99mTc-CCND1-Peptide nucleic acid (PNA)-IGF1 peptide chimeras (99mTc-WT4185) is a 99mTc-peptide-PNA-peptide chimera that was developed as a gene expression agent for gamma planar/single photon emission computed tomography (SPECT) imaging of breast cancer (1, 2). 99mTc-WT4185 is a radiohybridization probe for cyclin D1 (CCND1) proto-oncogene messenger RNA (mRNA) that is overexpressed in breast cancer cells.
Breast cancer is the second leading cause of cancer death in women in the United States women (3) The CCND1 oncogene exercises powerful control over the mechanisms that regulate the G1-S transition in the mitotic cell cycle (4, 5). Excessive CCND1 expression and /or activity is common in breast cancer and some other human cancers. Evidence from gene expression profiling has suggested that alterations in CCND1 mRNA are present in the preinvasive stage of breast cancer (1, 6). Studies have shown that CCND1 mRNA is a target that can be used to distinguish precancerous and invasive cancerous changes from benign breast diseases (1, 5).
Radiolabeled antisense oligonucleotides can be used to identify and image the presence of a particular mRNA in vivo (7). Some of the major obstacles in developing a clinically useful radiolabeled antisense probe include nonspecific affinity, ribonuclease destruction of the RNA target, and the lack of a receptor-targeting ligand. Peptide nucleic acids (PNAs) are DNA/RNA mimics in which the nucleobases are attached to a pseudopeptide backbone (1, 8, 9). The achiral, uncharged, and flexible PNA peptide backbone permits more stable hybridization to DNA and RNA oligomers with improved sequence selectivity. PNAs are also more stable against nuclease and protease attack, and the uncharged backbone is less likely to react with cellular proteins. However, relatively poor cellular uptake of PNAs requires additional design strategy such as the addition of a variety of ligands or coupling to different carriers (10). Tian et al. (3) demonstrated that adding a peptide analog specific for a cell surface receptor could be an effective way to increase the cellular uptake of PNAs in vitro and in vivo. One of the approaches is targeting to the insulin-like growth factor 1 (IGF1) receptor which is frequently overexpressed in breast cancer cells. Basu and Wickstrom (11) showed that a 5- to 10-fold uptake increase in cells expressing IGF1 receptors in vitro could be achieved by solid phase synthesis of a PNA sequence linked to a cyclized D-amino acid analog of IGF1. Tian et al. (1, 3) reported successful imaging of the CCND1 cancer gene in experimental human breast cancer xenografts with the 99mTc-WT4185 that was specific for both oncogene CCND1 and IGF1 receptor. The authors suggested that this probe could enter breast cancer cells overexpressing IGF1R, and then hybridize specifically with CCND1 mRNA.