In a recent publication from Minerva in collaboration with the University of Copenhagen, the preclinical biodistribution, tolerability, and efficacy of Terbium [161Tb]Tb-trastuzumab were evaluated in human epidermal growth factor receptor 2 (HER2) expressing breast cancer tumors.

Synthesis and in vitro evaluation of [161Tb]Tb-trastuzumab

Trastuzumab was successfully conjugated to DOTA with a labeling degree of 1–2 chelators per antibody. The conjugate was radiolabeled with 161Tb at a 97% ± 2% yield and >99% radiochemical purity. Both modified and unmodified trastuzumab retained nanomolar binding affinity (Kd 0.83 and 0.37 nM). The immunoreactive fraction of [161Tb]Tb‑trastuzumab was 89%. The radioconjugate showed <1% free ¹⁶¹Tb in buffer up to 168 h and remained >90% stable in human and mouse serum.

In vivo biodistribution

SPECT/CT imaging showed [161Tb]Tb‑trastuzumab uptake in tumor, liver, spleen, blood and kidney across all tested specific activities. Kidney and blood activity decreased over time, while liver uptake remained stable and spleen uptake increased. Tumor uptake peaked at 72 h, with lower uptake and higher blood retention at the lowest specific activity (0.005 MBq/mg), indicating partial blocking. Overall, specific activities of 0.5–0.05 MBq/mg provided the best tumor targeting and clearance profile.

Figure 1. In vivo biodistribution of [161Tb]Tb-trastuzumab. (A) Representative SPECT/CT images 168 h after injection of [161Tb]Tb-trastuzumab radiolabeled at 0.5, 0.05, and 0.005 MBq/mg. (B) Quantitative analysis of [161Tb]Tb-trastuzumab uptake in tumor, liver, spleen, blood, and kidney at 4, 24, 72, and 168 h after injection. From Cold et al, JNM 2026

Dose escalation of [161Tb]Tb-trastuzumab

The tolerability of [161Tb]Tb‑trastuzumab was assessed with a dose‑escalation study (5, 10, 20 MBq). All tested doses resulted in body weight loss, which became more significant at 10 and 20 MBq. The 20 MBq group showed clear radiotoxicity, and the 10 MBq group required euthanasia by day 21 due to continued weight loss. Blood analysis showed dose‑dependent reductions in red and white blood cells and platelets. All treated groups showed tumor‑growth delay. Based on these observations, a maximum dose of 10 MBq of [161Tb]Tb-trastuzumab was determined as tolerated.

In vivo antitumor efficacy

An efficacy study tested 5 and 10 MBq of [161Tb]Tb‑trastuzumab while keeping the antibody dose constant. Both dose groups showed no radiotoxicity and produced clear tumor‑growth delay compared with vehicle, trastuzumab, and trastuzumab deruxtecan. Treatment responses were consistent across mice receiving [161Tb]Tb‑trastuzumab, and both doses significantly reduced tumor volume throughout the study, with no significant difference between the two dose levels at study end.

Figure 2: (A) Body weight and (B) tumor growth in BT474 xenografts throughout the study. From Cold et al, JNM 2026.

These results support the potential of [161Tb]Tb-trastuzumab as an attractive treatment option for HER2-expressing cancers.

Interested in learning how Minerva Imaging can support studies of targeted radionuclide therapies in relevant HER2-expressing or other xenograft models?

Read more about our: 

References:

  • Sigrid Cold, Pragalath Sadasivam, Mette M. Wessel, Myrto Ischyropoulou, Jesper Fonslet, Trine B. Engel, Carsten H. Nielsen, Andreas Kjaer and Lotte K. Kristensen (2026). In Vivo Studies of [161Tb]Tb-Trastuzumab Radiopharmaceutical Therapy in Human Epidermal Growth Factor Receptor 2–Expressing Breast Tumors Show High Tumor Uptake and Tumor Growth Suppression. Journal of Nuclear Medicine. jnumed.125.269741; DOI: https://doi.org/10.2967/jnumed.125.269741