Synthesis and pharmacological evaluation of [18F]PBR316: a novel PET ligand targeting the translocator protein 18 kDa (TSPO) with low binding sensitivity to human single nucleotide polymorphism rs6971.

Radiopharmaceuticals that target the translocator protein 18 kDa (TSPO) have been investigated with positron emission tomography (PET) to study neuroinflammation, neurodegeneration and cancer. We have developed the novel, achiral, 2-phenylimidazo[1,2-a]pyridine, PBR316 that targets the translocator protein 18 kDa (TSPO) that addresses some of the limitations inherent in current TSPO ligands; namely specificity in binding, blood brain barrier permeability, metabolism and insensitivity to TSPO binding in subjects as a result of rs6971 polymorphism. PBR316 has high nanomolar affinity (4.7-6.0 nM) for the TSPO, >5000 nM for the central benzodiazepine receptor (CBR) and low sensitivity to rs6971 polymorphism with a low affinity binders (LABs) to high affinity binders (HABs) ratio of 1.5. [18F]PBR316 was prepared in 20 ± 5% radiochemical yield, >99% radiochemical purity and a molar activity of 160-400 GBq µmol-1. Biodistribution in rats showed high uptake of [18F]PBR316 in organs known to express TSPO such as heart (3.9%) and adrenal glands (7.5% ID per g) at 1 h. [18F]PBR316 entered the brain and accumulated in TSPO-expressing regions with an olfactory bulb to brain ratio of 3 at 15 min and 7 at 4 h. Radioactivity was blocked by PK11195 and Ro 5-4864 but not Flumazenil. Metabolite analysis showed that radioactivity in adrenal glands and the brain was predominantly due to the intact radiotracer. PET-CT studies in mouse-bearing prostate tumour xenografts indicated biodistribution similar to rats with radioactivity in the tumour increasing with time. [18F]PBR316 shows in vitro binding that is insensitive to human polymorphism and has specific and selective in vivo binding to the TSPO. [18F]PBR316 is suitable for further biological and clinical studies.

PET imaging of brain inflammation during early epileptogenesis in a rat model of temporal lobe epilepsy.

BACKGROUND: Recently, inflammatory cascades have been suggested as a target for epilepsy therapy. Positron emission tomography (PET) imaging offers the unique possibility to evaluate brain inflammation longitudinally in a non-invasive translational manner. This study investigated brain inflammation during early epileptogenesis in the post-kainic acid-induced status epilepticus (KASE) model with post-mortem histology and in vivo with [18F]-PBR111 PET. METHODS: Status epilepticus (SE) was induced (N = 13) by low-dose injections of KA, while controls (N = 9) received saline. Translocator protein (TSPO) expression and microglia activation were assessed with [125I]-CLINDE autoradiography and OX-42 immunohistochemistry, respectively, 7 days post-SE. In a subgroup of rats, [18F]-PBR111 PET imaging with metabolite-corrected input function was performed before post-mortem evaluation. [18F]-PBR111 volume of distribution (Vt) in volume of interests (VOIs) was quantified by means of kinetic modelling and a VOI/metabolite-corrected plasma activity ratio. RESULTS: Animals with substantial SE showed huge overexpression of TSPO in vitro in relevant brain regions such as the hippocampus and amygdala (P < 0.001), while animals with mild symptoms displayed a smaller increase in TSPO in amygdala only (P < 0.001). TSPO expression was associated with OX-42 signal but without obvious cell loss. Similar in vivo [18F]-PBR111 increases in Vt and the simplified ratio were found in key regions such as the hippocampus (P < 0.05) and amygdala (P < 0.01). CONCLUSION: Both post-mortem and in vivo methods substantiate that the brain regions important in seizure generation display significant brain inflammation during epileptogenesis in the KASE model. This work enables future longitudinal investigation of the role of brain inflammation during epileptogenesis and evaluation of anti-inflammatory treatments.

Radiation dosimetry of the translocator protein ligands [18F]PBR111 and [18F]PBR102.

INTRODUCTION: The translocator protein (TSPO) ligands [18F]PBR111 and [18F]PBR102 show promise for imaging neuroinflammation. Our aim was to estimate the radiation dose to humans from primate positron emission tomography (PET) studies using these ligands and compare the results with those obtained from studies in rodents. METHODS: [18F]PBR111 and [18F]PBR102 PET-computed tomography studies were carried out in baboons. The cumulated activity in the selected source organs was obtained from the volume of interest time-activity curves drawn on coronal PET slices and adjusted for organ mass relative to humans. Radiation dose estimates were calculated in OLINDA/EXM Version 1.1 from baboon studies and compared with those calculated from Sprague-Dawley rat tissue concentration studies, also adjusted for relative organ mass. RESULTS: In baboons, both ligands cleared rapidly from brain, lung, kidney and spleen and more slowly from liver and heart. For [18F]PBR111, the renal excretion fraction was 6.5% and 17% for hepatobiliary excretion; for [18F]PBR102, the renal excretion was 3.0% and 15% for hepatobiliary excretion. The estimated effective dose in humans from baboon data was 0.021 mSv/MBq for each ligand, whilst from rat data, the estimates were 0.029 for [18F]PBR111 and 0.041 mSv/MBq for [18F]PBR102. CONCLUSION: Biodistribution in a nonhuman primate model is better suited than the rat model for the calculation of dosimetry parameters when translating these ligands from preclinical to human clinical studies. Effective dose calculated from rat data was overestimated compared to nonhuman primate data. The effective dose coefficient for both these TSPO ligands determined from PET studies in baboons is similar to that for [18F]FDG.

A rapid solid-phase extraction method for measurement of non-metabolised peripheral benzodiazepine receptor ligands, [(18)F]PBR102 and [(18)F]PBR111, in rat and primate plasma.

OBJECTIVES: To develop a rapid and reliable method for estimating non-metabolised PBR ligands fluoroethoxy ([(18)F]PBR102)- and fluoropropoxy ([(18)F]PBR111)-substituted 2-(6-chloro-2-phenyl)imidazo[1,2-a]pyridine-3-yl)-N,N-diethylacetamides in plasma. METHODS: Rats and baboons were imaged with PET up to 2 h postinjection of [(18)F]PBR102 and [(18)F]PBR111 under baseline conditions, after pre-blocking or displacement with PK11195. Arterial plasma samples were directly analysed by reverse-phase solid-phase extraction (RP-SPE) and RP-HPLC and by normal-phase TLC. SPE cartridges were successively washed with acetonitrile/water mixtures. SPE eluant radioactivity was measured in a γ-counter to determine the parent compound fraction and then analysed by HPLC and TLC for validation. RESULTS: In SPE, hydrophilic and lipophilic radiolabelled metabolites were eluted in water and 20% acetonitrile/water. All non-metabolised [(18)F]PBR102 and [(18)F]PBR111 were in SPE acetonitrile fraction as confirmed by HPLC and TLC analysis. Unchanged (%) [(18)F]PBR102 and [(18)F]PBR111 from SPE analysis in rat and baboon plasma agreed with those from HPLC and TLC analysis. In rats and baboons, the fraction of unchanged tracer followed a bi-exponential decrease, with half-lives of 7 to 10 min for the fast component and >80 min for the slow component for both tracers. CONCLUSIONS: Direct plasma SPE analysis of [(18)F]PBR102 and [(18)F]PBR111 can reliably estimate parent compound fraction. SPE was superior to HPLC for samples with low activity; it allows rapid and accurate metabolite analysis of a large number of plasma samples for improved estimation of metabolite-corrected input function during quantitative PET imaging studies.

Discovery of [18F]N-(2-(diethylamino)ethyl)-6-fluoronicotinamide: a melanoma positron emission tomography imaging radiotracer with high tumor to body contrast ratio and rapid renal clearance.

The high melanoma uptake and rapid body clearance displayed by our series of [(123)I]iodonicotinamides prompted the development of [(18)F]N-(2-(diethylamino)ethyl)-6-fluoronicotinamide ([(18)F]2), a novel radiotracer for PET melanoma imaging. Significantly, unlike fluorobenzoates, [(18)F]fluorine incorporation on the nicotinamide ring is one step, facile, and high yielding. [(18)F]2 displayed high tumor uptake, rapid body clearance via predominantly renal excretion, and is currently being evaluated in preclinical studies for progression into clinical trials to assess the responsiveness of therapeutic agents.

In-vivo imaging characteristics of two fluorinated flumazenil radiotracers in the rat.

PURPOSE: [(11)C]Flumazenil shows promise as a clinical and research PET radiotracer to image changes in GABA(A) central benzodiazepine receptor (cBZR), but its widespread use has been limited by practical limitations of [(11)C]. This study evaluated the imaging characteristics of two fluorinated PET radiotracers in rats in vivo: [(18)F]fluoroflumazenil ([(18)F]FFMZ) and [(18)F]flumazenil ([(18)F]FMZ). METHODS: PET acquisitions were performed on a small-animal scanner following injection of [(18)F]FFMZ in nine rats and [(18)F]FMZ in eight rats. The following treatments were investigated: (1) injection of the tracer dose, (2) presaturation then injection of the tracer dose, and (3) injection of the tracer dose followed by a displacement injection. Unchanged tracer was measured in plasma and brain structures in four animals 10 and 30 min after injection, and ex-vivo autoradiography was also performed. RESULTS: For both [(18)F]FFMZ and [(18)F]FMZ maximal brain activity peaked rapidly, and was highest in the hippocampus (1.12+/-0.06 SUV, 1.24+/-0.10 SUV, respectively), and lowest in the pons (1.00+/-0.07 SUV, 1.03+/-0.09 SUV, respectively). By 50 min after injection, maximal uptake for [(18)F]FFMZ and [(18)F]FMZ had decreased in the hippocampus to 18+/-3% and 80+/-1% (p<0.01), respectively. The presaturation and displacement studies showed a higher nonspecific component for [(18)F]FFMZ than for [(18)F]FMZ. Metabolite studies showed that at 30 min only 10% of the signal was from [(18)F]FFMZ in the brain. This nonspecific binding was apparent on autoradiography. In contrast, [(18)F]FMZ accounted for >70% of the signal in the brain, which resulted in well-defined regional binding on autoradiography. CONCLUSION: These results demonstrate that [(18)F]FMZ is a superior radiotracer to [(18)F]FFMZ for in-vivo PET imaging of the GABA(A)/cBZR, having slower metabolism and leading to lower concentrations of metabolites in the brain that results in a substantially better signal-to-noise ratio.

Synthesis and evaluation of novel radioiodinated nicotinamides for malignant melanoma.

INTRODUCTION: A series of iodonicotinamides based on the melanin-binding iodobenzamide compound N-2-diethylaminoethyl-4-iodobenzamide was prepared and evaluated for the potential imaging and staging of disseminated metastatic melanoma. METHODS: [(123)I]Iodonicotinamides were prepared by iododestannylation reactions using no-carrier-added iodine-123 and evaluated in vivo by biodistribution and competition studies and by single photon emission computed tomography (SPECT) imaging in black and albino nude mice bearing B16F0 murine melanotic and A375 human amelanotic melanoma tumours, respectively. RESULTS: The iodonicotinamides displayed low-affinity binding for sigma(1)-sigma(2) receptors (K(i)>300 nM). In biodistribution studies in mice, N-(2-(diethylamino)ethyl)-5-[(123)I]iodonicotinamide ([(123)I]1) exhibited the fastest and highest uptake of the nicotinamide series in the B16F0 tumour at 1 h ( approximately 8% ID/g), decreasing slowly over time. No uptake was observed in the A375 tumour. Clearance from the animals by urinary excretion was more rapid for N-alkyl-nicotinamides than for piperazinyl derivatives. At 1 h postinjection, the urinary excretion was 66% ID for [(123)I]1, while the gastrointestinal tract amounted to 17% ID. Haloperidol was unable to reduce the uptake of [(123)I]1 in pigmented mice, indicating that this uptake was likely due to an interaction with melanin. SPECT imaging of [(123)I]1 in black mice bearing the B16F0 melanoma indicated that the radioactivity was predominately located in the tumour and eyes. No specific localisation was observed in nude mice bearing A375 amelanotic tumours. CONCLUSION: These findings suggest that [(123)I]1, which displays high tumour uptake with rapid clearance from the body, could be a promising imaging agent for the detection of melanotic tumours.

Synthesis and biological evaluation of substituted [18F]imidazo[1,2-a]pyridines and [18F]pyrazolo[1,5-a]pyrimidines for the study of the peripheral benzodiazepine receptor using positron emission tomography.

The fluoroethoxy and fluoropropoxy substituted 2-(6-chloro-2-phenyl)imidazo[1,2- a]pyridin-3-yl)- N, N-diethylacetamides 8 (PBR102) and 12 (PBR111) and 2-phenyl-5,7-dimethylpyrazolo[1,5- a]pyrimidin-3-yl)- N, N-diethylacetamides 15 (PBR099) and 18 (PBR146) were synthesized and found to have high in vitro affinity and selectivity for the peripheral benzodiazepine receptors (PBRs) when compared with the central benzodiazepine receptors (CBRs). The corresponding radiolabeled compounds [ (18)F] 8 [ (18)F] 12, [ (18)F] 15, and [ (18)F] 18 were prepared from their p-toluenesulfonyl precursors in 50-85% radiochemical yield. In biodistribution studies in rats, the distribution of radioactivity of the [ (18)F]PBR compounds paralleled the known localization of PBRs. In the olfactory bulbs, where the uptake of radioactivity was higher than in the rest of the brain, PK11195 and Ro 5-4864 were able to significantly inhibit [ (18)F] 12, while little or no pharmacological action of these established PBR drugs were observed on the uptake of [ (18)F] 8, [ (18)F] 15, and [ (18)F] 18 compared to control animals. Hence, [ (18)F] 12 appeared to be the best candidate for evaluation as an imaging agent for PBR expression in neurodegenerative disorders.

Synthesis and evaluation of novel radioiodinated benzamides for malignant melanoma.

The imaging potential of a series of [123I]benzamides was studied in mice bearing B16F0 melanoma tumors. Compound [123I]25 exhibited tumor uptake >8 %ID/g at 1 h, while that of [123I]14d and [123I]25 reached a maximum of 9-12 %ID/g at 6 h. Standardized uptake values of [123I]14d were higher than 100 between 24 and 72 h after injection. In haloperidol treated animals, the tumor uptake of [123I]14d was not significantly different to controls, while significant reduction of [123I]25 uptake was observed, supporting that [123I]14d uptake relates to melanin interaction, whereas part of the mechanism of [123I]25 uptake is related to its sigma 1-receptor affinity. Benzamides 14d and 25, which display rapid and high tumor uptake, appear to be promising imaging agents for melanoma detection, while 14d, which displays a long lasting and high melanoma/nontarget ratio, is more suitable for evaluation as a potential radiotherapeutic.

Preparation and biologic evaluation of a novel radioiodinated benzylpiperazine, 123I-MEL037, for malignant melanoma.

UNLABELLED: Radiopharmaceuticals that can target the random metastatic dissemination of melanoma tumors may present opportunities for imaging and staging the disease as well as potential radiotherapeutic applications. A novel molecule, 2-(2-(4-(4-(123)I-iodobenzyl)piperazin-1-yl)-2-oxoethyl)isoindoline-1,3-dione (MEL037), was synthesized, labeled with 123I, and evaluated for application in melanoma tumor scintigraphy and radiotherapy. METHODS: The tumor imaging potential of 123I-MEL037 was studied in vivo in C57BL/6J female mice bearing the B16F0 murine melanoma tumor and in BALB/c nude mice bearing the A375 human amelanotic melanoma tumor by biodistribution, competition studies, and SPECT. RESULTS: 123I-MEL037 exhibited high and rapid uptake in the B16F0 melanoma tumor at 1 h (13 %ID/g [percentage injected dose per gram]), increasing with time to reach 25 %ID/g at 6 h. A significant uptake was also observed in the eyes (2 %ID, at 3-6 h after injection) of black mice. No uptake was observed in the tumor or in the eyes of nude mice bearing the A375 tumor. Because of high uptake and long retention in the tumor and rapid body clearance, the mean contrast ratios (MCR) of 123I-MEL037 were 30 and 60, at 24 and 48 h after injection, respectively. At 24 h after injection of mice bearing the B16 melanoma, SPECT indicated that the radioactivity was located predominately in the tumor followed by the eyes, whereas no specific localization of the radioactivity was noted in mice bearing the A375 human amelanotic tumor. In competition experiments, uptake of 123I-MEL037 in brain, lung, heart, and kidney--organs known to contain sigma-receptors--was not significantly different in haloperidol-treated animals compared with control animals. Therefore, reduction of uptake in tumor and eyes of the pigmented mice bearing the B16F0 tumor suggested that the mechanism of tumor uptake was likely due to an interaction with melanin. CONCLUSION: These findings suggested that 123I-MEL037, which displays a rapid and very high tumor uptake, appeared to be a promising imaging agent for detection of most melanoma tumors with the potential for development as a therapeutic agent in melanoma tumor proliferation.

Pharmacological evaluation of an [(123)I] labelled imidazopyridine-3-acetamide for the study of benzodiazepine receptors.

In vitro binding of the iodinated imidazopyridine, N',N'-dimethyl-6-methyl-(4'-[(123)I]iodophenyl)imidazo[1,2-a]pyridine-3-acetamide [(123)I]IZOL to benzodiazepine binding sites on brain cortex, adrenal and kidney membranes is reported. Saturation experiments showed that [(123)I]IZOL, bound to a single class of binding site (n(H)=0.99) on adrenal and kidney mitochondrial membranes with a moderate affinity (K(d)=30 nM). The density of binding sites was 22+/-6 and 1.2+/-0.4 pmol/mg protein on adrenal and kidney membranes, respectively. No specific binding was observed in mitochondrial-synaptosomal membranes of brain cortex. In biodistribution studies in rats, the highest uptake of [(123)I]IZOL was found 30 min post injection in adrenals (7.5% ID/g), followed by heart, kidney, lung (1% ID/g) and brain (0.12% ID/g), consistent with the distribution of peripheral benzodiazepine binding sites. Pre-administration of unlabelled IZOL and the specific PBBS drugs, PK 11195 and Ro 5-4864 significantly reduced the uptake of [(123)I]IZOL by 30% (p<0.05) in olfactory bulbs and by 51-86% (p<0.01) in kidney, lungs, heart and adrenals, while it increased by 30% to 50% (p<0.01) in the rest of the brain and the blood. Diazepam, a mixed CBR-PBBS drug, inhibited the uptake in kidney, lungs, heart, adrenals and olfactory bulbs by 32% to 44% (p<0.01) but with no effect on brain uptake and in blood concentration. Flumazenil, a central benzodiazepine drug and haloperidol (dopamine antagonist/sigma receptor drug) displayed no effect in [(123)I]IZOL in peripheral organs and in the brain. [(123)I]IZOL may deserve further development for imaging selectively peripheral benzodiazepine binding sites.

A rapid and efficient preparation of [123I]radiopharmaceuticals using a small HPLC (Rocket) column.

A simplified method for the rapid and efficient preparation of [(123)I]radiopharmaceuticals is described. Three radiopharmaceuticals, [(123)I]beta-CIT, [(123)I]MIBG and [(123)I]clioquinol, were synthesised and purified as model compounds. The radiotracers were labelled with iodine-123 using electrophilic oxidative conditions and purified by a compact semi-preparative reverse phase column (C-18, 3 microm, 7 x 53 mm, Alltima Rocket, Alltech) using aqueous-ethanol as HPLC solvents that were directly used for radiopharmaceutical formulation. The radiochemical purity of the radioiodinated tracers as assessed by analytical HPLC was higher than 99% with specific activity higher than 3 GBq/nmol. The total preparation time of a radiotracer ranged from 40 to 60 min and, starting from 3.7 GBq of iodine-123, more than 2.5 GBq of formulated radiopharmaceuticals were available for clinical investigations.

Transplantation in Parkinson's disease: PET changes correlate with the amount of grafted tissue.

We compared the striatal uptake of [(18)F]fluorodopa with [(76)Br]-FE-CBT, a positron emission tomography (PET) ligand of the dopamine transporter (DAT), which estimates the density of dopamine nerve terminals, in 6 patients with Parkinson's disease grafted with fetal mesencephalic cells. There was no change in DAT ligand binding in the grafted putamen, despite a significant increase of [(18)F]fluorodopa uptake. This finding suggests that the clinical benefit induced by the graft is more related to increased dopaminergic activity than improved dopaminergic innervation in the host striatum and, therefore, that [(18)F]fluorodopa remains the optimal tracer to evaluate grafted PD patients. Further analysis showed that the clinical and [(18)F]fluorodopa uptake changes after the grafts were correlated with the number of ventral mesencephalae used for implantation.

Pharmacological evaluation of (S)-8-[123I]iodobretazenil: a radioligand for in vivo studies of central benzodiazepine receptors.

In vitro studies on cortical membranes indicated (S)-8-[(123)I]iodobretazenil bound saturably to a single population of binding sites (B(max) = 2.33 pmol/mg protein) with a dissociation constant K(d) = 1.9 nM. (R)-8-[(123)I]Iodobretazenil displayed only non-specific binding. In vivo biodistribution of (S)-8-[(123)I]iodobretazenil in rats indicated high accumulation in regions of high BZR density. Radioactivity was blocked by preadministration with iodobretazenil and flumazenil, while non-BZR drugs had no effect on the uptake of activity in any brain region. (S)-8-[(123)I]Iodobretazenil uptake was saturable in a dose dependent manner (ID(50) = 0.13 mg/kg) in all brain regions. With the (R)-enantiomer no specific uptake was observed. Metabolite studies at 1-3 h p.i. indicated that greater than 95% of activity extracted from brain tissue corresponded to unchanged radiotracer while that in plasma was over 70%. (S)-8-[(123)I]Iodobretazenil potently and selectively labels BZR in vivo and deserves further investigation as a possible SPECT radiotracer.

Pharmacological evaluation of a Br-76 analog of epibatidine: a potent ligand for studying brain nicotinic acetylcholine receptors.

[(76)Br]-Norchlorobromoepibatidine ([(76)Br]BrPH) is a specific and high affinity radioligand for the nicotinic acetylcholine receptors (nAChRs). In vitro, on rat thalamus membranes [(76)Br]BrPH bound to two sites with apparent affinities of 8 pM and 3 nM. The density of binding sites were 1.9 and 70 fmol/mg protein, respectively. In vivo, in biodistribution and autoradiographic studies in rats the regional distribution of [(76)Br]BrPH paralleled the neuroanatomical localization of nAChRs. Two hours postinjection, the highest concentration in the brain was found in thalamus and colliculi (4% ID/g). Competition experiments with specific nicotinic, muscarinic, dopaminergic, and serotoninergic drugs confirmed that the in vivo binding of [(76)Br]BrPH was consistent with neuronal nicotinic receptors. PET imaging of [(76)Br]BrPH in baboon demonstrated a rapid and high uptake in the brain. Peak uptake occurred at 30-40 min for the thalamus. Due to the constant washout in the cerebellum, the thalamus to cerebellum ratio was 5 at 2 h postinjection. Subcutaneous injection of cytisine (1 mg/kg), 3 h postinjection of [(76)Br]BrPH reduced the radioactivity concentration in thalamus and cortex by 58 and 50%, respectively, as observed 1 h later. Cytisine pretreatment (5 mg/kg s.c.) inhibited completely the radioligand accumulation in the thalamus. Chronic MPTP pretreatment resulted in reduction of [(76)Br]BrPH uptake in all brain regions except in cerebellum. These preliminary results suggest that [(76)Br]BrPH has the potential to be a useful radioligand for studying the pharmacology of nicotinic acetylcholine receptors in preclinical experiments.

Synthesis of a fluorine-18-labelled derivative of 6-nitroquipazine, as a radioligand for the in vivo serotonin transporter imaging with PET.

Considerable efforts have been engaged in the design, synthesis and pharmacological characterization of radioligands for imaging the serotonin transporter, based on its implication in several neuropsychiatric diseases, such as depression, anxiety and schizophrenia. In the 5-halo-6-nitroquipazine series, the fluoro derivative has been designed for positron emission tomography (PET). The corresponding 5-iodo-, 5-bromo- and 5-chloro N-Boc-protected quipazines as labelling precursors, as well as 5-fluoro-6-nitroquipazine as a reference compound have been synthesized. 5-[(18)F]Fluoro-6-nitroquipazine has been radiolabelled with fluorine-18 (positron-emitting isotope, 109.8 min half-life) by nucleophilic aromatic substitution from the corresponding N-Boc protected 5-bromo- and 5-chloro-precursors using K[(18)F]F-K(222) complex in DMSO by conventional heating (145 degrees C, 2 min) or microwave activation (50 W, 30-45 s), followed by removal of the protective group with TFA. Typically, 15-25 mCi (5.5-9.2 GBq) of 5-[(18)F]fluoro-6-nitroquipazine (1-2 Ci/micromol or 37-72 GBq/micromol) could be obtained in 70-80 min starting from a 550-650 mCi (20.3-24.0 GBq) aliquot of a cyclotron [(18)F]F(-) production batch (2.7-3.8% non decay-corrected yield based on the starting [(18)F]fluoride). Ex vivo studies (biodistribution in rat), as well as PET imaging (in monkey) demonstrated that 5-[(18)F]fluoro-6-nitroquipazine ([(18)F]-1d) readily crossed the blood brain barrier and accumulated in the regions rich in 5-HT transporter (frontal- and posterial cortex, striata). However, the low accumulation of the tracer in the thalamus (rat and monkey) as well as the comparable displacement of the tracer observed with both citalopram, a -HT re-uptake inhibitor and maprotiline, a norepinephrine re-uptake inhibitor (rat), indicate that 5-[(18)F]fluoro-6-nitroquipazine ([(18)F]-1d) does not have the suggested potential for PET imaging of the serotin transporter (SERT).

(+)-[76Br]A-69024: a non-benzazepine radioligand for studies of dopamine D1 receptors using PET.

(+)-[76Br]A-69024 is a specific and enantioselective dopamine D1 receptor radioligand. The Bmax of (+)-[76Br]A-69024 measured in vitro on rat striatum membranes was 320 +/- 25 fmoles/mg protein with an apparent dissociation constant of Kd = 0.6 +/- 0.1 nM. The inactive enantiomer, (-)-[76Br]A-69024, displayed no affinity in the same assay. In vivo, the biodistribution (+)-[76Br]A-69024 in rats showed a rapid and high uptake in the striatum (1% ID/g), followed by a slow wash out. The striatum/cerebellum concentration ratio (index of specific binding) reached a maximum value of 10 at 60 minutes post injection. A tissue to cerebellum ratio of 2.8 and 1.5 was also observed for frontal and posterior cortex respectively. With the pharmacologically inactive enantiomer, (-)-[76Br]A-69024, the brain uptake was determined to be non specific since a striatum/cerebellum ratio of approximately 1 was observed throughout the time course of the experiment. The selectivity of (+)-[76Br]A-69024 uptake was demonstrated in competition experiments. The specific uptake in the striatum and cortical regions was completely prevented after administration of the D1 antagonist SCH 23390. Pre-treatment of rats with unlabelled (+)A-69024 also displayed the same regional inhibition of (+)-[76Br]A-69024 uptake. Pre-administration of rats with spiperone (D2) and ketanserin (5-HT2/5-HT2C) showed no inhibitory effect on (+)-[76Br]A-69024 uptake in any brain region. Using (+)-[76Br]A-69024, PET study in baboon demonstrated a preferential accumulation of the radioactivity in the striatum, frontal and posterior cortex which was displaced to the level of the cerebellum by SCH 23390. These results suggest that (+)-[76Br]A-69024 may deserve further investigation as a potential radioligand for studying striatal and cortical dopamine D1 receptors using PET.

Dopaminergic function and dopamine transporter binding assessed with positron emission tomography in Parkinson disease.

BACKGROUND: Measuring progression of Parkinson disease (PD) using positron emission tomography may help demonstrate the efficacy of neuroprotective treatments. To date, (18)F-dopa has been the gold standard to measure presynaptic dopaminergic function in PD, but this tracer might overestimate the rate of neuronal death in PD because its uptake also depends on dopamine turnover rather than exclusively on the density of dopaminergic terminals in the striatum. The latter might be assessed using newly developed ligands of the membrane dopamine transporter. OBJECTIVE: To compare the striatal uptakes of (18)F-dopa and (76)Br-FE-CBT, a dopamine transporter ligand, in patients with PD. PATIENTS AND METHODS: The striatal uptakes of (76)Br-FE-CBT and (18)F-dopa were compared using positron emission tomography in 10 patients with early PD and 8 with advanced PD. Correlation of uptakes with motor performance was investigated. RESULTS: The reduction in (76)Br-FE-CBT binding to 43% of control values was more severe than the reduction in (18)F-dopa uptake (63% of control values) in the putamen of patients with early PD. No significant difference was found between either tracer's uptake in the putamen of patients with advanced PD. Motor performance was highly correlated to (18)F-dopa uptake, whereas correlation to (76)Br-FE-CBT binding was weak. CONCLUSIONS: Uptake of (18)F-dopa may be up-regulated in early PD, suggesting a compensatory increase of dopamine synthesis in surviving dopaminergic terminals. Positron emission tomography dopamine transporter ligands and (18)F-dopa give complementary information on the presynaptic status of the nigrostriatal dopaminergic system and might be associated to investigate the efficacy of neuroprotective treatments in PD.

In vivo quantification and parametric images of the cardiac beta-adrenergic receptor density.

UNLABELLED: Previous studies showed that the in vivo concentration of beta-adrenergic receptor sites can be estimated by PET using (-)-4-((S)-3-tert-butylamino-2-hydroxypropoxy)-1,3-dihydrobenzoimidazol-2-one (CGP 12177), a hydrophilic ligand. A graphic method was previously proposed and used by several groups. However, this approach was not completely validated. The purpose of this study was to improve and confirm the validity of this approach through a better knowledge of the associated ligand-receptor model, estimated for the first time using the multiinjection approach. METHODS: The concentration of beta-adrenergic receptor sites was estimated for mini pigs using 2 methods. The first was the usual multiinjection approach, which permits estimation of all model parameters, including receptor concentration. However, this approach needs a complex protocol, including blood sampling, thereby making it difficult to use for studies on patients. The second method was the CGP 12177 graphic method. This approach permits the estimation of only receptor concentration but has the advantage of not requiring blood sampling. Another advantage is the ability to generate parametric images easily. RESULTS: Using the multiinjection approach, we obtained for the first time a complete model describing interactions between CGP 12177 and beta-adrenergic receptors. Knowledge of all parameters of this model permitted good validation of the assumptions included in the graphic method. The concentration of beta-adrenergic receptor sites in mini pigs was estimated at 15.2 +/- 3.4 pmol/mL. CONCLUSION: The graphic method has been improved by taking into account various phenomena, such as protein binding and the nonlinearity between plasma concentration and injected dose. This method is now usable for patient studies and offers the ability to estimate the beta-adrenergic receptor concentration from a single PET experiment without blood sampling. Parametric imaging will enable screening of the receptor site location and observation of potential anomalies in patients.