To compare the drug delivery profile of the four doxorubicin formulations, we tried to detect the drug in tumor tissues using the fluorescence microscope. Images from all tumors showed that doxorubicin was visualized in the tumor nuclei one hour after SP5-2-LD was administered, but not after other formulations were injected (Fig. 5A) . Over time, areas of tumor sections with detectable doxorubicin increased. The areas with detectable doxorubicin were significantly larger in SP5-2-LD-treated tumors than that in FD, LD, and MP5-2-LD-treated tumors at each time point (Fig. 5A) . The MP5-2-LD formulation had the same distribution pattern as LD, but the regions of FD-treated tumors showed no detectable doxorubicin (Fig. 5A) .