[Table, 111In-Labeled affibody ABY-025 targeting epidermal growth factor receptor 2]. (2010)

An overexpressed or a mutated transmembrane epidermal growth factor receptor (EGFR; for details regarding this receptor family, see Hollmen and Elenius (1)) that modulates its physiological functions (cell growth, proliferation, migration, etc.) through an intracellular tyrosine kinase (TK) signaling pathway is known to be involved in the development of various cancers (2). As a consequence, one of the strategies used for the in vivo detection (by imaging) or treatment of cancers that are known to involve the EGFR includes the use of anti-EGFR antibodies (Abs) and monoclonal antibodies (mAbs) or their modified derivatives (3, 4). For use as imaging agents, Abs, mAbs, or their derivatives are designed to bind a radionuclide that serves as a source of the detection signal (4). However, their large size (~150 kDa) does not allow Abs and mAbs to penetrate deep into large tumors, and long blood circulating half-life times result in low sensitivity and the generation of a high background during imaging, thereby limiting their use for the detection or radiotherapy of malignant tumors (5, 6). To circumvent the limitations observed with the Abs and the mAbs, many small Ab or mAb derivatives such as the affibody molecules (ABs) have been generated (6, 7). The ABs can be up to ~7 kDa in size and have the same antigen specificity and binding characteristics as the native Ab from which they are derived. Various Abs have been labeled with radionuclides and evaluated for the detection of cancers (7).The ZHER2:342 affibody, which is based on the Z domain of the staphylococcus protein A, is directed against the human EGFR2 (HER2) and has been investigated extensively (5). The non-binding surface domain of ZHER2:342 was then altered to make it suitable for chemical synthesis and to enhance the stability and hydrophilicity of the molecule (5). The resulting AB, ZHER2:2891, which had 11 surface assessable amino acids substituted in the parent AB, was radioiodinated and shown to have an accumulation of ~10% of the injected activity per gram tissue (% IA/g) in tumors on mice at 4 h after an intravenous injection, indicating that it was a suitable tracer for the detection of malignant tumors in a murine model (8). In an effort to further improve the stability and hydrophilic properties of ZHER2:2891, a 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA) moiety was conjugated to the unique C-terminal cysteine of the parent AB to generate a second-generation AB designated ABY-025 (5). The new AB was then labeled with 111In ([111In]-ABY-025) and evaluated for its in vitro cell binding characteristics and for biodistribution and tumor imaging properties in mice bearing xenograft tumors (derived from SKOV-3, a human ovarian adenocarcinoma cell line that expresses the HER2 receptor) (5). These studies were performed to confirm that the various changes made to the native AB to produce ABY-025 had not changed the in vivo accuracy of the second generation AB for the detection of HER2 and also to confirm there was no change in the tumor and normal uptake properties of ABY-025.