Evaluating Radiotheranostic Targets for Endometrial Cancer

Endometrial cancer is the most common gynecologic malignancy worldwide, and its incidence and mortality rates have increased over the past decade. Although early-stage disease is effectively treated via hysterectomy, a dearth of molecularly targeted therapies means that prognoses are far poorer for those with disseminated or recurrent disease. Herein, we describe the exploration of 3 biomarkers-human epidermal growth factor receptor 2 (HER2), mucin-16 (MUC16), and CD24-as potential radiotheranostic targets for endometrial cancer. Methods: Immunocytochemical staining was used to evaluate the expression of HER2, MUC16, and CD24 in endometrial cancer and healthy uterine and healthy endometrial cell lines. Subsequently, similar immunofluorescence staining analyses were performed with patient-derived endometrial cancer, healthy cervix, and healthy endometrium tissue samples. Stochastic bioconjugation methods were used to append deferoxamine (DFO) to a trio of monoclonal antibodies: trastuzumab (αHER2), AR9.6 (αMUC16), and ATG-031 (αCD24). These immunoconjugates were labeled with [⁸⁹Zr]Zr⁴⁺, and the in vivo performance of the resulting immuno-PET probes-[⁸⁹Zr]Zr-DFO-trastuzumab, [⁸⁹Zr]Zr-DFO-AR9.6, and [⁸⁹Zr]Zr-DFO-ATG-031-was interrogated via PET imaging and biodistribution experiments in cell line and patient-derived murine models of endometrial cancer. Results: Immunofluorescent staining revealed that endometrial cancer cells and tissue samples expressed elevated levels of HER2, MUC16, and CD24 compared with healthy control cells and tissue samples. [⁸⁹Zr]Zr-DFO-trastuzumab, [⁸⁹Zr]Zr-DFO-AR9.6, and [⁸⁹Zr]Zr-DFO-ATG-031 were synthesized with high radiochemical conversion (>95%), radiochemical purity (>95%), specific activity (4-5 mCi/mg), and immunoreactivity (>80%). The 3 immuno-PET probes exhibited significantly different behavior in mice bearing subcutaneous endometrial cancer xenografts: [⁸⁹Zr]Zr-DFO-ATG-031 provided the highest tumor uptake (>30 %ID/g) and tumor-to-background contrast; [⁸⁹Zr]Zr-DFO-trastuzumab produced moderate yet promising results; and [⁸⁹Zr]Zr-DFO-AR9.6 yielded substandard images. Subsequent imaging experiments in mice bearing patient-derived xenografts reinforced the potential of the CD24- and HER2-targeted immuno-PET probes. Conclusion: [⁸⁹Zr]Zr-DFO-ATG-031 and [⁸⁹Zr]Zr-DFO-trastuzumab exhibit significant promise for immuno-PET imaging of endometrial cancer. These probes may prove beneficial in the clinic, not only for the noninvasive staging and monitoring of the disease but also as companion imaging agents for HER2- and CD24-targeted therapeutics.