Fluorodopa is a Promising Fluorine‐19 MRI Probe for Evaluating Striatal Dopaminergic Function in a Rat Model of Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra projecting to the striatum. It has been estimated that approximately 80% of the striatal dopamine and 50% of nigral dopaminergic neurons are lost before the onset of typical motor symptoms, indicating that early diagnosis of PD using noninvasive imaging is feasible. Fluorine‐19 (¹⁹F) magnetic resonance imaging (MRI) represents a highly sensitive, easily available, low‐background, and cost‐effective approach to evaluate dopaminergic function using non‐radioactive fluorine‐containing dopaminergic agents. The aim of this study was to find a potent ¹⁹F MRI probe to evaluate dopaminergic presynaptic function in the striatum. To select candidates for ¹⁹F MRI probes, we investigated the following eight non‐radioactive fluorine‐containing dopaminergic agents: fluorodopa (F‐DOPA), F‐tyrosine, haloperidol, GBR13069 duhydrochloride, GBR12909 duhydrochloride, 3‐bis‐(4‐fluorophenyl) methoxytropane hydrochloride, flupenthixol, and fenfluramine. In ¹⁹F nuclear magnetic resonance measurements, F‐tyrosine and F‐DOPA displayed a relatively higher signal‐to‐noise ratio value in brain homogenates than in others. F‐DOPA, but not F‐tyrosine, induced the rotational behavior in a 6‐hydroxydopamine (6‐OHDA)‐induced hemiparkinsonian rat model. In addition, a significantly high amount of F‐DOPA accumulated in the ipsilateral striatum of hemiparkinsonian rats after the injection. We performed ¹⁹F MRI in PC12 cells and isolated rat brain using a 7T MR scanner. Our findings suggest that F‐DOPA is a promising ¹⁹F MRI probe for evaluating dopaminergic presynaptic function in the striatum of hemiparkinsonian rats. © 2016 Wiley Periodicals, Inc.     

18F-dopa PET evidence that tolcapone acts as a central COMT inhibitor in Parkinson's disease.

Tolcapone is a potent, selective, and reversible inhibitor of cathecol-O-methyl-transferase (COMT). This enzyme plays a crucial role in the extraneural inactivation of catecholamine neurotransmitters. Tolcapone's ability to inhibit central COMT in humans at therapeutic concentrations is not yet clear. The aim was to determine the effect of tolcapone on central COMT activity in Parkinson's disease (PD) using (18)F-dopa positron emission tomography (PET). The study was a randomized two-way crossover study. Twelve PD patients were recruited. On the treatment days patients were given either tolcapone (200 mg) or placebo together with levodopa/carbidopa (100/125 mg) 1 h before the injection of (18)F-dopa. Data were acquired in 25 frames over 94 min for the first PET scan period. At the end of this period the patients were removed from the scanner for 90 min and subsequently repositioned and data acquired in six 10-min time frames over 60 min. Influx constants (Ki) were computed using a graphical approach with a plasma input function. Mean (18)F-dopa putamen Ki's for the first 30-90 min, primarily reflecting central dopa decarboxylase (DDC) activity, were similar in PD patients whether tolcapone was present (0.0078 +/- 0.0031 min(-1)) or absent (0.0078 +/- 0.0030 min(-1)). Mean putamen Ki values calculated 180-240 min after injection of (18)F-dopa, reflecting both central DDC and COMT activity, were unchanged from 30-90' values in the presence of tolcapone (0.0079 +/- 0.0030), implying blockade of central COMT, but were significantly reduced (0.0059 +/- 0.0028) in the absence of this drug. These findings are compatible with clinical doses of tolcapone having a significant blocking effect on peripheral and central COMT but not DDC activity in PD.     

PET study of striatal fluorodopa uptake and dopamine D2 receptor binding in a patient with juvenile parkinsonism

We studied pre-synaptic and post-synaptic function in the striatum of a patient with juvenile parkinsonism (JP) using positron emission tomography (PET). [¹⁸F]6-fluorodopa (¹⁸FDOPA), ¹¹C-YM-09151-2 and [¹⁸F]fluoro-2-deoxy- D -glucose (¹⁸FDG) were used to measure fluorodopa uptake, dopamine D₂ receptor binding and glucose metabolism, respectively. In this patient, ¹⁸FDOPA accumulation was decreased markedly in the caudate nucleus and the putamen bilaterally. In the images of ¹¹C-YM-09151-2 and ¹⁸FDG in contrast, no conspicuous changes were observed in the striatum. Thus our PET studies using ¹⁸FDOPA, ¹¹C-YM-09151-2 and ¹⁸FDG provide a useful approach for assisting the diagnosis of JP, because the present findings are different from the results in patients with dopa-responsive dystonia and hereditary progressive dystonia with marked diurnal fluctuation. Furthermore, our findings are of particular interest in relation to the pathogenesis of JP.     

Striatal 6-[18F]fluorodopa accumulation after combined inhibition of peripheral catechol-O-methyltransferase and monoamine oxidase type B: Differing response in relation to presynaptic dopaminergic dysfunction

The aim was to investigate the effects of inhibition of monoamine oxidase type B (MAO-B) with selegiline alone and the combined inhibition of peripheral catechol-O-methyltransferase (COMT) with entacapone and MAO-B with selegiline on striatal 6-[¹⁸F]fluorodopa (FDOPA) accumulation, and whether the effect of entacapone + selegiline on FDOPA uptake differed depending on the severity of the presynaptic dopaminergic dysfunction. Thus, eight healthy controls, eight de novo patients with Parkinson's disease (PD), and 18 levodopa-treated PD patients were investigated with positron emission tomography (PET). Half of the subjects in each population belonged to the selegiline group and half to the entacapone + selegiline group. Both groups were studied twice with PET using FDOPA. After the first (baseline) FDOPA PET investigation, both groups were on 2 weeks of selegiline treatment, 10 mg daily. Thereafter, the second FDOPA PET was performed for all subjects with a premedication administered 60 min before the PET imaging; one group received 10 mg of selegiline, and the other group received a single 400 mg dose of entacapone coadministered with 10 mg of selegiline. Selegiline treatment alone had no significant influence on striatal FDOPA metabolism. The FDOPA accumulation, expressed as striatal-to-occipital ratios and modified decarboxylation coefficients (k₃R₀), increased significantly after entacapone + selegiline administration in all subject populations. The FDOPA uptake rate constant (K_i) remained virtually unchanged in controls and in de novo patients but decreased significantly in levodopa-treated PD patients after entacapone + selegiline intake. Entacapone + selegiline administration did not influence significantly the unidirectional blood-to-brain clearance for FDOPA (K₁^D) or the relative dopadecarboxylase activity (k₃^D). The changes in the studied parameters after entacapone + selegiline administration probably reflect the effects of entacapone, since entacapone alone has caused similar changes in previous PET studies. Response in FDOPA accumulation to entacapone + selegiline was higher in controls and de novo patients compared with levodopa-treated PD patients. The milder response in levodopa-treated patients might reflect the reduced ability of the degenerated dopaminergic neurons to utilize the prolonged FDOPA availability, produced by entacapone. Synapse 27:336–346, 1997. © 1997 Wiley-Liss, Inc.     

Striatal subregional 6-[18F]fluoro-L-dopa uptake in early Parkinson's disease: a two-year follow-up study.

Thirty-one drug-naive patients with Parkinson's disease (PD) underwent 6-[18F]fluoro-L-dopa (F-dopa) positron emission tomography (PET) scan at the time of the diagnosis (baseline) and 2 years later in order to investigate F-dopa uptake in striatal and extrastriatal regions during the first years of early PD. Twenty-four healthy controls underwent one F-dopa PET scan. The regional differences in the striatal and extrastriatal regions were analyzed with statistical parametric mapping and automated region of interest analyses. Our study shows that the F-dopa uptake in unmedicated early PD is most severely decreased in the dorsal part of caudal putamen but significant decrease can be seen throughout the striatum compared with controls. During the first years of PD, there is a progressive regional decline in striatal F-dopa uptake, the dorsal part of caudal putamen being still the most severely affected region. The absolute decline is equal between the striatal subregions. This suggests that the decline of dopamine function starts from the dorsocaudal putamen, but once started, the rate of progression is equal between the subregions of the striatum. In contrast to the striatal decline, the increased cortical F-dopa uptake prevails at least during the first years of PD.     

Central dopamine deficiency in pure autonomic failure

Objective Pure autonomic failure (PAF) and Parkinson’s disease (PD) share several clinical laboratory abnormalities; however, PAF is not associated with parkinsonism. In this study, we tested the hypothesis that preservation of nigrostriatal dopaminergic innervation explains the absence of motor dysfunction in PAF. Methods Patients with PAF (N = 5) or PD (N = 21) and control subjects (N = 14) had brain 6-[¹⁸F]fluorodopa positron emission tomographic scanning and cerebrospinal fluid catechol measurements. A patient with PAF and another with PD had rapid postmortem striatal, nigral, and sympathetic ganglion sampling, with assays of catechols and tyrosine hydroxylase activity. Results The PAF and PD groups had similarly low mean substantia nigra (SN):occipital (OCC) ratios of 6-[¹⁸F]fluorodopa-derived radioactivity and similarly low cerebrospinal fluid dihydroxyphenylacetic acid and DOPA levels. Only the PD group, however, had low PUT:OCC, caudate:OCC, or PUT:SN ratios. The PAF and PD cases had similarly low SN tissue concentrations of dopamine and tyrosine hydroxylase activity, but the PD patient had tenfold lower PUT dopamine and the PAF patient 15-fold lower myocardial norepinephrine concentrations. Conclusions Surprisingly, PAF and PD entail similarly severe nigral and overall central dopaminergic denervation. There is more severe loss of striatal dopaminergic terminals in PD than in PAF and more severe loss of sympathetic noradrenergic terminals in PAF than in PD. These differences explain the distinctive clinical manifestations of the two Lewy body diseases. Parkinsonism appears to reflect striatal dopamine deficiency rather than loss of nigral dopaminergic neurons per se.     

Loss of metabolites from monkey striatum during PET with FDOPA

The decarboxylation of 6-[(18)F]fluorodopa (FDOPA) and retention of the product [(18)F]fluorodopamine within vesicles of catecholamine fibers results in the labeling of dopamine-rich brain regions during FDOPA/PET studies. However, this metabolic trapping is not irreversible due to the eventual diffusion of [(18)F]fluorodopamine metabolites from brain. Consequently, time-radioactivity recordings of striatum are progressively influenced by metabolite loss. In linear analyses, the net blood-brain clearance of FDOPA (K(D)(i), ml g(-1) min(-1)) can be corrected for this loss by the elimination rate constant k(Lin)(cl) (min(-1)). Similarly, the DOPA decarboxylation rate constant (k(D)(3), min(-1)) calculated by compartmental analysis can also be corrected for metabolite loss by the elimination rate constant k(DA)(9) (min(-1)). To compare the two methods, we calculated the two elimination rate constants using data recorded during 240 min of FDOPA circulation in normal monkeys and in monkeys with unilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesions. Use of the extended models increased the magnitudes of K(D)(i) and k(D)(3) in striatum; in the case of k(D)(3), variance of the estimate was substantially improved upon correction for metabolite loss. The rate constants for metabolite loss were higher in MPTP-lesioned monkey striatum than in normal striatum. The high correlation between individual estimates of k(Lin)(cl) and k(DA)(9) suggests that both rate constants reveal loss of decarboxylated metabolites from brain.     

Normalization of markers for dopamine innervation in striatum of MPTP-lesioned miniature pigs with intrastriatal grafts

As part of the DaNeX study, the uptake and binding of several positron emitting tracers was recorded in brain of healthy Göttingen minipigs, in minipigs with a syndrome of parkinsonism due to MPTP intoxication, and in parkinsonian minipigs which had received intrastriatal grafts of mesencephalic neurons from fetal pigs. The specific binding of [¹¹C]NS 2214 to catecholamine uptake sites was reduced by two thirds in striatum of the intoxicated animals, while the rate constant for the decarboxylation of [¹⁸F]fluorodopa was reduced by 50% in the intoxicated animals. Several months after grafting, both pre‐synaptic markers of dopamine fibres were normal in striatum. Dopamine depletion or grafting were without effect on the cerebral perfusion rate, measured with [¹⁵O]‐water, did not alter the rate of oxygen metabolism (CMRO₂) in brain, and did not alter the binding potential of tracers for dopamine D1 or D2 receptors in pig striatum. However, the grafting was associated with a local increase in the binding of [¹¹C]PK 11195, a tracer for reactive gliosis, suggesting that an immunological reaction occurs at the site of graft, which might potentially have reduced the graft patency. However, this apparent immunological response did not preclude the re‐establishment of normal [¹⁸F]fluorodopa and [¹¹C]NS 2214 uptake in the allografted striatum.     

Brain dopaminergic mechanisms in Parkinson's disease evaluated by positron emission tomography

Abstract– The meso-striatal dopamine neurons, essential for the automated control of movements, are primarily affected in patients with P.D. Direct study of the role of this pathway in states of disease has not been possible until recently and the application of PET for the in vivo investigation of dopaminergic mechanisms may serve to demonstrate the potential of the technique.
One basic idea has been to work out methods to investigate multiple aspects of dopaminergic function, i.e. presynaptic mechanisms such as re-uptake sites and synthesis of neurotransmitter as well as postsynaptic such as receptor properties. Furthermore, efforts have been made to evaluate dopamine degradating enzymes. Preclinical PET-investigations have regularly been performed in Rhesus monkeys and the hemiparkinsonian model produced by infusing MPTP into one internal carotid artery has been of great value to characterize new ¹¹C-labelled tracers. Today ¹¹C–(+)-nomifensine is used to give a measure of dopamine re-uptake sites, probably reflecting nerve terminals. ¹¹C-labelled L-dopa has now been introduced and can be expected to replace ¹⁸F-L-fluorodopa as a physiological tracer for precursor transport and transmitter synthesis. Several ligands are available for the quantitation of dopamine receptors – ¹¹C-N-methylspiperone and ¹¹C-raclopride have been used in our studies. ¹¹C-L-selegiline and its "inactive'D-form have been used in clinical PET-studies aimed to evaluate the enzyme MAO-B. A summary of in vivo information of dopaminergic mechanisms in P.D. obtained using the above-mentioned tracers and PET is presented.     

Evidence for impaired presynaptic dopamine function in parkinsonian patients with motor fluctuations

Summary. We used [¹⁸F]6-fluorodopa (FD) positron emission tomography (PET) to examine the severity of nigrostriatal dopaminergic dysfunction in 67 patients with Idiopathic Parkinsonism (IP), 52 with fluctuations and 15 with a stable response to levodopa. FD uptake (Ki) was reduced by 12% in the caudate (p = 0.08) and by 28% in the putamen (p = 0.0004) of patients with fluctuations compared to those with a stable response. However, there was considerable overlap of FD Ki values between the two groups. The fluctuators had a longer symptom duration (11.6 ± 5.7 years) than the patients with a stable response to levodopa (4.3 ± 2.4 years; p < 0.0001) and the age of onset of symptoms was earlier in the fluctuators (43.9 ± 8.9 versus 54.1 ± 10.4; p = 0.0004). Similar reductions in FD Ki in the fluctuators persisted following adjustment for these variables (7.5% in the caudate and 26% in the putamen; p = n.s. and 0.007, respectively). When smaller groups (n = 15 each) were matched for duration of symptoms, the reduction in caudate Ki in the fluctuators was only 1.9% (p = n.s.), but there was still a 24% reduction in putamen Ki (p = 0.05). These findings suggest that fluctuators and non-fluctuators may differ in the severity of their nigrostriatal damage and provide modest support for the hypothesis that fluctuations may in part reflect altered "buffering" capacity of dopaminergic nerve terminals. However, the considerable overlap between groups suggests that other factors such as altered postsynaptic mechanisms and/or increased turnover of dopamine may make a substantial contribution to the development of motor fluctuations. Received February 18, 1999; accepted October 4, 1999