Production of 6-[18F]fluoro-L-dopa and its metabolism in vivo--a critical review

Imaging of cardiac sympathetic dysfunction with 18F-FDOPA PET/CT in patients with heart failure: a pilot study

BACKGROUND

Routine use of cardiac sympathetic imaging in HF has been limited by the lower availability/sensitivity of radiotracers. This study was aimed to assess the feasibility of 18F-FDOPA (commonly available PET-radiotracer) in assessment of cardiac autonomic dysfunction.

METHODS

Twenty-four controls (46.5 ± 11.1 years, 16men) and 24 patients (43.5 ± 11.0 years, 18men) with diagnosed HF (Framingham-Criteria) underwent cardiac-PET/CT. Region(s) Of Interest were drawn over entire left ventricular myocardium (LV), individual walls, and mediastinum (M). Coefficient of Variation (CV) was calculated from individual wall counts.

RESULTS

HF patients had significantly lower myocardial 18F-FDOPA uptake (P < .001, independent t test) than controls [32.4% ± 9.5% global reduction; highest in apex (39.9% ± 7.0%)]. A cut-off of LV/M ≤ 1.68 could differentiate patients from controls with sensitivity and specificity of 100% and 95.8%, respectively. LV/M correlated positively with EF (Pearson coefficient = 0.460, P .031). During follow-up, 3 patients were lost to follow-up, 4 died (survival-20.5 ± 4 months), 2 worsened, and 15 remained stable/showed mild improvement. Patients who worsened/died during follow-up had higher CV than those with stable/improving symptoms [0.16 ± 0.05 vs 0.11 ± 0.05, P value .069 (independent t test); Cox regression P = .084].

CONCLUSION

Myocardial 18F-FDOPA uptake in patients with HF is significantly reduced. Higher reduction is seen in those with lower EF. CV, a maker of regional heterogeneity, is a potential prognostic marker.

Automatic Production of [18F]F-DOPA Using the Raytest SynChrom R&D Module

[¹⁸F]F-DOPA is widely used in PET diagnostics. Diseases diagnosed with this tracer are schizophrenia, Parkinson's disease, gliomas, neuroendocrine tumors, pheochromocytomas, and pancreatic adenocarcinoma. It should be noted that the [¹⁸F]F-DOPA tracer has been known for over 30 years. However, the methods of radiosynthesis applied in the past did not allow its clinical use due to low efficiency and purity. Currently, in the market, one encounters different types of radiosynthesis using the fluorine ¹⁸F isotope and variants of the same method. The synthesis and its modifications were carried out using a Raytest Synchrom R&D module. The synthesis consists of the following steps: (a) binding of the fluoride anion ¹⁸F^- on an anion exchange column; (b) elution with TBAHCO₃^-; (c) nucleophilic fluorination to the ABX 1336 precursor; (d) purification of the intermediate product on the C18ec column; (e) Baeyer-Villiger oxidation; (f) hydrolysis; and (gfinal purification of the crude product on a semipreparative column. The nucleophilic synthesis of [¹⁸F]F-DOPA was successfully performed in 120 min, using the ABX 1336 precursor on the Raytest SynChrom R&D module, with a radiochemical yield (RCY) of 15%, radiochemical purity (RCP) ≥ 97%, and enantiomeric purity (ee) ≥ 96%.

Assessment of myocardial sympathetic innervation with 18F-FDOPA-PET/CT in patients with autonomic dysfunction: feasibility study in IPD patients

BACKGROUND

Dysfunction and denervation of myocardial nor-adrenergic sympathetic neurons has been documented in IPD patients with dysautonomia. The aim of this study was to evaluate the feasibility of single tracer imaging of myocardial sympathetic and cerebral striatal involvement in these patients.

METHODS

Twenty-two controls (mean-age 59.09 ± 12.39 years, 15 men) with no clinical autonomic-dysfunction and normal striatal-uptake in 18F-FDOPA-PET/CT; and 28 patients (mean-age 58.18 ± 8.25 years, 18 men) with autonomic-dysfunction (in Autonomic Function Tests) and striatal dopaminergic-dysfunction were enrolled. Both cardiac-PET/CT (40 minutes post IV-injection of 185-259MBq 18F-FDOPA) and Brain-PET/CT (60 minutes post-IV) were acquired in same session. ROIs were drawn over the entire left ventricular myocardium, individual walls and mediastinum for quantification. Patients and controls were followed-up for 26.93 ± 5.43 months and 37.91 ± 8.63 months, respectively.

RESULTS

Striatal and myocardial-parameters were significantly lower in patients compared to controls; with Myocardium/mediastinal ratio (MwMR) yielding the area-under-the-curve of .941 (P < .001). MwMR correlated negatively with the drop in systolic blood pressure (SBP) during AFTs {Pearson-coefficient (-).565, P = .002}. Mean MwMR in patients with abnormal-AFTs was significantly lower than patients with borderline-AFTs (1.39 ± .12 vs 1.55 ± .10; P = .002). 9/20 patients with abnormal-AFTs showed functional worsening during follow-up, compared to 2/8 with borderline-AFTs.

CONCLUSION

Single tracer, single session imaging of striatal and cardiac sympathetic dysfunction in patients with advanced IPD is feasible with use of 18F-FDOPA. Significantly reduced 18F-FDOPA uptake is seen in the myocardium of the IPD patients with sympathetic dysfunction.

Overexpression of miR-375 and L-type Amino Acid Transporter 1 in Pheochromocytoma and Their Molecular and Functional Implications

Pheochromocytoma (Pheo) is a tumor derived from chromaffin cells. It can be studied using 18F-dihydroxyphenylalanine (DOPA)—positron emission tomography (PET) due to its overexpression of L-type amino acid transporters (LAT1 and LAT2). The oncogenic pathways involved are still poorly understood. This study examined the relationship between ¹⁸F-DOPA-PET uptake and LAT1 expression, and we explored the role of miR-375 and putative target genes. A consecutive series of 58 Pheo patients were retrospectively analyzed, performing ¹⁸F-DOPA-PET in 32/58 patients. Real-time quantitative PCR was used to assess the expression of LAT1, LAT2, phenylethanolamine N-methyltransferase (PNMT), miR-375, and the major components of the Hippo and Wingless/Integrated pathways. Principal germline mutations associated with hereditary Pheo were also studied. Pheo tissues had significantly higher LAT1, LAT2, and PNMT mRNA levels than normal adrenal tissues. MiR-375 was strongly overexpressed. Yes-associated protein 1 and tankyrase 1 were upregulated, while beta-catenin, axin2, monocarboxylate transporter 8, and Frizzled 8 were downregulated. A positive relationship was found between ¹⁸F-DOPA-PET SUV mean and LAT1 gene expression and for 24 h-urinary norepinephrine and LAT1. This is the first experimental evidence of ¹⁸F-DOPA uptake correlating with LAT1 overexpression. We also demonstrated miR-375 overexpression and downregulated (Wnt) signaling and identified the Hippo pathway as a new potentially oncogenic feature of Pheo.

Arene radiofluorination enabled by photoredox-mediated halide interconversion

Positron emission tomography (PET) is a powerful imaging technology that can visualize and measure metabolic processes in vivo and/or obtain unique information about drug candidates. The identification of new and improved molecular probes plays a critical role in PET, but its progress is somewhat limited due to the lack of efficient and simple labelling methods to modify biologically active small molecules and/or drugs. Current methods to radiofluorinate unactivated arenes are still relatively limited, especially in a simple and site-selective way. Here we disclose a method for constructing C-¹⁸F bonds through direct halide/¹⁸F conversion in electron-rich halo(hetero)arenes. [¹⁸F]F^- is introduced into a broad spectrum of readily available aryl halide precursors in a site-selective manner under mild photoredox conditions. Notably, our direct ¹⁹F/¹⁸F exchange method enables rapid PET probe diversification through the preparation and evaluation of an [¹⁸F]-labelled O-methyl tyrosine library. This strategy also results in the high-yielding synthesis of the widely used PET agent L-[¹⁸F]FDOPA from a readily available L-FDOPA analogue.

Characteristics of 18F-FDG and 18F-FDOPA PET in an 8-year-old neutered male Yorkshire Terrier dog with glioma: long-term chemotherapy using hydroxyurea plus imatinib with prednisolone and immunoreactivity for PDGFR-β and LAT1

An 8-year-old neutered male Yorkshire Terrier dog presented with head pressing, vestibular ataxia, neck tenderness, and no oculocephalic reflex. A demarcated lesion in the pons was identified on MRI. The patient was tentatively diagnosed with a glioma and was treated with hydroxyurea plus imatinib and prednisolone. After 30 days of therapeutic treatment, the patient showed a clear improvement in neurological signs, which lasted for 1117 days. On day 569 after the initiation of treatment, ¹⁸F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) was performed with no significant findings on visual analysis. The average and maximal standardized uptake values (SUVs) were 1.92 and 2.29, respectively. The tumor-to-normal-tissue (T/N) ratio was 0.97. The first evidence of clinical deterioration was noticed on day 1147. On day 1155, 3,4-dihydroxy-6-[¹⁸F]-fluoro-l-phenylalanine (¹⁸F-FDOPA)-PET was performed. High uptake of ¹⁸F-FDOPA was observed in the intracranial lesion. The mean and maximal SUVs of the tumor were 1.59 and 2.29, respectively. The T/N ratio was 2.22. The patient was euthanized on day 1155 and histopathologic evaluations confirmed glioma (astrocytoma). This case shows that chemotherapy with hydroxyurea plus imatinib may be considered in the treatment of canine glioma. Furthermore, this is the first case describing the application of ¹⁸F-FDG and ¹⁸F-FDOPA in a dog with glioma.

Advances in the automated synthesis of 6-[18F]Fluoro-L-DOPA

The neurotracer 6-[18F] FDOPA has been, for many years, a powerful tool in PET imaging of neuropsychiatric diseases, movement disorders and brain malignancies. More recently, it also demonstrated good results in the diagnosis of other malignancies such as neuroendocrine tumours, pheochromocytoma or pancreatic adenocarcinoma.The multiple clinical applications of this tracer fostered a very strong interest in the development of new and improved methods for its radiosynthesis. The no-carrier-added nucleophilic 18F-fluorination process has gained increasing attention, in recent years, due to the high molar activities obtained, when compared with the other methods although the radiochemical yield remains low (17-30%). This led to the development of several nucleophilic synthetic processes in order to obtain the product with molar activity, radiochemical yield and enantiomeric purity suitable for human PET studies.Automation of the synthetic processes is crucial for routine clinical use and compliance with GMP requirements. Nevertheless, the complexity of the synthesis makes the production challenging, increasing the chance of failure in routine production. Thus, for large-scale clinical application and wider use of this radiopharmaceutical, progress in the automation of this complex radiosynthesis is of critical importance.This review summarizes the most recent developments of 6-[18F]FDOPA radiosynthesis and discusses the key issues regarding its automation for routine clinical use.

Metabolism of Stem and Progenitor Cells: Proper Methods to Answer Specific Questions

Stem cells can stay quiescent for a long period of time or proliferate and differentiate into multiple lineages. The activity of stage-specific metabolic programs allows stem cells to best adapt their functions in different microenvironments. Specific cellular phenotypes can be, therefore, defined by precise metabolic signatures. Notably, not only cellular metabolism describes a defined cellular phenotype, but experimental evidence now clearly indicate that also rewiring cells towards a particular cellular metabolism can drive their cellular phenotype and function accordingly. Cellular metabolism can be studied by both targeted and untargeted approaches. Targeted analyses focus on a subset of identified metabolites and on their metabolic fluxes. In addition, the overall assessment of the oxygen consumption rate (OCR) gives a measure of the overall cellular oxidative metabolism and mitochondrial function. Untargeted approach provides a large-scale identification and quantification of the whole metabolome with the aim to describe a metabolic fingerprinting. In this review article, we overview the methodologies currently available for the study of invitro stem cell metabolism, including metabolic fluxes, fingerprint analyses, and single-cell metabolomics. Moreover, we summarize available approaches for the study of in vivo stem cell metabolism. For all of the described methods, we highlight their specificities and limitations. In addition, we discuss practical concerns about the most threatening steps, including metabolic quenching, sample preparation and extraction. A better knowledge of the precise metabolic signature defining specific cell population is instrumental to the design of novel therapeutic strategies able to drive undifferentiated stem cells towards a selective and valuable cellular phenotype.

Pancreatic uptake and radiation dosimetry of 6-[18F]fluoro-L-DOPA from PET imaging studies in infants with congenital hyperinsulinism

METHODS

After injecting 25.6 ± 8.8 MBq (0.7 ± 0.2 mCi) of 18F-Fluoro-L-DOPA intravenously, three static PET scans were acquired at 20, 30, and 40 min post injection in 3-D mode on 10 patients (6 male, 4 female) with congenital hyperinsulinism. Regions of interest (ROIs) were drawn over several organs visible in the reconstructed PET/CT images and time activity curves (TACs) were generated. Residence times were calculated using the TAC data. The radiation absorbed dose for the whole body was calculated by entering the residence times in the OLINDA/EXM 1.0 software.

RESULTS

The mean residence times for the 18F-Fluoro-L-DOPA in the liver, lungs, kidneys, muscles, and pancreas were 11.54 ± 2.84, 1.25 ± 0.38, 4.65 ± 0.97, 17.13 ± 2.62, and 0.89 ± 0.34 min, respectively. The mean effective dose equivalent for 18F-Fluoro-L-DOPA was 0.40 ± 0.04 mSv/MBq. The CT scan used for attenuation correction delivered an additional radiation dose of 5.7 mSv. The organs receiving the highest radiation absorbed dose from 18F-Fluoro-L-DOPA were the urinary bladder wall (2.76 ± 0.95 mGy/MBq), pancreas (0.87 ± 0.30 mGy/MBq), liver (0.34 ± 0.07 mGy/MBq), and kidneys (0.61 ± 0.11 mGy/MBq). The renal system was the primary route for the radioactivity clearance and excretion.

CONCLUSIONS

The estimated radiation dose burden from 18F-Fluoro-L-DOPA is relatively modest to newborns.

Amino acid PET and MR perfusion imaging in brain tumours

Purpose

Despite the excellent capacity of the conventional MRI to image brain tumours, problems remain in answering a number of critical diagnostic questions. To overcome these diagnostic shortcomings, PET using radiolabeled amino acids and perfusion-weighted imaging (PWI) are currently under clinical evaluation. The role of amino acid PET and PWI in different diagnostic challenges in brain tumours is controversial.

Methods

Based on the literature and experience of our centres in correlative imaging with PWI and PET using O-(2-[¹⁸F]fluoroethyl)-l-tyrosine or 3,4-dihydroxy-6-[¹⁸F]-fluoro-l-phenylalanine, the current role and shortcomings of amino acid PET and PWI in different diagnostic challenges in brain tumours are reviewed. Literature searches were performed on PubMed, and additional literature was retrieved from the reference lists of identified articles. In particular, all studies in which amino acid PET was directly compared with PWI were included.

Results

PWI is more readily available, but requires substantial expertise and is more sensitive to artifacts than amino acid PET. At initial diagnosis, PWI and amino acid PET can help to define a site for biopsy but amino acid PET appears to be more powerful to define the tumor extent. Both methods are helpful to differentiate progression or recurrence from unspecific posttherapeutic changes. Assessment of therapeutic efficacy can be achieved especially with amino acid PET, while the data with PWI are sparse.

Conclusion

Both PWI and amino acid PET add valuable diagnostic information to the conventional MRI in the assessment of patients with brain tumours, but further studies are necessary to explore the complementary nature of these two methods.

In Vitro and In Vivo Characterization of Selected Fluorine-18 Labeled Radioligands for PET Imaging of the Dopamine D3 Receptor

Cerebral dopamine D3 receptors seem to play a key role in the control of drug-seeking behavior. The imaging of their regional density with positron emission tomography (PET) could thus help in the exploration of the molecular basis of drug addiction. A fluorine-18 labeled D3 subtype selective radioligand would be beneficial for this purpose; however, as yet, there is no such tracer available. The three candidates [¹⁸F]1, [¹⁸F]2a and [¹⁸F]2b were chosen for in vitro and in vivo characterization as radioligands suitable for selective PET imaging of the D3 receptor. Their evaluation included the analysis of radiometabolites and the assessment of non-specific binding by in vitro rat brain autoradiography. While [¹⁸F]1 and [¹⁸F]2a revealed high non-specific uptake in in vitro rat brain autoradiography, the D3 receptor density was successfully determined on rat brain sections (n = 4) with the candidate [¹⁸F]2b offering a B_max of 20.38 ± 2.67 pmol/g for the islands of Calleja, 19.54 ± 1.85 pmol/g for the nucleus accumbens and 16.58 ± 1.63 pmol/g for the caudate putamen. In PET imaging studies, the carboxamide 1 revealed low signal/background ratios in the rat brain and relatively low uptake in the pituitary gland, while the azocarboxamides [¹⁸F]2a and [¹⁸F]2b showed binding that was blockable by the D3 receptor ligand BP897 in the ventricular system and the pituitary gland in PET imaging studies in living rats.

Overexpression of L-Type Amino Acid Transporter 1 (LAT1) and 2 (LAT2): Novel Markers of Neuroendocrine Tumors

Background

6-18F-fluoro-L-3,4-dihydroxyphenylalanine (¹⁸F-FDOPA) PET is a useful tool in the clinical management of pheochromocytoma (PHEO) and medullary thyroid carcinoma (MTC). ¹⁸F-FDOPA is a large neutral amino acid biochemically resembling endogenous L-DOPA and taken up by the L-type amino acid transporters (LAT1 and LAT2). This study was conducted to examine the expression of the LAT system in PHEO and MTC.

Methods

Real-time PCR and Western blot analyses were used to assess LAT1 and LAT2 gene and protein expression in 32 PHEO, 38 MTC, 16 normal adrenal medulla and 15 normal thyroid tissue samples. Immunohistochemistry method was applied to identify the proteins’ subcellular localization.

Results

LAT1 and LAT2 were overexpressed in both PHEO and MTC by comparison with normal tissues. LAT1 presented a stronger induction than LAT2, and their greater expression was more evident in PHEO (15.1- and 4.1-fold increases, respectively) than in MTC (9.9- and 4.1-fold increases, respectively). Furthermore we found a good correlation between LAT1/2 and GLUT1 expression levels. A positive correlation was also found between urinary noradrenaline and adrenaline levels and LAT1 gene expression in PHEO. The increased expression of LAT1 is also confirmed at the protein level, in both PHEO and MTC, with a strong cytoplasmic localization.

Conclusions

The present study is the first to provide experimental evidence of the overexpression in some NET cancers (such as PHEO or MTC) of L-type amino acid transporters, and the LAT1 isoform in particular, giving the molecular basis to explain the increase of the DOPA uptake seen in such tumor cells.

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.

Copper-catalyzed [18F]fluorination of (mesityl)(aryl)iodonium salts

A practical, rapid, and highly regioselective Cu-catalyzed radiofluorination of (mesityl)(aryl)iodonium salts is described. This protocol utilizes [(18)F]KF to access (18)F-labeled electron-rich, -neutral, and -deficient aryl fluorides under a single set of mild conditions. This methodology is applied to the synthesis of protected versions of two important radiotracers: 4-[(18)F]fluorophenylalanine and 6-[(18)F]fluoroDOPA.

Normal biodistribution pattern and physiologic variants of 18F-DOPA PET imaging

Dihydroxyphenylalanine (DOPA) is a neutral amino acid that resembles natural l-dopa (dopamine precursor). It enters the catecholamine metabolic pathway of endogenous l-DOPA in the brain and peripheral tissues. It is amenable to labeling with fluorine-18 (¹⁸F) for PET imaging and was originally used in patients with Parkinson’s disease to assess the integrity of the striatal dopaminergic system. The recent introduction and use of hybrid PET/CT scanners has contributed significantly to the management of a series of other pathologies including neuroendocrine tumors, brain tumors, and pancreatic cell hyperplasia. These pathologic entities present an increased activity of l-DOPA decarboxylase and therefore demonstrate high uptake of ¹⁸F-DOPA. Despite these potentially promising applications in several clinical fields, the role of ¹⁸F-DOPA has not been elucidated completely yet because of associated difficulties in synthesis and availability. Unfortunately, the available literature does not provide recommendations for procedures or administered activity, acquisition timing, and premedication with carbidopa. The aim of this paper is to outline the physiological biodistribution and normal variants, including possible pitfalls that may lead to misinterpretations of the scans in various clinical settings.

6-[18F]fluoro-L-DOPA: a well-established neurotracer with expanding application spectrum and strongly improved radiosyntheses

For many years, the main application of [(18)F]F-DOPA has been the PET imaging of neuropsychiatric diseases, movement disorders, and brain malignancies. Recent findings however point to very favorable results of this tracer for the imaging of other malignant diseases such as neuroendocrine tumors, pheochromocytoma, and pancreatic adenocarcinoma expanding its application spectrum. With the application of this tracer in neuroendocrine tumor imaging, improved radiosyntheses have been developed. Among these, the no-carrier-added nucleophilic introduction of fluorine-18, especially, has gained increasing attention as it gives [(18)F]F-DOPA in higher specific activities and shorter reaction times by less intricate synthesis protocols. The nucleophilic syntheses which were developed recently are able to provide [(18)F]F-DOPA by automated syntheses in very high specific activities, radiochemical yields, and enantiomeric purities. This review summarizes the developments in the field of [(18)F]F-DOPA syntheses using electrophilic synthesis pathways as well as recent developments of nucleophilic syntheses of [(18)F]F-DOPA and compares the different synthesis strategies regarding the accessibility and applicability of the products for human in vivo PET tumor imaging.

¹⁸F-DOPA PET/computed tomography imaging

18F-DOPA is a radiopharmaceutical with interesting clinical applications and promising performances in the evaluation of the integrity of dopaminergic pathways, brain tumors, NETs (especially MTCs, paragangliomas, and pheochromocytomas), and congenital hyperinsulinism. 18F-DOPA traces a very specific metabolic pathway and has a very precise biodistribution pattern. As for any radiopharmaceutical, the knowledge of the normal distribution of 18F-DOPA, its physiologic variants, and its possible pitfalls is essential for the correct interpretation of PET scans. Moreover, it is important to be aware of the potential false-positive and false-negative episodes that can occur in the various clinical settings.

Comparative diagnostic accuracy of contrast-enhanced MRI and (18)F-FDOPA PET-CT in recurrent glioma

OBJECTIVES

To compare the diagnostic accuracy of contrast enhanced magnetic resonance imaging (Ce-MRI) and (18)F-fluorodopa ((18)F-FDOPA) positron emission tomography (PET)-computed tomography (CT) for detecting recurrent glioma.

METHODS

In this prospective study, 35 patients (age, 36.62 ± 0.86 years; 80 % male) with histopathologically proven glioma with clinical suspicion of recurrence were evaluated using Ce-MRI and (18)F-FDOPA PET-CT. (18)F-FDOPA PET-CT images were evaluated qualitatively and semi-quantitatively. Combination of clinical follow-up (minimum 1 year), repeat imaging and/or biopsy (when available) was taken as the reference standard.

RESULTS

Based on the reference standard, 26 patients were positive and nine were negative for recurrence. The sensitivity, specificity and accuracy of Ce-MRI were 92.3 %, 44.4 % and 80 % respectively, whereas those of (18)F-FDOPA PET-CT were 100 %, 88.89 % and 97.1 % respectively. Results of Ce-MRI and (18)F-FDOPA PET-CT were concordant in 74.3 % (29/35) and discordant in 17.1 % of patients (6/35). On McNemar analysis the difference was not statistically significant overall (P = 0.687), for high-grade tumour (P = 0.5) or low-grade tumours (P = 1.0). However, (18)F-FDOPA PET-CT was more specific than Ce-MRI overall (P = 0.0002), for high-grade tumour (P = 0.006) and low-grade tumours (P = 0.004).

CONCLUSION

F-FDOPA PET-CT shows a high but comparable diagnostic accuracy to Ce-MRI for the detection of recurrent glioma. However, it is more specific than Ce-MRI.

KEY POINTS

• Recurrent glioma in the postoperative site remains a diagnostic dilemma. • (18) F-FDOPA PET-CT shows high diagnostic accuracy for detecting recurrent glioma. • Diagnostic accuracies for (18) F-FDOPA PET-CT and contrast enhanced MRI are comparable. • However, (18) F-FDOPA PET-CT is more specific than Ce-MRI for recurrent glioma.

18F-DOPA PET/CT biodistribution consideration in 107 consecutive patients with neuroendocrine tumours

OBJECTIVE

L-6-fluoro 3,4-dihydroxyphenylalanine (18F-DOPA), an amino acid-based radiopharmaceutical, is increasingly being used in the detection and management of neuroendocrine tumours. Knowledge of the normal biodistribution of this radiopharmaceutical is essential for the proper interpretation of such studies, but the literature available is scanty due to the rarity of these tumours. The aim of this study is to evaluate the biodistribution pattern and normal variants of 18F-DOPA in a cohort of patients with neuroendocrine tumours using semiquantitative analysis (maximum standardized uptake value).

METHODS

We analysed 107 consecutive 18F-DOPA PET/CT studies of patients referred with medullary carcinoma of the thyroid (43), phaeochromocytoma including cases of Von Hippel Lindau syndrome and multiple endocrine neoplasia type IIA cases (34), paraganglioma (14) and other neuroendocrine tumours (16). The study population were divided into two groups: those with negative 18F-DOPA PET/CT scans (32) and those with positive scans (75). The biodistribution of 18F-DOPA in each group was measured and compared between the two groups.

RESULTS

The physiological biodistribution in the basal ganglia and liver parenchyma showed no variability between the two groups. Conversely, uptake in the pancreas (particularly the uncinate process) and adrenals showed considerable variability between the groups. However, these differences were found not to be significant on statistical analysis.

CONCLUSION

The data presented may provide useful information in understanding the physiologic biodistribution of DOPA and its variants, for the purpose of improving the interpretation of 18F-DOPA PET/CT.

Radiopharmaceutical chemistry for positron emission tomography

Molecular imaging is an emerging technology that allows the visualization of interactions between molecular probes and biological targets. Molecules that either direct or are subject to homeostatic controls in biological systems could be labeled with the appropriate radioisotopes for the quantitative measurement of selected molecular interactions during normal tissue homeostasis and again after perturbations of the normal state. In particular, positron emission tomography (PET) offers picomolar sensitivity and is a fully translational technique that requires specific probes radiolabeled with a usually short-lived positron-emitting radionuclide. PET has provided the capability of measuring biological processes at the molecular and metabolic levels in vivo by the detection of the gamma rays formed as a result of the annihilation of the positrons emitted. Despite the great wealth of information that such probes can provide, the potential of PET strongly depends on the availability of suitable PET radiotracers. However, the development of new imaging probes for PET is far from trivial and radiochemistry is a major limiting factor for the field of PET. In this review, we provided an overview of the most common chemical approaches for the synthesis of PET-labeled molecules and highlighted the most recent developments and trends. The discussed PET radionuclides include ¹¹C (t₁(/)₂=20.4min), ¹³N (t₁(/)₂=9.9min), ¹⁵O (t₁(/)₂=2min), ⁶⁸Ga (t₁(/)₂=68min), ¹⁸F (t₁(/)₂=109.8min), ⁶⁴Cu (t₁(/)₂=12.7h), and ¹²⁴I (t₁(/)₂=4.12d).

18F-FDOPA PET/MRI fusion in patients with primary/recurrent gliomas: initial experience

BACKGROUND AND PURPOSE

(18)F-FDOPA PET demonstrates higher sensitivity and specificity for gliomas than traditional [(18)F] FDG PET imaging. However, PET provides limited anatomic localization. The purpose of this study was to determine whether (18)F-FDOPA PET/MRI fusion can provide precise anatomic localization of abnormal tracer uptake and how this activity corresponds to MR signal abnormality.

METHODS

Two groups of patients were analyzed. Group I consisted of 21 patients who underwent (18)F-FDOPA PET and MRI followed by craniotomy for tumor resection. Group II consisted of 70 patients with a pathological diagnosis of glioma that had (18)F-FDOPA PET and MRI but lacked additional pathologic follow-up. Fused (18)F-FDOPA PET and MRI images were analyzed for concordance and correlated with histopathologic data.

RESULTS

Fusion technology facilitated precise anatomical localization of (18)F-FDOPA activity. In group I, all 21 cases showed pathology-confirmed tumor. Of these, (18)F-FDOPA scans were positive in 9/10 (90%) previously unresected tumors, and 11/11 (100%) of recurrent tumors. Of the 70 patients in group II, concordance between MRI and (18)F-FDOPA was found in 49/54 (90.1%) of patients with sufficient follow-up; in the remaining 16 patients concordance could not be determined due to lack of follow-up. (18)F-FDOPA labeling was comparable in both high- and low-grade gliomas and identified both enhancing and non-enhancing tumor equally well. In some cases, (18)F-FDOPA activity preceded tumor detection on MRI.

CONCLUSION

(18)F-FDOPA PET/MRI fusion provides precise anatomic localization of tracer uptake and labels enhancing and non-enhancing tumor well. In a small minority of cases, (18)F-FDOPA activity may identify tumor not visible on MRI.

Tumor biology-guided radiotherapy treatment planning: gross tumor volume versus functional tumor volume

This issue of Seminars in Nuclear Medicine deals with a watershed event in cancer treatment -- the combined use of functional and anatomical information to guide therapeutic interventions. The use of positron emission tomography/computed tomography (PET/CT) in radiation treatment planning and tumor response evaluation brings a paradigm change in the development of image-guided therapies into routine clinical practice. The implications, as seen in the following articles, are not only promising but also groundbreaking. And, as in every new scientific breakthrough, each step forward generates a myriad of additional important clinical and research questions. Functional imaging takes advantage of the subtle differences between normal and malignant tissues at the cellular level to reveal in vivo unique functional characteristics of neoplasms. The ultimate goal of the partnership between nuclear medicine physicians and radiation oncologists is to use this information with absolute clarity in target definition for radiation treatment planning and therapy, as well as response evaluation. Functional imaging can provide metabolic information and behavioral correlation along with the anatomical imaging for correlative target delineation. Additionally, as a purely diagnostic instrument, PET/CT provides a tool for oncologists to make critical decisions regarding radiation treatment planning modifications secondary to changes in tumor staging (up or down), treatment field modifications, localized control, sites of residual and/or metastatic disease and post therapy response evaluation. The articles in this issue of the seminars provide insights into the current state-of-the-art of functional imaging techniques, mostly centered on the use of (18)F-fluorodeoxyglucose PET/CT in image guided oncologic therapies. Because it is a novel science, the future of image-guided functional treatment planning is bright with technologic and biologic innovations, translational research and new clinical applications.

(18)F-labeled positron emission tomographic radiopharmaceuticals in oncology: an overview of radiochemistry and mechanisms of tumor localization

Molecular imaging is the visualization, characterization, and measurement of biological processes at the molecular and cellular levels in a living system. At present, positron emission tomography/computed tomography (PET/CT) is one the most rapidly growing areas of medical imaging, with many applications in the clinical management of patients with cancer. Although [(18)F]fluorodeoxyglucose (FDG)-PET/CT imaging provides high specificity and sensitivity in several kinds of cancer and has many applications, it is important to recognize that FDG is not a "specific" radiotracer for imaging malignant disease. Highly "tumor-specific" and "tumor cell signal-specific" PET radiopharmaceuticals are essential to meet the growing demand of radioisotope-based molecular imaging technology. In the last 15 years, many alternative PET tracers have been proposed and evaluated in preclinical and clinical studies to characterize the tumor biology more appropriately. The potential clinical utility of several (18)F-labeled radiotracers (eg, fluoride, FDOPA, FLT, FMISO, FES, and FCH) is being reviewed by several investigators in this issue. An overview of design and development of (18)F-labeled PET radiopharmaceuticals, radiochemistry, and mechanism(s) of tumor cell uptake and localization of radiotracers are presented here. The approval of clinical indications for FDG-PET in the year 2000 by the Food and Drug Administration, based on a review of literature, was a major breakthrough to the rapid incorporation of PET into nuclear medicine practice, particularly in oncology. Approval of a radiopharmaceutical typically involves submission of a "New Drug Application" by a manufacturer or a company clearly documenting 2 major aspects of the drug: (1) manufacturing of PET drug using current good manufacturing practices and (2) the safety and effectiveness of a drug with specific indications. The potential routine clinical utility of (18)F-labeled PET radiopharmaceuticals depends also on regulatory compliance in addition to documentation of potential safety and efficacy by various investigators.

Selectfluor: Mechanistic Insight and Applications

The replacement of hydrogen atoms with fluorine substituents in organic substrates is of great interest in synthetic chemistry because of the strong electronegativity of fluorine and relatively small steric footprint of fluorine atoms. Many sources of nucleophilic fluorine are available for the derivatization of organic molecules under acidic, basic, and neutral conditions. However, electrophilic fluorination has historically required molecular fluorine, whose notorious toxicity and explosive tendencies limit its application in research. The necessity for an electrophilic fluorination reagent that is safe, stable, highly reactive, and amenable to industrial production as an alternative to very hazardous molecular fluorine was the inspiration for the discovery of selectfluor. This reagent is not only one of the most reactive electrophilic fluorinating reagents available, but it is also safe, nontoxic, and easy to handle. In this Review we document the many applications of selectfluor and discuss possible mechanistic pathways for its reaction.

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)).

The DaNeX study of embryonic mesencephalic, dopaminergic tissue grafted to a minipig model of Parkinson's disease: preliminary findings of effect of MPTP poisoning on striatal dopaminergic markers

A multicenter study is under way to investigate the efficacy of allografting of embryonic mesencephalic neurons in a pig model of Parkinson's disease. We have first established that a stable parkinsonian syndrome can be established by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication of adult male Göttingen minipigs. We are now using positron emission tomography (PET) methods for testing the physiological responses to MPTP intoxication and the time course of the response to several treatment strategies. We now report preliminary results obtained in 11 pigs employed in the initial phase of the study; the completed study shall ultimately include 30 pigs. Animals were randomly assigned to one of five groups: 1) Control, 2) MPTP intoxication, 3) MPTP intoxication followed by allograft, 4) MPTP intoxication followed by allograft with immunosuppression, and 5) MPTP intoxication followed by allograft with immunosuppression and co-grafting of immortalized HiB5 cells, which had been manipulated to secrete glia cell line-derived neurotrophic factor (GDNF) (approximately 2 ng GDNF/h/10(5) cells). MPTP was administered (1 mg/kg/day, SC) for 7-10 days until the pigs had developed mild parkinsonian symptoms of muscle rigidity, hypokinesia, and impaired coordination, especially of the hind limbs. Approximately 2 weeks after the last MPTP dose, animals received a T1-weighted magnetic resonance imaging (MRI) scan, and a series of dynamic PET recordings. After the first series of PET scans, four grafts of porcine embryonic mesencephalic tissue (E28 days) were placed in each striatum of some MPTP-intoxicated pigs, using MRI-based stereotactic techniques. Immunosuppression of some animals with cyclosporin and prednisolone began just prior to surgery. Two more series of PET scans were performed at 4-month intervals after surgery. After the last scans, pigs were killed and the brains were perfused for unbiased stereological examination of cytological and histochemical markers in striatum and substantial nigra. The behavioral impairment of the animals (the "Parkinson's score") had been evaluated throughout the 8-month period. Kinetic analysis of the first set of PET scans has indicated that the rate constant for the decarboxylation of FDOPA in catecholamine fibers was reduced by 33% in striatum of the mildly parkinsonian pigs. The rate of association of [11C]NS-2214 to catecholamine uptake sites was reduced by 62% in the same groups of pigs. No significant difference was found in the binding potential of [11C]raclopride to the dopamine D2-like receptors in striatum of the MPTP-intoxicated versus control pigs. These preliminary results are suggestive that the activity of DOPA decarboxylase may be upregulated in the partially denervated pig striatum.

Cerebral 6-[(18)F]fluoro-L-DOPA (FDOPA) metabolism in pig studied by positron emission tomography

We measured 6-[(18)F]fluoro-L-DOPA (FDOPA) uptake and metabolism in the brain of 4-month-old female pigs (n = 8) using a high-resolution positron emission tomograph (PET) in 3D mode. The mean net blood-brain clearance of FDOPA (K(i)(D)) to striatum was 0.011 ml g(-1) min(-1). Correcting for the elimination of decarboxylated metabolites from striatum (k(loss) = 0.004 min(-1)) increased the apparent magnitude of the estimate of K(i)(D) by 50%, at the expense of doubling the variance of the mean estimate. The mean decarboxylation rate of FDOPA in striatum relative to the cerebellum input (k(3)(s)) was 0.008 min(-1). For multicompartmental analyses, the FDOPA partition volume (V(e)(D)) was constrained to the individual value observed in cerebellum (mean = 0.53 ml g(-1)), with correction for the presence in brain of the plasma metabolite 3-O-methyl-FDOPA (OMFD). Using the first 60 min of the dynamic PET scans, the rate constant of FDOPA decarboxylation (k(3)(D)) was estimated to be 0.037 min(-1 )in striatum, but was not significantly different than zero in frontal cortex. Fitting of a compartmental model correcting for elimination of decarboxylated metabolites to the complete PET frame-sequence (120 min) increased the variance of the estimate of k(3)(D) in striatum. The magnitude of k(3)(D) in striatum of young pig was less than values estimated previously in neonatal piglet, adult monkey, and human. MRI-based simulations predicted that recovery of radioactivity from pig striatum was highly sensitive to the volume of interest. We conclude that the spatial resolution of our tomograph reduces the apparent magnitude of k(3)(D) in striatum. However, anaesthetised pigs are an appropriate experimental model for PET studies of DOPA decarboxylation in striatum.

Positron emission tomography in degenerative disorders of the dopaminergic system

21 patients who had Parkinson's disease (PD), PD plus dementia of Alzheimer type (PDAT) or progressive supranuclear palsy (PSP), were studied with positron emission tomography (PET) using (18F)-2-fluoro-2-deoxy-D-glucose (FDG). In one patient with strictly unilateral PD side differences in striatal dopa uptake were studied with 6-(18F)fluoro-L-dopa (F-dopa). In patients with PD PET with FDG did not show any significant change in regional cerebral metabolic rates for glucose (rCMR(Glu)). In PDAT glucose metabolism was generally reduced, the most severe decrease was found in parietal cortex. The metabolic pattern was similar to that typically found in patients with Alzheimer's disease (AD). In the patient with strictly unilateral PD rCMR(Glu) was normal, F-dopa PET, however, revealed a distinct reduction of dopa uptake in the contralateral putamen. In PSP glucose metabolism was significantly decreased in subcortical regions (caudatum, putamen and brainstem) and in frontal cortex. Thus PET demonstrated a clear difference of metabolic pattern between PDAT and PSP.