Manipulating the In Vivo Behaviour of ⁶⁸Ga with Tris(Hydroxypyridinone) Chelators: Pretargeting and Blood Clearance

• Main Authors: Cinzia Imberti, Pierre Adumeau, Julia E. Blower, Fahad Al Salemee, Julia Baguña Torres, Jason S. Lewis, Brian M. Zeglis, Samantha Y. A. Terry, Philip J. Blower
• Format: Article
• Language: English
• Published: MDPI AG 2020-02-01
• Series: International Journal of Molecular Sciences
• Subjects: metal chelation; radionuclide imaging; pretargeting; gallium-68; hydroxypyridinones; monoclonal antibodies; bifunctional chelators
• Online Access: https://www.mdpi.com/1422-0067/21/4/1496
• ISSN: 1422-0067

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^Me as a high-affinity pair capable of combining in vivo. After confirming the ability of THP^Me to bind ⁶⁸Ga in vivo at low concentrations, the bifunctional THP^Me-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^Me-NCS-huA33, followed after 24 h by 8−10 MBq of ⁶⁸Ga³⁺) with both a directly labelled radioimmunoconjugate (⁸⁹Zr-DFO-NCS-huA33, 88 μg, 7 MBq) and a ⁶⁸Ga-only negative control (8−10 MBq of ⁶⁸Ga³⁺). Imaging was performed 25 h after antibody administration (1 h after ⁶⁸Ga³⁺ 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” ⁶⁸Ga³⁺ in the tumour was found (12.9 %ID/g) even without prior administration of THP^Me-NCS-huA33, though tumour-to-background contrast was impaired by residual activity in the blood. Therefore, the ⁶⁸Ga-only experiment was repeated using THP^Me (20 μg, 1 h after ⁶⁸Ga³⁺ administration) to clear circulating ⁶⁸Ga³⁺, producing a three-fold improvement of the tumour-to-blood activity concentration ratio. Although preliminary, these results highlight the potential of THP^Me as a ⁶⁸Ga clearing agent in imaging applications with gallium citrate.