Single-Domain Antibody Probe with Low Renal Uptake for Claudin 18.2–Targeted PET Imaging of Gastric Cancer: Preclinical and Pilot Clinical Evaluations

High renal uptake limits the clinical translation of Claudin 18.2 (CLDN18.2) nanobody probes. We aimed to develop and identify a single-domain antibody–based molecular probe to minimize nonspecific renal accumulation while maintaining high tumor affinity and effective uptake. Methods: A CLDN18.2-targeted nanobody, SNA014, was radiolabeled with ⁶⁸Ga to yield [⁶⁸Ga]Ga-SNA014. The binding capability of [⁶⁸Ga]Ga-SNA014 was evaluated in vitro using flow cytometry, immunohistochemistry, and cell-binding assays. The biologic behavior of [⁶⁸Ga]Ga-SNA014 in vivo was assessed through small-animal PET imaging, biodistribution studies, and blood pharmacokinetic analysis in human gastric adenocarcinoma xenograft models (both wild-type and CLDN18.2-overexpressing). Furthermore, a preliminary clinical evaluation of [⁶⁸Ga]Ga-SNA014 was conducted in 3 patients with gastric cancer, including whole-body PET/CT imaging and radiation dosimetry analysis. Results: [⁶⁸Ga]Ga-SNA014 was successfully synthesized with high radiochemical purity (>95%) and excellent stability both in vitro and in vivo. The probe demonstrated strong binding affinity and specificity toward AGS^CLDN18.2 cells. Small-animal PET/CT images of AGS^CLDN18.2 tumor–bearing mice exhibited high tumor and stomach uptake and low kidney uptake, and pretreatment with succinylated gelatin further reduced kidney retention. Blood clearance revealed a rapid elimination profile, with a half-life of 47.68 ± 1.83 min. In human PET/CT studies, distinct visualization of lesions was achieved up to 0.5 h postinjection. Dosimetry analysis revealed that the effective radiation dose of [⁶⁸Ga]Ga-SNA014 was lower than that of standard [¹⁸F]FDG PET/CT. Conclusion: These findings demonstrated that [⁶⁸Ga]Ga-SNA014 exhibits high affinity and specificity and excellent targeting performance and safety, enabling precise detection of CLDN18.2-overexpressing tumors.