Peptide-based 68Ga-PET radiotracer for imaging CD133 expression in colorectal cancer

CD133 is a demonstrated cancer stem cell marker. A small peptide LS7, screened by a phage display technique, was identified to specifically target CD133. The purpose of this study was to develop a novel and specific peptide-based PET imaging agent for CD133 imaging in colorectal cancer.

The peptide LS7 was conjugated with 1,4,7,20-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and radiolabeled with 68Ga. The cellular uptake was assessed in vitro. In vivo small-animal PET/CT and ex vivo biodistribution evaluations were performed in mice bearing CD133-positive HCT116 and Lovo cell-derived tumors as well as CD133-negative DLD1 cell-derived tumors. Nonspecific uptake of the tracer in HCT116 cell-derived tumor cells and tumor models was determined by coincubation or coinjection with an excess of unlabeled DOTA-LS7 along with radiolabeled tracers.

68Ga-DOTA-LS7 was produced with 80.0% yield and the radiochemical purity was greater than 95.0%. In vitro, 68Ga-DOTA-LS7 was selectively taken up by HCT116 and Lovo cells but not by DLD1 cells. Small-animal PET/CT clearly revealed deposition of 68Ga-DOTA-LS7 in HCT116 and Lovo cell-derived tumors with excellent contrast. Biodistribution demonstrated that the tumor uptakes were 2.24 ± 0.16, 1.76 ± 0.42, and 0.69 ± 0.28% ID/g in HCT116, Lovo and DLD1 cell-derived tumors, respectively, at 90 min post-injection. Uptake of 68Ga-DOTA-LS7 in HCT116 tumors was significantly inhibited by coinjection of excess DOTA-LS7.

Rapid tumor CD133 detection and selectivity were demonstrated in vitro and in vivo with PET using the specific CD133 binding peptide 68Ga-DOTA-LS7. A robust correlation was detected in vivo between tumor signals from mouse xenograft models with different cell lines and CD133 expression. The favorable characteristics of 68Ga-DOTA-LS7, such as convenient synthesis and specific uptake, warrant its further investigation for CD133 expression imaging.