Targeted imaging of NK1 receptor-expressing cancer with ⁶⁴Cu-labeled new NK1 ligand

Objectives Neurokinin 1 receptor (NK1R) is expressed in gliomas and other malignancies and represents a promising target for molecular imaging and targeted radionuclide therapy. The goal of this study was to evaluate a novel ⁶⁴Cu-labeled ligand (NK1R-NOTA) for targeting NK1R expressing cells and xenografts.

Methods The new NK1R ligand was synthesized using a 6-step procedure and linked to NOTA for ⁶⁴Cu chelation. ⁶⁴Cu-NK1R-NOTA was prepared by incubation of NK1R-NOTA with ⁶⁴CuCl₂ in the presence of CH₃COONH₄. The radioligand uptake was evaluated for NK1R-transduced HEK293 cells (HEK293-NK1R) and for NK1R negative HEK293 wild type cells. Biodistribution and tumor uptake was assessed by small animal PET imaging of mice bearing HEK293-NK1R and HEK293-WT xenografts.

Results NK1R-NOTA was synthesized in an overall yield of ~55 %. NK1R-NOTA-⁶⁴Cu was obtained with a radiochemical purity of > 95%, and the specific activity was >3.7 MBq/nmol. HEK293-NK1R cells demonstrated a high uptake of NK1R-NOTA-⁶⁴Cu (15.9±0.6% after 2 h incubation at 37 °C); co-incubation with an excess of unlabeled NK1R-NOTA reduced uptake by 90%. PET imaging showed that NK1R-NOTA-⁶⁴Cu had a rapid high accumulation in HEK293-NK1R xenografts and low uptake in HEK293-WT xenografts (7.1±1.3 vs 0.7±0.2%ID/mL at 1h). A significant fraction of radioactivity was retained in HEK293-NK1R xenografts at 4 and 20 h pi (6.2±1.4 and 3.8±0.8%ID/mL, respectively). Radioactivity was rapidly cleared by the liver, which resulted in high contrast images. Biodistribution studies confirmed that the tumor-to-organ ratios were ≥ 5 for all studied organs at 1 h p.i, except liver and intestine; and the tumor-to-intestine ratio was also improved to 5 at 20 h.

Conclusions NK1R-NOTA-⁶⁴Cu is a promising ligand for PET imaging of NK1R-expressing cancer cells. Rapid tumor uptake and high tumor-to-organ ratios at 1 h are encouraging for imaging tumors outside the abdomen with Al¹⁸F or ⁶⁸Ga-labeled NK1R-NOTA.

Research Support Supported by NIH grant P50-CA84638, NIH Small-Animal Imaging Research Program grant R24 CA83084 and NIH Center grant P30 CA08748.