corresponding simultaneously fitted values were 2.1+/-0.3 using no constraints and 2.2+/-0.2 using a fixed k4. The second baboon had similar results. CONCLUSION: These findings suggest that the binding potential and corresponding kinetic rate constants can be reliably estimated in nonhuman primates with dynamic SPECT imaging of the dopamine transporter using a technetium-based tropane analogue. Furthermore, method II parameter vectors compare favorably to those produced using method I based on SEEs.

Limbic activation during cue-induced cocaine craving.

OBJECTIVE: Since signals for cocaine induce limbic brain activation in animals and cocaine craving in humans, the objective of this study was to test whether limbic activation occurs during cue-induced craving in humans. METHOD: Using positron emission tomography, the researchers measured relative regional cerebral blood flow (CBF) in limbic and comparison brain regions of 14 detoxified male cocaine users and six cocaine-naive comparison subjects during exposure to both non-drug-related and cocaine-related videos and during resting baseline conditions. RESULTS: During the cocaine video, the cocaine users experienced craving and showed a pattern of increases in limbic (amygdala and anterior cingulate) CBF and decreases in basal ganglia CBF relative to their responses to the non-drug video. This pattern did not occur in the cocaine-naive comparison subjects, and the two groups did not differ in their responses in the comparison regions (i.e., the dorsolateral prefrontal cortex, cerebellum, thalamus, and visual cortex). CONCLUSIONS: These findings indicate that limbic activation is one component of cue-induced cocaine craving. Limbic activation may be similarly involved in appetitive craving for other drugs and for natural rewards.

Synthesis and characterization of technetium-99m-labeled tropanes as dopamine transporter-imaging agents.

In the development of novel Tc-99m-labeled tropane derivatives as dopamine transporter (reuptake site)-imaging agents, a series of neutral and lipophilic complexes containing bis-(aminoethanethiol) as a neutral complexing moiety for a [99mTc]TcO3+ center core was successfully prepared. Biological evaluation of the Tc-99m-labeled complexes 13-16 as central nervous system (CNS) dopamine transporter-imaging agents was reported. Synthesis of the tropane derivatives was achieved by stepwise reactions adding two aminoethanethiol units. The final free thiol ligands were obtained by deblocking the 4-methoxybenzyl protecting group with Hg(OAc)2 to obtain trifluoroacetate salts. All of the Tc-99m complexes, with the exception of 16, displayed good initial brain uptake and selective uptake in the striatal area, where dopamine transporters are concentrated. One of the compounds, [2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo [3.2.1]oct-2-yl]methyl](2-mercaptoethyl)amino]ethy] amino]ethanethiolato-(3-)-N2,N2',S2,S2'] oxo-[1R-(exo-exo)]- [99mTc]technetium,[99mTc]TRODAT-1 (13), displayed the highest initial uptake in rat brain (0.4% at 2 min post iv injection); the striatal/cerebellar (ST/ CB) ratio reached 2.8 at 60 min after an iv injection. The specific uptake in rat brain can be blocked by pretreating rats with a competing dopamine transporter binding agent, beta-CIT (RTI-55, 2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane; iv, 1 mg/kg), which reduced the regional brain uptake ratio (ST/CB) to 1.2. In contrast, the specific striatal uptake was not affected by pretreating rats with a noncompeting ligand, haldol (iv, 1 mg/kg). After an iv injection of 9 mCi of [99mTc]TRODAT-1 (13), in vivo images of baboon brain using single-photon emission-computed tomography exhibited excellent localization in striatum (basal ganglia), where dopamine neurons are known to be concentrated. This series of compounds may provide a convenient source of short-lived imaging agents for routine diagnosis of CNS diseases (i.e., Parkinson's disease) in which changes in the dopamine transporter concentration are implicated.

Iodine-123-IPT SPECT imaging of CNS dopamine transporters: nonlinear effects of normal aging on striatal uptake values.

UNLABELLED: Iodine-123-labeled IPT (N-(3-iodopropen-2-yl)-2 beta-carbomethoxy-3 beta-(4-chlorophenyl)tropane) is an analog of cocaine that selectively binds the presynaptic dopamine transporter. This study sought to characterize changes in the striatal uptake of IPT with normal aging. METHODS: The sample included 18 healthy human volunteers. Their ages ranged from 19 to 67 yr. Dynamic SPECT scans of the brain were acquired with about 185 MBq (5 mCi) of IPT on a triple-headed camera. The images were reconstructed with a three-dimensional restorative filter and corrected for attenuation. The mean concentration of radioactivity [microCi/ml] was measured in the head of the caudate and body of the putamen. The remainder of the supratentorial brain was used as a reference. RESULTS: The specific uptake of IPT was higher in the caudate than in the putamen of each subject. It decreased significantly with age in both regions. The mean specific uptake in seven volunteers who were less than 30 yr old was 17.6 +/- 4.9 in the caudate and 13.3 +/- 4.0 in the putamen, compared to only 11.97 +/- 3.30 and 7.8 +/- 2.68, respectively, in the six middle-aged subjects (t = 2.53 and 2.90, df = 11, p = 0.027 and 0.014). However, there were no significant differences between the six middle-aged subjects and the five volunteers who were older than 60 yr, whose respective means were 9.0 +/- 1.6 and 6.2 +/- 0.7 (t = 1.83 and 1.28, df = 9, p = 0.10 and 0.23). The results were supported by regression analysis, which indicated that changes with age were not optimally described as a linear function when compared to several nonlinear alternatives. The fit improved when the models accounted for relatively rapid rates of decline during young adulthood, followed by less rapid decline throughout middle age. CONCLUSION: The results are consistent with the findings from previous studies that have shown that the specific uptake values for radiopharmaceuticals that bind the dopamine transporter decline with advancing age. However, results of this study suggest that the effects of aging may be nonlinear and regionally distinct.

In vivo SPECT imaging of 5-HT1A receptors with [123I]p-MPPI in nonhuman primates.

The in vivo imaging of a novel iodinated phenylpiperazine derivative for 5-HT1A receptors, [123I]p-MPPI (4-(2'-methoxy-)phenyl-1-[2'-(n-2"-pyridinyl)-p-iodobenzamido-] ethyl-piperazine), using single photon emission computed tomography (SPECT), was evaluated in nonhuman primates. After an i.v. injection, [123I]p-MPPI penetrated the blood-brain barrier quickly and localized in brain regions where 5-HT1A receptor density is high (hippocampus, frontal cortex, cingulate gyrus, entorhinal cortex). Maximum ratio of hippocampus to cerebellum was 3 to 1 at 50 min postinjection. The specific binding of the radioligand in the hippocampal region, an area rich in 5-HT1A receptor density, was blocked by a chasing dose of (+/-) 8-OH-DPAT (2 mg/kg, i.v.) or non-radioactive p-MPPI (1 mg/kg, i.v.), whereas the regional distribution of [123I]p-MPPI was unaffected by treatment with non 5-HT1A agents, such as ketanserin. Ex vivo and in vitro autoradiographic studies using monkey brain further confirmed that the specific binding of [123I]p-MPPI is associated with 5-HT1A receptor sites. However, the initial attempt at [123I]p-MPPI human imaging studies did not display specific localization of 5-HT1A receptors. This discrepancy observed for [123I]p-MPPI may be due to a dramatic difference in metabolic pathways between humans and monkeys.

Dosimetry of an iodine-123-labeled tropane to image dopamine transporters.

UNLABELLED: N-(3-Iodopropen-2-yl)-2 beta-carbomethoxy-3 beta-(4-chlorophenyl)tropane (IPT) is an analog of cocaine that selectively binds the presynaptic dopamine transporter. The present study sought to measure the radiation dosimetry of IPT in seven healthy human volunteers. METHODS: Dynamic renal scans were acquired immediately after the intravenous administration of 165 +/- 16 MBq (4.45 +/- 0.42 mCi) of [123I]IPT. Between 7 and 12 sets of whole-body scans were acquired over the next 24 hr. The 24-hr renal excretion fractions were measured from conjugate emission scans of 7-11 discreet voided urine specimens. The fraction of the administered dose in 11 organs and each urine specimen was quantified from the attenuation-corrected geometric mean counts in opposing views. Subject-specific residence times were calculated for each subject independently by fitting the time-activity curves to a multicompartmental model. The radiation doses were estimated with the MIRD technique from the residence times for each subject individually before any results were averaged. RESULTS: The findings showed that IPT was excreted rapidly by the renal system. There were no reservoirs of retained activity outside the basal ganglia, where SPECT images in these subjects showed that the mean ratio of caudate to calcarine cortex averaged 25:1 at 3 hr after injection (range 19.6-32 hr). The basal ganglia received a radiation dose of 0.028 mGy/MBq (0.10 rad/mCi). The dose-limiting organ in men was the stomach, which received an estimated 0.11 mGy/MBq (0.37 rad/mCi). In women, the critical organ was the urinary bladder at 0.14 mGy/MBq (0.51 rad/mCi). CONCLUSION: Relatively high-contrast images of the presynaptic dopamine transporters in the basal ganglia can be acquired with 185 MBq (5 mCi) of [123I]IPT. The radiation exposure that results is significantly less than the maximum allowed by current safety guidelines for research volunteers.

Striatal dopamine transporter imaging in nonhuman primates with iodine-123-IPT SPECT.

UNLABELLED: The regional distribution, kinetics and pharmacological specificity of a new radioiodinated cocaine analog, N-((E)-3-iodopropen-2-yl)-2 beta-carbomethoxy-3 beta-(4-chlorophenyl) tropane ([123I]IPT) were examined in brain SPECT studies (n = 20) of nonhuman primates. METHODS: Radiolabeling and purification of the iododestannylated trialkyltin percursor yielded the tracer at greater than 90% radiochemical purity and high (> 20,000 Ci/mmole) specific activity. Cynomologous monkeys were injected with 7.2 +/- 1.3 mCi (mean +/- s.d.) of the tracer, and serial 10-min images were acquired (total scan time = 177 +/- 22 min). Images were reconstructed as transaxial slices (2 mm) using restorative techniques (Wiener prefiltering). RESULTS: Radioactivity concentrated quickly in striatal regions (time of peak = 25 +/- 13 min) and cleared gradually thereafter (8.8 +/- 4.6% hr). Striatal-to-cerebellar ratios of 2.6 +/- 1.5 (n = 19), 6.7 +/- 3.2 (n = 20), 15.1 +/- 10.7 (n = 10) and 22.8 +/- 11.0 (n = 9) were observed at the time of peak and at 1, 2 and 3 hr p.i., respectively. In contrast, extrastriatal activity peaked earlier and at lower levels, cleared more rapidly and resembled cerebellar time-activity curves. Displacing doses of nonspecific antagonists of monoamine transporters (mazindol and beta-CIT) showed that 95% of specific [123I]IPT binding was reversible, while selective antagonists (e.g., paroxetine, nisoxetine and GBR 12909) demonstrated that striatal activity was specifically associated with dopamine transporters. CONCLUSION: These results indicate that [123I]IPT is a useful radioligand for in vivo SPECT imaging of striatal dopamine transporters.

In vitro and in vivo characterization of R(+)-FIDA2: a dopamine D2-like imaging agent.

UNLABELLED: R(+)-FIDA2, (R)-(+)-2,3-dimethoxy-5-iodo-N-[(1-(4'-fluorobenzyl)-2-pyrrolid iny l)- methyl]benzamide, is a new dopamine D2-like receptor imaging agent that can be labeled with either 123I or 18F for SPECT or PET imaging. The purpose of this study was to characterize its in vitro and in vivo binding properties. METHODS: In vitro binding studies using [125I]R(+)-FIDA2 were performed in Sf9 cells expressing dopamine D2 or D3 receptors and in rat basal forebrain homogenates, which contain a high density of dopamine D2-like receptors. A series of in vivo SPECT imaging studies in nonhuman primates (cynomologous monkeys) were performed by intravenously injecting 7.1 +/- 1.0 mCi of [123I]R(+)-FIDA2. At least one control study and one displacement experiment, in which a cold compound was injected intravenously 90 min after tracer injection, was performed in each monkey. Data were acquired in 10-min frames for 180 min, and the activity in regions of interest (basal ganglia and cerebellum) were plotted versus time. RESULTS: Iodine-125-R(+)-FIDA2 displayed Kd values for D2 and D3 receptor subtypes expressed in Sf9 cells of 0.11 and 0.04 nM, respectively. As expected, SPECT images of monkey brain (transaxial sections, 2 mm) showed that the radioactivity was localized in the area of the basal ganglia and reached peak concentrations in 11.5 +/- 5.8 min postinjection. An injection of R(+)7-OH-PIPAT, a new ligand that is selective for dopamine D3 receptors and the high affinity state of dopamine D2 receptors, did not show significant displacement of [123I]R(+)-FIDA2 binding in the basal ganglia. CONCLUSION: These studies indicate that R(+)-FIDA2 may be a useful ligand for in vitro pharmacological characterization and in vivo imaging of CNS dopamine D2-like receptors.

Dosimetry of a D2/D3 dopamine receptor antagonist that can be used with PET or SPECT.

UNLABELLED: FIDA-2 (R-(+)-2,3-dimethoxy-5-iodo-N-[(1-4'-fluorobenzyl)-2-pyrrolidinyl) methyl] benzamide) is a simultaneously fluorinated and iodinated D2/D3 dopamine receptor antagonist. The purpose of this study was to measure its biodistribution and radiation dosimetry in humans. METHODS: Whole-body emission scans were sequentially acquired in eight healthy volunteers 24-43 hr after the intravenous administration of 101-150 MBq 123I FIDA-2. Regions of interest (ROIs) were placed on the initial set of conjugate emission images and transposed as a single set onto all the other scans without manipulating any of the regions for solid organs independently. The counts in each ROI were corrected for attenuation with transmission scans and compared to the net counts in images of the injection syringe containing the administered dose. The radiation doses were estimated with the MIRD formalism from the residence times for both the 18F- and 123I-labeled ligands. RESULTS: There were no subjective or objective pharmacological effects of the tracer on any of the subjects. The findings showed that the dose-limiting organ for the 123I-labeled product was the thyroid gland in this sample. If the 18F-labeled product had been used, then the urinary bladder would have received 0.086 mGy/MBq (0.32 rads/mCi) and become the dose-limiting organ. The effective dose equivalents were 0.025 mSv/MBq (0.092 rem/mCi) for both the 123I- and the 18F-labeled versions of the tracer. CONCLUSION: The data suggest that FIDA-2 can be used to produce relatively high contrast images of the D2/D3 dopaminergic system with substantially less than the maximum allowable radiation dose for research volunteers.