solid-phase-11c-methylation-purification-formulation-for-production

Solid Phase <sup>11</sup>C-Methylation, Purification and Formulation for the Production of PET Tracers

Routine production of radiotracers used in positron emission tomography (PET) mostly relies on wet chemistry where the radioactive synthon reacts with a ...

McGill University

Tropomyosin receptor kinases (TrkA/B/C) are critically involved in the development of the nervous system, in neurological disorders as well as in multiple neoplasms of both neural and non-neural origins. The development of Trk radiopharmaceuticals would offer unique opportunities toward a more complete understanding of this emerging therapeutic target. To that end, we first developed [(11)C]GW441756 ([(11)C]9), a high affinity photoisomerizable pan-Trk inhibitor, as a lead radiotracer for our positron emission tomography (PET) program. Efficient carbon-11 radiolabeling afforded [(11)C]9 in high radiochemical yields (isolated RCY, 25.9% ± 5.7%). In vitro autoradiographic studies in rat brain and TrkB-expressing human neuroblastoma cryosections confirmed that [(11)C]9 specifically binds to Trk receptors in vitro. MicroPET studies revealed that binding of [(11)C]9 in the rodent brain was mostly nonspecific despite initial high brain uptake (SUVmax = 2.0). Modeling studies of the 4-aza-2-oxindole scaffold led to the successful identification of a small series of high affinity fluorinated and methoxy derivatized pan-Trk inhibitors based on our lead compound 9. Out of this series, the fluorinated compound 10 was selected for initial evaluation and radiolabeled with fluorine-18 (isolated RCY, 2.5% ± 0.6%). Compound [(18)F]10 demonstrated excellent Trk selectivity in a panel of cancer relevant kinase targets and a promising in vitro profile in tumors and brain sections but high oxidative metabolic susceptibility leading to nonspecific brain distribution in vivo. The information gained in this study will guide further exploration of the 4-aza-2-oxindole scaffold as a lead for Trk PET ligand development. 

Syntheses and evaluation of carbon-11- and fluorine-18-radiolabeled pan-tropomyosin receptor kinase (Trk) inhibitors: exploration of the 4-aza-2-oxindole scaffold as Trk PET imaging agents.

Translation of carbon-11 labeled PET tracers to clinical settings is currently impeded by the technical difficulties associated with [C]CO conversion into the highly reactive methylating agents [C]CHI and [C]CHOTf using automated modules relying on stationary valves. Here we describe development of the first in its kind "[C]kit" for production of carbon-11 radiotracer using disposable manifolds. This method proved to be very reliable and allows for consecutive production of PET tracers with minimal intervals between the syntheses.

Development of "[C]kits" for a fast, efficient and reliable production of carbon-11 labeled radiopharmaceuticals for Positron Emission Tomography.

Pittsburgh compound B ([ C]PiB) is the gold standard positron emission tomography (PET) tracer for the in vivo imaging of amyloid plaques. Currently, it is synthesized by either solution chemistry or using a "dry loop" approach followed by HPLC purification within 30 minutes starting from [ C]CO . Here, we report a novel, highly efficient solid phase supported carbon-11 radiolabeling procedure using commercially available disposable tC18 cartridge as a "3-in-1" entity: reactor, purifier, and solvent replacement system. [ C]PiB is synthesized by passing gaseous [ C]CH OTf through a tC18 cartridge preloaded with a solution of precursor. Successive elution with aqueous ethanol solutions allows for nearly quantitative separation of the reaction mixture to provide chemically and radiochemically pure PET tracer. [ C]PiB suitable for human injection is produced within 10 minutes starting from [ C]CH OTf (20 min from [ C]CO ) in 22% isolated yield not corrected for decay and molar activity of 190 GBq/μmol using 0.2 mg of precursor. This technique reduces the amount of precursor and other supplies, avoids use of preparative HPLC and toxic solvents, and decreases the time between consecutive production batches. Solid phase supported technique can facilitate [ C]PiB production compliant with Good Manufacturing Practice (GMP) and improve synthesis reliability.

Highly efficient solid phase supported radiosynthesis of [ C]PiB using tC18 cartridge as a "3-in-1" production entity.

Imaging agents capable of quantifying the brain's tau aggregates will allow a more precise staging of Alzheimer's disease (AD). The aim of the present study was to examine the in vitro properties as well as the in vivo kinetics, using gold standard methods, of the novel positron emission tomography (PET) tau imaging agent [F]MK-6240.

In vivo quantification of neurofibrillary tangles with [F]MK-6240.

[ F]MK-6240 (6-(fluoro)-3-(1H-pyrrolo[2,3-c]pyridin-1-yl)isoquinolin-5-amine) is a highly selective PET radiotracer for the in vivo imaging of neurofibrillary tangles (NFTs). [ F]MK-6240 was synthesized in one step from its bis-Boc protected precursor N-[(tert-butoxy)carbonyl]-N-(6-nitro-3-[1H-pyrrolo[2,3-c]pyridin-1-yl]isoquinolin-5-yl) carbamate in DMSO using [ F] fluoride with TEA HCO with step-wise heating up to 150°C, resulting in an isolated radiochemical yield of 9.8% ± 1.8% (n = 3) calculated from the end of bombardment (5.2% ± 1.0% calculated from the end of synthesis). This new synthetic approach eliminates the acidic deprotection of the bis-Boc  F-labeled intermediate, which reduces the number of operations necessary for the synthesis as well as losses, which occur during deprotection and neutralization of the crude product mixture prior to the HPLC purification. The synthesis was performed automatically with a single-use cassette on an IBA Synthera+ synthesis module. This synthesis method affords the radioligand with a reliable radiochemical yield, high radiochemical purity, and a high molar activity. [ F]MK-6240 synthesized with this method has been regularly (n > 60) used in our ongoing human and animal PET imaging studies.

A simplified radiosynthesis of [ F]MK-6240 for tau PET imaging.

Manual and automatic blood sampling at different time intervals is considered the gold standard to determine the arterial input function (AIF) in dynamic positron emission tomography (PET). However, blood sampling is characterized by poor time resolution and is an invasive procedure. The aim of this study was to characterize the scintillating fibers used to develop a non-invasive positron detector.

Article	Year
Syntheses and evaluation of carbon-11- and fluorine-18-radiolabeled pan-tropomyosin receptor kinase (Trk) inhibitors: exploration of the 4-aza-2-oxindole scaffold as Trk PET imaging agents. ACS chemical neuroscience\| PubMed ID: 25350780	2015
Development of "[C]kits" for a fast, efficient and reliable production of carbon-11 labeled radiopharmaceuticals for Positron Emission Tomography. Applied radiation and isotopes : including data, instrumentation and methods for use in agriculture, industry and medicine\| PubMed ID: 28038410	2017
Highly efficient solid phase supported radiosynthesis of [ C]PiB using tC18 cartridge as a "3-in-1" production entity. Journal of labelled compounds & radiopharmaceuticals\| PubMed ID: 28981146	2017
In vivo quantification of neurofibrillary tangles with [F]MK-6240. Alzheimer's research & therapy\| PubMed ID: 30064520	2018
A simplified radiosynthesis of [ F]MK-6240 for tau PET imaging. Journal of labelled compounds & radiopharmaceuticals\| PubMed ID: 30426529	2019
Characterization of scintillating fibers for use as positron detector in positron emission tomography. Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB)\| PubMed ID: 31450121	2019

Robert Hopewell

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