Targeted Bioimaging and Photodynamic Therapy Nanoplatform Using an Aptamer‐Guided G‐Quadruplex DNA Carrier and Near‐Infrared Light

Abstract

Photodynamic therapy (PDT) has recently emerged as an effective, noninvasive, and economical treatment for diseases including cancers. Traditional PDT suffers mainly from having an insufficient number of photons penetrating the tissue and preferentially targeting cancerous tissues with photosensitizers. Therefore, it is necessary to construct a method for controllable singlet-oxygen (O2) generation (SOG) with high selectivity and accurate localization to provide more efficient PDTwith fewer side effects. Several research groups have developed novel selective PDT agents, such as peptide or protein conjugates, and photosensitizer encapsulated nanocarriers. In recent years, single-stranded oligonucleotides called aptamers have emerged as a novel class of molecules which rival antibodies in both therapeutic and diagnostic applications. Compared to antibodies, aptamers offer significant advantages, such as flexible design, synthetic accessibility, easy modification, chemical stability, and rapid tissue penetration. Therefore, aptamers can potentially endow traditional PDT with high selectivity and accurate localization. The formation of a G-quadruplex (GQ) structure is results from a type of DNA self-assembly mode. This structure can be stabilized by a central monovalent metal cation such as potassium or sodium ions and small-molecule ligands including porphyrins. Cationic porphyrins usually bind to G-quadruplexes through p–p interactions with G-quartets and through electrostatic interactions with the anionic phosphate groups on G-quadruplexes. According to previous studies, some photosensitizers are porphyrin derivatives and are broadly used in PDT, such as 5,10,15,20-tetrakis-(1-methyl-4-pyridyl)-21H, 23H-porphine (TMPyP4). Therefore, the G-quadruplex DNA sequence can be a carrier for photosensitizers with porphyrin molecular structures. By taking advantage of the loading function of the G-quadruplex structure and the recognition function of aptamers, the photosensitizer can be delivered to a target cell with high affinity and selectivity. Up to now, most photosensitizers have been activated by visible light. As a consequence, the shallow penetration depth of incident light has limited their otherwise wide applications. If the photosensitizers are linked to a visible-light generator and the generator is remotely controlled (turned on/off) by near-infrared (NIR) light, then the problem of limited depth penetration by visible light could be easily overcome. To solve the problem of limited depth penetration of current PDT techniques, we used upconversion nanoparticles (UCNPs), in particular, lanthanide-doped rare-earth nanocrystals. These nanomaterials are able to emit shorter-wavelength photons under excitation by NIR light, thus making them good visiblelight generators with the ability to be remotely controlled by NIR light. Furthermore, owing to the ladderlike arrangement of energy levels in lanthanide ions, UCNPs show high efficiency of photon upconversion with a distinct set of sharp emission peaks under moderate excitation densities, thereby enabling targeted bioimaging when functionalized with biomolecular recognition moieties. Herein, we report a specific aptamer-guided G-quadruplex DNA nanoplatform for targeted bioimaging and PDT, and it is capable of selective recognition and imaging of cancer cells, controllable and effective activation of the photosensitizer, and improvement of the therapeutic effect. In particular, a guanine-rich DNA segment is linked to an aptamer to form a bifunctional DNA sequence, termed a G4aptamer. The G4-aptamer not only loads the photosensitizer but also specifically recognizes target cells. As shown in Scheme 1, the G4-aptamer is bioconjugated to a UCNP, thus placing the photosensitizer TMPyP4 at position near the UCNP for energy transfer between the UCNP and TMPyP4. Once the nanoplatform is delivered into cancer cells, the UCNPs are excited by NIR light to emit visible light to image cancer cells and, in turn, to activate TMPyP4, which, finally, generates sufficient ROS to efficiently kill cancer cells. [*] Prof. Q. Yuan, Y. Wu, D. Lu, Dr. Z. Zhao, T. Liu, Prof. X. Zhang, Prof. W. Tan Molecular Science and Biomedicine Laboratory State Key Laboratory for Chemo/Bio-Sensing and Chemometrics College of Chemistry and Chemical Engineering, College of Biology, and Collaborative Research Center for Chemistry and Molecular Medicine Hunan University, Changsha 410082 (China) E-mail: tan@chem.ufl.edu Prof. Q. Yuan, J. Wang Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education) College of Chemistry and Molecular Sciences Wuhan University, Wuhan 430072 (China) [] These authors contributed equally to this work.