A novel NTSR1-targeting diagnostic radioligand SKL35502 for noninvasive solid tumor imaging

Background: Neurotensin receptor 1 (NTSR1) is highly expressed in several aggressive malignancies, including pancreatic ductal adenocarcinoma, colorectal cancer, esophageal cancer, gastric cancer, biliary tract cancer, and head and neck squamous cell carcinoma. Given its role in tumor progression, NTSR1 represents a compelling target for molecular imaging and for selecting patients for NTSR1-directed radionuclide therapy. SKL35502, an 111 In-labeled small-molecule NTSR1 ligand, was developed to enable sensitive and selective visualization of NTSR1-positive tumors. This study evaluates the preclinical pharmacology, biodistribution, and imaging performance of SKL35502. Methods: The binding affinity of SKL35502 was determined using a cell-based saturation binding assay. Time-dependent uptake and internalization were assessed in HCT116 human colorectal carcinoma cells. In vivo biodistribution was evaluated in female mice bearing HCT116 or AsPC-1 xenografts by SPECT/CT imaging up to 168 hours post-injection, and ex vivo biodistribution and excretion were confirmed in HCT116 xenograft models up to 48 hours. Urine and feces were collected at 8, 24, 48, 72 and 96 hours post-dose. Results: SKL35502 exhibited high binding affinity to HCT116 cells (KD = 0.37 nM). Cellular uptake was time-dependent and NTSR1-specific, reaching 31.5% at 90 minutes and remaining at 30.5% at 240 minutes. Internalization accounted for approximately 56% of total uptake at both time points. In vivo SPECT/CT imaging revealed an initial widespread distribution followed by rapid clearance from non-target organs (heart, liver, kidney). Tumor uptake peaked at 4 hours post-injection and persisted through 168 hours. Ex vivo biodistribution confirmed strong tumor enrichment (44 %ID/g at 8 hours), with tumor exposure (AUC) approximately 22-fold higher than blood. By 96 hours (4 days) post-dose, the total excreted radioactivity was 84.12% of the administered dose Conclusions: SKL35502 demonstrates high affinity and specific uptake in NTSR1-expressing tumor cells, together with rapid systemic clearance that minimizes nonspecific radiation exposure to healthy tissues. These properties support its potential as a highly sensitive and selective imaging agent for detecting NTSR1-positive tumors and guiding patient selection in NTSR1-targeted theranostic approaches.