Targeting breast carcinoma with radioiodinated anti-HER2 Nanobody

Introduction: With a molecular weight an order of magnitude lower than antibodies but possessing comparable affinities, Nanobodies (Nbs) are attractive as targeting agents for cancer diagnosis and therapy. An anti-HER2 Nb could be utilized to determine HER2 status in breast cancer patients prior to trastuzumab treatment. This provided motivation for the generation of HER2-specific 5F7GGC Nb, its radioiodination and evaluation for targeting HER2 expressing tumors. Methods: 5F7GGC Nb was radioiodinated with 125I using Iodogen and with 131I using the residualizing agent Nε-(3-[131I]iodobenzoyl)-Lys5-Nα-maleimido-Gly1-GEEEK ([131I]IB-Mal-D-GEEEK) used previously successfully with intact antibodies. Paired-label internalization assays using BT474M1 cells and tissue distribution experiments in athymic mice bearing BT474M1 xenografts were performed to compare the two labeled Nb preparations. Results: The radiochemical yields for Iodogen and [131I]IB-Mal-D-GEEEK labeling were 83.6 ± 5.0% (n = 10) and 59.6 ± 9.4% (n = 15), respectively. The immunoreactivity of labeled proteins was preserved as confirmed by in vitro and in vivo binding to tumor cells. Biodistribution studies showed that Nb radiolabeled using [131I]IB-Mal-D-GEEEK, compared with the directly labeled Nb, had a higher tumor uptake (4.65 ± 0.61% ID/g vs. 2.92 ± 0.24% ID/g at 8 h), faster blood clearance, lower accumulation in non-target organs except kidneys, and as a result, higher concomitant tumor-to-blood and tumor-to-tissue ratios. Conclusions: Taken together, these results demonstrate that 5F7GGC anti-HER2 Nb labeled with residualizing [131I]IB-Mal-D-GEEEK had better tumor targeting properties compared to the directly labeled Nb suggesting the potential utility of this Nb conjugate for SPECT (129I) and PET imaging (124I) of patients with HER2-expressing tumors.