99mTc- N, N'-Bis(S-benzoyl-thioglycoloyl)diamidopropanoyl-KRAS-PNA- d(Cys-Ser-Lys-Cys)

Excerpt

^99mTc-N,N'-Bis(S-benzoyl-thioglycoloyl)diamidopropanoyl-KRAS-PNA-d(Cys-Ser-Lys-Cys) (^99mTc-WT4351) is a ^99mTc-peptide-PNA-peptide chimera that was developed as a gene expression agent for single-photon emission computed tomography (SPECT) imaging of pancreatic cancer (1, 2). ^99mTc-WT4351 is designed to bind to the insulin-like growth factor 1 (IGF1) receptor and to internalize and hybridize with oncogene KRAS oncogene messenger RNA (mRNA) that is overexpressed in pancreatic cancer.

Pancreatic cancer is the fourth leading cause of cancer death in the United States (3). The ras gene family encodes a 21-kDa membrane-bound protein, Ras, involved in cell proliferation and migration, and the KRAS is typically activated by point mutations in codon 12 as a “signature” of pancreatic cancer (4). About 90% of patients with pancreatic cancer carry activating mutations in their KRAS. Because KRAS mutation usually develops during the early phase of pancreatic carcinogenesis and elevated K-Ras protein levels have been found inside cancer cells, it has been suggested that the detection of this mutation may provide a diagnostic tool for the early detection of pancreatic cancer.

Radiolabeled antisense oligonucleotides can be used to identify and image the presence of a particular mRNA in vivo (5). Some of the major obstacles in the development of 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 (6-8). 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 an additional design strategy such as the addition of a variety of ligands or coupling to different carriers (9). Tian et al. (1) demonstrated that addition of a peptide analog that is 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 the IGF1R, which is frequently overexpressed in breast and pancreatic cancer cells. Basu and Wickstrom (10) showed that a 5- to 10-fold uptake increase in cells expressing IGF1Rs in vitro could be achieved by solid-phase synthesis of a PNA sequence linked to a cyclized D-amino acid analog of IGF1. Based on this concept, Tian et al. (1, 8) successfully imaged the breast cancer gene CCND1 in experimental human breast cancer xenografts. The authors suggested that the peptide-CCND1 PNA-peptide probe could enter cancer cells that overexpress IGF1R and then hybridize specifically with the oncogene mRNA. Similarly, Chakrabarti et al. (11) synthesized a chelator-KRAS PNA-peptide chimera labeled with ⁶⁴Cu (for PET) and ^99mTc (for SPECT) to target KRAS mRNA and image pancreatic cancer in human pancreas cancer xenografts. The mechanism of uptake was tested by IGF1 blocking of breast cancer xenograft imaging with a ⁶⁴Cu-DO3A-CCND1PNA-d(Cys-Ser-Lys-Cys) probe (12). Tian et al. (1, 2) reported successful SPECT imaging of human pancreas cancer xenografts with ^99mTc-4351.