Slow Internalization of Anti-HER2 Synthetic Affibody Monomer 111 In-DOTA-Z HER2:342-pep2 : Implications for Development of Labeled Tracers

Affibody molecules are a novel class of targeting proteins, demonstrating promising results in the molecular imaging of tumor markers. The aim of this study was to investigate the cellular processing of Affibody molecules bound to human epidermal growth-factor-receptor type 2 (HER2). Cellular processing of the synthetic Affibody molecule, DOTA-Z_{HER2:342-pep2} (K_D = 65 _pM) labeled with indium-111, was studied both during continuous and interrupted incubation with HER2-expressing cell lines (SKOV-3, SKBR-3, and BT474). The internalized and membrane bound fractions of Affibody molecule were discriminated by treatment with 4 M of urea solution in 0.2 M of glycine buffer (pH 2.0). Incubation media collected after an interrupted incubation was analyzed for the presence of radiocatabolites. Continuous incubation of tumor cells with ¹¹¹In-DOTA-Z_{HER2:342-pep2} led to the saturation of HER2 and slow internalization. Sixty (60)- to 80% of the radioactivity remained cell associated 24 hours after interrupted incubation. The rate of Affibody molecule internalization was the same after interrupted incubation, as in the continuous incubation experiments. Internalization of ¹¹¹ In-DOTA-Z_{HER2:342-pep2} was relatively slow. A high level of cellular retention of the tracer was provided by strong binding to cell-surface receptors. These data suggest that good tumor targeting with anti-HER Affibody molecules may be obtained by using short-lived, nonresidualizing labels.