Synthesis of⁶⁴Cu-,⁵⁵Co-, and⁶⁸Ga-Labeled Radiopharmaceuticals Targeting Neurotensin Receptor-1 for Theranostics: Adjusting In Vivo Distribution Using Multiamine Macrocycles

The development of theranostic radiotracers relies on their binding to specific molecular markers of a particular disease and the use of corresponding radiopharmaceutical pairs thereafter. This study reports the use of multiamine macrocyclic moieties (MAs), as linkers or chelators, in tracers targeting the neurotensin receptor-1 (NTSR-1). The goal is to achieve elevated tumor uptake, minimal background interference, and prolonged tumor retention in NTSR-1–positive tumors. Methods: We synthesized a series of neurotensin antagonists bearing MA linkers and metal chelators. The MA unit is hypothesized to establish a strong interaction with the cell membrane, and the addition of a second chelator may enhance water solubility, consequently reducing liver uptake. Small-animal PET/CT imaging of [⁶⁴Cu]Cu-DOTA-SR-3MA, [⁶⁴Cu]Cu-NT-CB-NOTA, [⁶⁸Ga]Ga-NT-CB-NOTA, [⁶⁴Cu]Cu-NT-CB-DOTA, and [⁶⁴Cu]Cu-NT-Sarcage was acquired at 1, 4, 24, and 48 h after injection using H1299 tumor models. [⁵⁵Co]Co-NT-CB-NOTA was also tested in HT29 (high NTSR-1 expression) and Caco2 (low NTSR-1 expression) colorectal adenocarcinoma tumor models. Saturation binding assay and internalization of [⁵⁵Co]Co-NT-CB-NOTA were used to test tracer specificity and internalization in HT29 cells. Results: In vivo PET imaging with [⁶⁴Cu]Cu-NT-CB-NOTA, [⁶⁸Ga]Ga-NT-CB-NOTA, and [⁵⁵Co]Co-NT-CB-NOTA revealed high tumor uptake, high tumor-to-background contrast, and sustained tumor retention (≤48 h after injection) in NTSR-1–positive tumors. Tumor uptake of [⁶⁴Cu]Cu-NT-CB-NOTA remained at 76.9% at 48 h after injection compared with uptake 1 h after injection in H1299 tumor models, and [⁵⁵Co]Co-NT-CB-NOTA was retained at 60.2% at 24 h compared with uptake 1 h after injection in HT29 tumor models. [⁶⁴Cu]Cu-NT-Sarcage also showed high tumor uptake with low background and high tumor retention 48 h after injection Conclusion: Tumor uptake and pharmacokinetic properties of NTSR-1–targeting radiopharmaceuticals were greatly improved when attached with different nitrogen-containing macrocyclic moieties. The study results suggest that NT-CB-NOTA labeled with either ⁶⁴Cu/⁶⁷Cu, ⁵⁵Co/58mCo, or ⁶⁸Ga (effect of ¹⁷⁷Lu in tumor to be determined in future studies) and NT-Sarcage labeled with ⁶⁴Cu/⁶⁷Cu or ⁵⁵Co/58mCo may be excellent diagnostic and therapeutic radiopharmaceuticals targeting NTSR-1–positive cancers. Also, the introduction of MA units to other ligands is warranted in future studies to test the generality of this approach.