Distribution and kinetics of 3-O-methyl-6-[18F]fluoro-L-dopa in the rhesus monkey brain

Monitoring of a progressive functional dopaminergic deficit in the A53T-AAV synuclein rats by combining 6-[18F]fluoro-L-m-tyrosine imaging and motor performances analysis

With the emergence of disease-modifying therapies for Parkinson's disease, reliable longitudinal markers are needed to quantify pathology and demonstrate disease progression. We developed the A53T-AAV rat model of synucleinopathy by combining longitudinal measures over 12 weeks. We first characterized the progression of the motor and dopaminergic deficits. Then, we monitored the disease progression using the [18F]FMT Positron Emission Tomography (PET) radiotracer. The nigral injection of A53T-AAV led to an increase in phosphorylated α-synuclein on S129, a progressive accumulation of α-synuclein aggregates, and a decrease of dopaminergic function associated with a deterioration of motor activity. The longitudinal monitoring of A53T-AAV rats with [18F]FMT PET showed a progressive reduction of the Kc outcome parameter in the caudate putamen from the lesioned side. Interestingly, the progressive reduction in the [18F]FMT PET signal correlated with defects in the stepping test. In conclusion, we established a progressive rat model of α-synuclein pathology which monitors the deficit longitudinally using both the [18F]FMT PET tracer and behavioral parameters, 2 features that have strong relevance for translational approaches.

O-(2-18F-fluoroethyl)-l-tyrosine (18F-FET) uptake in insulinoma: first results from a xenograft mouse model and from human

INTRODUCTION

Herein we have evaluated the uptake of O-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-FET) in insulinoma in comparison with those of 6-¹⁸F-fluoro-3,4-dihydroxy-l-phenylalanine (¹⁸F-FDOPA) providing first data from both murine xenograft model and one patient with proved endogenous hyperinsulinemic hypoglycemia.

METHODS

Dynamic ¹⁸F-FET and carbidopa-assisted ¹⁸F-FDOPA PET were performed on tumor-bearing nude mice after subcutaneous injection of RIN-m5F murine beta cells and on a 30-year-old man with type-1 multiple endocrine neoplasia and hyperinsulinemic hypoglycemia defined by a positive fasting test.

RESULTS

Seven and three nude mice bearing a RIN-m5F insulinoma xenograft were respectively studied by ¹⁸F-FET and ¹⁸F-FDOPA μPET. Insulinoma xenograft was detected in all the imaged animals. Xenograft was characterized by an early but moderate increase of ¹⁸F-FET uptake followed by a slight decline of uptake intensity during the 20 min dynamic acquisition. Tumoral radiotracer peak intensity and the highest tumor-to-background contrast were reached about 5 minutes after ¹⁸F-FET iv. injection (mean SUV: 1.21 ± 0.10). The biodistribution of ¹⁸F-FET and ¹⁸F-FDOPA and their dynamic tumoral uptake profile and intensity were similar. In the examined patient, ¹⁸F-FDOPA and ¹⁸F-FET PET/CT showed one concordant focal area of well-defined increased uptake in the pancreatic tail corresponding to 11 mm histologically proved insulinoma. The SUV_max tumor to liver ratio was 1.5, 1.1 for ¹⁸F-FDOPA, 1.1, 1 for ¹⁸F-FET at early (0-5 min post injection) and delayed (5-20 min post injection) PET/CT acquisition, respectively. Despite the relatively low tumoral uptake intensity, insulinoma was clearly identified due to the low background in the pancreas. At the contrary, no ¹⁸F-FDOPA or ¹⁸F-FET tumoral uptake was revealed on whole-body PET/CT images performed about 30 min after radiotracer administration. Note of worth, the dynamic uptake pattern of ¹⁸F-FET and ¹⁸F-FDOPA were similar between human insulinoma and mice xenograft tumor.

CONCLUSION

¹⁸F-FET PET compared equally to ¹⁸F-FDOPA PET in a preclinical RIN-m5F murine model of insulinoma and in one patient with insulinoma-related hypoglycemia. However, in both cases, the tumoral uptake intensity was moderate and the tumor was only visible until 20 min after radiotracer injection. Hence, caution should be taken before asserting the translational relevance of our results in the clinical practices. However, the structural analogies between ¹⁸F-FET and ¹⁸F-FDOPA as well as the limited pancreatic uptake of ¹⁸F-FET in human, encourage evaluating ¹⁸F-FET as diagnostic radiotracer for insulinoma detection in further prospective studies involving large cohorts of patients.

Comparative assessment of 6-[18 F]fluoro-L-m-tyrosine and 6-[18 F]fluoro-L-dopa to evaluate dopaminergic presynaptic integrity in a Parkinson's disease rat model

Because of the progressive loss of nigro-striatal dopaminergic terminals in Parkinson's disease (PD), in vivo quantitative imaging of dopamine (DA) containing neurons in animal models of PD is of critical importance in the preclinical evaluation of highly awaited disease-modifying therapies. Among existing methods, the high sensitivity of positron emission tomography (PET) is attractive to achieve that goal. The aim of this study was to perform a quantitative comparison of brain images obtained in 6-hydroxydopamine (6-OHDA) lesioned rats using two dopaminergic PET radiotracers, namely [¹⁸ F]fluoro-3,4-dihydroxyphenyl-L-alanine ([¹⁸ F]FDOPA) and 6-[¹⁸ F]fluoro-L-m-tyrosine ([¹⁸ F]FMT). Because the imaging signal is theoretically less contaminated by metabolites, we hypothesized that the latter would show stronger relationship with behavioural and post-mortem measures of striatal dopaminergic deficiency. We used a within-subject design to measure striatal [¹⁸ F]FMT and [¹⁸ F]FDOPA uptake in eight partially lesioned, eight fully lesioned and ten sham-treated rats. Animals were pretreated with an L-aromatic amino acid decarboxylase inhibitor. A catechol-O-methyl transferase inhibitor was also given before [¹⁸ F]FDOPA PET. Quantitative estimates of striatal uptake were computed using conventional graphical Patlak method. Striatal dopaminergic deficiencies were measured with apomorphine-induced rotations and post-mortem striatal DA content. We observed a strong relationship between [¹⁸ F]FMT and [¹⁸ F]FDOPA estimates of decreased uptake in the denervated striatum using the tissue-derived uptake rate constant K_c . However, only [¹⁸ F]FMT K_c succeeded to discriminate between the partial and the full 6-OHDA lesion and correlated well with the post-mortem striatal DA content. This study indicates that the [¹⁸ F]FMT could be more sensitive, with respect of [¹⁸ F]FDOPA, to investigate DA terminals loss in 6-OHDA rats, and open the way to in vivo L-aromatic amino acid decarboxylase activity targeting in future investigations on progressive PD models.

Evaluation of 6-11C-Methyl-m-Tyrosine as a PET Probe for Presynaptic Dopaminergic Activity: A Comparison PET Study with β-11C-l-DOPA and 18F-FDOPA in Parkinson Disease Monkeys

We recently developed a novel PET probe, 6-(11)C-methyl-m-tyrosine ((11)C-6MemTyr), for quantitative imaging of presynaptic dopamine synthesis in the living brain. In the present study, (11)C-6MemTyr was compared with β-(11)C-l-DOPA and 6-(18)F-fluoro-l-dopa ((18)F-FDOPA) in the brains of normal and Parkinson disease (PD) model monkeys (Macaca fascicularis).

METHODS

PD model monkeys were prepared by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration, and (11)C-β-CFT was applied to assess neuronal damage as dopamine transporter (DAT) availability. (11)C-6MemTyr, β-(11)C-l-DOPA, or (18)F-FDOPA was injected with and without carbidopa, a specific inhibitor of peripheral aromatic L-amino acid decarboxylase. In normal and PD monkeys, the dopamine synthesis rates calculated using PET probes were analyzed by the correlation plot with DAT availability in the striatum.

RESULTS

In normal monkeys, whole-brain uptake of β-(11)C-l-DOPA and (18)F-FDOPA were significantly increased by carbidopa at the clinical dose of 5 mg/kg by mouth. In contrast, (11)C-6MemTyr was not affected by carbidopa at this dose, and the metabolic constant value of (11)C-6MemTyr in the striatum was significantly higher than those of the other 2 PET probes. Significant reduction of the presynaptic DAT availability in the striatum was detected in MPTP monkeys, and correlation analyses demonstrated that (11)C-6MemTyr could detect dopaminergic damage in the striatum with much more sensitivity than the other PET probes.

CONCLUSION

(11)C-6MemTyr is a potential PET probe for quantitative imaging of presynaptic dopamine activity in the living brain with PET.

Influence of O-methylated metabolite penetrating the blood-brain barrier to estimation of dopamine synthesis capacity in human L-[β-(11)C]DOPA PET

O-methyl metabolite (L-[β-(11)C]OMD) of (11)C-labeled L-3,4-dihydroxyphenylalanine (L-[β-(11)C]DOPA) can penetrate into brain tissue through the blood-brain barrier, and can complicate the estimation of dopamine synthesis capacity by positron emission tomography (PET) study with L-[β-(11)C]DOPA. We evaluated the impact of L-[β-(11)C]OMD on the estimation of the dopamine synthesis capacity in a human L-[β-(11)C]DOPA PET study. The metabolite correction with mathematical modeling of L-[β-(11)C]OMD kinetics in a reference region without decarboxylation and further metabolism, proposed by a previous [(18)F]FDOPA PET study, were implemented to estimate radioactivity of tissue L-[β-(11)C]OMD in 10 normal volunteers. The component of L-[β-(11)C]OMD in tissue time-activity curves (TACs) in 10 regions were subtracted by the estimated radioactivity of L-[β-(11)C]OMD. To evaluate the influence of omitting blood sampling and metabolite correction, relative dopamine synthesis rate (kref) was estimated by Gjedde-Patlak analysis with reference tissue input function, as well as the net dopamine synthesis rate (Ki) by Gjedde-Patlak analysis with the arterial input function and TAC without and with metabolite correction. Overestimation of Ki was observed without metabolite correction. However, the kref and Ki with metabolite correction were significantly correlated. These data suggest that the influence of L-[β-(11)C]OMD is minimal for the estimation of kref as dopamine synthesis capacity.

Sensitivity of kinetic macro parameters to changes in dopamine synthesis, storage, and metabolism: a simulation study for [¹⁸F]FDOPA PET by a model with detailed dopamine pathway

Quantitative interpretation of brain [¹⁸F]FDOPA PET data has been made possible by several kinetic modeling approaches, which are based on different assumptions about complex [¹⁸F]FDOPA metabolic pathways in brain tissue. Simple kinetic macro parameters are often utilized to quantitatively evaluate metabolic and physiological processes of interest, which may include DDC activity, vesicular storage, and catabolism from (18) F-labeled dopamine to DOPAC and HVA. A macro parameter most sensitive to the changes of these processes would be potentially beneficial to identify impaired processes in a neurodegenerative disorder such as Parkinson's disease. The purpose of this study is a systematic comparison of several [¹⁸F]FDOPA macro parameters in terms of sensitivities to process-specific changes in simulated time-activity curve (TAC) data of [¹⁸F]FDOPA PET. We introduced a multiple-compartment kinetic model to simulate PET TACs with physiological changes in the dopamine pathway. TACs in the alteration of dopamine synthesis, storage, and metabolism were simulated with a plasma input function obtained by a non-human primate [¹⁸F]FDOPA PET study. Kinetic macro parameters were calculated using three conventional linear approaches (Gjedde-Patlak, Logan, and Kumakura methods). For simulated changes in dopamine storage and metabolism, the slow clearance rate (k(loss) ) as calculated by the Kumakura method showed the highest sensitivity to these changes. Although k(loss) performed well at typical ROI noise levels, there was large bias at high noise level. In contrast, for simulated changes in DDC activity it was found that K(i) and V(T), estimated by Gjedde-Patlak and Logan method respectively, have better performance than k(loss).

PET 6-[F]fluoro-L-m-tyrosine Studies of Dopaminergic Function in Human and Nonhuman Primates

Although positron emission tomography (PET) and the aromatic L-amino acid decarboxylase (AADC) tracer 6-[¹⁸F]fluoro-L-m-tyrosine (FMT) has been used to assess the integrity of the presynaptic dopamine system in the brain, relatively little has been published in terms of brain FMT uptake values especially for normal human subjects. Twelve normal volunteer subjects were scanned using PET and FMT to determine the range of normal striatal uptake values using Patlak graphical analysis. For comparison, seven adult rhesus monkeys were studied and the data analyzed in the same way. A subset of monkeys that were treated with a unilateral intracarotid artery infusion of the dopamine neurotoxin MPTP showed an 87% decrease in striatal FMT uptake. These findings support the use of PET and FMT to image AADC distribution in both normal and diseased brains using Patlak graphical analysis and tissue input functions.

Comparison of [18F]FDOPA, [18F]FMT and [18F]FECNT for imaging dopaminergic neurotransmission in mice

INTRODUCTION

The clinically established positron emission tomography (PET) tracers 6-[(18)F]-fluoro-l-DOPA ([(18)F]FDOPA), 6-[(18)F]-fluoro-l-m-tyrosine ([(18)F]FMT) and 2beta-carbomethoxy-3beta-(4-chlorophenyl)-8-(2-[(18)F]-fluoroethyl)-nortropane ([(18)F]FECNT) serve as markers of presynaptic integrity of dopaminergic nerve terminals in humans. This study describes our efforts to adopt the methodology of human Parkinson's disease (PD) PET studies to mice.

METHODS

The PET imaging characteristics of [(18)F]FDOPA, [(18)F]FMT and [(18)F]FECNT were analyzed in healthy C57BL/6 mice using the dedicated small-animal PET tomograph quad-HIDAC. Furthermore, [(18)F]FECNT was tested in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD.

RESULTS

[(18)F]FDOPA and [(18)F]FMT failed to clearly visualize the mouse striatum, whereas PET experiments using [(18)F]FECNT proved that the employed methodology is capable of delineating the striatum in mice with exquisite resolution. Moreover, [(18)F]FECNT PET imaging of healthy and MPTP-lesioned mice demonstrated that the detection and quantification of striatal degeneration in lesioned mice can be accomplished.

CONCLUSIONS

This study shows the feasibility of using [(18)F]FECNT PET to analyze noninvasively the striatal degeneration in the MPTP mouse model of PD. This methodology can be therefore considered as a viable complement to established in vivo microdialysis and postmortem techniques.

Small animal positron emission tomography in food sciences

Positron emission tomography (PET) is a 3-dimensional imaging technique that has undergone tremendous developments during the last decade. Non-invasive tracing of molecular pathways in vivo is the key capability of PET. It has become an important tool in the diagnosis of human diseases as well as in biomedical and pharmaceutical research. In contrast to other imaging modalities, radiotracer concentrations can be determined quantitatively. By application of appropriate tracer kinetic models, the rate constants of numerous different biological processes can be determined. Rapid progress in PET radiochemistry has significantly increased the number of biologically important molecules labelled with PET nuclides to target a broader range of physiologic, metabolic, and molecular pathways. Progress in PET physics and technology strongly contributed to better scanners and image processing. In this context, dedicated high resolution scanners for dynamic PET studies in small laboratory animals are now available. These developments represent the driving force for the expansion of PET methodology into new areas of life sciences including food sciences. Small animal PET has a high potential to depict physiologic processes like absorption, distribution, metabolism, elimination and interactions of biologically significant substances, including nutrients, 'nutriceuticals', functional food ingredients, and foodborne toxicants. Based on present data, potential applications of small animal PET in food sciences are discussed.

The influx of neutral amino acids into the porcine brain during development: a positron emission tomography study

Pigs of three different age groups (newborns, 1 week old, 6 weeks old) were used to study the transport of the large neutral amino acids 6-[18F]fluoro-L-DOPA ([18F]FDOPA) and 3-O-methyl-6-[18F]fluoro-L-DOPA ([18F]OMFD) across the blood-brain barrier (BBB) with positron emission tomography (PET). Compartmental modeling of PET data was used to calculate the blood-brain clearance (K1) and the rate constant for the brain-blood transfer (k2) of [18F]FDOPA and [18F]OMFD after i.v. injection. A 40-70% decrease of K1(OMFD), K1(FDOPA) and k2(OMFD) from newborns to juvenile pigs was found whereas k2(FDOPA) did not change. Generally, K1(OMFD) and k2(OMFD) are lower than K1(FDOPA) and k2(FDOPA) in all regions and age groups. The changes cannot be explained by differences in brain perfusion because the measured regional cerebral blood flow did not show major changes during the first 6 weeks after birth. In addition, alterations in plasma amino acids cannot account for the described transport changes. In newborn and juvenile pigs, HPLC measurements were performed. Despite significant changes of single amino acids (decrease: Met, Val, Leu; increase: Tyr), the sum of large neutral amino acids transported by LAT1 remained unchanged. Furthermore, treatment with a selective inhibitor of the LAT1 transporter (BCH) reduced the blood-brain transport of [18F]FDOPA and [18F]OMFD by 35% and 32%, respectively. Additional in-vitro studies using human LAT1 reveal a much lower affinity of FDOPA compared to OMFD or L-DOPA. The data indicate that the transport system(s) for neutral amino acids underlie(s) developmental changes after birth causing a decrease of the blood-brain barrier permeability for those amino acids during brain development. It is suggested that there is no tight coupling between brain amino acid supply and the demands of protein synthesis in the brain tissue.

Effect of dopamine loss and the metabolite 3-O-methyl-[18F]fluoro-dopa on the relation between the 18F-fluorodopa tissue input uptake rate constant Kocc and the [18F]fluorodopa plasma input uptake rate constant Ki

Parkinson disease is characterized by the loss of dopaminergic neurons, thus decreasing the system's ability to produce and store dopamine (DA). Such ability is often investigated using 18F-fluorodopa (FD) positron emission tomography. A commonly used model to investigate the DA synthesis and storage rate is the modified Patlak graphical approach. This approach allows for both plasma and tissue input functions, yielding the respective uptake rate constants K(i) and K(occ). This method requires the presence of an irreversible compartment and the absence of any nontrapped tracer metabolite. In the case of K(occ), this last assumption is violated by the presence of the FD metabolite 3-O-methyl-[18F]fluoro-dopa (3OMFD), which makes the K(occ) evaluation susceptible to a downward bias. It was found that both K(i) and K(occ) are influenced by DA loss and thus are not pure measures of DA synthesis and storage. In the case of K(occ), the presence of 3OMFD exacerbates the effect of DA egress, thus introducing a disease-dependent bias in the K(occ) determination. These findings imply that K(i) and K(occ) provide different assessments of disease severity and that, as disease progresses, K(i) and especially K(occ) become more related to DA storage capacity and less to the DA synthesis rate.

Brain uptake of the acid metabolites of F-18-labeled WAY 100635 analogs

The 5-HT1A ligands [ 18F]FPWAY and [ 18F]FCWAY are metabolized to [ 18F]fluorobenzoic acid (FB) and [ 18F]fluorocyclohexanecarboxylic acid (FC), respectively. To quantify the penetration of these acids into the brain, dynamic positron emission tomography studies were performed in rhesus monkeys with [ 18F]FB and [ 18F]FC. High-performance liquid chromatography analysis of arterial blood samples showed no metabolites for [ 18F]FB, whereas [ 18F]FC was rapidly metabolized to [ 18F]fluoride. A model with one tissue compartment and vascular radioactivity was used to analyze gray matter time-activity curves. For [ 18F]FC, an additional term was added to account for [ 18F]fluoride skull spillover into the brain; this term accounted for 70% to 90% of the measured radioactivity concentration at 90 minutes. For [ 18F]FB, mean gray matter parameters were as follows: K1, 10 +/- 3 micro L. min(-1). mL(-1); distribution volume, 0.052 +/- 0.006 (mL/mL). For [ 18F]FC, the values were as follows: K1, 15 +/- 4 micro L. min(-1). mL(-1); V, 0.29 +/- 0.06 mL/mL. The values were consistent with a physiologic model that included brain-to-blood pH difference and the plasma free fraction of the acid. Simulations based on [ 18F]FCWAY human data showed that [ 18F]FC uptake produces significant biases in estimates in regions with low specific binding. These results can be used to correct the tissue [ 18F]FCWAY time-activity data for brain uptake of [ 18F]FC using the measured [ 18F]FC input function.

FDOPA metabolism in the adult porcine brain: influence of tracer circulation time and VOI selection on estimates of striatal DOPA decarboxylation

Different methodologies for PET data analysis influence the magnitude of estimates of blood-brain transfer coefficients and rate constants for the metabolism of FDOPA in living striatum. We now test the effects on several kinetic parameters of automatic procedures for volume of interest (VOI) selection. We also tested the sensitivity of the estimates to dynamic frame sequence duration, and produced a standard method for minimizing the variations in physiological estimates for FDOPA kinetics in minipig brain. We used minipigs because our previous work has shown them to provide an appropriate animal model for study normal and pathological cerebral DOPA metabolism using PET. Time-activity curves in striatum of adult minipigs were acquired in VOIs defined manually on MR-images, or alternatively on the basis of the radioactivity concentration based on the most radioactive voxel in the last scan frame. For all frame sequences, the relative decarboxylase activity (k(3)(D)) declined significantly (P < 0.006) as the VOI threshold declined from 95 to 70% of the most radioactive voxel. Irrespective of VOI size, the magnitude of k(3)(D) declined significantly (P < 0.001) from 0.074+/-0.008 to 0.045+/-0.005 per min (mean+/-S.E.M.) as total sequence length increased from 60 to 120 min circulation. The method of VOI selection had no significant effect on the striatum decarboxylation index of FDOPA calculated relative to the radioactivity in cerebellum (k(3)(S)).

Distribution volume of 3-O-methyl-6

The distribution volume (DV) of 6-[F-18]fluoro-L-DOPA (FDOPA) in the cerebellum recently has been linked using positron emission tomography (PET) to plasma large neutral amino acid (LNAA) concentrations in monkeys. In this article the authors provide additional experimental support for this relation by directly measuring the DV as the steady-state tissue to plasma radioactivity ratio in rats using a labeled LNAA analog 3-O-methyl-6-[F-18]FDOPA (OMFD), a compound that has no known specific enzyme or receptor interactions in brain tissue. The measured DV for OMFD (tissue OMFD concentration/plasma OMFD concentration) was found to be inversely related to plasma LNAA concentrations. The relation (DV = 1.5-0.00094*[LNAA], R--2 = 0.79) resulted in an 8% DV decrease per 100 nmol/mL plasma LNAA increase within the observed range of 330 to 510 nmol/mL. This was similar to recent noninvasive observations with FDOPA PET in vervet monkeys and with 6-[F-18]Fluoro-m-tyrosine PET in squirrel monkeys. The OMFD striatum to cerebellum (Str/Cb) ratio was greater than 1.0 for all measurements, averaging 1.09 +/- 0.04, and was approximately equal to the Str/Cb LNAA ratio of 1.12 +/- 0.05. This current study verifies the variation of DV of OMFD or FDOPA as a function of plasma LNAA concentrations and suggests the possibility of using OMFD for measuring cerebral LNAA noninvasively with PET.

Striatal kinetic modeling of FDOPA with a cerebellar-derived constraint on the distribution of volume of 30MFD: a PET investigation using non-human primates

The peripherally born metabolite of FDOPA, 3-O-Methyl-FDOPA (3OMFD), crosses the blood-brain barrier, thus complicating positron emission tomography-FDOPA (PET-FDOPA) data analysis. In previous reports the distribution volume (DV) of 3OMFD was constrained to unity. We have recently shown that the forward transport rate-constant of FDOPA (K(S1)) and the cerebellum-to-plasma ratio (C(b)/C(p)), a measure for the DV of 3OMFD, are functions of plasma large neutral amino acid (LNAA) concentration. Given large interstudy and intersubject differences in plasma LNAA levels, variations in the DV of 3OMFD are significant. In this report, the authors propose a constraint on the DV of 3OMFD that accounts for these variations. Dynamic PET-FDOPA scans were performed on 12 squirrel monkeys and 12 vervet monkeys. Two sets of constraints were employed on the compartmental model--M1 or M2. In M1, the striatal DV of 3OMFD was constrained to unity; in M2, the striatal DV of 3OMFD was constrained to an estimate derived from the cerebellum. Striatal and cerebellar time-activity curves were fitted using FDOPA and 3OMFD plasma input functions. The estimate of K(S1) and that of the compartmental FDOPA uptake-constant (K(i)), both obtained using M2, were adjusted to values corresponding to average LNAA levels. Finally, K(i) was compared with the graphical uptake-constant (PK(j)). With the use of constraint M2, intersubject variability of squirrel monkey k(S3) and K(i) was reduced by 45% and 53%, respectively; and for vervet monkeys, by 54% and 44%, respectively. Intersubject variability of K(1) and K(i) was further reduced after correction for variations in intersubject plasma LNAA levels (for squirrel monkeys, by 67% and 41%; for vervet monkeys, by 40% and 36%, respectively). K(i) correlation to PK(i) was enhanced to identity. Finally, average cerebellar k(C2) estimates were more than 2.5-fold higher than striatal k(S2) estimates (P < 0.0001). In modeling of PET-FDOPA data, it cannot be assumed that the DV of 3OMFD is unity. The cerebellar-derived constraint furnishes a reliable estimate for the DV of 3OMFD. Invoking the constraint and correcting for variations in plasma LNAA significantly reduced interstudy and intersubject variations in parameter estimates.

The suitability of [11C]-alpha-methyl-L-tryptophan as a tracer for serotonin synthesis: studies with dual administration of [11C] and [14C] labeled tracer

The tracer [11C]-alpha-methyl-L-tryptophan (alphaMTP) has been used to measure brain serotonin synthesis rates with positron emission tomography (PET). To address questions about the accuracy of the kinetic model, [14C]alphaMTP was used to directly measure conversion to [14C]-alpha-methyl-serotonin (alphaM5HT) in monkeys that had been previously studied with PET and [11C]alphaMTP. Four male, fasted, isoflurane-anesthetized rhesus monkeys were studied with [11C]alphaMTP and PET. Immediately after the initial 3-hour scan, a second dose of [11C]alphaMTP was coinjected with 1 mCi of [14C]alphaMTP, and additional PET data were collected. Approximately 90 minutes after the second alphaMTP administration, the animals were killed with an overdose of phenobarbital, and brain samples from 21 regions were taken and analyzed by HPLC. Minimal conversion of alphaMTP to alphaM5HT occurred; HPLC analysis of 14C radioactivity showed that greater than 96% of the total counts were in fractions corresponding to the alphaMTP peak. Brain concentrations of serotonin, tryptophan, 5-hydroxyindole-3-acetic acid, and alphaMTP also were determined fluorometrically using external quantification. Patlak plots generated from PET images acquired over 3 hours showed no time period of linear increase, and final slopes were not significantly different from zero, consistent with the finding of minimal conversion to [14C]alphaM5HT. These data indicate that in the 3-hour period after injection, [11C]alphaMTP is acting predominantly as a tracer of tryptophan uptake, not serotonin synthesis.

Localization of trapping of 6-[(18)F]fluoro-L-m-tyrosine, an aromatic L-amino acid decarboxylase tracer for PET

The purpose of this study was to address four major questions regarding 6-FMT, a noncatecholic PET tracer for AAAD: 1) Where is the specific uptake of 6-FMT? 2) Why does it accumulate where and to the degree that it does? 3) How does its uptake differ from that of fluoroDOPA globally? and 4) Does its regional uptake differ significantly from that of fluoroDOPA? High-resolution PET scans were obtained in three rhesus monkeys using 6-FMT and in two of them using fluoroDOPA. Anatomic distribution was analyzed visually and quantitative uptake of 6-FMT was compared with published regional decarboxylase activity and monoamine neurotransmitter concentrations. In addition to high uptake in the dopamine-rich striatal nuclei, there was specific uptake of 6-FMT in brain regions which have little dopaminergic innervation but which have other amines in significant concentration. 6-FMT uptake correlated best with regional AAAD activity (r = 0.97). It correlated slightly less well with the sum of catecholamine and indolamine neurotransmitter concentrations, but does not correlate with dopamine concentration. The uptake of 6-FMT is greater than that of fluoroDOPA, with only slight differences in their regional distributions. Radiolabeled analogs of DOPA are often implicitly or explicitly regarded as tracers for presynaptic dopaminergic function. However, localization of these tracers more broadly includes many regions with relatively high concentrations of norepinephrine and serotonin. This may be especially important in diseases or experimental states in which dopaminergic neurons are selectively reduced, and may allow for the study of nondopaminergic neuronal systems in vivo with this tracer.

A comparison of 11C-labeled L-DOPA and L-fluorodopa as positron emission tomography tracers for the presynaptic dopaminergic system

11C-labeled 3,4-Dihydroxy-phenyl-L-alanine (L-DOPA) and L-fluorodopa were used as tracers for the functional state of the presynaptic dopamine system in anesthetized monkeys with positron emission tomography. The radiotracer disposition in brain tissue and plasma were studied and effects induced by pharmacologic challenges were evaluated. 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (6R-BH4) increased the striatal influx rate constant, e.g., striatal K(i) for L-[beta-11C]DOPA, but it induced no effect on the K(i)-value using L-[beta-11C]-6-fluorodopa. Studies of radiolabeled tracer and metabolites in plasma showed substantial differences between the two tracers. At baseline conditions, 60% unchanged L-[beta-11C]DOPA was detected in plasma 50 minutes after tracer injection and the 3-O-methylated fraction accounted for 25% of total radioactivity. For L-[beta-11C]-6-fluorodopa, the relation was inverse; about 25% unchanged tracer and 60% 3-O-methyl metabolite were present in plasma after 50 minutes. A site-specific 11C-labeling in the carboxylic position in the molecules revealed a significant specific retention of radioactivity in striatum with L-[car-boxy-11C]-6-fluorodopa but not with L-[carboxy-11C]DOPA. The 3-O-methyl metabolite of L-DOPA is known to pass the blood-brain barrier and may interfere with the calculation of the K(i)value using a brain reference region. Thus, extensive 3-O-methylation in circulation of the fluorinated analog could obscure the detectability of potential functional change in striatal K(i) of the tracer when using a reference tissue model for calculation.

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 (k(loss)), and an index of "effective turnover" of dopamine (k(loss)/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 k(loss) or k(loss)/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.

Simultaneous measurement of [18F]FDOPA metabolism and cerebral blood flow in newborn piglets

Available information on the dopamine (DA) metabolism of the immature brain is rare. In order to establish a useful animal model we have performed PET experiments in anesthetized neonatal pigs using 6-[18F]-fluoro-L-DOPA (FDOPA) as tracer. In this study, we have simultaneously determined the cerebral blood flow and the rate constant of FDOPA conversion by the aromatic amino acid decarboxylase, the ultimate enzyme in the synthesis of dopamine. The estimated values of FDOPA decarboxylation in the basal ganglia were similar to values calculated in adult animals and humans. However, in contrast to those studies a significant decarboxylation was also found in the frontal cortex and the cerebellum. HPLC analysis of brain samples also revealed extensive and rapid metabolism of FDOPA in the five investigated brain regions. At 8 min after tracer injection about 80% of FDOPA was already converted to FDA and its metabolites. Surprisingly, a rather high fraction (16-21%) of [18F]-fluoro-3-methoxytyramine was found which may indicate a low storage capacity of vesicular DA at this perinatal stage. It is suggested that the findings are related to the ontogenetic development of the dopaminergic system. The knowledge of the regulation of the DA metabolism in the immature brain may have implications for the understanding of neurodevelopmental effects of perinatal oxygen deprivation.

Striatal [18F]fluorodopa utilization after COMT inhibition with entacapone studied with PET in advanced Parkinson's disease

The effect of peripheral catechol-O-methyltransferase (COMT) inhibition with entacapone on striatal uptake of 6-[18F]fluoro-L-dopa (FDOPA) was studied with PET both without and with entacapone in fifteen advanced parkinsonian patients and six healthy controls. Entacapone significantly enhanced the fraction of unmetabolized FDOPA in plasma from 16% to about 50% at 80 minutes after FDOPA injection in all subjects. The striatal to occipital ratios and the striatal FDOPA uptake, expressed as a modified decarboxylation coefficient (k3R0), was significantly increased in healthy controls, whereas in parkinsonian patients the increase was significant only in the caudate. On the other hand, the influx constant (Ki) decreased significantly in the caudate and putamen in parkinsonian patients; in healthy controls the Ki remained virtually unchanged. Effective peripheral COMT inhibition markedly increased the fraction of FDOPA in plasma and thus its availability in the brain for decarboxylation both in patients and control subjects. However, the change in striatal FDOPA uptake was modest in the advanced parkinsonian patients as compared to that in control subjects, because of the advanced disease, decreased storage capacity, or both.

Compartmental analysis of dopa decarboxylation in living brain from dynamic positron emission tomograms

The trapping of decarboxylation products of radiolabelled dopa analogs in living human brain occurs as a function of the activity of dopa decarboxylase. This enzyme is now understood to regulate, with tyrosine hydroxylase, cerebral dopamine synthesis. Influx into brain of dopa decarboxylase substrates such as 6-[18F]fluorodopa and beta-[11C]dopa measured by positron emission tomography can be analyzed by solution of linear differential equations, assuming irreversible trapping of the decarboxylated products in brain. The isolation of specific physiological steps in the pathway for catecholamine synthesis requires compartmental modelling of the observed dynamic time-activity curves in plasma and in brain. The several approaches to the compartmental modelling of the kinetics of labelled substrates of dopa decarboxylase are now systematically and critically reviewed. Labelled catechols are extensively metabolized by hepatic catechol-O-methyltransferase yielding brain-penetrating metabolites. The assumption of a fixed blood-brain permeability ratio for O-methyl-6-[18F]fluorodopa or O-methyl-beta-[11C]dopa to the parent compounds eliminates several parameters from compartmental models. However, catechol-O-methyltransferase activity within brain remains a possible factor in underestimation of cerebral dopa decarboxylase activity. The O-methylation of labelled catechols is blocked with specific enzyme inhibitors, but dopa decarboxylase substrates derived from m-tyrosine may supplant the catechol tracers. The elimination from brain of decarboxylated tracer metabolites can be neglected without great prejudice to the estimation of dopa decarboxylase activity when tracer circulation is less than 60 minutes. However, elimination of dopamine metabolites from brain occurs at a rate close to that observed previously for metabolites of glucose labelled in the 6-position. This phenomenon can cause systematic underestimation of the rate of dopa decarboxylation in brain. The spillover of radioactivity due to the limited spatial resolution of tomographs also results in underestimation of dopa decarboxylase activity, but correction for partial volume effects is now possible. Estimates of dopa decarboxylase activity in human brain are increased several-fold by this correction. Abnormally low influx of dopa decarboxylase tracers in the basal ganglia is characteristic of Parkinson's disease and other movement disorders. Consistent with postmortem results, the impaired retention of labelled dopa is more pronounced in the putamen than in the caudate nucleus of patients with Parkinson's disease; this heterogeneity persists after correction for spillover. Current in vivo assays of dopa decarboxylase activity fail to discriminate clinically distinct stages in the progression of Parkinson's disease and are, by themselves, insufficient for differential diagnosis of Parkinson's disease and other subcortical movement disorders. However, potential new avenues for therapeutics can be tested by quantifying the rate of metabolism of exogenous dopa in living human brain.

Effects of monoamine oxidase and catechol-O-methyltransferase inhibition on dopamine turnover: a PET study with 6-[18F]L-DOPA

The consequences of monoamine oxidase and catechol-O-methyltransferase inhibition on the effective turnover of dopamine were investigated using 6-[18F]L-3-4-dihydroxyphenylalanine (6-[18F]L-DOPA) and positron emission tomography. The effective dopamine turnover was expressed as the ratio between the rate of reversibility of 6-[18F]L-DOPA trapping (k[loss]) and the rate of uptake of 6-[81F]L-DOPA (Ki) in the striatum of normal cynomolgus monkeys. The monkeys received 6-[18F]L-DOPA scans, untreated or after pretreatment with either the peripheral catechol-O-methyltransferase inhibitor nitecapone; the peripheral and central catechol-O-methyltransferase inhibitor tolcapone; the monoamine oxidase inhibitors deprenyl or pargyline; a combination of tolcapone and the monoamine oxidase inhibitors. Tolcapone alone or combined with the monoamine oxidase inhibitors produced a significant decrease in the dopamine turnover (55 to 65%). Neither nitecapone nor monoamine oxidase inhibition alone produced significant changes. These results may have implications for the use of central catechol-O-methyltransferase inhibitors added to routine levodopa therapy in parkinsonian patients.

Cerebral 6-[18F]fluoro-L-DOPA uptake in rhesus monkey: pharmacological influence of aromatic amino acid decarboxylase (AAAD) and catechol-O-methyltransferase (COMT) inhibition

FDOPA/PET scans were performed in one rhesus monkey to study the influence of three catechol-O-methyltransferase (COMT) inhibitors (CGP 28014, OR-611 and Ro 40-7592) on FDOPA pharmacokinetics. COMT inhibitors were administered in combination with carbidopa, a peripherally acting inhibitor of the aromatic amino acid decarboxylase (AAAD). FDOPA was administered intravenously and its metabolic fate in plasma was determined using an HPLC system with an on-line gamma-gamma coincidence detector. Cerebral tracer uptake was assessed in the striatum and in a non-dopaminergic brain region (occipital cortex). In the periphery, the pharmacokinetic efficiency of FDOPA was increased due to the combined inhibition of COMT and AAAD activity. All three COMT inhibitors reduced the FDOPA methylation rate constant in plasma, with complete suppression obtained in the case of Ro 40-7592. In the brain, specific 18F radioactivity (striatal minus brain reference radioactivity) increased as a result of the increase in FDOPA plasma availability following the administration of COMT and AAAD inhibitors. We established a significant linear correlation between striatal radioactivity and FDOPA plasma levels (r = 0.924 +/- 0.048, P < 0.0001 for total striatal and r = 0.948 +/- 0.054, P < 0.0001 for specific striatal radioactivity). Using plasma FDOPA radioactivity as input, we found that the striatal FDOPA uptake rate constant KiFD was not changed by any of the inhibitors. Thus, the enhancement of striatal radioactivity after application of enzyme inhibitors is a consequence of the increase in plasma FDOPA that becomes available for conversion to fluorodopamine in the striatal dopaminergic nerve terminals. By contrast, using the radioactivity in a non-dopaminergic region (cortex) as input, we found that the striatal FDOPA uptake rate constant Ki(ref) was significantly (P < 0.0001) increased following pretreatment with COMT inhibitors. Our analysis demonstrated that Ki(ref) and the 3-OMFD contribution to the cerebral radioactivity were inversely correlated.

6-[18F]fluoro-L-m-tyrosine: metabolism, positron emission tomography kinetics, and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine lesions in primates

The tracer 6-[18F]fluoro-L-m-tyrosine (FMT) was studied with regard to its biochemistry and kinetics, as well as its utility in evaluating brain dopaminergic function in primates before and after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment using positron emission tomography (PET). Plasma analysis of FMT and its F18-labeled metabolites 6-fluoro-3-hydroxyphenylacetic acid (FPAC) and 6-fluoro-3-hydroxyphenylethylamine (FMA) during PET scanning enabled kinetic analysis of FMT uptake. A separate study examined brain FMT metabolism in MPTP-naive monkeys euthanized 60 or 120 min after FMT injection. Almost 60% of total plasma F-18 activity was associated with FPAC and FMA 120 min after FMT injection. The FMT signal accumulated preferentially in dopaminergic areas such as caudate and putamen. This bilateral FMT signal was disrupted after unilateral intracarotid artery (ICA) MPTP infusion which reduced ipsilateral striatal activity. A three compartment three kinetic rate constant model for FMT uptake revealed reduced FMT decarboxylation (k3) in ipsilateral caudate and putamen after unilateral MPTP although a further decrease was not evident after intravenous MPTP. FPAC was the major F-18 species in all brain regions except in cerebellum where FMT was predominant 60 min post-mortem. FPAC was most concentrated in dopaminergic areas whereas lower levels occurred in areas containing few dopamine terminals. These data demonstrate preferential FMT metabolism and F-18 retention in dopaminergic tissue and support the use of FMT to evaluate normal and abnormal dopaminergic function.

Synthesis and evaluation of radioiodinated 6-iodo-L-DOPA as a cerebral L-amino acid transport marker

Regioselective radioiodination of N-trifluoroacetyl 3,4-dimethoxy-6-trifluoroacetoxymercurio-L-phenylalanine ethyl ester 1 under no-carrier-added condition gave 6-[125I]iodo protected L-DOPA with a labeling efficiency of more than 85%, and no-carrier-added 6-[125I]I-L-DOPA was obtained with a radio-chemical purity of over 95% after hydrolysis and chromatography. A nonradioactive standard of 6-iodo protected L-DOPA was synthesized by the iododemercuration of 6-mercuric trifluoroacetate protected L-DOPA 1 using I2 in chloroform. 6-[125I]I-L-DOPA showed high brain accumulation and rapid blood clearance in mice. The rat brain slice studies indicated high affinity of 6-[125I]I-L-DOPA for carrier-mediated and stereoselective active transport systems. The tissue homogenate analysis revealed that most of the accumulated radioactivity was intact 6-[125I]I-L-DOPA. Thus, 6-[123I]I-L-DOPA appears to be a suitable single photon emission computed tomography (SPECT) tracer for the selective measurement of cerebral L-amino acid transport, having no affinity for dopamine metabolism.

Synthesis and evaluation of an 18F-labeled dopa prodrug as a PET tracer for studying brain dopamine metabolism

In the quantitative studies of presynaptic dopamine metabolism by PET with 6-[18F]fluoro-L-dopa (6-[18F]FDOPA), metabolic analysis in the plasma is required to determine the precise input function because of susceptibility of the compound to peripheral metabolism. In this study, we prepared 6-[18F]-fluoro-O-pivaloyl-L-dopa (6-[18F]FPDOPA) as a prodrug of 6-[18F]FDOPA, and evaluated its potential as a PET tracer in mice. If the 6-[18F]FPDOPA is stable peripherally and is hydrolyzed to 6-[18F]FDOPA in the brain tissues, disadvantage of the 6-[18F]FDOPA will be overcome. Compared with the 6-[18F]FDOPA, the initial brain uptake of the 6-[18F]FPDOPA was lower; however, the uptake in the latter become comparable, and the uptake ratios of striatum to other reference regions were larger. Medication of mice with inhibitors of aromatic amino acid decarboxylase and catechol-O-methyl transferase greatly enhanced the striatal uptake of the two compounds. The reduced brain uptake of the compounds by L-phenylalanine-loading suggested transport through the blood-brain barrier by the neutral amino acid transporter. HPLC analysis showed the presence of 6-[18F]FPDOPA, 6-[18F]FDOPA and 6-[18F]fluorodopamine in the striatum; however, 6-[18F]fluoro-3-O-methyl-L-dopa was a predominate metabolite in the brain and plasma as in the case of [18F]FDOPA. Results suggested that 6-[18F]FPDOPA had characteristics as a prodrug of 6-[18F]FDOPA; however, the compound was also labile to metabolic alteration in vivo.

Positron emission tomography in drug evaluation: influence of three different catechol-O-methyltransferase inhibitors on metabolism of [NCA] 6-[18F]fluoro-L-dopa in rhesus monkey

We compared the influence of three different catechol-O-methyltransferase (COMT) inhibitors (CGP 28014, OR-611 and Ro 40-7592) on the metabolism of no-carrier-added (NCA) 6-[18F]fluoro-L-dopa (6-FDOPA) in one Rhesus monkey. All three COMT inhibitors improved 6-FDOPA availability in plasma, increased the specific uptake in the brain and thus improved 6-FDOPA uptake measurements using positron emission tomography (PET). Best results were obtained with Ro 40-7592.

The assessment of the non-equilibrium effect in the 'Patlak analysis' of Fdopa PET studies

'Patlak analysis' is a common approach used in Fdopa PET studies to calculate the uptake constant (Ki) of the tracer. It is assumed in the Patlak analysis that the reversible compartment of tissue radioactivity used is in effective equilibrium with the tracer in plasma. Therefore, using the data prior to equilibrium is in conflict with the assumption, and its effect on the estimate thus needs to be examined. In this study, we used simulations to investigate the errors due to the violation of the equilibrium assumption. Two factors affecting the estimate of Ki were examined--the eigenvalue of the model response function and the shape of the input function. The Ki estimate obtained from the Patlak analysis was found to be markedly biased because the system is not in equilibrium during the first 2 h post Fdopa injection. The magnitude of the bias was found to be dependent on the time interval used in the analysis (10% difference when comparing results from intervals 30-120 min and 60-120 min) and on the eigenvalue of the response function (10% change in Ki when the eigenvalue was changed by 20% around the nominal value). The estimates are also affected by the intersubject variations in the plasma time-activity curves. Therefore, Patlak analysis users should interpret their results with caution, particularly when examining small intersubject differences and small changes due to physiological or pharmacological interventions.

A probe for intracerebral aromatic amino-acid decarboxylase activity: distribution and kinetics of [18F]6-fluoro-L-m-tyrosine in the human brain

Positron tomography, using [18F]6-fluoro-L-dopa as a tracer, has been used for the study of Parkinson's disease. Unfortunately, the analysis of data obtained with this agent is bedeviled because it readily forms labeled methylated metabolites that enter the brain. We have evaluated [18F]6-fluoro-L-m-tyrosine (FmT) as an alternative tracer to study intracerebral dopamine metabolism with positron tomography. Imaging studies in humans showed specific accumulation of this tracer in the dopamine-rich striatal regions. Reduced striatal uptake of the tracer was demonstrated in a patient suffering from Parkinson's disease. Increased retention of the tracer was demonstrated in a subject pretreated with the peripheral decarboxylase inhibitor carbidopa. Analysis of plasma samples for labeled metabolites of FmT revealed no methylated metabolites. Results of compartmental analysis showed that a two-compartment three rate constant model described adequately the time course of radioactivity in the striatum after an injection of FmT. The FmT decarboxylation rate constant (k21) was found to be 0.0108 min-1. Because the peripheral metabolism of FmT is simpler than that of [18F]6-fluoro-L-dopa, we propose FmT as a superior agent with which to study intracerebral dopamine metabolism in health and disease in humans.

Routes of administration and effect of carbidopa pretreatment on 6-[18F]fluoro-L-dopa/PET scans in non-human primates

In 6-[18F]fluoro-L-dopa (Fdopa)/positron emission tomography (PET) studies, carbidopa pretreatment increases the Fdopa bioavailability to the brain and enhances the intensity of striatal PET images. Different PET research teams have used various carbidopa doses and routes of administration in non-human primate studies. The purpose of this study was to examine the plasma profiles of carbidopa and the effect of the route of administration of carbidopa on a Fdopa/PET scan. Cynomolgus monkeys were given carbidopa either orally (5 mg/kg), intraperitoneally (2.5 and 5 mg/kg) or intravenously (5 mg/kg) 60-90 min prior to the Fdopa injection. Carbidopa-treated monkeys were compared to monkeys without carbidopa treatment. No carbidopa was detected in the plasma samples when it was given orally, possibly due to poor absorption in the gastrointestinal tract. In addition, the striatal and cortical activities were not statistically different from those of the untreated monkeys, indicating that little or no inhibition of the peripheral decarboxylation of Fdopa by carbidopa had taken place. When carbidopa was given intraperitoneally at a dose of 2.5 and 5 mg/kg and intravenously at 5 mg/kg, plasma carbidopa concentrations at the time of Fdopa injection were 0.95 +/- 0.26, 2.22 +/- 0.23 and 2.79 +/- 0.26 micrograms/ml, respectively. Because of inhibition of peripheral decarboxylation of Fdopa by carbidopa, more Fdopa was available for transport into the brain and as a result, both the striatal and cortical activities were significantly higher than those of the untreated monkeys. Carbidopa administration had no effect on either the striatal-to-cortical activity ratio or the striatum uptake value.

The distribution and kinetics of [18F]6-fluoro-3-O-methyl-L-dopa in the human brain

The analysis of positron tomographic studies of 3,4-dihydroxyphenylethylamine (dopamine) metabolism in which [18F]6-fluoro-L-3,4-dihydroxyphenylalanine (F-dopa) is used as a tracer is confounded by the presence of [18F]6-fluoro-3-O-methyl-L-3,4-dihydroxyphenylalanine (OMFD). This labeled molecule, formed by the action of peripheral catechol-O-methyltransferase on F-dopa, crosses the blood-brain barrier and contributes to the radioactivity measured by the tomograph. Corrections for this radioactivity in the brain have been proposed. They rely upon the assumption that regional variations in the handling of this molecule by the brain are negligible. Although this assumption is pivotal for the proper quantification of dopamine metabolism using F-dopa, the distribution and kinetics of OMFD have never been studied in humans. We present results in humans that show that there is little selective regional 18F accumulation in the brain, that the distribution volume of OMFD is close to unity, and that a single, reversible compartment is adequate to model the measured time course of radioactivity after an OMFD injection. Analysis of plasma samples for labeled metabolites showed that more than 95% of the radioactivity was associated with OMFD at all times. Our results for OMFD kinetics are in accord with published results obtained in nonhuman primates and for the bidirectional transport of large neutral amino acids across the blood-brain barrier measured using a synthetic amino acid.(ABSTRACT TRUNCATED AT 250 WORDS)

6-[18F]fluoro-L-dihydroxyphenylalanine metabolism and positron emission tomography after catechol-O-methyltransferase inhibition in normal and hemiparkinsonian monkeys

Increased and sustained central delivery of L-dihydroxyphenylalanine (L-DOPA) is a desirable therapeutic strategy in Parkinson's disease. We investigated the effects of peripheral catechol-O-methyltransferase (COMT) inhibition, by the non-toxic drug nitecapone on the metabolism of 6-[18F]fluoro-L-dihydroxyphenylalanine (6FD) and on its positron emission tomography (PET) imaging in non-human primates. Nitecapone produced a dose-dependent inhibition in the formation of 3-O-methyl-6-[18F]fluorodihydroxyphenylalanine (OMFD). This inhibition of OMFD formation was attended by increased production of other metabolites, in particular 6-[18F]fluorodopamine (6FDA), 6-[18F]fluorodihydroxyphenylacetic acid (FDOPAC), 6-[18F]fluorohomovanillic acid (FHVA) and [18F]-sulfated conjugates (FSC). Although nitecapone had no effect on plasma 6FD pharmacokinetics, high-dose nitecapone increased contrast of cerebral 18F uptake and retention between regions with high (striatum) versus sparse (parieto-occipital lobes) dopaminergic innervation. 18F uptake contrast was also improved between structures known to possess an intermediate dopaminergic innervation, including the upper brainstem, frontal and temporal lobes, versus sparsely innervated regions. This increased contrast was secondary to decreased activity in sparsely innervated structures and not to increased activity in highly innervated structures. Contrast was correlated inversely with the plasma OMFD/6FD concentration ratio, OMFD being the main 6FD metabolite which can cross the blood brain barrier. We conclude that nitecapone is an effective inhibitor of COMT in non-human primates. This inhibition results in increased 6FD flux through other catabolic pathways. Because of decreased OMFD formation, however, COMT inhibition improves the specificity of 6FD-PET and facilitates in-vivo detection of a wide range of dopaminergic innervation densities in cerebral structures.

Pharmacokinetics of plasma 6-[18F]fluoro-L-3,4-dihydroxyphenylalanine ([18F]Fdopa) in humans

Like native DOPA, [18F]-6-fluoro-L-3,4-dihydroxyphenylalanine ([18F]FDOPA) is subject to methylation and decarboxylation. To determine the rates of formation and elimination of [18F]FDOPA metabolites, plasma from human subjects undergoing positron emission tomographic (PET) studies was analyzed by high-performance liquid chromatography (HPLC). In addition to the principal metabolite O-methyl-[18F]FDOPA (OMe[18F]FDOPA), two decarboxylated metabolites were detected in plasma from carbidopa pretreated subjects. The concentrations of each metabolite during 90 min following tracer injection could be described as a function of the concentration of [18F]FDOPA, and two rate constants; k0, the rate of formation, and k-1, the rate of clearance. Plasma metabolite time series generated from total plasma activity curves and measured rate constants were in close agreement with the actual concentrations determined by HPLC fractionation. Population means for k0 (0.011 +/- 0.002 min-1) and k-1 (0.010 +/- 0.003 min-1) were used to generate "simulated" plasma curves. The measured and generated plasma curves were used as inputs for estimation of partition and decarboxylation coefficients of [18F]FDOPA in brain. The use of generated input functions from normal population means of transfer coefficients did not introduce a systematic error into the estimate of the enzyme activity. However, the high variability of these estimates in patients precludes the use of this technique as an alterative to individual HPLC measurements.

Human striatal L-dopa decarboxylase activity estimated in vivo using 6-[18F]fluoro-dopa and positron emission tomography: error analysis and application to normal subjects

DOPA decarboxylase is the enzyme directly responsible for the synthesis of the neurotransmitters dopamine and serotonin, and indirectly of noradrenaline, in brain. We used the decarboxylation coefficient (k3D) of 6-[18F]fluoro-DOPA (FDOPA) to denote the relative activity of L-DOPA decarboxylase in vivo in the human brain. To determine the relative enzyme activity with positron emission tomography (PET), we evaluated the model that separates the metabolism into compartments of nondiffusible and diffusible (i.e., transient) tracer metabolites. Error analysis indicated that the least-squares optimization alone was not sufficient to yield accurate estimates of k3D in the presence of the inherent error of PET. To improve the accuracy of the k3D estimates by optimizing the number of parameters, we introduced biological constraints which included a tracer partition volume (Ve) common to frontal cortex and striatum, and a fixed ratio (q) between the blood-brain barrier transport coefficients of O-methyl-[18F]fluoro-DOPA and FDOPA, the two sources of radioactivity in plasma. We found that a two-step analysis yielded sufficiently accurate estimates of k3D. The two steps include the initial estimation of the partition volume in frontal cortex and the subsequent use of this value to determine k3D in striatum and other structures. We studied twelve healthy controls (age 45 +/- 15 years). The average k3D value was 0.081 +/- 0.024 min-1 (coefficient of variation (COV) 30%) for caudate nucleus, 0.074 +/- 0.013 min-1 (COV 18%) for putamen, and 0.010 +/- 0.005 min-1 (COV 50%) for cerebral cortex.

6-[18F]fluoro-L-dopa metabolism in living human brain: a comparison of six analytical methods

In 11 normal volunteers and six patients with Parkinson's disease, we compared six different analyses of dopaminergic function with L-3,4-dihydroxy-6-[18F]fluorophenylalanine (FDOPA) and positron emission tomography (PET). The caudate nucleus, putamen, and several reference regions were identified in PET images, using magnetic resonance imaging (MRI). The six analyses included two direct determinations of DOPA decarboxylase activity (k3D, k3*), the slope-intercept plot based on plasma concentration (K), two slope-intercept plots based on tissue content (k3r, k3s), and the striato-occipital ratio [R(T)]. For all analyses, the difference between two groups of subjects (normal volunteers and patients with Parkinson's disease) was larger in the putamen than in the caudate. For the caudate nucleus, the DOPA decarboxylase activity (k3D, k3*), tissue slope-intercept plots (kr3, ks3); and striato-occipital ratio [R(T)] analyses significantly discriminated between the normal volunteers and the patients with Parkinson's disease (p < 0.005) [with least significance for k3* (p < 0.05)], while the plasma slope-intercept plot (K) failed to do so. For the putamen, the values for k3D, k3*, K, k3r, k3s, and R(T) of normal volunteers were significantly higher than those of patients (p < 0.005) [with least significance for K (p < 0.025)]. Linear correlations were significant between k3D and k3s; k3D and k3r; k3D and R(T); and k3D and k3*, in this order of significance. We found no correlation between k3D and K values in the caudate nucleus.

Positron emission tomography with 18F-dopa: interpretation and biological correlates in nonhuman primates

Positron emission tomography (PET) was carried out, with 18F-DOPA as a ligand, in normal control monkeys and "parkinsonian" monkeys who had been treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The following approaches were used in data analysis: ratio of 18F accumulation in specific to nonspecific brain areas and 18F-DOPA influx constant obtained using either the actual plasma 18F-DOPA or the 18F activity in a nonspecific brain area as the input function. The results from these analyses were compared to one another and to biological parameters relevant to dopaminergic function. The striatum/cortex ratio and the rate constant calculated from plasma 18F-DOPA appeared to be the most sensitive analytic techniques.