Imaging of differential protease expression in breast cancers for detection of aggressive tumor phenotypes

PURPOSE: To determine if different expression levels of tumor cathepsin-B activity in well differentiated and undifferentiated breast cancers could be revealed in vivo with optical imaging. MATERIALS AND METHODS: A well differentiated human breast cancer (BT20, n = 8) and a highly invasive metastatic human breast cancer (DU4475, n = 8) were implanted orthotopically in athymic nude mice. Tumor-bearing animals were examined in vivo with near-infrared fluorescence (NIRF) imaging 24 hours after intravenous injection of an enzyme-sensing imaging probe. Immunohistochemistry, Western blotting (on cells and whole tumor samples), and correlative fluorescence microscopy were performed. RESULTS: Both types of breast cancers activated the NIRF probe so that tumors became readily detectable. However, in tumors of equal size, there was a 1.5-fold higher fluorescence signal in the highly invasive breast cancer (861 arbitrary units 88) compared with the well differentiated lesion (566 arbitrary units 36, P < .01). Western blotting confirmed a higher cathepsin-B protein content in the highly invasive breast cancer (DU4475) of about 1.4-fold (whole tumor samples) to 1.7-fold (cells). Immunohistochemistry and fluorescence microscopy findings confirmed the imaging findings. CONCLUSION: Cathepsin-B enzyme activity can be determined in vivo with NIRF optical imaging, while differences in tumoral expression may correlate with tumor aggressiveness. (C) RSNA, 2002.