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 I-125 using Iodogen and with I-131 using the residualizing agent AP-(3-[I-131]iodobenzoyl)-Lys(5)-N-alpha-maleimido-Gly(1)-GEEEK (I-113]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 [I-131]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 [I-131]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 [I-131]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 (I-129) and PET imaging (I-124) of patients with HER2-expressing tumors. (C) 2013 Elsevier Inc. All rights reserved.