Peptide-Based ROR1-Targeting PET Ligands for Melanoma Tumor Imaging: Design and Preclinical Evaluation

Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is overexpressed in multiple cancers while remaining largely absent in adult tissues, which makes it an attractive target for both tumor diagnosis and therapy. To enable noninvasive imaging of ROR1, four peptide-based PET ligands [⁶⁸Ga]<b>1</b>-<b>4</b> were rationally designed and evaluated for melanoma imaging. <i>In vitro</i> assays confirmed reasonable ROR1 binding affinity (<i>K</i>_D = 481.0 and 44.9 nM, respectively) and specific cellular uptake of [⁶⁸Ga]<b>2</b> and [⁶⁸Ga]<b>3</b>, which are functionalized with serum albumin-binding groups. Notably, microPET/CT imaging and biodistribution studies in B16F10, A375, and SK-MEL-28 tumor-bearing mice demonstrated that [⁶⁸Ga]<b>2</b> achieved the most favorable imaging performance, characterized by high tumor accumulation (up to 9.18% ID/g), sustained retention, and a relatively lower nonspecific background signal. These findings highlight [⁶⁸Ga]<b>2</b> as a promising candidate for ROR1-targeting PET imaging and underscore the potential of peptide-based ROR1 PET probes for tumor imaging and therapy guidance.