An improvement of 11C acetate synthesis--non-radioactive contaminants by irradiation-induced species emanating from the 11C carbon dioxide production target

Development and Optimization of 11C-Labeled Radiotracers: A Review of the Modern Quality Control Design Process

Introduction - Several 11C-tracers have demonstrated high potential in early diagnostic PET imaging applications of neurodegenerative diseases including Alzheimer's and Parkinson's disease. These radiotracers often track critical biomarkers in disease pathogenesis such as tau fibrils ([11C]PBB3) or β-amyloid plaques ([11C]PiB) associated with such diseases. Purpose - The short review aims to serve as a guideline in the future development of radiotracers for students, postdocs and/or new radiochemists who will be synthesizing clinical grade or novel research 11C-tracers, including knowledge of regulatory requirements. We aim to bridge the gap between novel and established 11C-tracer quality control (QC) processes through exploring the design process and regulatory requirements for 11C-pharmaceuticals. Methods - A literature survey was undertaken to identify articles with a detailed description of the QC methodology and characterization for each of the sections of the review. Overview - First a general summary of 11C-tracer production was presented; this was used to establish possible places for contamination or assurances for a sterile final product. The key mandated QC analyses for clinical use were then discussed. Further, we assessed the QC methods used for established 11C-tracers and then reviewed the routine QC tests for preclinical translational and validation studies. Therefore, both mandated QC methods for clinical and preclinical animal studies were reviewed. Last, some examples of optimization and automation were reviewed, and implications of the QC practices associated with such procedures were considered. Conclusion - All of the common QC parameters associated with 11C-tracers under clinical and preclinical settings (along with a few exceptions) were discussed in detail. While it is important to establish standard, peer-reviewed QC testing protocols for a novel 11C-tracer entering the clinical umbrella, equal importance is needed on preclinical applications to address credibility and repeatability for the study.

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

A new concept for the production of 11C-labelled radiotracers

Background

The GMP-compliant production of radiopharmaceuticals has been performed using disposable units (cassettes) with a dedicated synthesis module. To expand this “plug ‘n’ synthesize” principle to a broader scope of modules we developed a pressure controlled setup that offers an alternative to the usual stepper motor controlled rotary valves. The new concept was successfully applied to the synthesis of N-methyl-[¹¹C]choline, L-S-methyl-[¹¹C]methionine and [¹¹C]acetate.

Results

The target gas purification of cyclotron produced [¹¹C]CO₂ and subsequent conversion to [¹¹C]MeI was carried out on a TRACERlab Fx C Pro module. The labelling reactions were controlled with a TRACERlab Fx FE module. With the presented modular principle we were able to produce N-methyl-[¹¹C]choline and L-S-methyl-[¹¹C]methionine by loading a reaction loop with neat N,N'-dimethylaminoethanol (DMAE) or an ethanol/water mixture of NaOH and L-homocysteine (L-HC), respectively and a subsequent reaction with [¹¹C]MeI. After 18 min N-methyl-[¹¹C]choline was isolated with 52% decay corrected yield and a radiochemical purity of > 99%. For L-S-methyl-[¹¹C]methionine the total reaction time was 19 min reaction, yielding 25% of pure product (> 97%). The reactor design was used as an exemplary model for the technically challenging [¹¹C]acetate synthesis. The disposable unit was filled with 1 mL MeMgCl (0.75 M) in tetrahydrofuran (THF) bevore [¹¹C]CO₂ was passed through. After complete release of [¹¹C]CO₂ the reaction mixture was quenched with water and guided through a series of ion exchangers (H⁺, Ag⁺ and OH⁻). The product was retained on a strong anion exchanger, washed with water and finally extracted with saline. The product mixture was acidified and degassed to separate excess [¹¹C]CO₂ before dispensing. Under these conditions the total reaction time was 18 ± 2 min and pure [¹¹C]acetate (n = 10) was isolated with a decay corrected yield of 51 ± 5%.

Conclusion

Herein, we described a novel single use unit for the synthesis of carbon-11 labelled tracers for preclinical and clinical applications of N-methyl-[¹¹C]choline, L-S-methyl-[¹¹C]methionine and [¹¹C]acetate.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-022-00159-y.

Acetuino-A Handy Open-Source Radiochemistry Module for the Preparation of [1-11C]Acetate

Radiosynthesis of [1-¹¹C]acetate is well described in literature, but all syntheses either require adaptations in complex commercial synthesizers or rely on closed-source hardware and software control. Arduino microcontrollers are ideal for the compact, flexible, and inexpensive control of low-complexity hardware, making them particularly suited for radiochemistry where operation in space-limited shielded hot cells is mandatory. We established a [1-¹¹C]acetate radiosynthesis module for combination with a [¹¹C]MeI module available in almost every lab working with ¹¹C. Its small footprint even enables back-to-back production in a hot cell already occupied by other modules. Using this setup, we achieved a reliable and flexible supply of this tracer, with radiochemical yields of 51.4 ± 28.2% and radiochemical purities (RCPs) of 94.4 ± 6.7% ( n = 9) in a synthesis time of 15 minutes. Positron emission tomography (PET) and biodistribution analysis demonstrated low background uptake in healthy mice, with highest uptake in liver and kidneys. Arduino microcontrollers have become valuable and versatile tools in our lab for the automatization of low-complexity procedures not requiring full-blown commercial radiochemistry synthesizers, as showcased here for the production of [1-¹¹C]acetate.

In-house development of an optimized synthetic module for routine [11C]acetate production

[11C]Acetate, a radiotracer for PET imaging, is a promising radiopharmaceutical for overcoming the limitation of 2-deoxy-2-[18F]fluoro-D-glucose in a number of cancers. Here, the optimized automatic synthesis of [11C]acetate using an in-house-developed module under different conditions has been reported for routine production. [11C]CO2 was produced in a 16.4 MeV PETtrace cyclotron, and methyl magnesium chloride was used for synthesis. For product purification, ion-exchange solid-phase extraction cartridges were used, connected in series. High-performance liquid chromatography and gas chromatography were used to measure radiochemical and chemical purity. The Limulus amebocyte lysate test and the fluid thioglycollate medium test were performed for quality control of [11C]acetate. The total reaction time of [11C]acetate was within 15 min, and the overall decay-corrected radiochemical yield was 84.33±8.85%. Radiochemical purity was greater than 98% when evaluated on an analytical high-performance liquid chromatography system. No endotoxins or anaerobic bacteria were seen on quality control checks. Optimized production of [11C]acetate was achieved by the in-house module. Radiochemical and biological properties of the [11C]acetate produced were appropriate for clinical PET study.

Oxidative metabolism of astrocytes is not reduced in hepatic encephalopathy: a PET study with [(11)C]acetate in humans

In patients with impaired liver function and hepatic encephalopathy (HE), consistent elevations of blood ammonia concentration suggest a crucial role in the pathogenesis of HE. Ammonia and acetate are metabolized in brain both primarily in astrocytes. Here, we used dynamic [(11)C]acetate PET of the brain to measure the contribution of astrocytes to the previously observed reduction of brain oxidative metabolism in patients with liver cirrhosis and HE, compared to patients with cirrhosis without HE, and to healthy subjects. We used a new kinetic model to estimate uptake from blood to astrocytes and astrocyte metabolism of [(11)C]acetate. No significant differences of the rate constant of oxidation of [(11)C]acetate (k 3) were found among the three groups of subjects. The net metabolic clearance of [(11)C]acetate from blood was lower in the group of patients with cirrhosis and HE than in the group of healthy subjects (P < 0.05), which we interpret to be an effect of reduced cerebral blood flow rather than a reflection of low [(11)C]acetate metabolism. We conclude that the characteristic decline of whole-brain oxidative metabolism in patients with cirrhosis with HE is not due to malfunction of oxidative metabolism in astrocytes. Thus, the observed decline of brain oxidative metabolism implicates changes of neurons and their energy turnover in patients with HE.

Easy upgrade of the TRACERLab FX C Pro for [¹¹C]carboxylation reactions: application to the routine production of [1-¹¹C]acetate

Carbon-11-labeled acetate ([1-(11)C]acetate) is a radiopharmaceutical of importance in clinical practice as well as in preclinical research in cardiology and oncology. Its preparation is based on the [(11)C]carboxylation reaction of a Grignard reagent with [(11)C]CO2. Most of the commercially available synthesizers are only dedicated to the preparation of [(11)C]methyl iodide (or [(11)C]methyl triflate) for the radiomethylation of an appropriate precursor but not for the direct use of cyclotron-produced [(11)C]CO2. Based on the classical [(11)C]carboxylation reaction and SPE purification, we propose in this technical note a detailed, simple, easy-to-handle and fully reversible modification of the TRACERLab FX C Pro to operate, on demand, [(11)C]carboxylation reactions, exemplified herein by the production of [1-(11)C]acetate, or [(11)C]radiomethylation reactions. This also opens new prospects to other type of radiochemical reactions involving [(11)C]CO2.

Synthesis of oncological [11C]radiopharmaceuticals for clinical PET

Positron emission tomography (PET) is a nuclear medicine modality which provides quantitative images of biological processes in vivo at the molecular level. Several PET radiopharmaceuticals labeled with short-lived isotopes such as (18)F and (11)C were developed in order to trace specific cellular and molecular pathways with the aim of enhancing clinical applications. Among these [(11)C]radiopharmaceuticals are N-[(11)C]methyl-choline ([(11)C]choline), l-(S-methyl-[(11)C])methionine ([(11)C]methionine) and 1-[(11)C]acetate ([(11)C]acetate), which have gained an important role in oncology where the application of 2-[(18)F]fluoro-2-deoxy-d-glucose ([(18)F]FDG) is suboptimal. Nevertheless, the production of these radiopharmaceuticals did not reach the same level of standardization as for [(18)F]FDG synthesis. This review describes the most recent developments in the synthesis of the above-mentioned [(11)C]radiopharmaceuticals aiming to increase the availability and hence the use of [(11)C]choline, [(11)C]methionine and [(11)C]acetate in clinical practice.

Automated production of [11C]acetate and [11C]palmitate using a modified GE Tracerlab FX(C-Pro)

As researchers explore new applications for positron emission tomography radiopharmaceuticals, the demand for effective and readily available radiopharmaceuticals continues to increase. The syntheses of two such radiopharmaceuticals, [(11)C]acetate and [(11)C]palmitate, can be automated on the GE Tracerlab FX(C-Pro) by utilizing Grignard reactions. Radiochemical purities of the [(11)C]acetate and the [(11)C]palmitate products were high (>98% and >99.9%, respectively) with average non-corrected yields of 18% (n = 3) and 10% (n = 5), respectively. These data comprise the validation trials for site qualification of clinical production of both radiopharmaceuticals.

Development of an automated modular system for the synthesis of [11C]acetate

BACKGROUND

Carboxylation reactions offer a straightforward method for the synthesis of carbon-11 labelled carboxylic acids. Among these, the preparation of carbon-11 (C)-acetate is receiving increasing attention because of diagnostic applications in oncology in addition to its well-established use as a probe for myocardial oxidative metabolism. Although a number of dedicated modules are commercially available, the development of the synthesis on flexible platforms would be beneficial to widen the number of tracers, in particular for preclinical assessment and testing.

METHODS

In this study, the carboxylation reaction was implemented for the synthesis of sodium 1-[C]acetate after the classic route of carboxylation of methylmagnesium chloride by [C]carbon dioxide, followed by the acidic hydrolysis, purification and sterile filtration. This was performed using a commercially available kit of preassembled hardware units and fully compatible components of radiochemistry automation (VarioSystem).

RESULTS

The system proved be to highly versatile and inexpensive and allowed a quick translation of the radiochemistry project into a working system even by less experienced personnel, because of predefined interfaces between electronic parts and operating software (preloaded on a laptop and included in the kit). The automatic module proved to be a simple and reliable system for the production of 1-[C]acetate that was prepared in 24 min (total synthesis time) with stable radiochemical yields (20% nondecay corrected) and high radiochemical purity (>97%). The module is used routinely to produce 1-[C]acetate for preclinical studies and is being implemented for the production of the labelled fatty acids.

Incidental finding of an 11C-acetate PET-positive multiple myeloma

Multiple myeloma is a malignancy of plasma cells. The (18)F-FDG PET findings of multiple myeloma have been reported previously. However, the (11)C-acetate PET findings have not been clarified. Here, we report a case of multiple myeloma detected with (11)C-acetate PET in a 51-year-old male patient with known hepatocellular carcinoma. The patient was admitted for management of a pathologic fracture of the right tibia. Imaging workup including X-ray, magnetic resonance image, bone scintigraphy; (18)F-FDG led to a suspicion of metastatic bony lesions. Further, these lesions showed increased uptake on (11)C-acetate PET. Wide excision of the right tibia was performed, and histopathological examination of the lesion confirmed multiple myeloma. This case illustrates the characteristic (11)C-acetate PET findings of multiple myeloma.

A prospective evaluation of 18F-FDG and 11C-acetate PET/CT for detection of primary and metastatic hepatocellular carcinoma

Because (18)F-FDG PET has insufficient sensitivity for the detection of hepatocellular carcinoma (HCC), (11)C-acetate PET has been proposed as another technique for this use. We prospectively evaluated the value of PET/CT using these 2 tracers for the detection of primary and metastatic HCC.

METHODS

One hundred twelve patients (99 with HCC, 13 with cholangiocellular carcinoma) underwent biopsy and (18)F-FDG and (11)C-acetate PET/CT.

RESULTS

The overall sensitivities of (18)F-FDG, (11)C-acetate, and dual-tracer PET/CT in the detection of 110 lesions in 90 patients with primary HCC were 60.9%, 75.4%, and 82.7%, respectively. Elevated serum alpha-fetoprotein levels, an advanced tumor stage, portal vein tumor thrombosis, large tumors, and multiple tumors were significantly associated with positive (18)F-FDG PET/CT results. Uptake of (11)C-acetate was associated with large and multiple tumors. For (18)F-FDG, the sensitivities according to tumor size (1-2, 2-5, and >/=5 cm) were 27.2%, 47.8%, and 92.8%, respectively; for (11)C-acetate, these respective values were 31.8%, 78.2%, and 95.2%. (18)F-FDG was more sensitive in the detection of poorly differentiated HCC. Overall survival was lower in patients with (18)F-FDG PET/CT positive for all indexed lesions than in those with FDG negative or partially positive through the entire follow-up period. In analysis based on biopsied lesions, the sensitivity of (18)F-FDG PET/CT was 64.4% for primary HCC and 84.4% for (11)C-acetate PET/CT. The overall sensitivities of (18)F-FDG, (11)C-acetate, and dual-tracer PET/CT for 35 metastatic HCCs were 85.7%, 77.0%, and 85.7%, respectively. There was no significant difference in the sensitivity of tracers according to metastatic tumor size, location, or differentiation.

CONCLUSION

The addition of (11)C-acetate to (18)F-FDG PET/CT increases the overall sensitivity for the detection of primary HCC but not for the detection of extrahepatic metastases. (18)F-FDG, (11)C-acetate, and dual-tracer PET/CT have a low sensitivity for the detection of small primary HCC, but (18)F-FDG PET/CT has a relatively high sensitivity for the detection of extrahepatic metastases of HCC.

18F-choline and/or 11C-acetate positron emission tomography: detection of residual or progressive subclinical disease at very low prostate-specific antigen values (<1 ng/mL) after radical prostatectomy

OBJECTIVES

To assess the value of positron emission tomography (PET)/computed tomography (CT) with either (18)F-choline and/or (11)C-acetate, of residual or recurrent tumour after radical prostatectomy (RP) in patients with a prostate-specific antigen (PSA) level of <1 ng/mL and referred for adjuvant or salvage radiotherapy.

PATIENTS AND METHODS

In all, 22 PET/CT studies were performed, 11 with (18)F-choline (group A) and 11 with (11)C-acetate (group B), in 20 consecutive patients (two undergoing PET/CT scans with both tracers). The median (range) PSA level before PET/CT was 0.33 (0.08-0.76) ng/mL. Endorectal-coil magnetic resonance imaging (MRI) was used in 18 patients. Nineteen patients were eligible for evaluation of biochemical response after salvage radiotherapy.

RESULTS

There was abnormal local tracer uptake in five and six patients in group A and B, respectively. Except for a single positive obturator lymph node, there was no other site of metastasis. In the two patients evaluated with both tracers there was no pathological uptake. Endorectal MRI was locally positive in 15 of 18 patients; 12 of 19 responded with a marked decrease in PSA level (half or more from baseline) 6 months after salvage radiotherapy.

CONCLUSIONS

Although (18)F-choline and (11)C-acetate PET/CT studies succeeded in detecting local residual or recurrent disease in about half the patients with PSA levels of <1 ng/mL after RP, these studies cannot yet be recommended as a standard diagnostic tool for early relapse or suspicion of subclinical minimally persistent disease after surgery. Endorectal MRI might be more helpful, especially in patients with a low likelihood of distant metastases. Nevertheless, further research with (18)F-choline and/or (11)C-acetate PET with optimal spatial resolution might be needed for patients with a high risk of distant relapse after RP even at low PSA values.

A simple Tracerlab module modification for automated on-column [11C]methylation and [11C]carboxylation

A modification of commercial [11C]methylation module which can be implemented for both on-column [11C]methylation and [11C]carboxylation in the same automated system is described. This module configuration was applied to the solid-phase synthesis of N-[11C]methyl-choline ([11C]choline) and L-(S-methyl-[11C])methionine ([11C]methionine), using [11C]CH(3)I as methylating agent, as well as to the synthesis of [11C]acetate by [11C]carboxylation with [11C]CO2 of methylmagnesium chloride with high and reproducible radiochemical yields in short reaction time, demonstrating to be a fast and reliable tool for the production of these [11C]radiopharmaceuticals for clinical use.

Captive solvent [11C]acetate synthesis in GMP conditions

Reliable procedure for the production of 1-[(11)C]acetate in GMP conditions was developed based on a combination of the captive-solvent Grignard reaction conducted in the sterile catheter followed by the convenient solid-phase extraction purification on a series of ion-exchange cartridges. The described procedure proved to be reliable in more than 30 patient productions. The process provides stable radiochemical yields (65% EOB) of sodium acetate (1-[(11)C]) of the Ph.Eur. quality (radiochemical purity better than 95%) in a short time (5 min).

(11)C-acetate PET in the early evaluation of prostate cancer recurrence

PURPOSE

The first aim of the study was to investigate the diagnostic potential of (11)C-acetate PET in the early detection of prostate cancer recurrence. A second aim was the evaluation of early and late PET in this context.

METHODS

The study population comprised 32 prostate cancer patients with early evidence of relapse after initial radiotherapy (group A) or radical surgery (group B). The median PSA of group A (n=17) patients was 6 ng/ml (range 2.6-30.2) while that of group B (n=15) was 0.4 ng/ml (range 0.08-4.8). Pelvic-abdominal-thoracic PET was started 2 min after injection of (11)C-acetate and evaluated after fusion with CT.

RESULTS

Group A: Taking a SUV(max)> or =2 as the cut-off, PET showed local recurrences in 14/17 patients and two equivocal results. Distant disease was observed in six patients and an equivocal result was obtained in one. Endorectal MRI was positive in 12/12 patients. Biopsy confirmed local recurrence in six of six (100%) patients. PET was positive in five of the six patients with biopsy-proven recurrences, the result in the remaining patient being equivocal. Group B: Among the 15 patients, visual interpretation was positive for local recurrences in five patients and equivocal in four. One obturator lymph node was positive. Endorectal MRI was positive in 11/15 patients and equivocal in two. Positional correlation of positive/equivocal results on PET and endorectal MRI was observed in seven of nine patients. PSA decreased significantly after salvage radiotherapy in 8/14 patients, providing strong evidence for local recurrence. PET of the eight patients responding to RT was positive in three and equivocal in two.

CONCLUSION

(11)C-acetate PET was found to be valuable in the early evaluation of prostate cancer relapse. Optimising scanning time and use of modern PET-CT equipment might allow further improvement.

New aspects on the preparation of [11C]acetate--a simple and fast approach via distillation

[11C]Acetate, initially developed for nuclear cardiology has gained increased interest also for oncological problems. A conjoint problem of all preparation methods is the high sensitivity of the Grignard-precursor to moisture, demanding long cleaning and drying procedures of apparatus and reaction vials. Our rationale was to simplify and accelerate the preparation of [11C]acetate by the development of an inert, sterile and disposable system. The present publication deals with the remote-controlled preparation of [11C]acetate via distillation into a buffer ready to use.