Fluorescence optical imaging technologies are currently being developed to image specific molecular targets in vivo. Detection technologies range from those providing microscopic detail to whole body imaging systems with potential clinical use. A number of target-specific near-infrared imaging probes have recently been developed to image receptors,
antigens, and
enzymes. The goal of the current study was to evaluate a new near-infrared (NIR)
folate receptor (FR)-targeted imaging probe for its ability to improve detection of FR-positive
cancers. We hypothesized that modification of
folate would retain receptor affinity in vivo, despite the bulkier NIR
fluorochrome, NIR2 (em = 682 nm). Cellular uptake of the NIR conjugates was significantly higher in FR-positive nasopharyngeal
epidermoid carcinoma, KB cells, compared to FR-negative human
fibrosarcoma, HT1080 cells. When
tumors were implanted in vivo, equal-sized KB
tumors showed a 2.4-fold higher signal intensity compared to HT1080
tumors (24 h). The maximum signal-to-background ratio (3-fold) was observed at 24 h in KB
tumor. Injection of the unmodified NIR2
fluorochrome did not result in persistent contrast increases under similar conditions. Furthermore,
tumor enhancement with the
NIR2-folate probe persisted over 48 h and was inhibitable in vivo by administration of unlabeled
folate. These results indicate that
folate-modified NIR
fluorochrome conjugate can be used for improved detection of FR-positive
tumors.