Development of radiometal-based labelling techniques and tracers for non-invasive molecular imaging

Metallic radionuclides, radiometals, have an important role in nuclear medicine. Their straightforward coordination radiochemistry allows for a large variety of applications. The similarities and differences between the radiometals can be utilised to expand the window of diagnostic imaging or transfer diagnostic methods from one imaging modality to another. Radiometals from the same or from different elements (both therapeutic and diagnostic) may be coordinated to similar probes, as a theranostic pair.

Radionuclide-based molecular imaging is a non-invasive in vivo imaging technique that quantifies the concentrations of radioactive probes in biological processes occurring at cellular and subcellular levels in living organisms. The two major diagnostic in vivo imaging techniques used are Single-Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). In this thesis, radiometal production using a cyclotron solid target system and some fundamental aspects of radiometal labelling are explored, using two of the most common positron-emitting radiometals, gallium-68 (68Ga) and zirconium-89 (89Zr).

In paper I an albumin targeting Affibody molecule, ABY-028, was successfully developed, 68Ga-labelled and in vivo evaluated using a small animal PET camera. We showed that the biodistribution was consistent with the binding of [68Ga]Ga-ABY-028 to plasma albumin. Uptake patterns differed between tumours at different stages and of different phenotypes. Tracer uptake responses to permeability-altering therapeutics and during cerebral infarction could be observed. This novel radiotracer is a promising tool for in vivo molecular imaging of variations and alterations of vascular permeability and has the potential to function as a baseline control of the non-specific uptake of other albumin-binding domain (ABD)-based diagnostic or therapeutic agents.

In paper II cells were 89Zr-labelled, using two different metal complexes, with two distinctive labelling mechanisms, [89Zr]Zr-(oxine)4 and [89Zr]Zr-DFO-NCS. Synthesis protocols were successfully optimised to yield high radiochemical conversions of both 89Zr-complexes. Both radiotracers presented in this head-to-head study showed feasibility for universal radiolabellings of different cell types. The results suggested that [89Zr]Zr-(oxine)4 is most likely superior.

In papers III and IV methods to meet the generally increasing demand for 68Ga have been developed. In paper III a cyclotron-based solid target system was used for production and purification of the radionuclide. In paper IV a refinement method of the radionuclide’s quality (regarding content of competing metal ions) was developed for clinical applicability for use in radiolabelling of DOTA-based radiopharmaceuticals, [68Ga]Ga-DOTATOC and [68Ga]Ga-FAPI-46. Compared to generator-derived 68Ga, we successfully produced 10 times more product of both the radiopharmaceuticals using our solid target cyclotron-produced 68Ga.

The strategies and approaches investigated and developed in this thesis have potential for translation to more exotic radiometals in the future, to potentially expanding the palette of chemical properties that can be used in radiolabelling, as well as the decay characteristics and time-windows for imaging. The methods and techniques for radiometal labelling explored in this thesis might also be translated to other specific tissue targeting molecules or cell

List of scientific papers

I. Jussing, E.; Lu, L.; Grafstrom, J.; Tegnebratt, T.; Arnberg, F.; Rosik, H.W.; Wennborg, A.; Holmin, S.; Feldwisch, J.; Stone-Elander, S. [68Ga]ABY-028: an albumin-binding domain (ABD) protein-based imaging tracer for positron emission tomography (PET) studies of altered vascular permeability and predictions of albumin-drug conjugate transport. EJNMMI Research. 2020, 10, 106.
https://doi.org/10.1186/s13550-020-00694-2

II. Friberger, I.; Jussing, E.; Han, J.; Goos, J.; Siikanen, J.; Kaipe, H.; Lambert, M.; Harris, R.A.; Samen, E.; Carlsten, M.; Holmin, S.; Tran, T.A. Optimisation of the Synthesis and Cell Labelling Conditions for [89Zr]Zr-oxine and [89Zr]Zr-DFO-NCS: a Direct In Vitro Comparison in Cell Types with Distinct Therapeutic Applications. Molecular Imaging and Biology. 2021.
https://doi.org/10.1007/s11307-021-01622-z

III. Siikanen, J.; Jussing, E.; Milton, S.; Steiger, C.; Ulin, J.; Jonsson, C.; Samén, E.; Tran, T.A. Cyclotron-produced 68Ga from enriched 68Zn foils. Applied Radiation and Isotopes. 2021, 109825.
https://doi.org/10.1016/j.apradiso.2021.109825

IV. Jussing, E.; Milton, S.; Samen, E.; Moein, M.M.; Bylund, L.; Axelsson, R.; Siikanen, J.; Tran, T.A. Clinically Applicable Cyclotron-Produced Gallium-68 Gives High-Yield Radiolabeling of DOTA-Based Tracers. Biomolecules. 2021, 11.
https://doi.org/10.3390/biom11081118