C/Ga-Labeled sel-tagged anti-epidermal growth factor receptor 2 affibody Z

The human epidermal growth factor receptor-2 (HER2, ErbB2) modulates its activity through a tyrosine kinase signaling pathway and is involved in the development of various types of cancers (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 are not 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 imaging agent is not able to 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 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-HER2 Affibody molecule (Z) with a picomolar affinity for the receptor and showed that I-labeled Z can be used with gamma planar imaging to visualize xenograft SKOV-3 cell tumors (these cells express the HER2) in mice (6). The Z Affibody has also been labeled with In or Tc and shown to be suitable for the detection of HER2 tumors with single-photon emission computed tomography (SPECT) in preclinical and clinical studies (7). Probes labeled with nuclides such as F (half-life = 109.8 min and 97% β-decay) and C (half-life = 20.4 min and 100% β-decay) are commonly used with PET imaging in the clinic because this technique generates images that have high spatial resolution and sensitivity and produces superior quantitative data compared with SPECT (7). Recently, F-labeled Z was shown to be suitable for the detection of HER2 pulmonary metastasis with PET in mice (1). However, the only known procedure to label proteins with C is by introducing a tetrapeptide tag (sel-tag; known as Sec or U; sequence –Gly-Cys-Sec-Gly-COOH) on the C-terminus of the molecule as described by Cheng et al. (8). The properties of sel-tag are described elsewhere (9). In an effort to develop an alternate Affibody that can be used to detect HER2 tumors with PET, a recombinant Z containing the sel-tag was developed (Z-ST) and labeled with C/Ga ([C/Ga]Z-ST) (7). The biodistribution and use of [C/Ga]Z-ST for the detection of HER2 xenograft tumors was then investigated in severe combined immunodeficient (SCID) mice and mice bearing SKOV-3 cell tumors that have a high expression of HER2 (7).