225Ac/89Zr-Labeled N4MU01 Radioimmunoconjugates as Theranostics Against Nectin-4–Positive Triple-Negative Breast Cancer

Nectin-4 is an overexpressed biomarker in 60%-70% of triple-negative breast cancer (TNBC) cases and an ideal target for radiotherapy and PET imaging. In this study, theranostic radioimmunoconjugates were developed using a fully human anti-nectin-4 antibody (N4MU01). The imaging properties and therapeutic effectiveness of the radioimmunoconjugates were evaluated using TNBC models. Methods: N4MU01 was radiolabeled with ⁸⁹Zr and ²²⁵Ac for imaging and radiotherapy, respectively, using TNBC xenograft and syngeneic models. Biodistribution and PET imaging of the [⁸⁹Zr]Zr-deferoxamine (DFO)-N4MU01 radioimmunoconjugate was studied in mice bearing nectin-4-positive xenografts. Dosimetry and toxicity of [²²⁵Ac]Ac-Macropa-N4MU01 were studied in naïve BALB/c mice, and the therapeutic efficacy was evaluated with two doses of 13 or two doses of 18.6 kBq, administered 10 d apart in athymic BALB/c nude mice bearing either a human TNBC MDA-MB-468 xenograft or a human nectin-4-transfected 4T1 (4T1._nectin-4) syngeneic allograft. Results: The pharmacokinetic profile of the [⁸⁹Zr]Zr-DFO-N4MU01 radioimmunoconjugate showed biphasic distribution with a moderate elimination half-life of 63 h. PET imaging and biodistribution of [⁸⁹Zr]Zr-DFO-N4MU01 in mice bearing the MDA-MB-468 xenograft showed high tumor uptake of 13.2 ± 1.12 percent injected activity per gram at 120 h. [²²⁵Ac]Ac-Macropa-N4MU01 was effectively internalized in MDA-MB-468 and was cytotoxic to the cells with a 50% inhibition concentration of 1.2 kBq/mL. Toxicity studies revealed that 15 kBq of [²²⁵Ac]Ac-Macropa-N4MU01 was generally well tolerated, as indicated by hematologic, blood chemistry, and histopathologic analysis. Mice bearing MDA-MB-468 and 4T1._nectin-4 xenografts treated with 13 kBq of [²²⁵Ac]Ac-Macropa-N4MU01 had 100% (6/6) and 83.3% (5/6) complete tumor remissions, respectively. Conclusion: The specific tumor uptake and remarkable effectiveness against aggressive TNBC tumors are very promising and warrant the clinical development of N4MU01 radioimmunoconjugates.