Synthesis and evaluation of Ga-HBED-CC-EDBE-folate for positron-emission tomography imaging of overexpressed folate receptors on CT26 tumor cells.

The Ga is a positron-emitting radionuclide that can be combined with bifunctional chelating agents and bioactive substances for use as positron-emission tomography (PET) diagnostic agents. The HBED-CC is an acyclic chelating agent that is rapidly labeled with Ga under mild conditions. To target cancer cells, bioactive substances can be conjugated to the carboxyl terminus of HBED-CC. Because folic acid strongly binds to folate receptors that are overexpressed on the surfaces of many types of cancer cells, it was coupled with HBED-CC through a small polyethylene glycol-based linker (EDBE) to generate an active, receptor-selective targeting system. The HBED-CC-EDBE-folate (HCEF) precursor was readily labeled with Ga in 5 minutes at room temperature (98% radiochemical yield; 99% radiochemical purity after isolation). In cellular uptake tests, higher uptakes of Ga-HCEF were observed for the CT26 and KB cell lines (which express folate receptors) than for the A549 cell line (which does not). Finally, in vivo micro-PET measurements over 2 hours of binding in BALB/c mice into which CT26 tumors had been transplanted showed the selective accumulation of Ga-HCEF in the folate receptor-expressing CT26 tumors. These results confirmed the potential of Ga-HCEF as a PET diagnostic agent for tumors that express folate receptors.