Design and Evaluation of ⁶⁸ Ga-Labeled Peptide-Based PET Probes for Noninvasive Imaging of ROR1 Expression in Tumor Models

The receptor tyrosine kinase-like orphan receptor 1 (ROR1) is aberrantly overexpressed in multiple malignancies and has emerged as a clinically relevant biomarker for tumor staging and therapeutic decision-making. Here, we developed a series of PR7-derived ROR1-targeted radiotracers, including linear and cyclic analogues, and systematically evaluated their specificity in vitro and in vivo. All radiotracers showed high stability in saline and fetal bovine serum for at least 90 min and rapid tumor accumulation within 30 min post-injection in MC38 tumor-bearing mice. Among them, [68Ga]Ga-LP4 achieved favorable tumor targeting and tumor-to-nontumor ratios in PET imaging with predominant renal clearance. Notably, compared with [18F]AlF-NP1 reported in our previous study, [68Ga]Ga-LP4, which incorporates N-methylation and carboxyl amidation, demonstrated improved in vivo metabolic stability. Collectively, these findings identify [68Ga]Ga-LP4 as a promising ROR1-targeted imaging probe and highlight useful peptide optimization strategies.