A cell permeable peptide analog as a potential-specific PET imaging probe for prostate cancer detection

Non-invasive detection of prostate cancer or metastases still remains a challenge in the field of molecular imaging. In our recent work of screening arginine- or lysine-rich peptides for intracellular delivery of a therapeutic agent into prostate cancer cells, an arginine-rich cell permeable peptide (NH(2)GR(11)) was found with an unexpectedly preferential uptake in prostate cancer cell lines. The goal of this work was to develop this peptide as a positron emission tomography (PET) imaging probe for specific detection of distant prostate cancer metastases. The optimal length of arginine-rich peptides was evaluated by the cell uptake efficiency of three fluorescein isothiocyanate (FITC)-tagged oligoarginines (NHGR(9), NHGR(11), and NHGR(13)) in four human prostate cell lines (LNCaP, PZ-HPV-7, DU145, and PC3). Of the three oligoarginines, NH(2)GR(11) showed the highest cell uptake and internalization efficiency with its subcellular localization in cytosol. The biodistribution of FITC-NHGR(9), FITC-NHGR(11), and FITC-NHGR(13) performed in control nude mice displayed the unique preferential accumulation of FITC-NHGR(11) in the prostate tissue. Further in vivo evaluation of FITC-NHGR(11) in PC3 tumor-bearing nude mice revealed elevated uptake of this peptide in tumors as compared to other organs. In vivo pharmacokinetics evaluated with (64)Cu-labeled NH(2)GR(11) showed that the peptide was rapidly cleared from the blood (t(1/2) = 10.7 min) and its elimination half-life was 17.2 h. The PET imaging specificity of (64)Cu-labled NH(2)GR(11) was demonstrated for the detection of prostate cancer in a comparative imaging experiment using two different human cancer xenograft models.