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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2023)

Positron emission tomography (PET) is a widely-used nuclear medicine imaging technique for assessing biodistribution of drugs, diagnosing diseases, and monitoring therapy response. The rapid development of new PET tracers in both research and clinical applications (to image new targets) demands new advances in radiolabeling techniques to facilitate the frequent production of diverse tracers. Recent developments in droplet-based radiochemistry have shrunk reaction volumes 100x (i.e. to <10 µL), offering advantages like minimal reagent use, rapid synthesis, high yields, and increased molar activity; they also enable high-throughput optimization and scalable production, with great potential to revolutionize radiopharmaceutical production. While our group has successfully utilized microdroplet reactors (a small Teflon-coated silicon chip containing hydrophilic reaction sites) for multiple different radiopharmaceuticals, exploration into metal-mediated radiosynthesis remains limited, primarily due to concerns about the sensitivity of metal reagents to environmental moisture in droplet-based reactors. As a proof-of-concept, I conducted the first microscale copper (Cu)-mediated synthesis of [18F]FDOPA (a clinical PET probe used for imaging dopaminergic function). Substantial enhancement in yield and time was achieved while utilizing only nanomole quantities of precursors and other reagents. Later, I explored the versatility of this method in optimizing additional tracers employing similar Cu-mediated 18F-radiolabeling routes on a high-throughput microdroplet reactor. For example, across 5 days, I conducted 117 reactions, exploring 36 conditions with <15 mg of precursor, and achieved 12x yield improvement for a novel monoacylglycerol lipase (MAGL) probe ([18F]YH149). Leveraging an automated robotic platform for high-throughput studies, we optimized the production of [18F]FBnTP, a potentiometric radiopharmaceutical, with 64 simultaneous droplet reactions in one morning. In addition, on the technology side, many researchers have wondered whether the droplet-based optimized conditions can guide reaction conditions in conventional vial-based reactors, and I demonstrated for the first time that this indeed can be done. This suggests a rapid and economical approach for novel tracer development, i.e., optimizing radiochemistry on a high-throughput microdroplet platform (rapidly, with minimal reagents) and then performing straightforward translation to vial-based systems to enable wider applicability to the existing install base of radiosynthesizer technology. Furthermore, to assess the adaptability of droplet-based radiochemistry in handling exceptionally complex syntheses, I undertook the investigation of a highly intricate three-step radiosynthesis of [18F]FMAU (imaging cell proliferation), encompassing radiofluorination, coupling, and deprotection reactions all within a microdroplet reactor. Compared the lengthy (~150 min) and low-yielding conventional production, the microdroplet-based radiosynthesis of [18F]FMAU provided significant improvement, completing the production in <60 min and achieving >2x higher radiochemical yield and >3x activity yield, while consuming 34-200x less reagents. Moreover, to establish the clinical relevance of droplet-based radiochemistry, we developed various droplet-based scale-up approaches including (i) iteratively loading and evaporating [18F]fluoride aliquots in a single droplet reaction, (ii) pre-concentrating [18F]fluoride in a miniature cartridge compatible with a single reaction site, and (iii) pooling multiple droplet reactions for on-demand dose. These methods, validated for reliability and versatility, successfully delivered clinically-relevant doses of [18F]FET (an amino acid tracer), [18F]Florbetaben (an amyloid imaging agent), [18F]FBnTP, isotopic exchange fluorinated compounds, and aluminum-[18F]fluoride probes. Apart from droplet-based radiosynthesis techniques, I also pursued other novel radiochemistry systems. I helped to develop a platform for microvolume reactions, featuring a pipettor on an XYZ motion gantry and a disposable cassette with integrated micro-vial. The versatile setup performs operations like trapping/releasing [18F]fluoride, liquid transfers, and lid installation/removal for reactor. Comprehensive experiments have been conducted to characterize the system and demonstrate the radiosynthesis feasibility, using [18F]Fallypride as an example. I also helped develop a novel electrochemical radiofluorination (ECRF) technique using a spilt bipolar electrode (s-BPE) for electron-rich compounds such as thioether derivatives. Unlike traditional ECRF which requires high salt concentration, this s-BPE system, with its dual conductive materials, facilitates anodic and cathodic reactions at lower salt concentrations. We achieved a 5x increase in molar activity for [18F]fluoromethyl (methylthio)acetate compared to conventional ECRF approaches, mainly attributed to reduced [19F]F- contamination from less salt. Radiochemistry in droplets and electrochemistry for [18F]fluoride labeling showcased an innovative optimization approach and scalable method for clinically-relevant production, surpassing conventional methods. The methodologies outlined in this dissertation provide a comprehensive pathway to speed up the transition of both established and novel PET tracers from the laboratory to clinical application swiftly and cost-effectively.

    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2023)
    The growing discovery and development of novel radiopharmaceuticals and radiolabeling methods requires an increasing capacity for radiochemistry experiments. However, such studies typically rely on radiosynthesizers designed for clinical batch production rather than research, greatly limiting throughput. Two general solutions are being pursued to address this: developing new synthesis optimization algorithms to minimize how many experiments are needed, and developing apparatus with enhanced experiment throughput. We describe here a novel high-throughput system based on performing arrays of droplet-based reactions at 10 μL volume scale in parallel. The automatic robotic platform can perform a set of 64 experiments in ~3 h (from isotope loading to crude product, plus sampling onto TLC plates), plus ~1 h for off-line radio-TLC analysis and radioactivity measurements, rather than the weeks or months that would be needed using a conventional system. We show the high repeatability and low crosstalk of the platform and demonstrate optimization studies for two 18F-labeled tracers. This novel automated platform greatly increases the practicality of performing arrays of droplet reactions by eliminating human error, vastly reducing tedium and fatigue, minimizing radiation exposure, and freeing up radiochemist time for other intellectually valuable pursuits.
      Cover page: Accelerating radiochemistry development: Automated robotic platform for performing up to 64 droplet radiochemical reactions in a morning
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2023)
      Radiopharmaceutical analysis is limited by conventional methods. Radio-HPLC may be inaccurate for some compounds (e.g., 18F-radiopharmaceuticals) due to radionuclide sequester. Radio-TLC is simpler, faster, and detects all species but has limited resolution. Imaging-based readout of TLC plates (e.g., using Cerenkov luminescence imaging) can improve readout resolution, but the underlying chromatographic separation efficiency may be insufficient to resolve chemically similar species such as product and precursor-derived impurities. This study applies a systematic mobile phase optimization method, PRISMA, to improve radio-TLC resolution. The PRISMA method optimizes the mobile phase by selecting the correct solvent, optimizing solvent polarity, and optimizing composition. Without prior knowledge of impurities and by simply observing the separation resolution between a radiopharmaceutical and its nearest radioactive or non-radioactive impurities (observed via UV imaging) for different mobile phases, the PRISMA method enabled the development of high-resolution separation conditions for a wide range of 18F-radiopharmaceuticals ( [18F]PBR-06, [18F]FEPPA, [18F]Fallypride, [18F]FPEB, and [18F]FDOPA). Each optimization required a single batch of crude radiopharmaceutical and a few hours. Interestingly, the optimized TLC method provided greater accuracy (compared to other published TLC methods) in determining the product abundance of one radiopharmaceutical studied in more depth ( [18F]Fallypride) and was capable of resolving a comparable number of species as isocratic radio-HPLC. We used the PRISMA-optimized mobile phase for [18F]FPEB in combination with multi-lane radio-TLC techniques to evaluate reaction performance during high-throughput synthesis optimization of [18F]FPEB. The PRISMA methodology, in combination with high-resolution radio-TLC readout, enables a rapid and systematic approach to achieving high-resolution and accurate analysis of radiopharmaceuticals without the need for radio-HPLC.
        Cover page: A rapid and systematic approach for the optimization of radio thin-layer chromatography resolution
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2023)
        Copper-mediated radiofluorination has demonstrated remarkable potential in forming aromatic C-18F bonds of radioligands for positron emission tomography (PET). Achieving optimal results often requires optimization efforts, requiring a substantial amount of radiolabeling precursor and time, severely limiting the experimental throughput. Recently, we successfully showcased the feasibility of performing and optimizing Cu-mediated radiosynthesis on a high-throughput microdroplet platform using the well-known and clinically used radioligand [18F]FDOPA as an illustrative example. In our current work, we optimized the Cu-mediated synthesis of a novel monoacylglycerol lipase (MAGL) PET tracer ([18F]YH149), showing the versatility of droplet-based techniques for early stage tracer development. Across 5 days, we conducted a total of 117 experiments, studying 36 distinct conditions, while utilizing <15 mg of total organoboron precursor. Compared to the original report in which the radiochemical yield (RCY) was 4.4 ± 0.5% (n = 5), the optimized droplet condition provided a substantial improvement in RCY (52 ± 8%, n = 4) and showed excellent radiochemical purity (100%) and molar activity (77-854 GBq μmol-1), using a starting activity of 0.2-1.45 GBq. Furthermore, we showed for the first time a translation of the optimized microscale conditions to a vial-based method. With similar starting activity (0.2-1.44 GBq), the translated synthesis exhibited a comparable RCY of 50 ± 10% (n = 4) while maintaining excellent radiochemical purity (100%) and acceptable molar activity (20-46 GBq μmol-1). The successful translation to vial-based reactions ensures wider applicability of the optimized synthesis by leveraging widely available commercial vial-based synthesis modules.
          Cover page: Proof-of-concept optimization of a copper-mediated 18 F-radiosynthesis of a novel MAGL PET tracer on a high-throughput microdroplet platform and its macroscale translation
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2024)
          The radiometal gallium-68 (Ga-68) has garnered significant interest due to its convenient production via compact and widely available generators and the high performance of 68Ga-labeled compounds for positron-emission tomography (PET) imaging for cancer diagnosis and management of patients undergoing targeted radionuclide therapy. Given the short half life of Ga-68 (68 min), microfluidic-based radiosynthesis is a promising avenue to establish very rapid, efficient, and routine radiolabeling with Ga-68; however, the typical elution volume of Ga-68 from a generator (4-10 mL) is incompatible with the microliter reaction volumes of microfluidic devices. To bridge this gap, we developed a microscale cartridge-based approach to concentrate Ga-68. By optimizing cartridge design, resin type, resin mass, and eluent composition, Ga-68 was reliably concentrated from ~6 mL to ~80 µL with high recovery efficiency (>97%, n = 14). Furthermore, this method is suitable for both single- and dual-generator setups. To demonstrate suitability of the concentrated radiometal for radiolabeling, we performed microdroplet synthesis of [68Ga]Ga-PSMA-11, achieving high radiochemical yield (83 ± 11%, n = 3), excellent radiochemical purity (>99%), and high apparent specific activity (255-320 MBq/μg). The entire process, including Ga-68 concentration, radiosynthesis, purification, and formulation, was completed in 12 min. Starting with activity of 0.81-0.84 GBq, 0.51-0.64 GBq of product was produced, sufficient for multiple patient doses. This work paves the way to clinical-scale production of other 68Ga-labeled compounds using droplet microreactor methods, or high-throughput labeling optimization or compound screening of 68Ga-labeled probes using droplet reaction arrays.
            Cover page: Rapid Concentration of Ga-68 and Proof-of-Concept Microscale Labeling of [68Ga]Ga-PSMA-11 in a Droplet Reactor
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2021)
            Current equipment and methods for preparation of radiopharmaceuticals for positron emission tomography (PET) are expensive and best suited for large-scale multi-doses batches. Microfluidic radiosynthesizers have been shown to provide an economic approach to synthesize these compounds in smaller quantities, but can also be scaled to clinically-relevant levels. Batch microfluidic approaches, in particular, offer significant reduction in system size and reagent consumption. Here we show a simple and rapid technique to concentrate the radioisotope, prior to synthesis in a droplet-based radiosynthesizer, enabling production of clinically-relevant batches of [18F]FET and [18F]FBB. The synthesis was carried out with an automated synthesizer platform based on a disposable Teflon-silicon surface-tension trap chip. Up to 0.1 mL (4 GBq) of radioactivity was used per synthesis by drying cyclotron-produced aqueous [18F]fluoride in small increments directly inside the reaction site. Precursor solution (10 µL) was added to the dried [18F]fluoride, the reaction chip was heated for 5 min to perform radiofluorination, and then a deprotection step was performed with addition of acid solution and heating. The product was recovered in 80 µL volume and transferred to analytical HPLC for purification. Purified product was formulated via evaporation and resuspension or a micro-SPE formulation system. Quality control testing was performed on 3 sequential batches of each tracer. The method afforded production of up to 0.8 GBq of [18F]FET and [18F]FBB. Each production was completed within an hour. All batches passed quality control testing, confirming suitability for human use. In summary, we present a simple and efficient synthesis of clinically-relevant batches of [18F]FET and [18F]FBB using a microfluidic radiosynthesizer. This work demonstrates that the droplet-based micro-radiosynthesizer has a potential for batch-on-demand synthesis of 18F-labeled radiopharmaceuticals for human use.
              Cover page: Economical droplet-based microfluidic production of [18F]FET and [18F]Florbetaben suitable for human use
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