Rhodamine-marked bombesin: a novel means for prostate cancer fluorescence imaging

The gastrin releasing peptide receptor (GRPR) has been found to be strongly expressed in various types of cancers such as prostate and breast carcinomas. The GRPR ligands gastrin releasing peptide and bombesin can play a very significant role in cancer therapy and diagnostics. In this study we synthesized unlabeled bombesin BBN along with two conjugates in which the correct bombesin (BBN-Rhd) and a mutant bombesin (mBBN-Rhd) sequence was coupled to rhodamine, a fluorescent dye. These novel rhodamine fluorescent conjugates were used to study the targeting and uptake of bombesin on a cellular level. Nine different human cell lines including both tumor and healthy cells were examined using flow cytometry and confocal laser scanning microscopy. GRPR mRNA expression analysis was performed and it was found that the receptor is highly expressed in LNCaP and PC3 cells compared to the rest of other cell lines. Competition experiments were performed to verify the receptor dependence of the labeled conjugates using unmarked bombesin. The present study is a first attempt at direct fluorescence imaging of living cells using bombesin and its target, the GRPR. A rhodamine bombesin conjugate can be used as marker to differentiate between healthy cells and malignant cells such as prostate hyperplasia and prostate carcinoma in the early detection of cancer.