Development of zirconium-89 chelators for use in positron emission tomography imaging

Jowanaridhi, Boon-Uma

Abstract

Positron emission tomography (PET) is used for non-invasive high sensitivity molecular imaging to diagnose diseases, to follow biological processes or to monitor treatment response. Immuno-PET is an imaging technique used to improve the visualisation of a target site by combining the high sensitivity of PET with the specificity of monoclonal antibodies (mAbs). Due to the relatively slow pharmacokinetics of antibodies, the half-life of the radioisotope and the stability of radioisotope attachment are key factors in the development of immuno-PET imaging agents. Zirconium-89 is an excellent candidate to be the immuno-PET radionuclide of choice as it decays with a half-life of 78.41 hours.

DFO (acyclic structure) is the most common chelator used to bind to [⁸⁹Zr]Zr(IV) for imaging applications. However, it has been reported that the [⁸⁹Zr]ZrDFO-mAb conjugate showed decomplexation and significant uptake of radioactivity in bones. The bone uptake of free [⁸⁹Zr]Zr(IV) results in increased radiation dose to the bone marrow and a decrease in target-to-background ratio. Therefore, the development of a more suitable chelator for [⁸⁹Zr]Zr(IV) is of high interest to biomedical researchers.

The tetraazamacrocycle cyclen was functionalised with three different pendant arms; phosphonic acid, phosphinic acid and picolinic acid, to offer at least eight binding sites for Zr(IV) to form complexes with the metal ion either “in cavity” or “out of cavity”. A preliminary study of [⁸⁹Zr]Zr(IV) radiolabelling of compounds 1, 5 and 9 was performed in acetate buffer at pH 7, with [⁸⁹Zr]zirconium(IV) oxalate incubated at 95oC for 2 hours to obtain 5.0%, 6.1% and 8.4% labelling yields respectively. Labelling with [⁸⁹Zr]zirconium(IV) chloride was performed either in water or acetate buffer to form [⁸⁹Zr]Zr9 in a 49.3% labelling yield when using 0.5 MBq of [⁸⁹Zr]Zr(IV) in 0.2 M acetate buffer at pH 6 incubated at 95oC for 2 hours but achieving this yield required a higher concentration of the chelator. Extended picolinic acid arm chelators were synthesised with both cyclen and cyclam backbones. Compounds 16 and 20 were synthesised and labelled with [⁸⁹Zr]zirconium(IV) chloride. Unexpectedly [⁸⁹Zr]Zr9 gives higher radiolabelling yields than [⁸⁹Zr]Zr16 and [⁸⁹Zr]Zr20. It was possible that 16 and 20 formed lower stability complexes with Zr(IV) with a higher chelator to metal ratio than the anticipated 1:1 ratio, and so the chelators are not octadentate.

Novel macrocycle-linked DFO chelators were designed as the linear chelator DFO rapidly forms a complex with Zr(IV) at ambient temperature and then the metal could transfer to the cyclic ring giving a higher stability thermodynamic product (the “in-cavity” complex). Compounds 21, 22 and 23 were synthesized and labelled with [⁸⁹Zr]Zr(IV). The labelled compounds were stable in 100-fold excess EDTA and fetal bovine serum but did not show any clear improvement over DFO.

Citation

Jowanaridhi, B. (2017). Development of zirconium-89 chelators for use in positron emission tomography imaging. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4220815

• Thesis Type: Thesis
• Deposit Date: Oct 19, 2018
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4220815
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Nov 1, 2017