Light-Driven Radiochemistry with Fluorine-18, Carbon-11 and Zirconium-89.

This review discusses recent advances in light-driven radiochemistry for three key isotopes: fluorine-18, carbon-11, and zirconium-89, and their applications in positron emission tomography (PET). In the case of fluorine-18, the predominant approach involves the use of cyclotron-produced [18F]fluoride or reagents derived thereof. Light serves to activate either the substrate or the fluorine-18 labeled reagent. Advancements in carbon-11 photo-mediated radiochemistry have been leveraged for the radiolabeling of small molecules, achieving various transformations, including 11C-methylation, 11C-carboxylation, 11C-carbonylation, and 11C-cyanation. Contrastingly, zirconium-89 photo-mediated radiochemistry differs from fluorine-18 and carbon-11 approaches. In these cases, light facilitates a postlabeling click reaction, which has proven valuable for the labeling of large biomolecules such as monoclonal antibodies (mAbs). New technological developments, such as the incorporation of photoreactors in commercial radiosynthesizers, illustrate the commitment the field is making in embracing photochemistry. Taken together, these advances in photo-mediated radiochemistry enable radiochemists to apply new retrosynthetic strategies in accessing novel PET radiotracers.

Automated radiosynthesis of [18 F]CETO, a PET radiotracer for imaging adrenal glands, on Synthra RNplus.

Primary aldosteronism (PA) is the leading secondary cause of hypertension. Determining whether one (unilateral) or both (bilateral) adrenal glands are the source of PA in a patient remains challenging, and yet it is a critical step in the decision whether to recommend potentially curative surgery (adrenalectomy) or lifelong medical therapy (typically requiring multiple drugs). Recently, we have developed a fluorine-18 radiopharmaceutical [18 F]CETO to permit greater access to PA molecular imaging. Herein, we report an automated synthesis of this radiotracer. To manufacture the radiopharmaceutical routinely for clinical PET studies, we implemented an automated radiosynthesis method on a Synthra RNplus© synthesiser for which Cl-tosyletomidate was used as the precursor for radiolabelling via nucleophilic [18 F]fluorination. [18 F]CETO was produced with 35 ± 1% (n = 7), decay corrected and 25 ± 4% (n = 7) non-decay corrected radiochemical yield with molar activities ranging from 150 to 400 GBq/µmol. The GMP compliant manufacturing process produces a sterile formulated [18 F]CETO injectable solution for human use as demonstrated by the results of quality control. Automation of the radiosynthesis of [18 F]CETO should facilitate uptake by other adrenal centres and increase access to molecular imaging in PA.

Fully automated dual-run manufacturing of [11C]PIB on FASTlab.

This report describes an updated, fully automated method for the production of [11C]PIB on a cassette-based automated synthesis module. The method allows for two separate productions of [11C]PIB, both of which meet all specification for use in clinical studies. The GE FASTlab developer system was used to create the cassette design as well as the controlling tracer package. The method takes 16 min from the delivery of [11C]MeOTf to the FASTlab, or 35 min from the End of Bombardment; and reliably produces 3547 ± 586 MBq of [11C]PIB in high radiochemical purity (> 98 %). This methodology increases the production capacity of radiopharmaceutical facilities for [11C]PIB, and can easily produce 4 batches in a single day with limited infrastructure footprint.

Production, isolation, and shipment of clinically relevant quantities of astatine-211: A simple and efficient approach to increasing supply.

The alpha emitter astatine-211 (211At) is a promising candidate for cancer treatment based on Targeted Alpha (α) Therapy (TAT). A small number of facilities, distributed across the United States, are capable of accelerating α-particle beams to produce 211At. However, challenges remain regarding strategic methods for shipping 211At in a form adaptable to advanced radiochemistry reactions and other uses of the radioisotope. PURPOSE: Our method allows shipment of 211At in various quantities in a form convenient for further radiochemistry. PROCEDURES: For this study, a 3-octanone impregnated Amberchrom CG300M resin bed in a column cartridge was used to separate 211At from the bismuth matrix on site at the production accelerator (Texas A&M) in preparation for shipping. Aliquots of 6 M HNO3 containing up to ≈2.22 GBq of 211At from the dissolved target were successfully loaded and retained on columns. Exempt packages (<370 MBq) were shipped to a destination radiochemistry facility, University of Texas MD Anderson Cancer Center, in the form of a convenient air-dried column. Type A packages have been shipped overnight to University of Alabama at Birmingham. MAIN FINDINGS: Air-dried column hold times of various lengths did not inhibit simple and efficient recovery of 211At. Solution eluted from the column was sufficiently high in specific activity to successfully radiolabel a model compound, 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (1), with 211At. The method to prepare and ship 211At described in this manuscript has also been used to ship larger quantities of 211At a greater distance to University of Alabama at Birmingham. PRINCIPAL CONCLUSIONS: The successful proof of this method paves the way for the distribution of 211At from Texas A&M University to research institutions and clinical oncology centers in Texas and elsewhere. Use of this simple method at other facilities has the potential increase the overall availability of 211At for preclinical and clinical studies.

Alpha-induced production and robust radiochemical separation of 43Sc as an emerging radiometal for formulation of PET radiopharmaceuticals.

Scandium-43 is an emerging PET radiometal that was produced by α-particle bombardment on natural CaCO3 target via natCa (α,p) 43Sc and natCa (α,n) 43Ti→43Sc reactions using K-130 cyclotron at VECC. A robust radiochemical procedure based on selective precipitation of 43Sc as Sc(OH)3 was developed for separation of the radioisotope from the irradiated target. The overall yield of the separation process was >85% and it was obtained in a form suitable for preparation of target specific radiopharmaceuticals for PET imaging of cancer.

Cyclotron production of 103Pd using a liquid target.

INTRODUCTION: In this work, we present the first feasibility study on the production of the medically important radionuclide 103Pd via the 103Rh(p,n)103Pd reaction by cyclotron irradiation of a liquid target. Using a liquid target removes the time consuming and complex dissolution process of rhodium post-irradiation due to its chemically inactive nature and thereby will improve the accessibility of this radioisotope. METHODS: Liquid targets made from Rh(NO3)3·×H2O salt dissolved in de-ionized water were irradiated using a 12 MeV beam at the TR13 cyclotron at TRIUMF, Vancouver. RESULTS: A maximum EOB activity of 1.03 ± 0.05 MBq was achieved with the tested conditions, sufficient for basic radiochemistry studies. An effective separation method using anion exchange chromatography is reported using 1 M HNO3 as an eluent for rhodium (90.1 ± 2.1 % recovery) and a 1:1 mixture of 0.5 M NH3 + NH4Cl palladium eluent (103.8 ± 2.3 % recovery). The solution showed good in-target pressure stability. However, the production efficiency decreased significantly with higher solution concentrations and irradiation lengths which puts into question the scaling potential of this method. CONCLUSION: This proof-of-concept study has demonstrated the potential for using liquid targets as complementary production method of 103Pd for research purposes. The liquid target route faces several scaling challenges but can nonetheless improve the availability of 103Pd and consequently aid in widening its utility for radiopharmaceuticals.

Recent Developments in Carbon-11 Chemistry and Applications for First-In-Human PET Studies.

Positron emission tomography (PET) is a molecular imaging technique that makes use of radiolabelled molecules for in vivo evaluation. Carbon-11 is a frequently used radionuclide for the labelling of small molecule PET tracers and can be incorporated into organic molecules without changing their physicochemical properties. While the short half-life of carbon-11 (11C; t½ = 20.4 min) offers other advantages for imaging including multiple PET scans in the same subject on the same day, its use is limited to facilities that have an on-site cyclotron, and the radiochemical transformations are consequently more restrictive. Many researchers have embraced this challenge by discovering novel carbon-11 radiolabelling methodologies to broaden the synthetic versatility of this radionuclide. This review presents new carbon-11 building blocks and radiochemical transformations as well as PET tracers that have advanced to first-in-human studies over the past five years.

High-yield cyclotron production of 203Pb using a sealed 205Tl solid target.

INTRODUCTION: 203Pb (t1/2 = 51.9 h, 279 keV (81 %)) is a diagnostic SPECT imaging radionuclide ideally suited for theranostic applications in combination with 212Pb for targeted alpha particle therapy. Our objectives were to develop a high-yield solid target 203Pb cyclotron production route using isotopically enriched 205Tl target material and the 205Tl(p,3n)203Pb reaction as an alternative to lower energy production via the 203Tl(p,n)203Pb reaction. METHODS: 250 mg 205Tl metal (99.9 % isotopic enrichment) was pressed using a hardened stainless steel die. Aluminum target discs were machined with a central depression and annulus groove. The flattened 205Tl pellet was placed into the central depression of the Al disc and a circle of indium wire was laid in the machined annulus surrounding the pellet. An aluminum foil cover was then pressed onto the target disc to create an airtight bond. Targets were irradiated at 23.3 MeV for up to 516 min on a TR-24 cyclotron at currents up to 60 µA to produce 203Pb via the 205Tl(p,3n)203Pb nuclear reaction. Following a cool-down period of >12 h, the target was removed and 205Tl dissolved in 4 M HNO3. A NEPTIS Mosaic-LC synthesis unit performed automated separation using Eichrom Pb resin, and 203Pb was eluted using 8 M HCl or 1 M NH4OAc. 205Tl was diverted to a vial for recovery in an electrolytic cell. 203Pb product radionuclidic purity was assessed by HPGe gamma spectroscopy, while elemental purity was assessed by ICP-OES. Radiolabeling and stability studies were performed with PSC, TCMC, and DOTA chelators, and 203Pb incorporation was verified by radio-TLC analysis. RESULTS: Cyclotron irradiations performed at 60 µA proton beam current and 23.3 MeV (205Tl incident energy) had a 203Pb saturated yield of 4658 ± 62 MBq/µA (n = 3). Automated NEPTIS separation took <4 h from the start of target dissolution to product elution, yielding >85 % decay-corrected [203Pb]PbCl2 with a radionuclidic purity of >99.9 %. Purified [203Pb]PbCl2 yields of up to 12 GBq 203Pb were attained (15.8 GBq at EOB). The [203Pb]PbCl2 and [203Pb]Pb(OAc)2 products contained no detectable radionuclidic impurities besides 201Pb (<0.1 %), and <0.4 ppm stable Pb. 205Tl metal was recovered with a 92 % batch yield. Aliquots of 100 µL [203Pb]Pb(OAc)2 were used for radiolabeling PSC-Bn-NCS, TCMC-NCS, and DOTA-NCS chelators at pH 4.5 and 22 °C for 30 min, with maximum respective molar activities of 461 ± 30 GBq/µmol, 195 ± 37 GBq/µmol, and 83 ± 12 GBq/µmol. PSC, TCMC, and DOTA chelators exhibited >99.9 % incorporation after a 120-hour incubation in human serum at 37 °C. CONCLUSIONS: Nuclear medicine centers with access to higher energy cyclotrons can produce large 203Pb activities sufficient for clinical applications, with a convenient separation technique producing highly pure [203Pb]PbCl2 or [203Pb]Pb(OAc)2 for direct radiolabeling. This represents an attractive route to produce 203Pb for diagnostic SPECT imaging alongside 212Pb targeted alpha particle therapy. ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: Our high-yield 203Pb production technique significantly enhances 203Pb production capabilities to meet the growing preclinical and clinical demand for 203Pb radiopharmaceuticals alongside 212Pb target alpha particle therapy.

An optimized radiosynthesis of [18 F]DK222, a PET radiotracer for imaging PD-L1.

A radiochemical synthesis of [18 F]DK222, a peptide binder of programmed death ligand 1 protein, suitable for human PET studies is described, and results from validation productions are presented. The high specific activity radiotracer product is prepared as a sterile, apyrogenic solution that conforms to current Good Manufacturing Practice (cGMP) requirements. In addition, the production is extended to use a commercial synthesizer platform (General Electric FASTlab 2).

An Investigation into the In Vitro Metabolic Stability of Aryl Sulfonyl Fluorides for their Application in Medicinal Chemistry and Radiochemistry.

Molecules that feature a sulfonyl fluoride (SO2F) moiety have been gaining increasing interest due to their unique reactivity and potential applications in synthetic chemistry, medicinal chemistry, and other biological uses. A particular interest is towards 18F-radiochemistry where sulfonyl fluorides can be used as a method to radiolabel biomolecules or can be used as radiofluoride relay reagents that facilitate radiolabeling of other molecules. The low metabolic stability of sulfonyl fluoride S-F bonds, however, presents an issue and limits the applicability of sulfonyl fluorides. The aim of this work was to increase understanding of what features contribute to the metabolic instability of the S-F bond in model aryl sulfonyl fluorides and identify approaches to increasing sulfonyl fluoride stability for 18F-radiochemistry and other medicinal, synthetic chemistry and biological applications. To undertake this, 14 model aryl sulfonyl fluorides compounds with varying functional groups and substitution patterns were investigated, and their stabilities were examined in various media, including phosphate-buffered saline and rat serum as a model for biological conditions. The results indicate that both electronic and steric factors affect the stability of the S-F bond, with the 2,4,6-trisubstituted model aryl sulfonyl fluorides examined displaying the highest in vitro metabolic stability.

An Experimental Generator for Production of High-Purity 212Pb for Use in Radiopharmaceuticals.

The feasibility, performance, and radiation safety of an experimental generator were evaluated to efficiently produce 212Pb intended for radiopharmaceuticals. Methods: The generator consisted of a flask with a removable cap containing a source of 224Ra or 228Th absorbed on quartz wool. Gaseous 220Rn emanated from the decaying source, which subsequently decayed to 212Pb, which was adsorbed on the flask's interior surface. The 212Pb was collected by washing the flask with 0.5-1 mL of 0.1 M HCl. Results: The generator collector flask trapped 62%-68% of the 212Pb, of which more than 87% (tested up to 26 MBq) could be harvested. The obtained 212Pb solution had a high purity (>99.98%) and could be used for the preparation of radioconjugates with more than 97% radiochemical purity. Future designs of the generator should aim to further reduce the risk of radon and γ-energy exposure to operators. Conclusion: The presented technology is a promising method for easy and convenient 212Pb production.

Next Generation Copper Mediators for the Efficient Production of 18 F-Labeled Aromatics.

Cu-mediated radiofluorination is a versatile tool for the preparation of 18 F-labeled (hetero)aromatics. In this work, we systematically evaluated a series of complexes and identified several generally applicable mediators for highly efficient radiofluorination of aryl boronic and stannyl substrates. Utilization of these mediators in nBuOH/DMI or DMI significantly improved 18 F-labeling yields despite use of lower precursor amounts. Impressively, application of 2.5 µmol aryl boronic acids was sufficient to achieve 18 F-labeling yields of up to 75 %. The practicality of the novel mediators was demonstrated by efficient production of five PET-tracers and transfer of the method to an automated radiosynthesis module. In addition, (S)-3-[18 F]FPhe and 6-[18 F]FDOPA were prepared in activity yields of 23±1 % and 30±3 % using only 2.5 µmol of the corresponding boronic acid or trimethylstannyl precursor.

Production of medical isotope 68Ge based on a novel chromatography separation technique and assembling of 68Ge/68Ga generator.

A double-column chromatography separation technique was involved for isolation of 68Ge from a bombarded Ga-Ni alloy target. About 185 MBq 68Ge obtained was used for assembling SnO2-based 68Ge/68Ga generator. Approximately 70% of 68Ga in high radioactivity concentration was eluted from generator with excellent radionuclidic, radiochemical and chemical purity. 68Ga was quite adequate for radiolabeling with DOTATATE or PSMA-617 with a high labelling efficiency of >92%. The double-column chromatography technique possessed a potential application prospect of 68Ge/68Ga production, aiding the development of 68Ga in nuclear medicine.

Rapid Purification and Formulation of Radiopharmaceuticals via Thin-Layer Chromatography.

Before formulating radiopharmaceuticals for injection, it is necessary to remove various impurities via purification. Conventional synthesis methods involve relatively large quantities of reagents, requiring high-resolution and high-capacity chromatographic methods (e.g., semi-preparative radio-HPLC) to ensure adequate purity of the radiopharmaceutical. Due to the use of organic solvents during purification, additional processing is needed to reformulate the radiopharmaceutical into an injectable buffer. Recent developments in microscale radiosynthesis have made it possible to synthesize radiopharmaceuticals with vastly reduced reagent masses, minimizing impurities. This enables purification with lower-capacity methods, such as analytical HPLC, with a reduction of purification time and volume (that shortens downstream re-formulation). Still, the need for a bulky and expensive HPLC system undermines many of the advantages of microfluidics. This study demonstrates the feasibility of using radio-TLC for the purification of radiopharmaceuticals. This technique combines high-performance (high-resolution, high-speed separation) with the advantages of a compact and low-cost setup. A further advantage is that no downstream re-formulation step is needed. Production and purification of clinical scale batches of [18F]PBR-06 and [18F]Fallypride are demonstrated with high yield, purity, and specific activity. Automating this radio-TLC method could provide an attractive solution for the purification step in microscale radiochemistry systems.

Automated Optimized Synthesis of [18F]FLT Using Non-Basic Phase-Transfer Catalyst with Reduced Precursor Amount.

3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT) is a positron emission tomography (PET) tracer useful for tumor proliferation assessment for a number of cancers, particularly in the cases of brain, lung, and breast tumors. At present [18F], FLT is commonly prepared by means of the nucleophilic radiofluorination of 3-N-Boc-5'-O-DMT-3'-O-nosyl thymidine precursor in the presence of a phase-transfer catalyst, followed by an acidic hydrolysis. To achieve high radiochemical yield, relatively large amounts of precursor (20−40 mg) are commonly used, leading to difficulties during purification steps, especially if a solid-phase extraction (SPE) approach is attempted. The present study describes an efficient method for [18F]FLT synthesis, employing tetrabutyl ammonium tosylate as a non-basic phase-transfer catalyst, with a greatly reduced amount of precursor employed. With a reduction of the precursor amount contributing to lower amounts of synthesis by-products in the reaction mixture, an SPE purification procedure using only two commercially available cartridges­OASIS HLB 6cc and Sep-Pak Alumina N Plus Light­has been developed for use on the GE TRACERlab FX N Pro synthesis module. [18F]FLT was obtained in radiochemical yield of 16 ± 2% (decay-corrected) and radiochemical purity >99% with synthesis time not exceeding 55 min. The product was formulated in 16 mL of normal saline with 5% ethanol (v/v). The amounts of chemical impurities and residual solvents were within the limits established by European Pharmacopoeia. The procedure described compares favorably with previously reported methods due to simplified automation, cheaper and more accessible consumables, and a significant reduction in the consumption of an expensive precursor.

Solid-phase isotope harvesting of 88Zr from a radioactive ion beam facility.

During routine operation of the Facility for Rare Isotope Beams (FRIB), radionuclides will accumulate in both the aqueous beam dump and along the beamline in the process of beam purification. These byproduct radionuclides, many of which are far from stability, can be collected and purified for use in other scientific applications in a process called isotope harvesting. In this work, the viability of 88Zr harvesting from solid components was investigated at the National Superconducting Cyclotron Laboratory. A secondary 88Zr beam was stopped in a series of collectors comprised of Al, Cu, W, and Au foils. This work details irradiation of the collector foils and the subsequent radiochemical processing to isolate the deposited 88Zr (and its daughter 88Y) from them. Total average recovery from the Al, Cu, and Au collector foils was (91.3 ± 8.9) % for 88Zr and (95.0 ± 5.8) % for 88Y, respectively, which is over three times higher recovery than in a previous aqueous-phase harvesting experiment. The utility of solid-phase isotope harvesting to access elements such as Zr that readily hydrolyze in near-neutral pH aqueous conditions has been demonstrated for application to harvesting from solid components at FRIB.

Radiosynthesis and validation of [18 F]fluoroestradiol in a Synthra plus research platform for use in routine clinical practice.

In this practitioner protocol, the optimization of the radiochemical synthesis of [18 F]fluoroestradiol (FES) on the Synthra RNplus research automated platform is described in detail and a quality control (QC) summary of three validation productions is presented. In comparison with published synthesis methods developed on other platforms, the yield was considerably improved (40%-45% ndc). The other important improvement is the reduction of the required concentration of H2 SO4 avoiding the production of high concentrations of acidic vapors that can deteriorate the module. Purification was achieved by solid phase extraction, and the required adaptation of an external heating plate to the module to evaporate the ethanol is also described. The product was obtained with high radiochemical purity and fulfilled all the requirements of current Good Manufacturing Practice (cGMP). The final product is formulated as a sterile, pyrogen-free solution suitable for human injection. To the best of our knowledge, this is the first report of FES production using this type of module.

An improved radiosynthesis of [18 F]FAraG, a PET radiotracer for imaging T-cell activation.

In this concise practitioner protocol, the radiochemical synthesis of 2'-deoxy-2'-[18 F]fluoro-9-ß-d-arabinofuranosylguanine ([18 F]FAraG) suitable for human positron emission tomography (PET) studies is described and the results from validation productions are presented. The high specific activity (sometimes referred to as molar activity) radiotracer product is prepared as a sterile, apyrogenic solution that conforms to current Good Manufacturing Practice (cGMP) requirements established by the U.S. Food and Drug Administration.

Good manufacturing procedure production of [18 F]SynVesT-1, a radioligand for in vivo positron emission tomography imaging of synaptic vesicle glycoprotein 2A.

[18 F]SynVesT-1 (also known as [18 F]SDM-8 or [18 F]MNI-1126) is a potent and selective synaptic vesicle glycoprotein 2 (SV2A) positron emission tomography (PET) imaging agent. In order to fulfill the increasing clinical demand of an 18 F-labeled SV2A PET ligand, we have developed a fully automated procedure to provide a sterile and pyrogen-free good manufacturing procedure (GMP)-compliant product of [18 F]SynVesT-1 suitable for clinical studies in humans. [18 F]SynVesT-1 is synthesized via a rapid copper-mediated radiofluorination protocol. The procedure was developed and established on a commercially available module, TracerMaker (ScanSys Laboratorieteknik ApS, Copenhagen, Denmark), a synthesis platform originally developed to conduct carbon-11 radiochemistry. From ~130 GBq (end-of-bombardment), our newly developed procedure enabled us to prepare [18 F]SynVesT-1 in an isolated radioactivity yield of 14,220 ± 800 MBq (n = 3), which corresponds to a radiochemical yield (RCY) of 19.5 ± 0.5%. The radiochemical purity (RCP) and enantiomeric purity of each of the final formulated batches exceeded 98%. The overall synthesis time was 90 min and the molar activity was 330 ± 60 GBq/µmol (8.9 ± 1.6 Ci/µmol). The produced [18 F]SynVesT-1 was stable over 8 h at room temperature and is suitable for in vivo PET imaging studies in human subjects.