Synthesis of a Fluorescently Labeled 68Ga-DOTA-TOC Analog for Somatostatin Receptor Targeting

Fluorescently labeled imaging agents can identify surgical margins in real-time to help achieve complete resections and minimize the likelihood of local recurrence. However, photon attenuation limits fluorescence-based imaging to superficial lesions or lesions that are a few millimeters beneath the tissue surface. Contrast agents that are dual-labeled with a radionuclide and fluorescent dye can overcome this limitation and combine quantitative, whole-body nuclear imaging with intraoperative fluorescence imaging. Using a multimodality chelation (MMC) scaffold, IRDye 800CW was conjugated to the clinically used somatostatin analog, Ga-68-DOTATOC, to produce the dual-labeled analog, Ga-68-M.MC (IRDye 800CW)-TOC, with high yield and specific activity. In vitro pharmacological assays demonstrated retention of receptor-targeting properties for the dual-labeled compound with robust internalization that was somatostatin receptor (SSTR) 2-mediated. Biodistribution studies in mice identified the kidneys as the primary excretion route for Ga-68-MMC(IRDye 800CW)-TOC, along with clearance via the reticuloendothelial system. Higher uptake was observed in most tissues compared to Ga-68-DOTA-TOC but decreased as a function of time. The combination of excellent specificity for SSTR2-expressing cells and suitable biodistribution indicate potential application of Ga-68-MMC(IRDye 800CW)-TOC for "intraoperative detection of SSTR2expressing tumors.