The
metalloexopeptidase CD13/
aminopeptidase N (APN) has been shown to be involved in
cancer angiogenesis, invasion, and
metastasis. Therefore, a CD13/APN-targeted
NGR-peptide was labeled with the cyanine
dye Cy 5.5 and applied to image
tumor xenografts with different APN-expression levels using both planar and tomographic optical imaging methods. In vitro, the
peptide-
dye conjugate showed a clear binding affinity to APN-positive HT-1080 cells, while negative MCF-7 cells and predosing with the free
NGR-peptide revealed little to no fluorescence. In vivo,
tumor xenografts (n>or=5) were clearly visualized by two-dimensional (2-D) planar fluorescence reflectance imaging (FRI) and three-dimensional (3-D) fluorescence mediated tomography (FMT) up to 24 h after injection. FMT also allowed us to quantify
fluorochrome distribution in deeper tissue sections, showing an average
fluorochrome concentration of 306.7+/-54.3 nM Cy 5.5 (HT-1080) and 116.0+/-18.3 nM Cy 5.5 (MCF-7) in the target tissue after 5 h. Competition with the free
NGR-peptide resulted in a reduction of
fluorochrome concentration in HT-1080
tumor tissue (195.3+/-21.9 nM; 5 h). We thus conclude that NGR-Cy 5.5 combined with novel tomographic optical imaging methods allows us to image and quantify
tumor-associated CD13/APN expression noninvasively. This may be a promising strategy for a sensitive evaluation of
tumor angiogenesis in vivo.