AbstractOver the past decades, Ga3+ application in medicine has become significant, based in many respects on its similarity in some aspects to Fe3+. The difference between the two metals renders gallium interesting as potential therapeutic agent to disturb iron demand in cells and pathogens. The availability of radioactive gallium (68Ga and 67Ga) helps facilitate the use of gallium as an iron mimic for radionuclide imaging purposes. The current work is composed of two main projects dealing with different aspects of 68Ga/67Ga in medical imaging, with the common theme of involving gallium-specific biology.
Fe3+ siderophore complexes are taken up by bacteria making their isostructural Ga3+ complexes applicable, in principle, for imaging infection. Desferrioxamine-B (DFO) is a siderophore and a clinically accepted drug, hence, it can be radiolabelled with 68Ga and tested clinically for imaging graft infection with low regulatory barriers for use in humans. In the first part of this project, [68Ga]Ga-DFO was radiolabelled using GMP-graded reagents. [68Ga]Ga-DFO stability in human serum and urine was assessed in vitro. [68Ga]Ga-DFO in vivo biodistribution was studied in healthy animal models after i.v. injection. [68Ga]Ga-DFO was labelled with high radiochemical purity (≥ 95%). [68Ga]Ga-DFO demonstrated no binding to serum proteins in vitro and rapid renal excretion and low blood retention in vivo. Ex vivo biodistribution showed most of the activity resided in urine. However, incubating [68Ga]Ga-DFO in urine samples in vitro showed formation of metabolites which increase with time. We concluded that [68Ga]Ga-DFO is stable in blood during the timescale investigated and degradation occurred in urine. Overall, [68Ga]Ga-DFO can be easily radiolabelled for evaluation clinically as infection imaging agent.
Tris(8-hydroxyquinoline) gallium (III) (KP46) has been investigated as an orally administered anti-cancer drug. KP46 was developed on the rationale of better gut absorption and tumour effect compared to orally administered gallium salts. Despite preclinical and clinical evaluation of its efficacy, its trafficking to tumours remains poorly understood. In the second part of this project, [68Ga/67Ga]KP46 was used to elucidate the biological behaviour of KP46 in vivo post oral and i.v. administration as tracer or combined with a pharmacologically relevant dose of KP46. [68Ga/67Ga]KP46 was synthesised and its binding to apo-transferrin and albumin was measured in vitro by size exclusion chromatography. [68/67Ga]KP46 was giving i.v. or orally, as a tracer or combined with KP46. Mice were PET/CT scanned and culled for ex-vivo biodistribution. Octanol extraction and tissue digestion was performed on tissue samples to determine the form of 68/67Ga and to measure stable 69Ga by ICP-MS. [68/67Ga]KP46 showed no binding to apo-transferrin and minimal binding to albumin indicating the stability of [68/67Ga]KP46. Post i.v. injection of [68Ga]KP46 as tracer, most of the activity was seen in the liver, owing to its lipophilicity. Post oral administration of [68/67Ga]KP46 as tracer or with KP46, radioactivity remained in the large intestine with minimal trafficking to other tissues at 4 hours. At 24 hours post oral administration, the group administered with [67Ga]KP46 combined with KP46 showed better gut absorption compared to the group administered with [67Ga]KP46 as a tracer. However, delivery of 67Ga to tumour and other tissues was still low. All measurable radioactivity in tissue samples was no longer in the form of [68/67Ga]KP46. ICP-MS measurement of 69Ga in tissue samples was consistent with 68/67Ga trafficking results. We concluded that KP46 does not enhance gallium gut absorption compared to gallium salts after single oral dose and does not deliver gallium significantly to tumour. However, KP46 trafficking could be further investigated in the future after prolonged oral administration. In addition, the delivery of 8-hydroxyquinoline to tumour cells could be investigated as an active component to KP46 mode of action.
Finally, in the third part of this project, we investigated multiple conditions for radiolabelling 68Ga with 8-hydroxyquinoline and some of its derivatives aiming to find the best chemical form to deliver 67Ga into cells and study its radiobiology as an Auger electron emitter therapeutic radionuclide. Among ligands investigated, thiooxine complex of 68Ga showed the highest cellular uptake. However, more studies such as protein binding and cellular efflux should be conducted to understand the complex biological behaviour.
|Date of Award
|1 Sept 2021
|Philip Blower (Supervisor) & Michelle Ma (Supervisor)