Optical Imaging of Tumors with Copper-Labeled Rhodamine Derivatives by Targeting Mitochondria

In this study, we evaluated Cu(L1) in two xenografted tumor-bearing (U87MG and MDA-MB-435) animal models to prove the concept that Cu(II)-labeled rhodamine derivatives, Cu(L) (L = L1 - L4) are useful as selective fluorescent probes for tumor imaging. We found that both multidrug resistance (MDR) negative U87MG gliomas and MDR-positive MDA-MB-435 breast tumors could be visualized. Because of tissue attenuation, accurate quantification of tumor uptake was difficult by optical methods. Therefore, 64Cu(L) (L = L1 - L4) were evaluated to compare their biodistribution properties. It was found that all four 64Cu radiotracers had a high glioma uptake (64Cu(L1): 5.71± 1.43 %ID/g; 64Cu(L2): 5.98 ± 2.75 %ID/g; 64Cu(L3): 4.28 ± 1.45 %ID/g; and 64Cu(L4): 6.25 ± 3.42 %ID/g) with 64Cu(L1) showing the highest tumor/background ratios. In athymic nude mice bearing MDA-MB-435 breast cancer xenografts, 64Cu(L4) showed almost identical normal organ uptake to that in the glioma-bearing animals, but its breast tumor uptake (1.26 ± 0.10% ID/g) was significantly lower (p < 0.001) than that in the glioma (6.25 ± 3.42% ID/g) because of MDR Pgps (P-glycoproteins) and MRPs (multidrug resistance-associated proteins) overexpressed in the xenografted MDA-MB-435 breast tumors. Results from cellular staining assays showed that both Cu(L2) and Cu(L4) were able to localize in mitochondria of U87MG cells, and their tumor selectivity was caused by the elevated negative mitochondrial potential in U87MG glioma cells as compared to that in human fibroblast cells. On the basis of these results, it was concluded that Cu(L) (L = L1 - L4) are useful as selective fluorescent probes for cellular staining assays and optical tumor imaging while 64Cu(L) (L = L1 - L4) have the potential as PET radiotracers for tumor imaging. This study represents a good example of dual modality imaging (PET and optical) using two agents, 64Cu(L) and Cu(L), with identical chemical composition. Future research will focus on developing new fluorescent probes with longer wavelength and reduced liver uptake.