Tc-Affibody Z-Cys

Epidermal growth factor (EGF) is a cytokine of 53 amino acids (6.2 kDa) and is secreted by ectodermic cells, monocytes, kidneys, and duodenal glands (1). EGF stimulates growth of epidermal and epithelial cells. EGF and at least seven other growth factors and their transmembrane receptor kinases play important roles in cell proliferation, survival, adhesion, migration, and differentiation. The EGF receptor (EGFR) family consists of four transmembrane receptors: EGFR (HER1/erbB-1), HER2 (erbB-2/neu), HER3 (erbB-3), and HER4 (erbB-4) (2). HER1, HER3, and HER4 comprise three major functional domains: an extracellular ligand-binding domain, a hydrophobic transmembrane domain, and a cytoplasmic tyrosine kinase domain. No ligand has been clearly identified for HER2. However, HER2 can be activated as a result of ligand binding to other HER receptors with the formation of receptor homodimers and/or heterodimers (3). HER1 and HER2 are overexpressed on many solid tumor cells such as breast, non-small cell lung, head and neck, and colon cancers (4-6). The high levels of HER1 and HER2 expression on cancer cells are associated with a poor prognosis (7-10). Trastuzumab is a humanized IgG monoclonal antibody (mAb) against the extracellular domain of recombinant HER2 with an affinity constant () of 0.1 nM (11). In-Trastuzumab, Cy5.5-trastuzumab, and Ga-trastuzumab-F(ab') have been developed for imaging of human breast cancer (12-16). However, the pharmacokinetics of the intact radiolabeled mAb, with high liver uptake and slow blood elimination, are generally not ideal for imaging. Smaller antibody fragments, such as Fab or F(ab´), have better imaging pharmacokinetics because they are rapidly excreted by the kidneys. A novel class of recombinant affinity ligands (Affibody molecules) for HER2 was constructed on the basis of the Z-domain residues (58 amino acids) from one of the IgG-binding domains of staphylococcal protein A (17). Affibody molecules exhibit high binding affinity to HER2 with values <100 pM. Various radiolabeled Affibody molecules have been studied in terms of their ability to image HER2 in tumors [PubMed]. A cysteine molecule was introduced at the C-terminus of Affibody molecules for site-specific coupling of Tc to Z Affibody to form Tc-Z-Cys (Tc-Z-C), which has been evaluated in nude mice bearing human colon adenocarcinoma tumors (18).