Chelator boosted tumor-retention and pharmacokinetic properties: development of 64Cu labeled radiopharmaceuticals targeting neurotensin receptor.

Purpose: Accumulating evidence suggests that neurotensin (NTS) and neurotensin receptors (NTSRs) play key roles in lung cancer progression by triggering multiple oncogenic signaling pathways. This study aims to develop Cu-labeled neurotensin receptor 1 (NTSR1)-targeting agents with the potential for both imaging and therapeutic applications.

Method: A series of neurotensin receptor antagonists (NRAs) with variable propylamine (PA) linker length and different chelators were synthesized, including [⁶⁴Cu]Cu-CB-TE2A-iPA-NRA ([⁶⁴Cu]Cu-4a-c, i = 1, 2, 3), [⁶⁴Cu]Cu-NOTA-2PA-NRA ([⁶⁴Cu]Cu-4d), [⁶⁴Cu]Cu-DOTA-2PA-NRA ([⁶⁴Cu]Cu-4e, also known as [⁶⁴Cu]Cu-3BP-227), and [⁶⁴Cu]Cu-DOTA-VS-2PA-NRA ([⁶⁴Cu]Cu-4f). The series of small animal PET/CT were conducted in H1299 lung cancer model. The expression profile of NTSR1 was also confirmed by IHC using patient tissue samples.

Results: For most of the compounds studied, PET/CT showed prominent tumor uptake and high tumor-to-background contrast, but the tumor retention was strongly influenced by the chelators used. For previously reported 4e, [⁶⁴Cu]Cu-labeled derivative showed initial high tumor uptake accompanied by rapid tumor washout at 24 h. The newly developed [⁶⁴Cu]Cu-4d and [⁶⁴Cu]Cu-4f demonstrated good tumor uptake and tumor-to-background contrast at early time points, but were less promising in tumor retention. In contrast, our lead compound [⁶⁴Cu]Cu-4b demonstrated 9.57 ± 1.35, 9.44 ± 2.38 and 9.72 ± 4.89%ID/g tumor uptake at 4, 24, and 48 h p.i., respectively. Moderate liver uptake (11.97 ± 3.85, 9.80 ± 3.63, and 7.72 ± 4.68%ID/g at 4, 24, and 48 h p.i.) was observed with low uptake in most other organs. The PA linker was found to have a significant effect on drug distribution. Compared to [⁶⁴Cu]Cu-4b, [⁶⁴Cu]Cu-4a had a lower background, including a greatly reduced liver uptake, while the tumor uptake was only moderately reduced. Meanwhile, [⁶⁴Cu]Cu-4c showed increased uptake in both the tumor and the liver. The clinical relevance of NTSR1 was also demonstrated by the elevated tumor expression in patient tissue samples.

Conclusions: Through the side-by-side comparison, [⁶⁴Cu]Cu-4b was identified as the lead agent for further evaluation based on its high and sustained tumor uptake and moderate liver uptake. It can not only be used to efficiently detect NTSR1 expression in lung cancer (for diagnosis, patient screening, and treatment monitoring), but also has the great potential to treat NTSR-positive lesions once chelating to the beta emitter ⁶⁷Cu.