Colonic Adenocarcinomas: Near-Infrared Microcatheter Imaging of Smart Probes for Early Detection—Study in Mice

Abstract

To prospectively evaluate the ability of micro-fiberoptic catheters, which simultaneously record white light and near-infrared (NIR) images, to reveal colonic neoplasms after the intravenous administration of activatable "smart" probes that increase in NIR fluorescence subsequent to protease activation. The institutional animal care committee approved all animal experiments. CT26 tumor cells were orthotopically implanted into the descending colon of C57BL6/J mice (n=10). Thirteen days later, mice intravenously received either 2 nmol of a protease-sensing probe that had cathepsin B as a major activator (n=5) or saline (control animals [n=5]). One day later, animals were noninvasively examined to the point of the splenic flexure by using microcatheter imaging. Excised colons were subsequently evaluated with epifluorescence imaging, histologic examination, and cathepsin B immunohistochemistry. Student t test was used for statistical analysis, with P<.05 considered to indicate a significant difference. Results with fiberoptic imaging demonstrated that all tumors were visible with the protease-activatable probe, even when they were not readily apparent at white light imaging. A target-to-background ratio (TBR) of 8.86 for tumor to adjacent normal mucosa was achieved in the NIR channel after probe administration (P=.001), whereas white light images resulted in a TBR of 1.14 (P>.5) based on luminosity. The tumoral NIR fluorescence intensity was more than 30-fold greater in probe-injected animals than in control animals, indicating that essentially all of the signal recorded in lesions was from activatable probe administration. Results of immunohistochemistry confirmed cathepsin B overexpression in the tumor compared with adjacent mucosa. The use of NIR imaging microcatheters combined with protease-activatable smart probes results in a beacon effect that highlights tumors with high TBRs; this technique thus may be a potentially useful adjunct to white light colonoscopy in the future.