Volume 12, Issue 3
Recent Advances of PET Chemistry in Flow
Recent Advances of PET Chemistry in Flow
Lee Collier, PhD, Advion Inc., Ithaca, NY
Microfluidics have revolutionized the radiopharmaceuticals field, particularly in both pre-clinical and clinical production of labeled radiotracers. Microfluidics reduce reaction times, consumption of reagents, and the complexity of reaction mixtures while they increase radiochemical yields and optimize reaction parameters for labeling. In short, they allow for the rapid optimization of chemical processes and increase productivity.
There are three broadly defined types of microfluidics (Figure 1): Continuous-flow microfluidics, microreactors and droplet/digital microfluidics. This review covers the various uses and advantages of continuous-flow microfluidics.
The first reported synthesis PET radiotracers in a microfluidic device were published nearly two decades ago and demonstrated that this technology is capable of providing multi-doses of fluorine-18 (18F; t1/2 = 109.7 min) labeled compounds.4 Initially, reactions were limited to simple nucleophilic displacements of tosylates, mesylates and triflates to prepare such compounds as 18F-FDG. However, the choice of substrates now includes fluorodenitration,5 ammonium salts,5 iodonium salts,6-9 halides, etc.
A number of papers and reviews have been published on the use of radiotracers in drug development10-15 and radiotracer production.9,16-34 Continuous-flow microfluidics have exciting advantages over traditional radiochemical synthesis, including low sample and reagent consumption, faster kinetics, high throughput screening of reaction conditions, high reproducibility, easy automation of the processes and enhanced product yields, such as the ability to synthesize materials that cannot be prepared in conventional apparatus due to the rapid degradation of precursor or product under the reaction conditions. Implementation of such a method has significantly accelerated the process of synthesizing labeled compounds and positron emission tomography (PET) radiopharmaceuticals.1, 24 To date, over a hundred radiolabeled compounds have been synthesized in continuous-flow microfluidic devices for PET applications.
One major consideration of [18F]fluoride is the method used to prepare the fluoride for reaction. A review of the major methods observed that in the microfluidic environment, radiofluorinations can tolerate up to ~5 percent water for a number of precursors and conditions tested in the paper.47
Many reports have shown that there are significant improvements using microfluidics compared to classic reaction methodologies.
It is possible to have an array or matrix of different concentrations and amounts of reagents and run these at different temperatures, monitoring them over time by removing aliquots. The downside of this type of reaction set-up is that each reaction will consume precursors/reagents, and the automated system must be cleaned between each reaction.
From a single setup of the system, more than 25 reaction conditions, including temperature, residence time (equivalent to the reaction time), precursor concentration and multistep reactions, may be optimized during these operations.24 This reduces the utilization of reagents, including isotopes, to a minimum and can normally be performed in less than 2 hours. The analysis of the reaction mixture typically is the rate-limiting step.
Autoradiolysis is the degradation of radioactively labeled compounds due to the activity of the labeled compounds and typically increases with activity concentration. Microfluidic devices can be designed to reduce this autoradiolysis by utilizing shapes that exclude the positron interaction, which typically causes the autoradiolysis effects. Rensch et al. noted that about 85 percent of the positron’s kinetic energy is deposited in the first 1 mm and only 13 percent within the first 100 µm.48 Microfluidic synthesis of radiotracers such as [18F]FPEB show no significant reduction in radiochemical yield over the range of activities from 20 mCi or ~45 mCi/mL to 4.5 Ci or >10,000 mCi/mL, using a 100 µm ID reactor.25
In vial/reaction vessel systems the loss of either precursor and/or product due to the prolonged heating during the radiochemical synthesis is a major cause of low radiochemical yield. The microfluidic systems have shown significant improvement in yield by only heating the portion of the reagents/product that is currently in the microfluidic reactor. Once the reaction mixture leaves the heated area of the microfluidic environment, it can be quickly cooled, decreasing the degradation of the product. This is a result of the large surface-area-to-volume ratio in the microfluidic environment (Figure 2).
One such example of this was shown by Philippe et al. in the synthesis of [18F]FE@SNAP (2) (Figure 3).49 The highest radiochemical incorporation yields were found to be zero percent for the vessel-based reaction and ~44 percent for the microfluidic reaction.
In a typical vial/reaction-based system, multistep reactions are performed as discrete steps. For example, the incorporation of the isotope is completed in one step followed by hydrolysis or coupling to a precursor, protein or peptide. In the microfluidic environment, two (or more) steps may be performed in a continuous manner,50-53 such as in the formation of a pendant group, which is later attached to a substrate (either a precursor, a protein or peptide). For example, Cummin et al. demonstrated the microfluidic synthesis of the [18F]F-Py-TFP prosthetic group with radiochemical yields of up to 97 percent and a synthesis time of 3 minutes. This pendant group was then directly transferred to second microfluidic reactor and then coupled to prepare [18F]F-Py-YGGFL (Figure 4). The overall radiochemical yield within 8 minutes, starting from anhydrous [18F]fluoride, was determined to be 28 percent.51
Recently, the combination of microfluidics for the incorporation of the radioisotope and “flow hydrogenation” for the rapid (<3 minutes, using pressures and temperatures of up to 100 bar and 150˚C) reduction or deprotection of PET radiotracers has been shown in the synthesis of [18F]CABS13 (9) (Figure 5).54,55
While the main isotope used with microfluidics has been limited to [18F] fluoride, there has been a recent explosion in the last few years of other useful isotopes.
The next most common isotope used is [11C]carbon and in a number of forms, such as [11C]methyl iodide, [11C]methyl triflate and [11C]carbon monoxide. The reactions performed with [11C]methyl iodide and [11C]methyl triflate include O-methylation,56 N-methylation of amino species,57 S-methylation of thiol species and carbonylation reactions using [11C]carbon monoxide.32,58 Kealey et al. found that the major benefit to using microfluidics, compared to the conventional vial-based systems, was the elimination of unwanted radioactive emissions and the reduction in yield due to the partitioning of the volatile radioactive agent to the headspace during the heating process of a conventional system (Figure 6).58
Most researchers have found that the microfluidic approach generally outperforms conventional radiosynthetic methods when labeling common chelated peptides and proteins. Numerous metals have been used to label proteins, such as 64Cu, 68Ga, 89Zr, 90Y, 99mTc, 111In and 211At. The initial studies were performed using classic silica surfaces, either etched surfaces or silica capillaries. However, there were noted losses related to the binding of some of the metals to a differing level depending on the isotope and the solvents and temperatures being used. Recently, the glass surfaces have been replaced by polymers, such as PEEK59 or poly(dimethylsiloxane) (PDMS)-based microfluidic devices,60 with reductions in the binding of metals to the microfluidic surfaces. It has been noted that increasing the reaction temperature can enhance the reaction rates and improve the incorporation yield. Microfluidics, using peptides, have been performed at temperatures of up to 100˚C without significant reduction in activity of most of the peptides31.
Typically, the purification of the products prepared by microfluidics is performed by classical semi-preparative HPLC, usually followed by solid phase extraction (SPE) reformulations or by using biocompatible HPLC solvents.25,40 There are three main methods of improving purification simplicity – critical for rodent animal study. Typically, 100-200 µCi must be injected in a volume of 100 µl or less because of the low blood volume of the animal. Normal SPE reformulation methods will yield ~10 mLs of a 10 percent ethanol/saline solution but require high starting activities to possess a sufficient radioactive concentration.
The first method is to substitute the HPLC column/SPE reformulation methodology with a simple method using SPE columns to purify the final radiotracer, since microfluidic methodologies generally lead to much cleaner crude reaction mixtures,25 and this allows for a much simpler purification methodology53,61,62.
The second method involves the use of analytical columns instead of the standard semi-prep columns, as well as the replacement of the standard loop with a self-packed trapping cartridge or monolithic column. The solvents used in the purification are normally biocompatible solvents, such as ethanol with water or saline solutions. The product is normally collected in a volume of 0.25-1 mL and because of the higher resolution of analytical columns, impurity peaks may be separated from the product that cannot be removed using classical semi-preparative columns.63,64
The third method is the use of a microfluidic concentration system for the reduction of the total solvent in the solution.65 Chao et al. described a compact microfluidic device based on membrane distillation.66 The system is compact in size and can concentrate the solutions from ~10 mL down to ~1-1.5 mL in ~9-14 minutes. The initial solvents can contain up to 80 percent acetonitrile/water or up to 40 percent ethanol/water.
Further development of microchip fabrication and materials will improve the availability and reduce the cost of individual chips of a more complex design.10,67-73 The proven reduction in radiolysis and expansion of reaction types that are able to be performed in a microfluidic environment, including photochemistry,74 direct use of gases in flow,75 microchip quality control processes,76-78 metabolite analysis79 and the direct measurement of reaction kinetics80 will continue to drive the use of microfluidics. An interesting and expanding area of research is the use of organs on a chip (OC), which has the ability to mimic the response of real organs and has the potential to substitute for animal models, reducing the cost and time required to study radiotracers in controlled microenvironments.68,71,72,81
I would like to thank all of the researchers who have performed pioneering work in the area of microfluidics, which we have built upon, as well as the researchers and collaborators with the vision to look to the future and embrace new technologies for radiotracer development.
During the SNMMI Annual Meeting in Philadelphia, the following CMIIT awards were presented by CMIIT 2016-2018 President Buck E. Rogers, PhD
|2018 CMIIT Laboratory Professionals Award
Shelley Acuff, CNMT, RT(R)(CT), clinical research leader of Molecular Imaging and Translational Research/Radiology at the University of Tennessee Medical Center, received the 2018 CMIIT Laboratory Professionals Award, which recognizes a laboratory professional who has developed innovative and high-impact tools, techniques, and/or practices in molecular imaging. The CMIIT Laboratory Professionals Award winner receives free registration to the SNMMI Annual Meeting and a $1,000 travel reimbursement. This award was made possible through a grant from the Education and Research Foundation for Nuclear Medicine and Molecular Imaging.
CMIIT 2018-2019 Board of Directors
Congratulations to the board members selected during the 2018 elections, whose terms began during the SNMMI Annual Meeting in June:
Vice President – Cathy S. Cutler, PhD
Secretary/Treasurer – Dustin Osborne, PhD, DABSNM
Board Member, Nuclear – Hossein Jadvar, MD, PhD, MPH, MBA, FACNM, FSNMMI and Katherine Binzel, PhD
Board Member, non-Nuclear – Ali Azhdarinia, PhD and Kayvan Rahimi Keshari, PhD
The board also thanked the following three outgoing board members for their service to CMIIT:
Jonathan McConathy, MD, PhD, Walter Akers, DVM, PhD, and Thomas Reiner, PhD
The full 2018-2019 CMIIT Board:
PRESIDENT – Kimberly Kelly, PhD
VICE-PRESIDENT – Cathy Cutler, PhD
SECRETARY/TREASURER – Dustin Osborne, PhD, DABSNM
IMMEDIATE PAST PRESIDENT – Buck Rogers, PhD
Ali Azhdarinia, PhD
Elizabeth A. Bailey, PhD
Katherine Binzel, PhD
Delphine L. Chen, MD
Georges N. El Fakhri, PhD
Hossein Jadvar, MD, PhD, MPH, MBA, FACNM, FSNMMI
Kayvan Rahimi Keshari, PhD
Suzanne E. Lapi, PhD
Mark “Marty” Pagel, PhD
Todd E. Peterson, PhD
Neil A. Petry, MS, RPh, BCNP
Albert J. Sinusas, MD, FACC, FAHA
BOARD INTERN – Thomas Ng, MD, PhD
Ex-Officio Members – David Dick, PhD – RPSC President and Terrence Ruddy, MD – CVC President
|2018 CMIIT Young Investigators Awards
The Young Investigator Awards (YIA) recognize the best abstracts presented during the CMIIT Young Investigators Session. Young Investigators are defined as individuals within five years of completing a residency training program or a PhD program (five years from the YIA presentation date).
Abstract:“Evans Blue Attachment Enhances Somatostatin Receptor Subtype-2 Imaging and Radiotherapy”
Abstract: “An Improved Prostate Specific Membrane Antigen Targeting Agent for Prostate Cancer Imaging and Therapy”
Abstract: “Vasculature-based Differential Tumor Uptake of Radiolabeled Ultrasmall Mesoporous Silica Nanoparticles in Breast Cancers Models”
MI in the Literature
Each month, the CMIIT Editorial Board selects some of the top molecular imaging research papers from all papers indexed by PubMed. Below are links to these papers (free full text available).
A Dual-Modality Hybrid Imaging System Harnesses Radioluminescence and Sound to Reveal Molecular Pathology of Atherosclerotic Plaques
Zaman RT, Yousefi S, Long SR, Saito T, Mandella M, Qiu Z, Chen R, Contag CH, Gambhir SS, Chin FT, Khuri-Yakub BT, McConnell MV, Shung KK, Xing L. PMID: 29895966
The human somatostatin receptor type 2 as an imaging and suicide reporter gene for pluripotent stem cell-derived therapy of myocardial infarction
Neyrinck K, Breuls N, Holvoet B, Oosterlinck W, Wolfs E, Vanbilloen H, Gheysens O, Duelen R, Gsell W, Lambrichts I, Himmelreich U, Verfaillie CM, Sampaolesi M, Deroose CM. PMID: 29774076
Comparative Evaluation of Radioiodine and Technetium-Labeled DARPin 9_29 for Radionuclide Molecular Imaging of HER2 Expression in Malignant Tumors
Vorobyeva A, Bragina O, Altai M, Mitran B, Orlova A, Shulga A, Proshkina G, Chernov V, Tolmachev V, Deyev S. PMID: 29977173
Molecular Imaging of endometrial sentinel lymph nodes utilizing fluorescent-labeled Tilmanocept during robotic-assisted surgery in a porcine model
Anderson KM, Barback CV, Qin Z, Hall DJ, Hoh CK, Vera DR, McHale MT. PMID: 29965996
First-in-human study of PET and optical dual-modality image-guided surgery in glioblastoma using 68Ga-IRDye800CW-BBN
Li D, Zhang J, Chi C, Xiao X, Wang J, Lang L, Ali I, Niu G6, Zhang L, Tian J, Ji N, Zhu Z, Chen X. PMID: 29721096
A Novel Metal-Based Imaging Probe for Targeted Dual-Modality SPECT/MR Imaging of Angiogenesis
Tsoukalas C, Psimadas D, Kastis GA, Koutoulidis V, Harris AL, Paravatou-Petsotas M, Karageorgou M, Furenlid LR, Moulopoulos LA, Stamopoulos D, Bouziotis P. PMID: 29974048
Radionuclide-fluorescence Reporter Gene Imaging to Track Tumor Progression in Rodent Tumor Models
Volpe A, Man F, Lim L, Khoshnevisan A, Blower J, Blower PJ, Fruhwirth GO. PMID: 29608157
Tumor-targeted Dual-modality Imaging to Improve Intraoperative Visualization of Clear Cell Renal Cell Carcinoma: A First in Man Study
Hekman MC, Rijpkema M, Muselaers CH, Oosterwijk E, Hulsbergen-Van de Kaa CA, Boerman OC, Oyen WJ, Langenhuijsen JF, Mulders PF. PMID: 29721070
MI in the News
MI Gateway presents a sampling of research and news of interest to the community of molecular imaging scientists.
Hybrid heart imaging can foresee major cardiac events
Health Data Management
Researchers examined whether hybrid imaging, performed by fusing coronary computed tomography angiography (CCTA) with nuclear stress testing by use of myocardial perfusion imaging with single photon emission tomography (SPECT) could provide more information of a heart’s anatomy and function in a non-invasive manner. Study results indicate that it can improve treatment decision-making and help avoid unnecessary invasive angiographies.
AI enables stress-first SPECT MPI protocol
A multi-institutional and multinational research group found that a machine-learning algorithm outperformed visual diagnosis or automated total perfusion deficit analysis for predicting the presence of CAD — including high-risk CAD — without increasing false-positive results. As a result, patients with a normal score from the machine-learning algorithm could safely be discharged and not have to receive rest imaging.
Novel theranostic agent designed for precise tumor diagnosis and therapy
A novel, intelligent theranostic agent for precise tumor diagnosis and therapy has been developed that remains as small molecules while circulating in the bloodstream, can then self-assemble into larger nanostructures in the tumor, and be activated by the tumor microenvironment for therapy guided by photoacoustic imaging.
Novel PET, SPECT techniques help track, modify T cells for immunotherapy
With PET and SPECT, researchers have demonstrated that T cells can be modified with the DOTA antibody reporter 1 (DAbR1) gene to enable in vivo tracking for immunotherapy.
TOF-PET/MRI allows lower FDG dose for breast cases
Clinicians can reduce a standard dose of FDG by as much as 90 percent and still achieve clinically acceptable PET image quality in time-of-flight (TOF) PET/MRI scans for breast cancer.
Nuclear Imaging Moves Toward Digital Detector Technology
Diagnostic and Interventional Cardiology
Nuclear imaging technology for both SPECT and PET have made advancements in the past couple years. The main drivers for this have been a movement to digital imaging detectors to improve image quality and address radiation dose concerns, reimbursement and radiotracer supply issues. Other advancements have come in the areas of software to improve image reconstruction quality, offer better clinical qualification and analytics data.
Laser-sonic scanner aims to replace mammograms for finding breast cancer
Caltech researchers have developed a laser-sonic scanning system, known as photoacoustic computed tomography, or PACT, that can find tumors in as little as 15 seconds by shining pulses of light into the breast.
Study demonstrates promise of new imaging method for precise brain cancer diagnostics
A research team in China and Singapore reports the first NIR-II fluorescent molecule with aggregation-induced-emission (AIE) characteristics for dual fluorescence and photoacoustic imaging to diagnosis brain cancer.
Robot-assisted imaging may hasten treatment for prostate cancer patients
Researchers have successfully used robot-assisted multispectral-fluorescence imaging to distinguish between healthy and diseased lymphatic flow patterns in prostate and lower limb-draining lymph nodes. The method may reduce the use of invasive extended pelvic lymph node dissections for prostate cancer.
Scientists develop a new approach for efficient cancer therapy
Two synergistic drug components combined in a dimer within polymeric nanotransporters are activated when the dimer is split within the tumor. High concentrations of the drug are delivered to the tumor with few side effects. In addition, the photosensitizer is a fluorescent dye, and it can bind the radioisotope copper-64, which enables visualization with both fluorescence imaging and PET. Quantitative PET allows for precise monitoring of the dimer, as well as confirmation of its pharmacokinetics and biodistribution in vivo.
International Conference on Advanced Microbiology and Research
September 19 – 20, 2018
Amsterdam, The Netherlands
Hong Kong Hybrid PET-Imaging Symposium and Workshop 2018
September 29 – October 1, 2018
88th Annual Meeting of the American Thyroid Association
October 3 – 7, 2018
Pharmaceutics & Novel Drug Delivery Systems 2018
October 4 – 6, 2018
The 18th International Cancer Imaging Society
October 7 – 9, 2018
EANM'18: 31st Annual Congress of the European Association of Nuclear Medicine
October 13 – 17, 2018
World Congress on Breast Cancer 2018
October 15 – 16, 2018
7th European Clinical Microbiology Congress
November 1 –2, 2018
World Congress on Bio-organic and Medicinal Chemistry
November 12 – 13, 2018
Joint Meeting of the British Nuclear Medicine Society and Irish Nuclear Medicine Association
November 19, 2018