Numerous normal and tumors cells are well-known to express the somatostatin receptors (SSTRs) on their surface which makes the receptor be useful for tumor scintigraphy. Thus, the identification of SSTRs is beneficial, especially SSTR2. The somatostatin analog, Octreotide (OCT), was chosen as a ligand, as it is known to selectively bind to SSTR2. Moreover, polyethylene glycol (PEG), 8armPEG, was used as a branched PEG to provide a low nonspecific cell binding and easily chemical modification. OCT and fluorescein (Flu) were conjugated to branched PEG using a water-soluble carbodiimide (EDC) and N-hydroxy succinimide (NHS) so as to activate its carboxylic acid group. 8armPEG-tagged Flu and OCT was characterized by gel permeation chromatography (GPC) to proof the conjugation of OCT to 8armPEG. Finally, cellular uptake was studied using pancreatic cancer cells with well-expressed somatostatin receptors using a confocal laser scanning microscope (CLMS) and fluorescence activated cell sorting (FACS). GPC showed increases in molecular mass since it showed a difference in elution time of 8armPEG itself and 8armPEG labeled with Flu. CLMS and FACS showed high binding with the positive SSTR2 cells expression and showed negative results with negative expressing SSTR2. These bindings were decreased when the receptors were occupied with free OCT which confirms the specific binding to SSTR2. Therefore, we formulated a novel model to easily identify SSTR2 and other receptors which serves as a promising platform for identification of tumor cells overexpressing the SSTR2, which would be a hopeful target for cancer therapy and tumor scintigraphy.
Keywords: 8arm polyethylene glycol; Octreotide; fluorescein; somatostatin receptor.