Manipulating the In Vivo Behaviour of Ga with Tris(Hydroxypyridinone) Chelators: Pretargeting and Blood Clearance.

Pretargeting is widely explored in immunoPET as a strategy to reduce radiation exposure of non-target organs and allow the use of short-lived radionuclides that would not otherwise be compatible with the slow pharmacokinetic profiles of antibodies. Here we investigate a pretargeting strategy based on gallium-68 and the chelator THP as a high-affinity pair capable of combining in vivo. After confirming the ability of THP to bind Ga in vivo at low concentrations, the bifunctional THP-NCS was conjugated to a humanised huA33 antibody targeting the A33 glycoprotein. Imaging experiments performed in nude mice bearing A33-positive SW1222 colorectal cancer xenografts compared pretargeting (100 μg of THP-NCS-huA33, followed after 24 h by 8-10 MBq of Ga) with both a directly labelled radioimmunoconjugate (Zr-DFO-NCS-huA33, 88 μg, 7 MBq) and a Ga-only negative control (8-10 MBq of Ga). Imaging was performed 25 h after antibody administration (1 h after Ga administration for negative control). No difference between pretargeting and the negative control was observed, suggesting that pretargeting via metal chelation is not feasible using this model. However, significant accumulation of "unchelated" Ga in the tumour was found (12.9 %ID/g) even without prior administration of THP-NCS-huA33, though tumour-to-background contrast was impaired by residual activity in the blood. Therefore, the Ga-only experiment was repeated using THP (20 μg, 1 h after Ga administration) to clear circulating Ga, producing a three-fold improvement of the tumour-to-blood activity concentration ratio. Although preliminary, these results highlight the potential of THP as a Ga clearing agent in imaging applications with gallium citrate.