Synthesis and PET Imaging Biodistribution Studies of Radiolabeled Iododiflunisal, a Transthyretin Tetramer Stabilizer, Candidate Drug for Alzheimer's Disease.

The small-molecule iododiflunisal (IDIF) is a transthyretin (TTR) tetramer stabilizer and acts as a chaperone of the TTR-Amyloid beta interaction. Oral administration of IDIF improves Alzheimer's Disease (AD)-like pathology in mice, although the mechanism of action and pharmacokinetics remain unknown. Radiolabeling IDIF with positron or gamma emitters may aid in the in vivo evaluation of IDIF using non-invasive nuclear imaging techniques. In this work, we report an isotopic exchange reaction to obtain IDIF radiolabeled with 18F. [19F/18F]exchange reaction over IDIF in dimethyl sulfoxide at 160 °C resulted in the formation of [18F]IDIF in 7 ± 3% radiochemical yield in a 20 min reaction time, with a final radiochemical purity of >99%. Biodistribution studies after intravenous administration of [18F]IDIF in wild-type mice using positron emission tomography (PET) imaging showed capacity to cross the blood-brain barrier (ca. 1% of injected dose per gram of tissue in the brain at t > 10 min post administration), rapid accumulation in the liver, long circulation time, and progressive elimination via urine. Our results open opportunities for future studies in larger animal species or human subjects.

Tandem Iridium-Catalyzed C-H Borylation/Copper-Mediated Radiofluorination of Aromatic C-H Bonds with [18F]TBAF.

Direct C-H functionalization of (hetero)aromatic C-H bonds with iridium-catalyzed borylation followed by copper-mediated radiofluorination of the in situ generated organoboronates affords fluorine-18 labeled aromatics in high radiochemical conversions and meta-selectivities. This protocol describes the benchtop reaction assembly of the C-H borylation and radiofluorination steps, which can be utilized for the fluorine-18 labeling of densely functionalized bioactive scaffolds.

Strategies for the Production of [11C]LY2795050 for Clinical Use.

This report describes a comparison of four different routes for the clinical-scale radiosynthesis of the κ-opioid receptor antagonist [11C]LY2795050. Palladium-mediated radiocyanation and radiocarbonylation of an aryl iodide precursor as well as copper-mediated radiocyanation of an aryl iodide and an aryl boronate ester have been investigated. Full automation of all four methods is reported, each of which provides [11C]LY2795050 in sufficient radiochemical yield, molar activity, and radiochemical purity for clinical use. The advantages and disadvantages of each radiosynthesis method are compared and contrasted.

Copper-Mediated Radiocyanation of Unprotected Amino Acids and Peptides.

This report describes a copper-mediated radiocyanation of aryl halides that is applicable to complex molecules. This transformation tolerates an exceptionally wide range of functional groups, including unprotected amino acids. As such, it enables the site-specific introduction of [11C]CN into peptides at an iodophenylalanine residue. The use of a diamine-ligated copper(I) mediator is crucial for achieving high radiochemical yield under relatively mild conditions, thus limiting racemization and competing side reactions of other amino acid side chains. The reaction has been scaled and automated to deliver radiolabeled peptides, including analogues of adrenocorticotropic hormone 1-27 (ACTH) and nociceptin (NOP). For instance, this Cu-mediated radiocyanation was leveraged to prepare >40 mCi of [11C]cyano-NOP to evaluate biodistribution in a primate using positron emission tomography. This investigation provides preliminary evidence that nociceptin crosses the blood-brain barrier and shows uptake across all brain regions (SUV > 1 at 60 min post injection), consistent with the known distribution of NOP receptors in the rhesus brain.

Synthesis of 68Ga-radiopharmaceuticals using both generator-derived and cyclotron-produced 68Ga as exemplified by [68Ga]Ga-PSMA-11 for prostate cancer PET imaging.

[68Ga]Ga-PSMA-11, a urea-based peptidomimetic, is a diagnostic radiopharmaceutical for positron emission tomography (PET) imaging that targets the prostate-specific membrane antigen (PSMA). The recent Food and Drug Administration approval of [68Ga]Ga-PSMA-11 for PET imaging of patients with prostate cancer, expected follow-up approval of companion radiotherapeutics (e.g., [177Lu]Lu-PSMA-617, [225Ac]Ac-PSMA-617) and large prostate cancer patient volumes requiring access are poised to create an unprecedented demand for [68Ga]Ga-PSMA-11 in nuclear medicine clinics around the world. Meeting this global demand is going to require a variety of synthesis methods compatible with 68Ga eluted from a generator or produced on a cyclotron. To address this urgent need in the PET radiochemistry community, herein we report detailed protocols for the synthesis of [68Ga]Ga-PSMA-11, (also known as HBED-CC, Glu-urea-Lys(Ahx)-HBED-CC and PSMA-HBED-CC) using both generator-eluted and cyclotron-produced 68Ga and contrast the pros and cons of each method. The radiosyntheses are automated and have been validated for human use at two sites (University of Michigan (UM), United States; Royal Prince Alfred Hospital (RPA), Australia) and used to produce [68Ga]Ga-PSMA-11 for patient use in good activity yields (single generator, 0.52 GBq (14 mCi); dual generators, 1.04-1.57 GBq (28-42 mCi); cyclotron method (single target), 1.47-1.89 GBq (40-51 mCi); cyclotron method (dual target), 3.63 GBq (98 mCi)) and high radiochemical purity (99%) (UM, n = 645; RPA, n > 600). Both methods are appropriate for clinical production but, in the long term, the method employing cyclotron-produced 68Ga is the most promising for meeting high patient volumes. Quality control testing (visual inspection, pH, radiochemical purity and identity, radionuclidic purity and identity, sterile filter integrity, bacterial endotoxin content, sterility, stability) confirmed doses are suitable for clinical use, and there is no difference in clinical prostate cancer PET imaging using [68Ga]Ga-PSMA-11 prepared using the two production methods.

Sequential Ir/Cu-Mediated Method for the Meta-Selective C-H Radiofluorination of (Hetero)Arenes.

This article describes a sequential Ir/Cu-mediated process for the meta-selective C-H radiofluorination of (hetero)arene substrates. In the first step, Ir-catalyzed C(sp2)-H borylation affords (hetero)aryl pinacolboronate (BPin) esters. The intermediate organoboronates are then directly subjected to copper-mediated radiofluorination with [18F]tetrabutylammonium fluoride to afford fluorine-18 labeled (hetero)arenes in high radiochemical yield and radiochemical purity. This entire process is performed on a benchtop without Schlenk or glovebox techniques and circumvents the need to isolate (hetero)aryl boronate esters. The reaction was automated on a TracerLab FXFN module with 1,3-dimethoxybenzene and a meta-tyrosine derivative. The products, [18F]1-fluoro-3,5-dimethoxybenzene and an 18F-labeled meta-tyrosine derivative, were obtained in 37 ± 5% isolated radiochemical yield and >99% radiochemical purity and 25% isolated radiochemical yield and 99% radiochemical purity, and 0.52 Ci/µmol (19.24 GBq/µmol) molar activity (Am), respectively.

Use of 55 PET radiotracers under approval of a Radioactive Drug Research Committee (RDRC).

BACKGROUND: In the US, EU and elsewhere, basic clinical research studies with positron emission tomography (PET) radiotracers that are generally recognized as safe and effective (GRASE) can often be conducted under institutional approval. For example, in the United States, such research is conducted under the oversight of a Radioactive Drug Research Committee (RDRC) as long as certain requirements are met. Firstly, the research must be for basic science and cannot be intended for immediate therapeutic or diagnostic purposes, or to determine the safety and effectiveness of the PET radiotracer. Secondly, the PET radiotracer must be generally recognized as safe and effective. Specifically, the mass dose to be administered must not cause any clinically detectable pharmacological effect in humans, and the radiation dose to be administered must be the smallest dose practical to perform the study and not exceed regulatory dose limits within a 1-year period. In our experience, the main barrier to using a PET radiotracer under RDRC approval is accessing the required information about mass and radioactive dosing. RESULTS: The University of Michigan (UM) has a long history of using PET radiotracers in clinical research studies. Herein we provide dosing information for 55 radiotracers that will enable other PET Centers to use them under the approval of their own RDRC committees. CONCLUSIONS: The data provided herein will streamline future RDRC approval, and facilitate further basic science investigation of 55 PET radiotracers that target functionally relevant biomarkers in high impact disease states.

Manual and automated Cu-mediated radiosynthesis of the PARP inhibitor [18F]olaparib.

Positron emission tomography (PET) is a diagnostic nuclear imaging modality that relies on automated protocols to prepare agents labeled with a positron-emitting radionuclide (e.g., 18F). In recent years, new reactions have appeared for the 18F-labeling of agents that are difficult to access by applying traditional radiochemistry, for example those requiring 18F incorporation into unactivated (hetero)arenes. However, automation of these new methods for translation to the clinic has progressed slowly because extensive modification of manual protocols is typically required when implementing novel 18F-labeling methodologies within automated modules. Here, we describe the workflow that led to the automated radiosynthesis of the poly(ADP-ribose) polymerase (PARP) inhibitor [18F]olaparib. First, we established a robust manual protocol to prepare [18F]olaparib from the protected N-[2-(trimethylsilyl)ethoxy]methyl (SEM) arylboronate ester precursor in a 17% ± 5% (n = 15; synthesis time, 135 min) non-decay-corrected (NDC) activity yield, with molar activity (Am) up to 34.6 GBq/µmol. Automation of the process, consisting of copper-mediated 18F-fluorodeboronation followed by deprotection, was achieved on an Eckert & Ziegler Modular-Lab radiosynthesis platform, affording [18F]olaparib in a 6% ± 5% (n = 3; synthesis time, 120 min) NDC activity yield with Am up to 319 GBq/µmol.

Copper-Mediated Late-stage Radiofluorination: Five Years of Impact on Pre-clinical and Clinical PET Imaging.

PURPOSE: Copper-mediated radiofluorination (CMRF) is emerging as the method of choice for the formation of aromatic C-18F bonds. This minireview examines proof-of-concept, pre-clinical, and in-human imaging studies of new and established imaging agents containing aromatic C-18F bonds synthesized with CMRF. An exhaustive discussion of CMRF methods is not provided, although key developments that have enabled or improved upon the syntheses of fluorine-18 imaging agents are discussed. METHODS: A comprehensive literature search from April 2014 onwards of the Web of Science and PubMed library databases was performed to find reports that utilize CMRF for the synthesis of fluorine-18 radiopharmaceuticals, and these represent the primary body of research discussed in this minireview. Select conference proceedings, previous reports describing alternative methods for the synthesis of imaging agents, and preceding fluorine-19 methodologies have also been included for discussion. CONCLUSIONS: CMRF has significantly expanded the chemical space that is accessible to fluorine-18 radiolabeling with production methods that can meet the regulatory requirements for use in Nuclear Medicine. Furthermore, it has enabled novel and improved syntheses of radiopharmaceuticals and facilitated subsequent PET imaging studies. The rapid adoption of CMRF will undoubtedly continue to simplify the production of imaging agents and inspire the development of new radiofluorination methodologies.

Derisking the Cu-Mediated 18F-Fluorination of Heterocyclic Positron Emission Tomography Radioligands.

Molecules labeled with fluorine-18 (18F) are used in positron emission tomography to visualize, characterize and measure biological processes in the body. Despite recent advances in the incorporation of 18F onto arenes, the development of general and efficient approaches to label radioligands necessary for drug discovery programs remains a significant task. This full account describes a derisking approach toward the radiosynthesis of heterocyclic positron emission tomography (PET) radioligands using the copper-mediated 18F-fluorination of aryl boron reagents with 18F-fluoride as a model reaction. This approach is based on a study examining how the presence of heterocycles commonly used in drug development affects the efficiency of 18F-fluorination for a representative aryl boron reagent, and on the labeling of more than 50 (hetero)aryl boronic esters. This set of data allows for the application of this derisking strategy to the successful radiosynthesis of seven structurally complex pharmaceutically relevant heterocycle-containing molecules.

Synthesis and Reactivity of 18F-Labeled α,α-Difluoro-α-(aryloxy)acetic Acids.

In this work, we describe the 18F-labeling of α,α-difluoro-α-(aryloxy)acetic acid derivatives and demonstrate that these building blocks are amenable to post-18F-fluorination functionalization. Protodecarboxylation offers a new entry to 18F-difluoromethoxyarene, and the value of this approach is further demonstrated with coupling processes leading to representative 18F-labeled TRPV1 inhibitors and TRPV1 antagonists.

Radiosynthesis of SPECT tracers via a copper mediated 123I iodination of (hetero)aryl boron reagents.

A general method for the copper mediated nucleophilic 123I-iodination of (hetero)aryl boronic esters and acids has been developed. The broad substrate scope of this radiosynthetic approach allows access to [123I]DPA-713, [123I]IMPY, [123I]MIBG and [123I]IPEB that are four commonly used SPECT radiotracers. Our results infer that aryl boronic reagents can now be employed as common precursors for both fluorine-18 and iodine-123 radiolabelling.

Enhanced copper-mediated (18)F-fluorination of aryl boronic esters provides eight radiotracers for PET applications.

[(18)F]FMTEB, [(18)F]FPEB, [(18)F]flumazenil, [(18)F]DAA1106, [(18)F]MFBG, [(18)F]FDOPA, [(18)F]FMT and [(18)F]FDA are prepared from the corresponding arylboronic esters and [(18)F]KF/K222 in the presence of Cu(OTf)2py4. The method was successfully applied using three radiosynthetic platforms, and up to 26 GBq of non-carrier added starting activity of (18)F-fluoride.

(18)F-Labeling of Arenes and Heteroarenes for Applications in Positron Emission Tomography.

Diverse radiochemistry is an essential component of nuclear medicine; this includes imaging techniques such as positron emission tomography (PET). As such, PET can track diseases at an early stage of development, help patient care planning through personalized medicine and support drug discovery programs. Fluorine-18 is the most frequently used radioisotope in PET radiopharmaceuticals for both clinical and preclinical research. Its physical and nuclear characteristics (97% ß(+) decay, 109.8 min half-life, 635 keV positron energy) and high specific activity make it an attractive nuclide for labeling and molecular imaging. Arenes and heteroarenes are privileged candidates for (18)F-incorporation as they are metabolically robust and therefore widely used by medicinal chemists and radiochemists alike. For many years, the range of (hetero)arenes amenable to (18)F-fluorination was limited by the lack of chemically diverse precursors, and of radiochemical methods allowing (18)F-incorporation in high selectivity and efficiency (radiochemical yield and purity, specific activity, and radio-scalability). The appearance of late-stage fluorination reactions catalyzed by transition metal or small organic molecules (organocatalysis) has encouraged much research on the use of these activation manifolds for (18)F-fluorination. In this piece, we review all of the reactions known to date to install the (18)F substituent and other key (18)F-motifs (e.g., CF3, CHF2, OCF3, SCF3, OCHF2) of medicinal relevance onto (hetero)arenes. The field has changed significantly in the past five years, and the current trend suggests that the radiochemical space available for PET applications will expand rapidly in the near future.

Silyl phosphorus and nitrogen donor chelates for homogeneous ortho borylation catalysis.

Ir catalysts supported by bidentate silyl ligands that contain P- or N-donors are shown to effect ortho borylations for a range of substituted aromatics. The substrate scope is broad, and the modular ligand synthesis allows for flexible catalyst design.

A general copper-mediated nucleophilic 18F†fluorination of arenes.

Molecules labeled with fluorine-18 are used as radiotracers for positron emission tomography. An important challenge is the labeling of arenes not amenable to aromatic nucleophilic substitution (SNAr) with [(18)F]F(-). In the ideal case, the (18)F fluorination of these substrates would be performed through reaction of [(18)F]KF with shelf-stable readily available precursors using a broadly applicable method suitable for automation. Herein, we describe the realization of these requirements with the production of (18)F arenes from pinacol-derived aryl boronic esters (arylBPin) upon treatment with [(18)F]KF/K222 and [Cu(OTf)2(py)4] (OTf = trifluoromethanesulfonate, py = pyridine). This method tolerates electron-poor and electron-rich arenes and various functional groups, and allows access to 6-[(18)F]fluoro-L-DOPA, 6-[(18)F]fluoro-m-tyrosine, and the translocator protein (TSPO) PET ligand [(18)F]DAA1106.

A traceless directing group for C-H borylation.

Not a trace: Borylation of the nitrogen in nitrogen heterocycles or anilines provides a traceless directing group for subsequent catalytic C-H borylation. Selectivities that previously required Boc protection can be achieved; furthermore, the NBpin directing group can be installed and removed in-situ, and product yields are substantially higher. Boc=tert-butoxycarbonyl, pin=pinacolato.

High-throughput optimization of Ir-catalyzed C-H borylation: a tutorial for practical applications.

With the aid of high-throughput screening, the efficiency of Ir-catalyzed C-H borylations has been assessed as functions of precatalyst, boron reagent, ligand, order of addition, temperature, solvent, and substrate. This study not only validated some accepted practices but also uncovered unconventional conditions that were key to substrate performance. We anticipate that insights drawn from these findings will be used to design reaction conditions for substrates whose borylations are difficult to impossible using standard catalytic conditions.

Electronic effects in iridium C-H borylations: insights from unencumbered substrates and variation of boryl ligand substituents.

Experiment and theory favour a model of C-H borylation where significant proton transfer character exists in the transition state.