Nanotechnology has allowed the construction of various nanostructures for applications, including biomedicine. However, a simple target-specific, economical, and biocompatible drug delivery platform with high maximum tolerated doses is still in demand. Here, we report aptamer-tethered DNA nanotrains (aptNTrs) as carriers for targeted drug transport in cancer therapy. Long aptNTrs were selfassembled from only two short DNA upon initiation by modified aptamers, which worked like locomotives guiding nanotrains toward target cancer cells. Meanwhile, tandem "boxcars" served as carriers with high payload capacity of drugs that were transported to target cells and induced selective cytotoxicity. aptNTrs enhanced maximum tolerated dose in nontarget cells. Potent antitumor efficacy and reduced side effects of drugs delivered by biocompatible aptNTrs were demonstrated in a mouse xenograft tumor model. Moreover, fluorophores on nanotrains and drug fluorescence dequenching upon release allowed intracellular signaling of nanotrains and drugs. These results make aptNTrs a promising targeted drug transport platform for cancer theranostics. A lthough chemotherapeutic drugs are widely used in cancer therapy, they lack specificity and can induce cytotoxicity in both cancerous and healthy cells, causing side effects (1), limited maximum tolerated dose (MTD), and reduced therapeutic efficacy (2, 3). A theranostic (4) platform with targeted and efficient drug transport would solve these problems, and, by its programmability, DNA nanotechnology has been used for the rational assembly of one-, two-, and three-dimensional nanostructures (5-8), which have been further studied for biomedical applications, including the passive targeted transport of theranostic agents (9-17). In addition, aptamers, as specific recognition elements, have been studied for active targeted transport of conventional chemotherapeutic drugs (11,12,(18)(19)(20)(21). Nucleic acid aptamers are single-stranded oligonucleotides with unique intramolecular conformations and specific recognition abilities to cognate targets, including mammalian cancer cells (22-26). Recent biotechnological advancements have led to a variety of targeted drug transport (TDT) strategies based on aptamer-drug conjugates or aptamer-nanomaterial assemblies (11,12,(18)(19)(20)(21)27). However, these strategies have unique limitations that could hamper the transition to clinical application, including (i) complicated design, laborious and uneconomical bulky preparation of myriad ssDNA as building blocks to construct sophisticated nucleic acid-based nanomaterials, or laborious and inefficient preparation of aptamer-drug conjugates (9,11,14,15,17,18); (ii) limited drug payload capacity and the attendant high cost, hampering production scale-up (9,11,14,15,17,18,20,27); (iii) poor biodegradability, causing chronic accumulation of nanomaterials in vivo (28, 29); and (iv) limited universality by the requirement of specific aptamer for drug loading (20).However, we have designed and engineered a DNA ...

An aptamer switch probe (ASP) linking chlorin e6 (Ce6), a photosensitizer molecule, to the surface of gold nanorods (AuNRs) was used to target cancer cells for photodynamic therapy (PDT) and photothermal therapy (PTT). In the presence of target cancer cells, the ASP changes conformation to drive Ce6 away from the gold surface, thereby producing singlet oxygen for PDT upon light irradiation. Since each AuNR is modified with many ASP/Ce6 molecules, the AuNR/ASP/Ce6 conjugate yields enhanced binding and therapeutic effect by the added ability to carry many photosensitizers. In addition, absorption of radiation by the gold nanorods enables further cell destruction by the photothermal effect. Consequently, this multimodal AuNR/ASP/Ce6 conjugate offers a remarkably improved and synergistic therapeutic effect compared to PTT or PDT alone, providing high specificity and therapeutic efficiency, which can be generalized to other types of cancer therapies.