Hayden T. Ravert

In Vivo Imaging of Amyloid Deposition in Alzheimer Disease Using the Radioligand 18F-AV-45 (Flobetapir F 18)

An 18F-labeled PET amyloid-β (Aβ) imaging agent could facilitate the clinical evaluation of late-life cognitive impairment by providing an objective measure for Alzheimer disease (AD) pathology. Here we present the results of a clinical trial with (E)-4-(2-(6-(2-(2-(2-18F-fluoroethoxy)ethoxy)ethoxy)pyridin-3-yl)vinyl)-N-methyl benzenamine (18F-AV-45 or flobetapir F 18). Methods: An open-label, multicenter brain imaging, metabolism, and safety study of 18F-AV-45 was performed on 16 patients with AD (Mini-Mental State Examination score, 19.3 ± 3.1; mean age ± SD, 75.8 ± 9.2 y) and 16 cognitively healthy controls (HCs) (Mini-Mental State Examination score, 29.8 ± 0.45; mean age ± SD, 72.5 ± 11.6 y). Dynamic PET was performed over a period of approximately 90 min after injection of the tracer (370 MBq [10 mCi]). Standardized uptake values and cortical-to-cerebellum standardized uptake value ratios (SUVRs) were calculated. A simplified reference tissue method was used to generate distribution volume ratio (DVR) parametric maps for a subset of subjects. Results: Valid PET data were available for 11 AD patients and 15 HCs. 18F-AV-45 accumulated in cortical regions expected to be high in Aβ deposition (e.g., precuneus and frontal and temporal cortices) in AD patients; minimal accumulation of the tracer was seen in cortical regions of HCs. The cortical-to-cerebellar SUVRs in AD patients showed continual substantial increases through 30 min after administration, reaching a plateau within 50 min. The 10-min period from 50 to 60 min after administration was taken as a representative sample for further analysis. The cortical average SUVR for this period was 1.67 ± 0.175 for patients with AD versus 1.25 ± 0.177 for HCs. Spatially normalized DVRs generated from PET dynamic scans were highly correlated with SUVR (r = 0.58–0.88, P < 0.005) and were significantly greater for AD patients than for HCs in cortical regions but not in subcortical white matter or cerebellar regions. No clinically significant changes in vital signs, electrocardiogram, or laboratory values were observed. Conclusion: 18F-AV-45 was well tolerated, and PET showed significant discrimination between AD patients and HCs, using either a parametric reference region method (DVR) or a simplified SUVR calculated from 10 min of scanning 50–60 min after 18F-AV-45 administration.

Dopamine D2 and D3 Receptor Occupancy in Normal Humans Treated with the Antipsychotic Drug Aripiprazole (OPC 14597): A Study Using Positron Emission Tomography and [11C]Raclopride

Aripiprazole (OPC 14597) is an antipsychotic drug that has high affinity for dopamine D2 and D3 receptors and the dopamine autoreceptor. It is being developed for treatment of patients with schizophrenia. The purpose of this study was to determine whether a dose response following graduated doses of aripiprazole could be quantified and correlated with its occupancy of the D2 and D3 dopamine receptors in the brain of living humans. Dopamine D2 and D3 receptor occupancy in fifteen normal male human brains was measured using positron emission tomography (PET) with [11C]raclopride. PET studies were performed before and after two weeks of administration of aripiprazole. The dopamine D2 receptor occupancy was quantified with two kinetic modeling methods without using a blood input function. Administration of aripiprazole for 14 days resulted in a dose-dependent receptor occupancy between 40 – 95% after the administration of 0.5mg, 1 mg, 2 mg, 10 mg, and 30 mg per day. These results suggest that an adequate occupancy can be obtained, and this may be useful to predict an appropriate therapeutic dose for an individual patient. Interestingly, even at striatal D2 receptor occupancy values above 90%, which occurred with the higher doses, extrapyramidal side effects (EPS) were not observed. This underlines aripiprazoles unique mechanism of action as a partial dopamine receptor agonist, which might become a novel principle in the treatment of schizophrenia.

Imaging opiate receptors in the human brain by positron tomography

Carfentanil is a potent, synthetic opiate that binds to mu opiate receptors with very high affinity (K1 = 0.051 n M, 37°). In rat brain, carfentanil is 90 and 250 times more selective for mu opiate receptors compared with delta and kappa opiate receptors, respectively. Carbon-11-carfentanil was synthesized by reacting [11C]methyliodide with the carboxylic acid precursor of carfentanil. Carbon-11-carfentanil was injected intravenously in man and a baboon and the distribution of the radioactivity in the brain was monitored using positron emission tomography. High concentrations of radioactivity were observed in the basal ganglia and thalamus, intermediate concentrations in the frontal and parietal cerebral cortex, and low concentrations in the cerebellum and occipital cortex; this distribution corresponds to the known regional density of opiate receptors measured using in vitro techniques. This heterogeneous distribution could be abolished by pretreatment with naloxone (1 mg/kg), an opiate antagonist. The percent inhibition of binding by naloxone is approximately 90% in the caudate nucleus and medial thalamus for the period 30–60 min after injection; therefore, this method is associated with a high level of specific binding to opiate receptors compared with nonspecific binding sites. The ability to measure opiate receptors in vivo in man makes it possible to study a variety of neurologic and psychiatric disorders in which opiate receptors are thought to be abnormal and to study physiologic role of opiate receptors in the central nervous system.

Radiolabeled Small-Molecule Ligands for Prostate-Specific Membrane Antigen: In vivo Imaging in Experimental Models of Prostate Cancer

Purpose: Prostate-specific membrane antigen (PSMA) is a cell surface protein that is overexpressed in prostate cancer, including hormone-refractory and metastatic disease. Our goal in this study was to develop a series of PSMA-based imaging agents for clinical use. Experimental Design: We have synthesized and evaluated the in vivo biodistribution of two radiolabeled urea derivatives that have high affinity for PSMA in severe combined immunodeficient mice harboring MCF-7 (breast, PSMA-negative), PC-3 (prostate, PSMA-negative), and LNCaP (prostate, PSMA-positive) xenografts. Radiopharmaceutical binding selectivity and tumor uptake were also evaluated in vivo using dedicated small animal positron emission tomography, single photon emission computed tomography, and gamma scintigraphic imaging devices. N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-S-[11C]methyl-l-cysteine ([11C]DCMC Ki, 3.1 nmol/L) and N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-S-3-[125I]iodo-l-tyrosine ([125C]DCIT Ki, 1.5 nmol/L) were synthesized using [11C]CH3I and with [125I]NaI/Iodogen, respectively. Results: At 30 minutes postinjection, [11C]DCMC and [125I]DCIT showed tumor/muscle ratios of 10.8 and 4.7, respectively, with clear delineation of LNCaP-derived tumors on imaging. MCF-7- and PC-3-derived tumors showed significantly less uptake of [11C]DCMC or [125I]DCIT. Conclusion: These results show the feasibility of imaging PSMA-positive prostate cancer using low molecular weight agents.

Quantitative PET studies of the serotonin transporter in MDMA users and controls using [11C]McN5652 and [11C]DASB

(±)3,4-Methylenedioxymethamphetamine (MDMA, ‘Ecstasy’) is a widely used illicit drug that produces toxic effects on brain serotonin axons and axon terminals in animals. The results of clinical studies addressing MDMAs serotonin neurotoxic potential in humans have been inconclusive. In the present study, 23 abstinent MDMA users and 19 non-MDMA controls underwent quantitative positron emission tomography (PET) studies using [11C]McN5652 and [11C]DASB, first- and second-generation serotonin transporter (SERT) ligands previously validated in baboons for detecting MDMA-induced brain serotonin neurotoxicity. Global and regional distribution volumes (DVs) and two additional SERT-binding parameters (DVspec and DVR) were compared in the two subject populations using parametric statistical analyses. Data from PET studies revealed excellent correlations between the various binding parameters of [11C] McN5652 and [11C]DASB, both in individual brain regions and individual subjects. Global SERT reductions were found in MDMA users with both PET ligands, using all three of the above-mentioned SERT-binding parameters. Preplanned comparisons in 15 regions of interest demonstrated reductions in selected cortical and subcortical structures. Exploratory correlational analyses suggested that SERT measures recover with time, and that loss of the SERT is directly associated with MDMA use intensity. These quantitative PET data, obtained using validated first- and second-generation SERT PET ligands, provide strong evidence of reduced SERT density in some recreational MDMA users.

N-[N-[(S)-1,3-Dicarboxypropyl]Carbamoyl]-4-[18F]Fluorobenzyl-l-Cysteine, [18F]DCFBC: A New Imaging Probe for Prostate Cancer

Purpose: Previously, we showed successful imaging of xenografts that express the prostate-specific membrane antigen (PSMA) using small-animal positron emission tomography (PET) and the radiolabeled PSMA inhibitor N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-S-[11C]methyl-l-cysteine. Herein, we extend that work by preparing and testing a PSMA inhibitor of the same class labeled with fluorine-18. Experimental Design:N-[N-[(S)-1,3-Dicarboxypropyl]carbamoyl]-4-[18F]fluorobenzyl-l-cysteine ([18F]DCFBC) was prepared by reacting 4-[18F]fluorobenzyl bromide with the precursor (S)-2-[3-[(R)-1-carboxy-2-mercaptoethyl]ureido]-pentanedioic acid in ammonia-saturated methanol at 60°C for 10 min followed by purification using C-18 reverse-phase semipreparative high-performance liquid chromatography. Severe combined immunodeficient mice bearing a s.c. PSMA+ PC-3 PIP tumor behind one shoulder and a PSMA− PC-3 FLU tumor behind the other shoulder were injected via the tail vein with either 1.85 MBq (50 μCi) of [18F]DCFBC for ex vivo biodistribution or 7.4 MBq (200 μCi) for imaging. For biodistribution, mice were sacrificed at 5, 15, 30, 60, and 120 min. Tumor, blood, and major organs were harvested and weighed, and radioactivity was counted. Imaging was done on the GE eXplore Vista small-animal PET scanner by collecting 12 consecutive 10-min frames. Results: Radiochemical yield for [18F]DCFBC averaged 16 ± 6% (n = 8) from 4-[18F]fluorobenzyl bromide. Specific radioactivities ranged from 13 to 133 GBq/μmol (350-3,600 Ci/mmol) with an average of 52 GBq/μmol (1,392 Ci/mmol; n = 6). Biodistribution and imaging studies showed high uptake of [18F]DCFBC in the PIP tumors with little to no uptake in FLU tumors. High radiopharmaceutical uptake was also seen in kidneys and bladder; however, washout of radioactivity from these organs was faster than from the PIP tumors. The maximum PIP tumor uptake was 8.16 ± 2.55% injected dose per gram, achieved at 60 min after injection, which decreased to 4.69 ± 0.89 at 120 min. The PIP tumor to muscle ratio was 20 at 120 min after injection. Based on the mouse biodistribution, the dose-limiting organ is the kidneys (human estimated absorbed dose: 0.05 mGy/MBq; 0.2 rad/mCi). Conclusion: [18F]DCFBC localizes to PSMA+-expressing tumors in mice, permitting imaging by small-animal PET. This new radiopharmaceutical is an attractive candidate for further studies of PET imaging of prostate cancer.

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

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

Column-switching HPLC for the analysis of plasma in PET imaging studies

A column-switch high performance liquid chromatography method for the analysis of 4 mL of plasma is described with six examples of chromatography of [(11)C]-labeled positron-emission tomography imaging agents. Complete extraction of all but the most polar metabolites by the reverse phase capture column is achieved by disruption of plasma protein binding by 8 M urea.

Positron Emission Tomographic Studies of Brain Dopamine and Serotonin Transporters in Abstinent (±) 3,4-Methylenedioxymethamphetamine (“Ecstasy”) Users: Relationship to Cognitive Performance

Background(±)3,4-Methylenedioxymethamphetamine (MDMA, “ecstasy”) is a recreational drug and brain serotonin (5-HT) neurotoxin. Under certain conditions, MDMA can also damage brain dopamine (DA) neurons, at least in rodents. Human MDMA users have been found to have reduced brain 5-HT transporter (SERT) density and cognitive deficits, although it is not known whether these are related. This study sought to determine whether MDMA users who take closely spaced sequential doses, which engender high plasma MDMA concentrations, develop DA transporter (DAT) deficits, in addition to SERT deficits, and whether there is a relationship between transporter binding and cognitive performance.Materials and methodsSixteen abstinent MDMA users with a history of using sequential MDMA doses (two or more doses over a 3- to 12-h period) and 16 age-, gender-, and education-matched controls participated. Subjects underwent positron emission tomography with the DAT and SERT radioligands, [11C]WIN 35,428 and [11C]DASB, respectively. Subjects also underwent formal neuropsychiatric testing.ResultsMDMA users had reductions in SERT binding in multiple brain regions but no reductions in striatal DAT binding. Memory performance in the aggregate subject population was correlated with SERT binding in the dorsolateral prefrontal cortex, orbitofrontal cortex, and parietal cortex, brain regions implicated in memory function. Prior exposure to MDMA significantly diminished the strength of this relationship.ConclusionsUse of sequential MDMA doses is associated with lasting decreases in brain SERT, but not DAT. Memory performance is associated with SERT binding in brain regions involved in memory function. Prior MDMA exposure appears to disrupt this relationship. These data are the first to directly relate memory performance to brain SERT density.

Radiosynthesis of an opiate receptor binding radiotracer: [11C]carfentanil

Carfentanil, a potent opiate receptor agonist, was labeled with 11C for in vivo studies using positron emission tomography. This synthesis was completed in approximately 30 min using iodomethane as the 11C precursor. The synthesis, purification, characterization, and determination of specific activity are presented and discussed.