In-2-(-isothiocyanatobenzyl)-6-methyl-diethylenetriamine-,,´,´´,´´-pentaacetic acid-trastuzumab (In-1B4M-DTPA-humAb4D5-8), which is formed by the conjugation of In with an intact recombinant humanized monoclonal antibody (MAb) against the antigen of human epidermal growth factor receptor 2 (HER2), has been developed for imaging of human breast cancer (1). In is a gamma emitter with a physical half-life () of 2.8 days. The epidermal growth factor receptors (HER/erbB) with tyrosine kinase activity are involved in transmission of signals controlling normal cell development, growth, differentiation, and survival (2-4). This cell-surface receptor family consists of four distinct members, the epidermal growth factor receptors HER1, HER2, HER3, and HER4. These receptors are found in various combinations in different tissues. There are nine ligands that bind directly to HER1, HER3, or HER4. HER2 can be activated as a result of ligand binding to other HER receptors. The ligandless HER2 receptor is encoded by the human gene HER2/c-erbB2 (HER2/neu). It has been found to be overexpressed in a wide variety of human cancers and in 20-30% of primary breast cancers (5). This overexpression is associated with aggressive tumor growth and metastatic activity. In breast cancer, HER2/neu gene amplification can result in 10- to 100-fold higher HER2 protein levels in tumor cells than that in normal breast epithelium. HER2 overexpression is considered a negative prognostic indicator for breast cancer. Radiolabeled MAbs have been developed for both the diagnosis and treatment of tumors (6-8). Anti-HER2 MAbs have been generated for inhibiting the growth of tumor cells that possess activated HER2/neu receptors (9). Murine 4D5 MAb was raised against human breast and ovarian tumor cell lines overexpressing p185. This MAb was found to recognize an extracellular epitope (amino acids 529 to 627) in the cysteine-rich II domain that resides very close to the transmembrane region (9). Murine 4D5 MAb was fully humanized by Carter et al. (10) to generate the humAb4D5-8 MAb in an effort to improve the safety and tolerance of the antibody for clinical applications. Trastuzumab is of the IgG kappa class and binds to the recombinant HER2 extracellular domain with an affinity constant () of 0.1 nM. Trastuzumab is approved by the Food and Drug Administration to treat breast cancer that is HER2 positive and has metastasized to other areas of the body. It has also been studied in the treatment of other types of cancer. Cardiotoxicity is the most serious complication of using trastuzumab in humans (5, 11). One potential application of a radiolabeled anti-HER2 MAb is the pretreatment imaging of breast cancer patients to predict the cardiotoxicity and therapeutic efficacy of unlabeled trastuzumab. Various anti-HER2 MAbs have been labeled with I, I, In, Y, Y, Cu, Br, or Gd for potential imaging and therapeutic applications (1, 12-18). The labeling with radiometals is generally achieved by indirect labeling with use of a bifunctional chelating agent (19, 20). In, a photon emitter with a physical half-life () of 2.8 days, is considered one of the radionuclides of choice for antibody imaging studies.