Cyanine Dye Labeled Vasoactive Intestinal Peptide and Somatostatin Analog for Optical Detection of Gastroenteropancreatic Tumors

Many gastroenteropancreatic (GEP) tumors express vasoactive intestinal peptide (VIP) and/or somatostatin (SST) receptors in high densities. 1 These receptors can, therefore, be used as targets for the tumor directed delivery of contrast agents for detection of primary tumors as well as metastases. Radiolabeled SST analogues are used in daily clinical routine for receptor scintigraphy of neuroendocrine GEP tumors, whereas radiolabeled VIP has recently been introduced for the detection of GEP adenocarcinomas (for a review see Ref. 2). In the last few years optical (nearinfrared) imaging, a method that does not expose patients to ionizing radiation, has emerged as a new diagnostic modality for oncological applications. 3 The method is based on the detection of differences in the absorption and/or fluorescence of normal and tumor tissue. Due to the limited penetration depth of the excitation light, optical imaging is mainly applied to the detection of superficial lesions. A potential application is the detection of gastrointestinal tumors by fluorescence guided endoscopy after administration of a tumor specific, fluorescent contrast agent. We previously reported the synthesis and characterization of cyanine dyes as fluorescent contrast agents for optical imaging. 4 Conjugates consisting of a cyanine dye and a peptide that binds with high affinity to receptors on tumor cells may provide new tumor specific contrast agents for the optical detection of superficial GEP tumors.