Preparation and In Vitro and In Vivo Characterization of the Tumor-specific Antigen-derived Peptide as a Potential Candidate for Targeting Human Epidermal Growth Factor Receptor 2-positive Breast Carcinomas.

Abstract

The human epidermal growth factor receptor (HER2) is considered as one of the most well-characterized tumor-associated antigens for cancer therapy and plays an important role in the growth and progression of breast cancer. Overexpression of HER2 in various cancers and the availability of its extracellular region makes it a clinically useful target for the development of tumor-antigen specific agents. In this study, we have prepared a HER2-targeted hybrid peptide as a single-photon emission computed tomography (SPECT) imaging probe and evaluated its tumor targeting potential in subcutaneous HER2-positive breast cancer xenograft models. The HER2-targeted hybrid peptide was prepared by solid-phase peptide synthesis and radiolabeled with 99mTc by the ligand exchange method. In vitro tumor cell binding properties of 99mTc-HER2 were evaluated in HER2-positive (SKBR3) and ER-positive (MCF7 and T47D) breast cancer cell lines. In vivo tumor targeting characteristics were investigated in both SKBR3 (HER2-positive) and MDA-MB-231 (HER2-negative) xenografted animal models. A high labeling efficiency of greater than 95% was achieved when HER2 peptide was radiolabeled with 99mTc by the standard ligand exchange method. 99mTc-HER2 displayed a high binding affinity (Kd=49.95±14.11 nM) to HER2-positive SKBR3 cell line whereas in the case of the ER-positive cell lines (MCF-7 and T47D), the binding affinity was found to be 2-3-fold lower than SKBR3. In vivo tumor uptake in nude mice with SKBR3 tumor xenografts was 2.81±0.79% ID/g as early as 60 min p.i. The uptake in SKBR3 tumors was always higher than the uptake in the blood and muscle, with good tumor-to-blood and tumor-to-muscle ratios. In contrast, low accumulation in ER-positive tumors (MCF7 and T47D) was observed compared to HER2-positive SKBR3 tumor mice. A low to moderate (less than 5% ID/g) accumulation and retention of 99mTc-HER2 was found in most of the major organs excluding the kidneys in both healthy and tumor-bearing mice. In view of its ability to detect HER2-positive breast cancer cells in vivo, 99mTc-HER2-targeted peptide may be a promising tumor imaging probe and warrants further investigation.