Background

[PubMed]

Epidermal growth factor (EGF) is a cytokine that comprises 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 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 eveloped 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 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 (K_d) 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]. Mercaptoacetyl-Gly-Gly-Gly (MAG3) was used a chelating linker for coupling ^99mTc to Z_HER2:342 Affibody (18). ^99mTc-MAG3-Z_HER2:342 has been evaluated in nude mice bearing human colon adenocarcinoma tumors, resulting in high tumor/blood and tumor/muscle ratios. However, ^99mTc-MAG3-Z_HER2:342 exhibited a high hepatobiliary clearance, which resulted in a high radioactivity in the intestines at 4 h after injection (19). Therefore, glutamic acid was used as the chelating linker to decrease the lipophilicity of Z_HER2:342 Affibody to suppress hepatobiliary clearance (20). ^99mTc-Mercaptoacetyl-Glu-Glu-Glu-Affibody Z_HER2:342 (^99mTc-MaEEE-Z_HER2:342) was found to a have favorable biodistribution in nude mice bearing SKOV-3 tumors, resulting in a 3-fold lower hepatobiliary clearance than ^99mTc-MAG3-Z_HER2:342.

Synthesis

[PubMed]

Z_HER2:342 Affibody was prepared by standard solid phase peptide synthesis. MaEEE-Z_HER2:342 was prepared by addition of MaEEE to the N-terminal of the Z-Affibody (20). ^99mTc as pertechnetate was added to a solution of MaEEE-Z_HER2:342 containing SnCl₂. The mixture was incubated for 60 min at room temperature. The labeling efficiency of ^99mTc incorporation was 90 ± 2% with >98% purity. ^99mTc- MaEEE-Z_HER2:342 was purified by size-exclusion chromatography. Specific activities of the preparations were not reported. ^99mTc-MaEEE-Z_HER2:342 was found to be stable after incubation in murine blood plasma (>99%) at 37°C for 1 h or solution containing 300-fold excess cysteine (93 ± 3%) at 37°C for 2 h.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Tran et al. (20) performed binding experiments with MaEEE-Z_HER2:342 with use of a Biacore sensor chip immobilized with extracellular domain of HER2 protein. The K_d values were 410 and 80 pM for MaEEE-Z_HER2:342 and Z_HER2:342, respectively. Hence, the binding affinity of the synthetic Affibody molecule MaEEE-Z_HER2:342 was 4.1-fold lower than the parent Affibody molecule Z_HER2:342. In vitro binding specificity tests showed that binding of ^99mTc-MaEEE-Z_HER2:342 to SKOV-3 cells expressing HER2 was receptor-mediated because saturation of receptors by preincubation with non-labeled Z_HER2:342 significantly decreased binding of ^99mTc-MaEEE-Z_HER2:342. The antigen binding capacity of ^99mTc-MaEEE-Z_HER2:342 was 67%. The cell-bound radioactivity remained constant at ~70% of the initially bound activity for up to 24 h when the cells were incubated with ^99mTc-MaEEE-Z_HER2:342. In contrast, cells incubated with ¹²⁵I-Z_HER2:342 retained ~40% of the original cell-associated radioactivity.

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

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