Targeting of δ-opioid receptors in xenografted colon cancer using ¹¹¹In-labeled deltorphin-II ligand

Objectives δ-opioid receptors (DOR) are highly expressed in a variety of human tumors. We have developed an ¹¹¹In-labeled deltorphin-II ligand for in vivo imaging of DOR. The purpose of this study was to investigate the feasibility of this radioligand for tumor targeting and to characterize its in vivo kinetic profile in xenografted human HCT116 colon cancer.

Methods The deltorphin-II ligand was generated by solid-phase peptide synthesis and linked with DOTA chelator for ¹¹¹In labeling. Specific competitive cell binding studies were performed in HCT116 human cancer cells expressing DOR. Xenografted tumors were grown for 10-18 days after subcutaneous implantation of 1 x 10⁶ HCT116 cells in the right shoulder of nude mice. High-resolution SPECT imaging was performed in 6 mice expressing DOR (DOR+) and 5 wild-type control mice (DOR-). Immediately after i.v. injection of 7.4 MBq of radiotracer, dynamic images were acquired for 1 hr using a stationary small-animal SPECT imager. The animals were imaged again at 3 and 24 hrs post-injection. Tissue samples were harvested at the end of the imaging session for postmortem analysis.

Results In vitro competitive binding assays revealed that the ¹¹¹In-labeled ligand bound to cells expressing DOR with an IC₅₀ value of 18 nM. DOR+ tumors were detectable at 15-30 minutes post-injection by SPECT imaging and became well visualized at 3 hrs and increasingly prominent at 24 hrs. The DOR- tumors were initially visualized, but became less visible at 3 hrs and faintly visible at 24 hrs. The tumor/non-tumor ratios determined by ROI analysis were 1.62±0.21 for DOR- and 7.91±0.19 for DOR+ (P < 0.01) at 24 hrs.

Conclusions The deltorphin-II radioligand may provide a new approach to study the expression of δ-opioid receptors in tumor growth and metastasis.

Research Support NCI grant RO1-CA123547 & RO1-CA97360; NIBIB grant P41-EB002035; ABRC-070