Salma Jivan

Biochemical variations in the synaptic level of dopamine precede motor fluctuations in Parkinson's disease: PET evidence of increased dopamine turnover

Motor fluctuations are a major disabling complication in the treatment of Parkinsons disease. To investigate whether such oscillations in mobility can be attributed to changes in the synaptic levels of dopamine, we studied prospectively patients in the early stages of Parkinsons disease with a follow‐up after at least 3 years of levodopa treatment. At baseline, 3 positron emission tomography (PET) scans using [11C]raclopride before and after (1 hour and 4 hours) orally administered levodopa were performed on the same day for each patient. Patients who developed “wearing‐off” fluctuations during the follow‐up period had a different pattern of levodopa‐induced changes in [11C]raclopride binding potential (BP) from that observed in patients who were still stable by the end of the follow‐up. Thus, 1 hour post‐levodopa the estimated increase in the synaptic level of dopamine was 3 times higher in fluctuators than in stable responders. By contrast, only stable responders maintained increased levels of synaptic dopamine in the PET scan performed after 4 hours. These results indicate that fluctuations in the synaptic concentration of dopamine precede clinically apparent “wearing‐off” phenomena. The rapid increase in synaptic levels of dopamine observed in fluctuators suggests that increased dopamine turnover might play a relevant role in levodopa‐related motor complications. Ann Neurol 2001;49:298–303

In vivo PET studies of the dopamine D1 receptors in rhesus monkeys with long-term MPTP-induced Parkinsonism.

Studies of dopamine (DA) receptor binding in early parkinsonian patients, or in models of Parkinsons disease, have revealed a supersensitivity of the D2‐like receptor subtype as compared to age‐matched controls. The lack of upregulation in advanced patients is often attributed to the effects of prolonged antiparkinsonian therapy, but the impact of therapy vs. intrinsic mechanisms in untreated patients or animals with long‐term lesions of the DA nigrostriatal pathway has been difficult to address. We studied, in vivo, by PET using the DA D2 receptor ligand raclopride, the status of the DA receptors in normal rhesus monkeys and those with acute (3 months) or long‐term (10 years) MPTP‐induced nigrostriatal lesions. Compared to age‐matched controls, there was no change in raclopride binding in MPTP‐treated animals without parkinsonian symptoms. There was a significant increase in raclopride binding in the putamen (but not caudate nucleus) of all the animals displaying rigidity, hypo‐ and bradykinesia. This increase was greater in the animals with acute lesions (32%) than with established, long‐term lesions (18%). There was no correlation between the postmortem striatal DA concentrations and in vivo raclopride binding but there was a correlation between PET raclopride binding and [3H]raclopride binding in vitro. Complex changes in D2 receptor binding occur in various stages of parkinsonism. Antiparkinsonian therapy is unlikely to be solely responsible for the lack of upregulation found in advanced parkinsonian patients but may be a contributing factor. Synapse 38:105–113, 2000.

Synthesis and purification of l-6-[18F]fluorodopa

This work describes the synthesis and purification of l-6-[18F]fluorodopa synthesized via the reaction of acetyl hypofluorite with a mercury-dopa derivative. The mercurated starting material and the mercury compounds produced in the reaction are efficiently removed by passage of the reaction mixture through a mercaptan modified silica gel. After evaporation the residue was treated with HI, and purified by HPLC to give the pure product in 12% radiochemical yield (decay corrected) and containing <10 ppb of mercury.

Fluorination of aromatic compounds with F2 and acetyl hypofluorite: synthesis of 18F-aryl fluorides by cleavage of aryl-tin bonds [1]

Abstract Elemental fluorine and acetyl hypofluorite, labeled with 18 F, were added to eight simple aryl-tin derivatives. Both reagents gave good radiochemical yields of labeled aryl fluorides. Overall, acetyl hypofluorite gave more consistent yields (∼70%), while F 2 gave more variable yields (54% – 95%).

Effect of age on markers for monoaminergic neurons of normal and MPTP-lesioned rhesus monkeys: a multi-tracer PET study.

The binding of three tracers for monoaminergic terminals was mapped in the brain of healthy young (N=6) and healthy old rhesus monkeys (N=4), aged monkeys with mild unilateral intracarotid MPTP lesions (N=3), and monkeys of intermediate age with severe systemic MPTP lesions (N=6). The ligand for monoaminergic vesicles (+)-[(11)C]dihydrotetrabenazine (+DTBZ) had a mean binding potential (pB) of 1.4 in striatum of the healthy young monkeys, which was reduced by 20% in putamen of the old monkeys. The catecholamine transporter ligand (+)-[(11)C]methylphenidate (+MP) had a mean pB of 1.3 in striatum of the young monkeys, which was reduced by 40% in caudate and putamen of the old monkeys. The DOPA decarboxylase substrate [(18)F]fluoro-l-DOPA (FDOPA) had a mean decarboxylation coefficient (k(3)(S)) of 0.4 h(-1) in striatum of the young group, and was not significantly reduced in the aged group. Of the three ligands, only +DTBZ pB was significantly reduced in striatum of the small group of animals with mild unilateral lesions. In the group with systemic MPTP lesions, the mean reduction of the binding of the three ligands was 80% in the caudate and putamen. However, the decline in +MP pB in the ventral striatum (-75%) exceeded the declines of +DTBZ pB and FDOPA k(3)(S) in that region (-65%), suggesting that compensatory down-modulation of uptake sites may occur in the striatal regions with the least dopamine depletion. Binding of all three ligands was reduced by 50% in the anterior cingulate cortex and in the thalamus, suggesting toxicity of MPTP for extrastriatal catecholamine innervations. +DTBZ binding in the hypothalamus, presumably mainly in serotonin fibers, was unaffected by systemic MPTP treatment. Of the three tracers, +DTBZ was most sensitive for detecting MPTP-induced dopamine depletion in monkey striatum.

Evaluation of Dopaminergic Presynaptic Integrity: 6-[18F]Fluoro-L-Dopa Versus 6-[18F]Fluoro-L-m-Tyrosine

The effectiveness of 6-[18F]fluoro-L-m-tyrosine (6FMT) to evaluate dopamine presynaptic integrity was compared to that of 6-[18F]fluoro-L-dopa (6FDOPA) in vivo by positron emission tomography (PET). Six normal and six 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) -lesioned monkeys received 6FDOPA and 6FMT PET scans on separate occasions with identical scanning protocols. Four measures, the rate of uptake of tracer into striatum using either the arterial input function (Ki) or the activity in the occipital cortex as the input function (Kc), the rate of loss of striatal radioactivity (kloss), and an index of “effective turnover” of dopamine (kloss/Ki), were obtained for both tracers during extended PET studies. 6-[18F]Fluoro-L-m-tyrosine was as effective as 6FDOPA in separating normals from MPTP-lesioned subjects on the basis of the uptake rate constants Ki and Kc. However, in contrast to 6FDOPA, it was not possible to differentiate the normal from the lesioned animal using kloss or kloss/Ki for 6FMT. Thus, FMT appears to be a reasonable, highly specific tracer for studying the activity of aromatic dopa decarboxylase enzyme as an index of presynaptic integrity. However, if one is interested in investigating further the metabolic pathway and obtaining an in vivo estimate of the effective turnover of dopamine (after pharmacologic manipulation, for example), 6FDOPA remains the tracer of choice.

Reaction of [18F]acetyl hypofluorite with derivatives of dihydroxyphenylalanine: Synthesis of l-[18F]6-fluorodopa

Twelve derivatized dihydroxyphenylalanine (dopa) substrates were treated with gaseous [18F]acetyl hypofluorite. Radiochemical yields of [18F]6-fluorodopa, obtained after HI hydrolysis, ranged from < 1–8% (decay corrected). The highest yield was obtained using l-methyl-N-acetyl-[β-(3-methoxy-4-acetoxyphenyl]alaninate.

Quantitative in vitro phosphor imaging using [3H] and [18F] radioligands: the effects of chronic desipramine treatment on serotonin 5-HT2 receptors

Traditionally, autoradiography of neuroreceptors is performed in vitro using tritiated ligands and low sensitivity X-ray film, requiring long exposure times. In vivo imaging of neuroreceptors using positron emission tomography (PET) suffers poor spatial resolution, but in vitro PET autoradiography is difficult with film due to the short half-life of the isotopes. Storage phosphor screens provide an extremely sensitive alternative to film. To demonstrate and validate quantitative in vitro phosphor imaging with PET and tritiated ligands, we treated rats chronically with the antidepressant desipramine, which results in decreased binding to serotonin 5-HT(2) receptors. Serotonin 5-HT(2) binding decreased significantly in all cortical regions examined as measured by both [(3)H]ketanserin and [(18)F]setoperone. The data from the two radioligands were not significantly different, and the distribution of the receptors was in agreement with previous reports. We also present data on the reusability of tritium-sensitive phosphor screens, and show that the use of simple corrections allows receptor binding data with PET ligands to be compared across different days. The results indicate that phosphor imaging is a valid, fast, and quantifiable technique for measuring neuroreceptor regulation, and that it provides an excellent tool to corroborate in vivo PET data in vitro at higher resolution.

A high-affinity [(18)F]-labeled phosphoramidate peptidomimetic PSMA-targeted inhibitor for PET imaging of prostate cancer.

INTRODUCTION In this study, a structurally modified phosphoramidate scaffold, with improved prostate-specific membrane antigen (PSMA) avidity, stability and in vivo characteristics, as a PET imaging agent for prostate cancer (PCa), was prepared and evaluated. METHODS p-Fluorobenzoyl-aminohexanoate and 2-(3-hydroxypropyl)glycine were introduced into the PSMA-targeting scaffold yielding phosphoramidate 5. X-ray crystallography was performed on the PSMA/5 complex. [(18)F]5 was synthesized, and cell uptake and internalization studies were conducted in PSMA(+) LNCaP and CWR22Rv1 cells and PSMA(-) PC-3 cells. In vivo PET imaging and biodistribution studies were performed at 1 and 4 h post injection in mice bearing CWR22Rv1 tumor, with or without blocking agent. RESULTS The crystallographic data showed interaction of the p-fluorobenzoyl group with an arene-binding cleft on the PSMA surface. In vitro studies revealed elevated uptake of [(18)F]5 in PSMA(+) cells (2.2% in CWR22Rv1 and 12.1% in LNCaP) compared to PSMA(-) cells (0.08%) at 4 h. In vivo tumor uptake of 2.33% ID/g and tumor-to-blood ratio of 265:1 was observed at 4 h. CONCLUSIONS We have successfully synthesized, radiolabeled and evaluated a new PSMA-targeted PET agent. The crystal structure of the PSMA/5 complex highlighted the interactions within the arene-binding cleft contributing to the overall complex stability. The high target uptake and rapid non-target clearance exhibited by [(18)F]5 in PSMA(+) xenografts substantiates its potential use for PET imaging of PCa. ADVANCES IN KNOWLEDGE The only FDA-approved imaging agent for PCa, Prostascint®, targets PSMA but suffers from inherent shortcomings. The data acquired in this manuscript confirmed that our new generation of [(18)F]-labeled PSMA inhibitor exhibited promising in vivo performance as a PET imaging agent for PCa and is well-positioned for subsequent clinical trials. Implications for Patient Care Our preliminary data demonstrate that this tracer possesses the required imaging characteristics to be sensitive and specific for PCa imaging in patients at all stages of the disease.

In Vivo Receptor Assay with Multiple Ligand Concentrations: An Equilibrium Approach

The ligand-receptor binding potential determined by in vivo PET studies at high ligand-specific radioactivity reflects both the receptor density and ligand-receptor affinity. This ambiguity has been resolved by various methods based on the administration of multiple unlabeled ligand concentrations. The authors aimed to implement and refine an approach to multiple ligand concentration receptor assay that combined maximum simplicity and a minimum of assumptions and model dependence that would nonetheless reliably distinguish density from affinity effects. The approach uses administration by bolus followed by infusion to obtain a true equilibrium between bound ligand and the other components of the ligand concentration, and does not require measurements of ligand in blood plasma. Four approaches to the optimization of the desired density and affinity parameters from the measured equilibrium data were implemented and compared in the analysis of raclopride studies performed in both normal control and MPTP-lesioned nonhuman primates. The authors conclude that the method is simple enough for routine use and yet reliable enough to apply in ongoing studies of both chronic and acute drug effects in the dopamine system.