N-[2-(4-[18F]Fluorobenzamido)ethyl]maleimide-Cys-ZEGFR:1907

Excerpt

Epidermal growth factor (EGF) is a growth factor composed of 53 amino acids (6.2 kDa) that 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 poor patient prognosis because high levels are related to increased proliferation (7-10).

Trastuzumab is a humanized IgG₁ monoclonal antibody (mAb) against the extracellular domain of recombinant HER2 with an affinity constant (K_d) of 0.1 nM (11). Trastuzumab is approved for clinical use for anti-cancer therapies in both Europe and North America. ¹¹¹In-Trastuzumab, Cy5.5-trastuzumab, and ⁶⁸Ga-trastuzumab-F(ab')₂ have been developed for imaging 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')₂, have better imaging pharmacokinetics because they are rapidly excreted by the kidneys. A novel class of recombinant affinity ligands (Affibody molecules) for HER2, based on the Z-domain residues (58 amino acids) from one of the IgG-binding domains of staphylococcal protein A, was constructed (17). Affibody molecules exhibit high binding affinity to HER2 with K_d values <100 pM. Various radiolabeled Affibody molecules have been studied in terms of their ability to image HER2 in tumors [PubMed]. The EGFR-specific Affibody Z_EGFR:1907 (6.7 kDa) was identified to have good affinity (K_d = 5.4 nM) for EGFR (HER1) and was labeled with ¹¹¹In via isothiocyanate-benzoyl-diethylenetriamine pentaacetic acid (DTPA) to form ¹¹¹In-Bz-DTPA-Z_EGFR:1907 for use with single-photon emission computed tomography (18) imaging in nude mice bearing human tumors (19). To facilitate site-specific conjugation, Ac-Cys-Z_EGFR:1907 (Cys at the N terminal) was synthesized and conjugated with 1,4,7,10-tetraazacyclododecane-1,4,7-tris-acetic acid-10-maleimidethylacetamide (maleimido-mono-amide-DOTA) to form Ac-Cys(DOTA)-Z_EGFR:1907 (DOTA-Z_EGFR:1907), and the product was labeled with ⁶⁴Cu (t_1/2, 12.7 h) to form ⁶⁴Cu-DOTA-Z_EGFR:1907 for positron emission tomography imaging studies in tumor-bearing mice (20). Miao et al. (21) reported the site-specific labeling of Ac-Cys-Z_EGFR:1907 with N-[2-(4-[¹⁸F]fluorobenzamido)ethyl]maleimide ([¹⁸F]FBEM) to form [¹⁸F]FBEM-Cys-Z_EGFR:1907 for use with in vivo imaging of EGFR-positive tumors.