Figure 2.

General scheme of drug delivery to tumors. (A) Schematic illustration of drug delivery via passive targeting. Abnormal tumor microenvironment and EPR effect result in passive drug accumulation and retention in tumors. This passive tumor-targeting strategy exhibits limitations of poor cancer cell targeting and inadequate drug uptake. (B) Schematic illustration of aptamer-guided active targeting of tumor. Aptamers undergo endocytosis after binding to cell surface targets, along with aptamer-conjugated therapeutic agents or drug-encapsulated nanoparticles. Following the entrance and escape from endosomes and/or lysosomes, the released therapeutic agents or small RNAs engage their cytoplasmic or nucleic targets to eliminate cancer cells.