F-Labeled Cys-Z, an anti-epidermal growth factor receptor-2 Affibody

The epidermal growth factor receptor-2 (HER2, ErbB2) modulates its activity through a tyrosine kinase signaling pathway and is involved in the development of various cancers, including those of the lung and the breast (1, 2). Overexpression or amplification of the HER2 gene is known to occur in a high percentage of cancer cases (e.g., 20% of breast cancer (BC)) and predicts a poor prognosis for the patient. Invasive methods such as biopsies in conjunction with immunohistochemistry and fluorescence hybridization are often employed to assess the HER2 status of the primary and metastasized neoplastic tumors; however, because of sampling bias and tumor heterogeneity, results obtained with these procedures may not be reliable (2). In the clinic, F-labeled fluorodeoxyglucose ([F]-FDG) is commonly used with positron emission tomography (PET) to detect and determine the tumor burden of a patient, but this technique does not distinguish between benign and malignant lesions, cannot differentiate tumors that overexpress HER2 from those that have a low expression or do not express the receptor, and often identifies inflammation as a false-positive neoplasm (1). An Affibody molecule is a chain of 58 amino acids (6.5 kDa) that contains a modified B domain of the staphylococcal protein A and can be obtained by chemical synthesis or produced in bacteria with the use of recombinant DNA technology (3). Because of their small size and high chemical and thermal stability, there is much interest to radiolabel these molecules and use them for the targeted detection and treatment of malignant tumors as discussed in detail elsewhere (3-5). Orlova et al. generated an anti-EGFR2 Affibody molecule (Z) with a picomolar affinity for the receptor and showed that I-labeled Z can be used to visualize xenograft SKOV-3 cell tumors (these cells express the HER2) with gamma planar imaging in mice (6). Kramer-Marek et al. envisioned that, because F is routinely available in the clinic, F-labeled Z can be used to detect and monitor (after therapy) cancerous lesions that express HER2 (7). For the site-specific labeling of Z with F, a cysteine residue was attached to the C-terminus of the Affibody (Cys-Z), and the derivative was conjugated with -(2-(4-[F]fluorobenzamido)ethyl)maleimide ([F]FBEM) to obtain [F]-Cys-Z (7). Subsequently, the radiofluorinated conjugate was characterized and evaluated to assess the expression of HER2 in xenograft tumors in mice (7).