Ion chromatographic analysis of high specific activity 18FDG preparations and detection of the chemical impurity 2-deoxy-2-chloro-D-glucose

Determination of Total Radiochemical Purity of [18F]FDG and [18F]FET by High-Performance Liquid Chromatography Avoiding TLC Method

The goal of this work was to present two high-performance liquid chromatography (HPLC) method that could be applied for the determination of the total radioactive purity of 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) and O-(2-[18F]fluoroethyl)-L-tyrosine ([18F]FET). The separation of [18F]fluoride ions, [18F]FET and [18F]FET intermediate was accomplished on LiChrosper RP-18, 250 × 4 mm, 5 µm (Merck) analytical column. For mobile phase 10 mM potassium dihydrogen phosphate buffer at pH7 (A) and acetonitrile (B) was used: 0–2 min: 15% B; 2–12 min: 85% B; 12–15 min: 15% B, respectively. Analysis of [18F]FDG was performed using LiChrosper 100 NH2, 250 × 4.5 mm, 5 µm (Merck) analytical column. The initial mobile phase composition was 10 mM KH2PO4 buffer (pH7) and acetonitrile (15:85, v/v) and the acetonitrile ratio was decreased to 15% at 2 min after the sample injection and held for 5 min. Complete elution of [18F]fluoride ions from stationary phases could be achieved by adding 10 mg/mL K[19F]F to radioactive samples in a ratio 1:1 during the sample preparation. Recovery of [18F]fluoride ions ranged from 99.5 to 100.6%. The validation of the developed methods showed good results for linearity (r2 = 0.9981–0.9996), specificity (RS = 3.7–10.2), repeatability (%Area RSD% = 1.2–4.3%) and limit of quantitation (LOQ = 1.6–4.5 kBq). During the cross-validation similar radiochemical purity values were obtained by the novel HPLC methods and thin layer chromatography performed according to the recommendations of the Ph. Eur. monographs.

An Exposition of 11C and 18F Radiotracers Synthesis for PET Imaging

The development of new radiolabeled Positron emission tomography tracers has been extensively utilized to access the increasing diversity in the research process and to facilitate the development in research methodology, clinical usage of drug discovery and patient care. Recent advances in radiochemistry, as well as the latest techniques in automated radio-synthesizer, have encouraged and challenged the radiochemists to produce the routinely developed radiotracers. Various radionuclides like 18F, 11C, 15O, 13N 99mTc, 131I, 124I and 64Cu are used for incorporating into different chemical scaffolds; among them, 18F and 11C tagged radiotracers are mostly explored such as 11C-Methionine, 11C-Choline, 18F-FDG, 18F-FLT, and 18F-FES. This review is focused on the development of radiochemistry routes to synthesize different radiotracers of 11C and 18F for clinical studies.

On-chip electrochemical detection of glucose towards the miniaturised quality control of carbohydrate-based radiotracers

We have developed two microfluidic platforms for the electrochemical detection of glucose, using either a screen-printed electrode or wire electrodes, towards the quality control testing of carbohydrate-based radiotracers used in medical imaging.

In vitro and in vivo characterization of 2-deoxy-2-18F-fluoro-D-mannose as a tumor-imaging agent for PET

2-Deoxy-2-(18)F-fluoro-d-mannose ((18)F-FDM) is an (18)F-labeled mannose derivative and a stereoisomer of (18)F-FDG. Our preliminary study demonstrated that (18)F-FDM accumulated in tumors to the same extent as (18)F-FDG, with less uptake in the brain and faster clearance from the blood. However, detailed studies on the uptake of (18)F-FDM in tumors have not been conducted. We undertook this study to establish a practical method of (18)F-FDM synthesis based on an (18)F-nucleophilic substitution (SN2) reaction and to advance the biologic characterization of (18)F-FDM for potential application as a tumor-imaging agent.

METHODS

We synthesized 4,6-O-benzylidene-3-O-ethoxymethyl-1-O-methyl-2-O-trifluoromethanesulfonyl-β-D-glucopyranoside as a precursor for the nucleophilic synthesis of (18)F-FDM. The precursor was radiofluorinated with (18)F-KF/Kryptofix222, followed by removal of the protecting groups with an acid. (18)F-FDM was purified by preparative high-performance liquid chromatography and then subjected to in vitro evaluation regarding phosphorylation by hexokinase as well as uptake and metabolism in AH109A tumor cells. The in vivo properties of (18)F-FDM were examined in Donryu rats bearing AH109A tumor cells by biodistribution studies and imaging with a small-animal PET system.

RESULTS

We radiosynthesized (18)F-FDM in sufficient radiochemical yields (50%-68%) with excellent purities (97.6%-98.7%). (18)F-FDM was phosphorylated rapidly by hexokinase, resulting in 98% conversion into (18)F-FDG-6-phosphate within 30 min. Tumor cells showed significant uptake of (18)F-FDM with time in vitro, and uptake was dose-dependently inhibited by D-glucose. (18)F-FDM injected into tumor-bearing rats showed greater uptake in tumors (2.17 ± 0.32 percentage injected dose per gram [%ID/g]) than in the brain (1.42 ± 0.10 %ID/g) at 60 min after injection. PET studies also revealed the tumor uptake of (18)F-FDM (quasi-standardized uptake value, 2.83 ± 0.22) to be the same as that of (18)F-FDG (2.40 ± 0.30), but the brain uptake of (18)F-FDM (1.89 ± 0.13) was ≈ 30% lower than that of (18)F-FDG (2.63 ± 0.26).

CONCLUSION

We prepared (18)F-FDM with good radiochemical yield and purity by an SN2 reaction. We demonstrated that (18)F-FDM had adequate tumor cell uptake by a metabolic trapping mechanism and can afford high-contrast tumor images with less uptake in the brain, indicating that (18)F-FDM has almost the same potential as (18)F-FDG for PET tumor imaging, with better advantages with regard to the imaging of brain tumors.

Imaging of VMAT2 binding sites in the brain by (18)F-AV-133: the effect of a pseudo-carrier

OBJECTIVES

Recently, 9-[(18)F]fluoropropyl-(+)-dihydrotetrabenazine ((18)F-AV-133) was reported as a new vesicular monoamine transporter (VMAT2) imaging agent for diagnosis of Parkinson's disease (PD). To shorten the preparation of (18)F-AV-133 and to make it more widely available, we evaluated a simple, rapid purification with a solid-phase extraction method (SPE) using an Oasis HLB cartridge instead of high pressure liquid chromatography (HPLC). The SPE method produced doses containing a pseudo-carrier, 9-hydroxypropyl-(+)-dihydrotetrabenazine (AV-149).

METHODS

To test the possible side effects of this pseudo-carrier, comparative dynamic PET scans of the brains of normal monkeys (2 each) and uni-laterally 6-OH-dopamine-lesioned PD monkeys (2 each) were performed using (18)F-AV-133 doses prepared by either SPE (containing pseudo-carrier) or HPLC (containing no pseudo-carrier). Autoradiographs of post mortem monkey brain sections were evaluated to confirm the relative (18)F-AV-133 uptake in the PD monkey brains and the effects of the pseudo-carrier on VMAT2 binding.

RESULTS

The radiochemical purity of the (18)F-AV-133, whether prepared by SPE or by HPLC, was excellent (>99%). PET scans of normal and PD monkey brains showed an expected reduction of VMAT2 in the lesioned areas of the striatum. It was not affected by the presence of the pseudo-carrier, AV-149 (maximally 250 μg/dose). The reduced uptake in the striatum of the lesioned monkey brains was confirmed by autoradiography. Ex vivo inhibition studies of (18)F-AV-133 binding in rat brains, conducted with increasing amounts of AV-149, suggested that at the highest concentration (3.5mg/kg) the VMAT2 binding in the striatum was only moderately blocked (20% reduction).

CONCLUSIONS

The pseudo-carrier, AV-149, did not affect the (18)F-AV-133/PET imaging of VMAT2 binding sites in normal or uni-laterally lesioned monkey brains. The new streamlined SPE purification method will enable (18)F-AV-133 to be widely available for routine clinical application in determining changes in monoamine neurons for patient with movement disorders or other psychiatric illnesses.

A simple method for the quality control of [(18)F]FDG

Most automated synthesis modules produce [(18)F]FDG within half an hour, but the quality control involving up to three separate methods and three different analytical systems is time consuming. The use of HPLC, TLC, and GC for the quality control of [(18)F]FDG is both time consuming and expensive (high purchase costs). Presented here is a method using a single HPLC system for all three analyses.

Synthesis, radiolabeling, and biological evaluation of (R)- and (S)-2-amino-3-[(18)F]fluoro-2-methylpropanoic acid (FAMP) and (R)- and (S)-3-[(18)F]fluoro-2-methyl-2-N-(methylamino)propanoic acid (NMeFAMP) as potential PET radioligands for imaging brain tumors

The non-natural amino acids (R)- and (S)-2-amino-3-fluoro-2-methylpropanoic acid 5 and (R)- and (S)-3-fluoro-2-methyl-2-N-(methylamino)propanoic acid 8 were synthesized in shorter reaction sequences than in the original report starting from enantiomerically pure (S)- and (R)-alpha-methyl-serine, respectively. The reaction sequence provided the cyclic sulfamidate precursors for radiosynthesis of (R)- and (S)-[(18)F]5 and (R)- and (S)-[(18)F]8 in fewer steps than in the original report. (R)- and (S)-[(18)F]5 and(R)- and (S)-[(18)F]8 were synthesized by no-carrier-added nucleophilic [(18)F]fluorination in 52-66% decay-corrected yields with radiochemical purity over 99%. The cell assays showed that all four compounds were substrates for amino acid transport and enter 9L rat gliosarcoma cells in vitro at least in part by system A amino acid transport. The biodistribution studies demonstrated that in vivo tumor to normal brain ratios for all compounds were high with ratios of 20:1 to115:1 in rats with intracranial 9L tumors. The (R)-enantiomers of [(18)F]5 and [(18)F]8 demonstrated higher tumor uptake in vivo compared to the (S)-enantiomers.

Simultaneous analysis of FDG, ClDG and Kryptofix 2.2.2 in [18F]FDG preparation by high-performance liquid chromatography with UV detection

A practical, sensitive and rapid analytical method was established and validated for chemical impurity tests of 2-deoxy-2-fluoro-d-glucose (FDG), 2-deoxy-2-chloro-d-glucose (ClDG) and Kryptofix 2.2.2 (K-222) in [18F]FDG. This method was based on precolumn derivatization with ultraviolet (UV) detection. FDG and ClDG were rapidly derivatized with 1-phenyl-3-methyl-5-pyrazolone in the presence of borate buffer at 40 degrees C, and the labeled derivatives and K-222 were separated by reversed-phase high-performance liquid chromatography and monitored by UV absorbance at 210 nm. After optimization of the conditions, FDG, ClDG and K-222 could be determined within 15 min and showed good performance in terms of sensitivity, linearity and reproducibility. This method could be successfully applied to the quality control test of [18F]FDG produced by a commercially available apparatus.

Quality control of PET radiopharmaceuticals using HPLC with electrochemical detection

The usefulness of high-performance liquid chromatography with electrochemical detection (HPLC/ECD) in the quality control of positron emission tomography (PET) radiopharmaceuticals was evaluated for a number of substances. Chromatographic separation was performed using a reversed phase column and acetonitrile or 20 mM sodium phosphate buffer as the mobile phase. The effluent from the column was introduced into an electrochemical detector equipped with a glassy carbon electrode versus Ag/AgCl electrode operated in the direct current mode. In 19 of 21 PET radiopharmaceuticals studied, the compounds and corresponding precursors used in the synthesis of the radiopharmaceuticals could be successfully detected by the HPLC/ECD method. For 17 compounds with electroactive functional groups, such as aliphatic amines, phenols and aromatic amines, the detection limits were ppb levels for a 20-mul injection volume; this was significantly better compared with ultraviolet (UV) detection. This method could be applied to the analysis of [11C]MP4A, useful PET radiopharmaceutical for measuring acetylcholinesterase activity in the brain with no available UV absorbance.

Improved quality control of [18F]FDG by HPLC with UV detection

A conventional high-performance liquid chromatographic (HPLC) method for the analysis of 2-fluoro-2-deoxy-d-glucose (FDG) and 2-deoxy-2-chloro-d-glucose (ClDG) in [18F]FDG preparations is described. This method was based on a postcolumn derivatization with 2-cyanoacetamide (2-CA) and UV detection. FDG and ClDG were separated on a normal-phase column using acetonitrile/water as the mobile phase. The eluate was mixed with 2-CA in sodium borate buffer solution at the outlet of a PTFE coil (10 m x 0.5 mm id) from the column, and the reaction was carried out at 100 degrees C during the passage through the coil. The UV absorbance of the resultant product was monitored at 276 nm. Under optimum conditions, the detection limits [signal-to-noise (S/N) ratio=3] for FDG and ClDG were 0.31 and 0.17 microg/ml for a 20-microl injection volume, respectively, and the linearity ranges were 0.5-100 microg/ml for both compounds. The intra- and interday reproducibilities were better than 2.2% [relative standard deviation (R.S.D.)]. This HPLC separation procedure is also useful for determining the radiochemical purity of [18F]FDG preparations since it allows the analysis of 2-[18F]fluoro-1,3,4,6-tetra-O-acetyl-d-glucose ([18F]TAG), partially hydrolyzed [18F]TAG and [18F]F-. This method can be used at many positron emission tomography (PET) facilities since it does not require an expensive, sophisticated electrochemical detector.

Factors affecting quality control of [18F]FDG injection: bacterial endotoxins test, aluminum ions test and HPLC analysis for FDG and CIDG

High concentrations of citrates and phosphates which are often used in the manufacturing of [18F] fluro-D-glucose (FDG) preparations and wide deviation in the pH value from the neutral level often disturb the detection of endotoxins and aluminum ions using the turbidimetric and aluminum ion paper test method. The column temperature was found to be a major factor influencing the sensitivity of ClDG detection with the HPAEC/PAD method.

Increased [18F]2-fluoro-2-deoxy-D-glucose ([18F]FDG) yield with recycled target [18O]water: factors affecting the [18F]FDG yield

The reuse of [18O] water after being purified by distillation has been reported to give lower [18F]2-fluoro-2-deoxy-D-glucose ([18F]FDG) yields, probably due to the presence of organic impurities. In our routine production of [18F]FDG, however, we observed increased [18F]FDG yields with recycled [18O]water. Thus, factors affecting [18F]FDG yield were examined using as-purchased (virgin) and recycled (by photochemical combustion and distillation) [18O]water. [18F]FDG was synthesized by nucleophilic 18F-fluorination on a quaternary 4-aminopyridinium resin. The recycled [18O]water gave an [18F]FDG yield significantly higher than did the virgin water, without any significant difference in the [18F]fluoride yield. Levels of several ionic impurities including Cl- and Ca2+ were significantly higher in the virgin [18O]water than in the recycled water, while significantly larger amounts of organic impurities were detected in the former. Hence, trace amounts of organic impurities were not responsible for the lower [18F]FDG yield. Chloride anion in the [18O]water may compete with [18F]fluoride to lower the [18F]FDG yield.

Chemical impurities in [18F]FDG preparations produced by solid-phase 18F-fluorination

[18F]FDG was produced by solid-phase 18F-fluorination (resin method) and chemical impurities were determined in the [18F]FDG preparations by ion chromatography. The major chemical impurities were D-glucose (90.5 +/- 6.4 microg/mL), 2-chloro-2-deoxy-D-glucose (11.8 +/- 2.7 microg/mL), and D-mannose (1.7 +/- 0.7 microg/mL), which were expected to be present by considering the synthetic routes. An FDG mass (0.5 +/- 0.2 microg/mL) was also detected in the preparations. No notable radiochemical impurities, including 2-[18F]fluoro-2-deoxy-D-mannose, were detected in the [18F]FDG preparations. Thus, the levels of several chemical impurities were determined in the [18F]FDG preparations produced by solid-phase 18F-fluorination.

Initial and subsequent approach for the synthesis of 18FDG

2-deoxy-2-[18F] fluoro-D-glucose (18FDG) was developed in 1976 in a collaboration between scientists at the National Institutes of Health, the University of Pennsylvania, and Brookhaven National Laboratory. It was developed for the specific purpose of mapping brain glucose metabolism in living humans, thereby serving as a tool in the basic human neurosciences. With 18FDG it was possible for the first time to measure regional glucose metabolism in the living human brain. Around the same time, the use of 18FDG for studies of myocardial metabolism and as a tracer for tumor metabolism were reported. After the first synthesis of 18FDG via an electrophilic fluorination with 18F gas (produced via the 20Ne(d,alpha)18F reaction), small volume enriched water targets were developed that made it possible to produce large quantities of [18F]fluoride ion via the high-yield 18(p,n)18F reaction. This was followed by a major milestone, the development of a nucleophilic fluorination method that produced 18FDG in very high yield. These advances and the remarkable properties of 18FDG have largely overcome the limitations of the 110-minute half-life of 18F so that 18FDG is now available to most regions of the United States from a number of central production sites. This avoids the need for an on-site cyclotron and chemistry laboratory and has opened up the use of 18FDG to institutions that have a positron emission tomography (PET) scanner (or other imaging device) but no cyclotron or chemistry infrastructure. Currently, 18FDG is used by many hospitals as an off the shelf radiopharmaceutical for clinical diagnosis in heart disease, seizure disorders, and oncology, the area of most rapid growth. However, it remains an important tool in human neuroscience and in drug research and development.

Analysis of residual solvents in 2-[(18)F]FDG by GC

A gas chromatography method has been developed for the measurement of the residual acetone, ethanol and acetonitrile in 2-deoxy-2-[(18)F] fluoro-D-glucose (2-[(18)F]FDG), in accordance with the pending FDA revision on the drug. The detections limits were 0.1 ppm for all three solvents. Good precision and linearity were obtained over ranges spanning the allowable concentration levels proposed by FDA. The amounts of the three solvents in our routine 2-[(18)F]FDG products have been found well below the maximum permissible levels. The method is very amenable to quality control testing for the radiopharmaceutical.

Preparation of fluorine-18 labelled sugars and derivatives and their application as tracer for positron-emission-tomography

The usefulness of 18F-labelled carbohydrates, especially 2-deoxy-2-[18F]fluoro-D-glucose, to study pathophysiological processes in man non-invasively using positron-emission-tomography (PET) led to a widespread investigation of different 18F-labelled sugars and sugar derivatives. In consideration of the short half-life of fluorine-18 (T(1/2) = 110 min) synthetic strategies concerning precursor design, labelling conditions and deprotection of the intermediate compounds were developed to guarantee an efficient high radiochemical yield synthesis for diagnostic purposes. Besides some aspects of medical application of 2-deoxy-2-[18F]fluoro-D-glucose, a few synthetic strategies are described reflecting development work on promising 18F-labelled sugars for diagnostic purposes during the last two decades.

Simple rapid hydrolysis of acetyl protecting groups in the FDG synthesis using cation exchange resins

A new solid phase method for hydrolysis of acetyl groups from the intermediate 2-[18F]fluoro-2-deoxyglucose tetraacetate ([18F]FDG-Ac4) was developed. Fast cleavage occurs with Dowex 50 sulfonic acid resin (H+ form) at approximately 100 degrees C in the absence of bulk water. [18F]FDG-Ac4 reaction mixtures were efficiently converted to neutral aqueous solutions of pure FDG in 10-15 min. The method avoids problems associated with liquid acids and can be easily integrated into existing [18F]FDG procedures.