Fully-automated synthesis of 16β-(18)F-fluoro-5α-dihydrotestosterone (FDHT) on the ELIXYS radiosynthesizer

State of the art procedures towards reactive [18F]fluoride in PET tracer synthesis

BACKGROUND

Positron emission tomography (PET) is a powerful, non-invasive preclinical and clinical nuclear imaging technique used in disease diagnosis and therapy assessment. Fluorine-18 is the predominant radionuclide used for PET tracer synthesis. An impressive variety of new 'late-stage' radiolabeling methodologies for the preparation of 18F-labeled tracers has appeared in order to improve the efficiency of the labeling reaction.

MAIN BODY

Despite these developments, one outstanding challenge into the early key steps of the process remains: the preparation of reactive [18F]fluoride from oxygen-18 enriched water ([18O]H2O). In the last decade, significant changes into the trapping, elution and drying stages have been introduced. This review provides an overview of the strategies and recent developments in the production of reactive [18F]fluoride and its use for radiolabeling.

CONCLUSION

Improved, modified or even completely new fluorine-18 work-up procedures have been developed in the last decade with widespread use in base-sensitive nucleophilic 18F-fluorination reactions. The many promising developments may lead to a few standardized drying methodologies for the routine production of a broad scale of PET tracers.

Direct incorporation of [18F] into Aliphatic Systems: A promising Mn-catalysed Labelling Technique for PET Imaging

BACKGROUND

One of the challenges in positron emission tomography (PET) is labelling complex aliphatic molecules.

OBJECTIVE

This study aimed to develop a method of metal-catalysed radiofluorination that is site-selective and works in moderate to good yields under facile conditions.

METHODS

Herein, we report on the optimisation of an aliphatic C-H to C-18F bond transformation catalysed by a Mn(porphyrin) complex.

RESULTS

The successful oxidation of 11 aliphatic molecules, including progesterone, is reported. Radiochemical Incorporations (RCIs) up to 69% were achieved within 60 min without the need for pre-activation or special equipment.

CONCLUSION

The method features mild conditions (60 °C) and promises to constitute a valuable approach to labelling of biomolecules and drug substances.

18F-labeled anti-human CD20 cys-diabody for same-day immunoPET in a model of aggressive B cell lymphoma in human CD20 transgenic mice

PURPOSE

Metabolic imaging using [¹⁸F]FDG is the current standard for clinical PET; however, some malignancies (e.g., indolent lymphomas) show low avidity for FDG. The majority of B cell lymphomas express CD20, making it a valuable target both for antibody-based therapy and imaging. We previously developed PET tracers based on the humanised anti-CD20 antibody obinutuzumab (GA101). Preclinical studies showed that the smallest bivalent fragment, the cys-diabody (GAcDb, 54.5 kDa) with a peak uptake at 1-2 h post-injection and a biological half-life of 2-5 h, is compatible with short-lived positron emitters such as fluorine-18 (¹⁸F, t_1/2 110 min), enabling same-day imaging.

METHODS

GAcDb was radiolabeled using amine-reactive N-succinimidyl 4-[¹⁸F]-fluorobenzoate ([¹⁸F]SFB), or thiol-reactive N-[2-(4-[¹⁸F]-fluorobenzamido)ethyl]maleimide ([¹⁸F]FBEM) for site-specific conjugation to C-terminal cysteine residues. Both tracers were used for immunoPET imaging of the B cell compartment in human CD20 transgenic mice (hCD20TM). [¹⁸F]FB-GAcDb immunoPET was further evaluated in a disseminated lymphoma (A20-hCD20) syngeneic for hCD20TM and compared to [¹⁸F]FDG PET. Tracer uptake was confirmed by ex vivo biodistribution.

RESULTS

The GAcDb was successfully ¹⁸F-radiolabeled using two different conjugation methods resulting in similar specific activities and without impairing immunoreactivity. Both tracers ([¹⁸F]FB-GAcDb and [¹⁸F]FBEM-GAcDb) specifically target human CD20-expressing B cells in transgenic mice. Fast blood clearance results in high contrast PET images as early as 1 h post injection enabling same-day imaging. [¹⁸F]FB-GAcDb immunoPET detects disseminated lymphoma disease in the context of normal tissue expression of hCD20, with comparable sensitivity as [¹⁸F]FDG PET but with added specificity for the therapeutic target.

CONCLUSIONS

[¹⁸F]FB-GAcDb and [¹⁸F]FBEM-GAcDb could monitor normal B cells and B cell malignancies non-invasively and quantitatively in vivo. In contrast to [¹⁸F]FDG PET, immunoPET provides not only information about the extent of disease but also about presence and localisation of the therapeutic target.

Automation of a Positron-emission Tomography (PET) Radiotracer Synthesis Protocol for Clinical Production

The development of new positron-emission tomography (PET) tracers is enabling researchers and clinicians to image an increasingly wide array of biological targets and processes. However, the increasing number of different tracers creates challenges for their production at radiopharmacies. While historically it has been practical to dedicate a custom-configured radiosynthesizer and hot cell for the repeated production of each individual tracer, it is becoming necessary to change this workflow. Recent commercial radiosynthesizers based on disposable cassettes/kits for each tracer simplify the production of multiple tracers with one set of equipment by eliminating the need for custom tracer-specific modifications. Furthermore, some of these radiosynthesizers enable the operator to develop and optimize their own synthesis protocols in addition to purchasing commercially-available kits. In this protocol, we describe the general procedure for how the manual synthesis of a new PET tracer can be automated on one of these radiosynthesizers and validated for the production of clinical-grade tracers. As an example, we use the ELIXYS radiosynthesizer, a flexible cassette-based radiochemistry tool that can support both PET tracer development efforts, as well as routine clinical probe manufacturing on the same system, to produce [¹⁸F]Clofarabine ([¹⁸F]CFA), a PET tracer to measure in vivo deoxycytidine kinase (dCK) enzyme activity. Translating a manual synthesis involves breaking down the synthetic protocol into basic radiochemistry processes that are then translated into intuitive chemistry "unit operations" supported by the synthesizer software. These operations can then rapidly be converted into an automated synthesis program by assembling them using the drag-and-drop interface. After basic testing, the synthesis and purification procedure may require optimization to achieve the desired yield and purity. Once the desired performance is achieved, a validation of the synthesis is carried out to determine its suitability for the production of the radiotracer for clinical use.

Production of diverse PET probes with limited resources: 24 18F-labeled compounds prepared with a single radiosynthesizer

New radiolabeled probes for positron-emission tomography (PET) are providing an ever-increasing ability to answer diverse research and clinical questions and to facilitate the discovery, development, and clinical use of drugs in patient care. Despite the high equipment and facility costs to produce PET probes, many radiopharmacies and radiochemistry laboratories use a dedicated radiosynthesizer to produce each probe, even if the equipment is idle much of the time, to avoid the challenges of reconfiguring the system fluidics to switch from one probe to another. To meet growing demand, more cost-efficient approaches are being developed, such as radiosynthesizers based on disposable "cassettes," that do not require reconfiguration to switch among probes. However, most cassette-based systems make sacrifices in synthesis complexity or tolerated reaction conditions, and some do not support custom programming, thereby limiting their generality. In contrast, the design of the ELIXYS FLEX/CHEM cassette-based synthesizer supports higher temperatures and pressures than other systems while also facilitating flexible synthesis development. In this paper, the syntheses of 24 known PET probes are adapted to this system to explore the possibility of using a single radiosynthesizer and hot cell for production of a diverse array of compounds with wide-ranging synthesis requirements, alongside synthesis development efforts. Most probes were produced with yields and synthesis times comparable to literature reports, and because hardware modification was unnecessary, it was convenient to frequently switch among probes based on demand. Although our facility supplies probes for preclinical imaging, the same workflow would be applicable in a clinical setting.

Evaluation of Castration-Resistant Prostate Cancer with Androgen Receptor-Axis Imaging

Castration-resistant prostate cancer (CRPC) is the lethal form of prostate cancer, and more than 26,000 men will die from this disease in 2016. The pathophysiology of CRPC is clearly multifactorial, but most often, androgen receptor (AR) upregulation is associated with its earliest beginnings and the AR increase is part of the multimolecular complex including downstream effector proteins linked to AR (AR-axis) responsible for rapid proliferation and malignant features of the malignant cell. In both animal models and patients, glycolysis (Warburg effect) is also an early manifestation of CRPC transformation. At Memorial Sloan Kettering Cancer Center, we have focused our energies on imaging studies of the AR-axis in CRPC, using ¹⁸F-FDG, ¹⁸F-16β-fluoro-5α-dihydrotestosterone (¹⁸F-FDHT), and a variety of radiolabeled antibodies targeting downstream effectors, such as prostate-specific membrane antigen (PSMA). Small-molecular-weight PSMA-targeting agents are not part of this review. In this review, we will focus on molecular imaging of the AR-axis in metastatic CRPC (mCRPC) and discuss our personal experience with these tracers. Our goal is to put these radiopharmaceuticals in the context of mCRPC biology and diagnosis (e.g., ¹⁸F-FDHT).

Fully automated synthesis of [(18) F]fluoro-dihydrotestosterone ([(18) F]FDHT) using the FlexLab module

Imaging of androgen receptor expression in prostate cancer using F-18 FDHT is becoming increasingly popular. With the radiolabelling precursor now commercially available, developing a fully automated synthesis of [(18) F] FDHT is important. We have fully automated the synthesis of F-18 FDHT using the iPhase FlexLab module using only commercially available components. Total synthesis time was 90 min, radiochemical yields were 25-33% (n = 11). Radiochemical purity of the final formulation was > 99% and specific activity was > 18.5 GBq/µmol for all batches. This method can be up-scaled as desired, thus making it possible to study multiple patients in a day. Furthermore, our procedure uses 4 mg of precursor only and is therefore cost-effective. The synthesis has now been validated at Austin Health and is currently used for [(18) F]FDHT studies in patients. We believe that this method can easily adapted by other modules to further widen the availability of [(18) F]FDHT.