In Vivo Studies of [ ¹⁶¹ Tb]Tb-Trastuzumab Radiopharmaceutical Therapy in Human Epidermal Growth Factor Receptor 2–Expressing Breast Tumors Show High Tumor Uptake and Tumor Growth Suppression

Human epidermal growth factor receptor 2 (HER2) has emerged as attractive for targeted radionuclide therapy of HER2-expressing breast cancer. The present study aimed to investigate the image-derived biodistribution, tolerability, and efficacy of [¹⁶¹Tb]Tb-trastuzumab as a potential therapeutic alternative or supplement to trastuzumab and trastuzumab deruxtecan in HER2-expressing tumors. <b>Methods:</b> Trastuzumab was functionalized with DOTA by non-site-specific conjugation and radiolabeled with ¹⁶¹Tb. In vitro assays were performed to evaluate binding affinity and immunoreactivity of [¹⁶¹Tb]Tb-trastuzumab. The influence of mass dose on the biodistribution of [¹⁶¹Tb]Tb-trastuzumab was assessed in mice bearing subcutaneous BT474 tumors by SPECT/CT imaging performed 4, 24, 72, and 168 h after injection of 20 MBq of [¹⁶¹Tb]Tb-trastuzumab labeled at 3 specific activities. Tolerability was evaluated with escalating doses of 5, 10, and 20 MBq of [¹⁶¹Tb]Tb-trastuzumab, and the efficacy of [¹⁶¹Tb]Tb-trastuzumab was compared with that of the vehicle, trastuzumab, and trastuzumab deruxtecan groups. <b>Results:</b> Functionalization and radiolabeling resulted in a radiochemical yield of 97% ± 2% and a radiochemical purity above 99%. The binding affinity of functionalized and native trastuzumab was 0.83 and 0.39 nM, respectively, with an immunoreactive fraction of 89%. SPECT/CT imaging demonstrated peak tumor uptake 72 h after injection, with tumor uptake of 10.8 ± 1.1, 12.8 ± 2.2, and 6.3 ± 0.03 %ID/g of tissue for mice dosed with 0.5, 0.05, and 0.005 MBq/µg, respectively. Radiotoxicity was evident in mice dosed with 20 MBq of [¹⁶¹Tb]Tb-trastuzumab, whereas 5 and 10 MBq of [¹⁶¹Tb]Tb-trastuzumab were well tolerated. Antitumor efficacy was observed in mice dosed with 10 MBq of [¹⁶¹Tb]Tb-trastuzumab, which presented tumor volumes statistically significantly smaller than those of the vehicle group 104 d after treatment (<i>P</i> = 0.0007). <b>Conclusion:</b> Functionalization and radiolabeling did not affect binding affinity. Biodistribution of [¹⁶¹Tb]Tb-trastuzumab revealed high tumor uptake of more than 10 %ID/g of tissue, peaking at 72 h. [¹⁶¹Tb]Tb-trastuzumab in doses of up to 10 MBq was tolerated and highly effective at inhibiting tumor growth (<i>P</i> = 0.0007). These results support the potential of [¹⁶¹Tb]Tb-trastuzumab as an attractive treatment option for HER2-expressing cancers.