Targeted Radionuclide Therapy of CLDN18.2-Positive Gastric Cancer with [¹³¹I]I-Zolbetuximab: An <i>In Vitro</i> and <i>In Vivo</i> Study

Radiopharmaceuticals are a promising therapeutic strategy for tumors with less reduced resistance and minimal side effects. In our previous study, we radiolabeled the monoclonal antibody zolbetuximab, which targets CLDN18.2, with ¹²⁵I, and the labeled compound showed favorable tumor targeting and retention. In this research article, [¹³¹I]I-zolbetuximab was prepared to investigate its therapeutic effect on gastric cancer using <i>in vitro</i> and <i>in vivo</i> studies. The zolbetuximab was radiolabeled with ¹³¹I using the Iodogen method. The uptake mechanism of [¹³¹I]I-zolbetuximab was investigated through an endocytosis experiment using MKN45-CLDN18.2 and MKN45 cells. The safety assessment of [¹³¹I]I-zolbetuximab was conducted in normal mice using hematoxylin/eosin (H&E) staining. The tumor uptake, biodistribution, and therapeutic efficacy of [¹³¹I]I-zolbetuximab were evaluated in nude mice bearing MKN45-CLDN18.2 tumors, while the hematological analysis, immunohistochemistry, Western blotting (WB), immunofluorescence, and H&E assays were used to further assess the treatment response and toxicity. [¹³¹I]I-Zolbetuximab exhibited a high labeling efficiency of (96.05 ± 0.23)%, a specific activity of 1.75 × 10² GBq/μmol, and good <i>in vitro</i> stability. The binding of [¹³¹I]I-zolbetuximab to the membrane surface receptors of MKN45-CLDN18.2 cells resulted in significant tumor uptake, retention, and favorable biodistribution in CLDN18.2-positive tumor-bearing nude mice. Safety assessments, including H&E staining, indicated no significant damage to normal mouse organs caused by the labeled compounds. In a therapeutic study involving MKN45-CLDN18.2 tumor-bearing nude mice, increasing drug dose led to notable enhancements in therapeutic efficacy and survival rates. H&E staining of mouse organs at the end of treatment showed no significant toxicity was observed across all dose groups throughout the treatment period. Furthermore, immunohistochemistry, immunoblotting, and immunofluorescence analyses conducted on mouse tumors at the treatment end point demonstrated a reduction in CLDN18.2 expression following treatment. Altogether, [¹³¹I]I-zolbetuximab displayed exceptional targeting capability, tumor retention property, significant therapeutic efficacy, and safety. These findings suggest its potential to serve as a targeted radiopharmaceutical for the treatment of CLDN18.2-positive gastric cancer.