Radiosynthesis and Optimization of 18F-Fluorinated GW2580 Derivatives as A Dual Trk/CSF-1R Radiotracer for Potential Use in PET Imaging

• Author / Creator

  Sang Ho Choi

• As one of the medical imaging modalities applied in clinical diagnosis of various diseases, positron emission tomography (PET) is used to non-invasively visualize cellular and molecular changes that are occurring in the human body. Fluorine-18 [18F] is the most widely used radionuclide for development of radiotracers due to its favorable decay properties. A key process in developing radiotracers for PET imaging is selecting a target with a particular set of cellular and molecular properties that is expected to have therapeutic utility. As a member of the tyrosine kinase family, tropomyosin receptor kinase (Trk) and colony stimulating factor-1 receptor (CSF-1R) have been demonstrated to be valuable for cancer prognosis and are potential therapeutic targets for cancer treatment due to their diverse involvement in tumor regulation. As a result, development of radiotracers for these targets would allow identification of patients suitable for Trk/CSF-1R targeted therapy, improve prognosis, and offer a non-invasive method for in vivo visualization of treatment response. Being able to differentiate patients based on tumor characteristic, such as NTRK gene fusion, would ultimately lead to an improved personalized therapy reducing toxic side effects and low drug responses that are often associated with generalized chemotherapy.

  In this M.Sc thesis, two new fluorinated analogues 3.0 and 3.1 were synthesized based on the structure of a selective pan-Trk and CSF-1R inhibitor, GW2580. Initial molecular docking studies of 3.0 and 3.1 against TrkB crystal structure suggested similar binding potency as GW2580 (TrkB: IC50 = 56 nM). However, in vitro binding assay demonstrated a 2-fold decrease in binding potency for 3.0 (TrkB: IC50 = 123 nM) and a 30-fold decrease in binding potency for 3.1 (TrkB: IC50 = 1690 nM). The decrease in binding potency for both 3.0 and 3.1 was attributed to unfavorable electrostatic interactions between the methyl/fluorine substituents and amino acid residues within the binding site. However, when the binding potency of 3.0 was compared to a previously studied derivative of GW2580, 1.23 (TrkB: IC50 = 132 nM), by the Schirrmacher Group, both compounds demonstrated a similar binding potency.

  Due to the significant decrease in binding potency of 3.1, only 3.0 was considered for radiosynthesis. The boronic ester precursor 4.8a was synthesized and subjected to a Cu-catalyzed two-step (labelling/deprotecting) radiofluorination method to access [18F]3.0. However, unfavorable steric interactions between the ortho-methoxy substituent and the Cu-catalyst prevented radiofluorination of 4.8a, therefore preventing the development of [18F]3.0.

  In order to avoid a methoxy substituent ortho to the boronic ester, radiosynthesis of [18F]1.23 lacking the ortho-methoxy group was chosen. Even though radiosynthesis of [18F]1.23 was previously achieved by the Schirrmacher Group, it was accessed via a multi-step radiosynthesis unsuitable for automatic synthesis. In order to avoid the multi-step radiosynthesis, boronic ester precursor 4.8b was utilized. Radiofluorination of 4.8b and subsequent Boc-deprotection led to a two-step radiosynthesis of [18F]1.23. Due to inseparable protodeboronated side product 4.21 and unexpected Boc-deprotection during the initial radiofluorination, radiosynthesis [18F]1.23 afforded radiochemical yields (RCY) of only 2.5 ± 0.5%, > 99% radiochemical purity (RCP), and molar activity (Am) of 3.4 – 4.5 GBq/µmol. Even though radiosynthesis of [18F]1.23 was successfully developed for potential use in preliminary studies such as autoradiography, further investigation to improve the RCY and Am is required for human studies.

• Subjects / Keywords

  • PET
  • Radiosynthesis
  • GW2580
  • F-18

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3251G21R

• License

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