Aptamers as drug delivery agents. Prostate specific membrane antigen (PSMA) is internalized into cells via endocytosis. PSMA-binding RNA aptamers serve as drug delivery vehicles for small interfering RNAs (siRNAs, (a)). The siRNAs escape from the endosome, are processed by Dicer and RISC, and lead to the degradation of target mRNAs. Furthermore, the aptamers can escort cytotoxic molecules into prostate cancer cells like the ribosomal toxin gelonin (b), cisplatin encapsulated in organic PLGA-PEG-NPs (c), nanoparticles (NPs, gold, iron oxide and quantum dots; (d)), or doxorubicin as a physical conjugate (e). For references see text.