Synthesis, preclinical evaluation and pilot clinical translation of [${}^{68}$Ga]Ga-PMD22, a novel nanobody PET probe targeting CLDN18.2 of gastrointestinal cancer

Claudin18.2 (CLDN18.2) is a novel target for diagnosis and therapy of gastrointestinal cancer. This study aimed to evaluate the safety and feasibility of a novel CLDN18.2-targeted nanobody, PMD22, labeled with gallium-68 ([${}^{68}$Ga]Ga, for detecting CLDN18.2 expression in patients with gastrointestinal cancer using PET/CT imaging. [${}^{68}$Ga]Ga-PMD22 was synthesized based on the nanobody, and its cell binding properties were assayed. Preclinical pharmacokinetics were determined in CLDN18.2-positive xenografts using microPET/CT. Effective dosimetry of [${}^{68}$Ga]Ga-PMD22 was evaluated in 5 gastrointestinal cancer patients, and PET/CT imaging of [${}^{68}$Ga]Ga-PMD22 and [${}^{18}$F]FDG were performed head-to-head in 16 gastrointestinal cancer patients. Pathological tissues were obtained for CLDN18.2 immunohistochemical (IHC) staining and comparative analysis with PET/CT findings. Cell binding assay showed that [${}^{68}$Ga]Ga-PMD22 had a higher binding ability to AGS${}^{CLDN18.2}$ and BGC823${}^{CLDN18.2}$ cells than to AGS and BGC823 cells (p < 0.001). MicroPET/CT images showed that [${}^{68}$Ga]Ga-PMD22 rapidly accumulated in AGS${}^{CLDN18.2}$ and BGC823${}^{CLDN18.2}$ tumors, and high contrast tumor to background imaging was clearly observed. In the pilot study, the effective dose of [${}^{68}$Ga]Ga-PMD22 was 1.68E-02 ± 1.45E-02 mSv/MBq, and the CLDN18.2 IHC staining result was highly correlated with the SUV${}_{max}$/BKG${}_{stomach}$ of [${}^{68}$Ga]Ga-PMD22 (r${}_s$ = 0.848, p < 0.01). A novel [${}^{68}$Ga]Ga-labeled nanobody probe targeting CLDN18.2, [${}^{68}$Ga]Ga-PMD22, was established and preliminarily proved to be safe and effective in revealing CLDN18.2-positive gastrointestinal cancer, providing a basis for the clinical translation of the agent.