The nonradiative conversion of light energy into heat (photothermal therapy, PTT) or sound energy (photoacoustic imaging, PAI) has been intensively investigated for the treatment and diagnosis of cancer, respectively. By taking advantage of nanocarriers, both imaging and therapeutic functions together with enhanced tumour accumulation have been thoroughly studied to improve the pre-clinical efficiency of PAI and PTT. In this review, we first summarize the development of inorganic and organic nano photothermal transduction agents (PTAs) and strategies for improving the PTT outcomes, including applying appropriate laser dosage, guiding the treatment via imaging techniques, developing PTAs with absorption in the second NIR window, increasing photothermal conversion efficiency (PCE), and also increasing the accumulation of PTAs in tumours. Second, we introduce the advantages of combining PTT with other therapies in cancer treatment. Third, the emerging applications of PAI in cancer-related research are exemplified. Finally, the perspectives and challenges of PTT and PAI for combating cancer, especially regarding their clinical translation, are discussed. We believe that PTT and PAI having noteworthy features would become promising next-generation non-invasive cancer theranostic techniques and improve our ability to combat cancers.

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Photothermal therapy and photoacoustic imaging via nanotheranostics in fighting cancer

The reactive oxygen species (ROS)-mediated mechanism is the major cause underlying the efficacy of photodynamic therapy (PDT). The PDT procedure is based on the cascade of synergistic effects between light, a photosensitizer (PS) and oxygen, which greatly favors the spatiotemporal control of the treatment. This procedure has also evoked several unresolved challenges at different levels including (i) the limited penetration depth of light, which restricts traditional PDT to superficial tumours; (ii) oxygen reliance does not allow PDT treatment of hypoxic tumours; (iii) light can complicate the phototherapeutic outcomes because of the concurrent heat generation; (iv) specific delivery of PSs to sub-cellular organelles for exerting effective toxicity remains an issue; and (v) side effects from undesirable white-light activation and self-catalysation of traditional PSs. Recent advances in nanotechnology and nanomedicine have provided new opportunities to develop ROS-generating systems through photodynamic or non-photodynamic procedures while tackling the challenges of the current PDT approaches. In this review, we summarize the current status and discuss the possible opportunities for ROS generation for cancer therapy. We hope this review will spur pre-clinical research and clinical practice for ROS-mediated tumour treatments.

Reactive oxygen species generating systems meeting challenges of photodynamic cancer therapy

Reactive oxygen species (ROS) play an essential role in regulating various physiological functions of living organisms. The intrinsic biochemical properties of ROS, which underlie the mechanisms ne...

Reactive Oxygen Species (ROS)-Based Nanomedicine.

Photodynamic therapy (PDT) has been applied to treat a wide range of medical conditions, including wet age-related macular degeneration psoriasis, atherosclerosis, viral infection and malignant cancers. However, the tissue penetration limitation of excitation light hinders the widespread clinical use of PDT. To overcome this “Achilles' heel”, deep PDT, a novel type of phototherapy, has been developed for the efficient treatment of deep-seated diseases. Based on the different excitation sources, including near-infrared (NIR) light, X-ray radiation, and internal self-luminescence, a series of deep PDT techniques have been explored to demonstrate the advantages of deep cancer therapy over conventional PDT excited by ultraviolet-visible (UV-Vis) light. In particular, the featured applications of deep PDT, such as organelle-targeted deep PDT, hypoxic deep PDT and deep PDT-involved multimodal synergistic therapy are discussed. Finally, the future development and potential challenges of deep PDT are also elucidated for clinical translation. It is highly expected that deep PDT will be developed as a versatile, depth/oxygen-independent and minimally invasive strategy for treating a variety of malignant tumours at deep locations.

Overcoming the Achilles' heel of photodynamic therapy.

The chemistry of the less familiar elements is a fascinating topic especially for the inorganic minded. The lanthanides, or rare earths, comprise the 5d block of the periodic table and represent a huge array of applications from catalysis to lasers, and of course, imaging agents.1

Recent advances in luminescence and magnetic resonance microscopy have, in part, been stimulated by extraordinary success in the development of new lanthanide probes. The unique properties of the lanthanides provide for a deep tool chest for the chemist, biologist and the imaging scientist to exploit, and that exploitation is in full swing.

In this review we focus on two classes of lanthanide probes that are subsets of the larger area of metalloimaging: luminescent and magnetic lanthanides. In Section 2 we discuss the general design and photophysical properties of lanthanides and how these parameters are tuned to develop bioresponsive probes for optical imaging. In Section 3 we provide a brief description of how MR images are acquired and the how MRI contrast agents are engineered to respond to biological events of interest. These guiding principles have driven research that has produced a truly diverse number of new agents that are target specific and bioresponsive (or bioactivatable). While other imaging modalities utilize lanthanide-based probes, these topics are beyond the scope of this review. We direct the reader to explore some excellent reviews in the important areas of radiometals and multimodal imaging.2–5

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Lanthanide probes for bioresponsive imaging.

Bioactive species, including reactive oxygen species (ROS, including O2˙-, H2O2, HOCl, 1O2, ˙OH, HOBr, etc.), reactive nitrogen species (RNS, including ONOO-, NO, NO2, HNO, etc.), reactive sulfur species (RSS, including GSH, Hcy, Cys, H2S, H2S n , SO2 derivatives, etc.), ATP, HCHO, CO and so on, are a highly important category of molecules in living cells. The dynamic fluctuations of these molecules in subcellular microenvironments determine cellular homeostasis, signal conduction, immunity and metabolism. However, their abnormal expressions can cause disorders which are associated with diverse major diseases. Monitoring bioactive molecules in subcellular structures is therefore critical for bioanalysis and related drug discovery. With the emergence of organelle-targeted fluorescent probes, significant progress has been made in subcellular imaging. Among the developed subcellular localization fluorescent tools, ROS, RNS and RSS (RONSS) probes are highly attractive, owing to their potential for revealing the physiological and pathological functions of these highly reactive, interactive and interconvertible molecules during diverse biological events, which are rather significant for advancing our understanding of different life phenomena and exploring new technologies for life regulation. This review mainly illustrates the design principles, detection mechanisms, current challenges, and potential future directions of organelle-targeted fluorescent probes toward RONSS.

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Fluorescent probes for organelle-targeted bioactive species imaging

Photodynamic therapy (PDT) is ineffective against deeply seated metastatic tumors due to poor penetration of the excitation light. Herein, we developed a biomimetic nanoreactor (bio-NR) to achieve synergistic chemiexcited photodynamic-starvation therapy against tumor metastasis. Photosensitizers on the hollow mesoporous silica nanoparticles (HMSNs) are excited by chemical energy in situ of the deep metastatic tumor to generate singlet oxygen (1O2) for PDT, and glucose oxidase (GOx) catalyzes glucose into hydrogen peroxide (H2O2). Remarkably, this process not only blocks the nutrient supply for starvation therapy but also provides H2O2 to synergistically enhance PDT. Cancer cell membrane coating endows the nanoparticle with biological properties of homologous adhesion and immune escape. Thus, bio-NRs can effectively convert the glucose into 1O2 in metastatic tumors. The excellent therapeutic effects of bio-NRs in vitro and in vivo indicate their great potential for cancer metastasis therapy.

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A biomimetic nanoreactor for synergistic chemiexcited photodynamic therapy and starvation therapy against tumor metastasis.

Cancer is a leading cause of death worldwide and accounts for several millions of deaths every year. The survival of cancer patients is strongly associated with the stage of the tumor at the time of diagnosis. Identifying the cancer at the cellular level in an early stage before metastasis holds great promise for increasing the survival of cancer patients. A major focus of research towards this goal is on the estimation of abnormalities in gene expression in living cells. Tumor-related mRNA has been widely used as a specific marker to assess the migration of tumor cells locally or in the bloodstream. Changes in the level of tumor-related mRNA expression are correlated with tumor burden and malignant progression. The detection of tumor-related mRNA markers in intact cancer cells provides new tools for identifying cancer cells in clinical samples. Recently, a variety of techniques have been exploited to monitor tumor-related mRNAs. Among these methods, fluorescence imaging analysis offers an appealing approach for the detection of cancer at the cellular level, which may be of prognostic significance. Many fluorescence probes have been synthesized for the detection and imaging of mRNA in cancer cells and most of the research focuses on detecting a single type of mRNA, which may yield false positive results and limits the development of intracellular mRNA imaging and detection. Notably, cancer is associated with multiple tumor-related mRNAs, and some mRNA markers are expressed in normal cells. Simultaneous detection of multiple targets brings new opportunities for improving the accuracy of early cancer detection over the single-marker assay. Although various types of platforms for the detection of multiple targets have been developed, none have been designed for imaging three or more markers in living cells. Such intracellular imaging techniques for multiple tumorrelated mRNAs could promote the progress of early cancer detection. Herein, we describe a multicolor fluorescence nanoprobe based on nanoflares, which simultaneously detects three intracellular tumor-related mRNAs. The nanoprobe consists of gold nanoparticles (Au NPs) functionalized with a dense shell of recognition sequences (synthetic oligonucleotides) hybridized to three short dye-terminated reporter sequences by gold–thiol bond formation (Figure 1). The recognition sequences contain 21-base recognition elements for three specific mRNA transcripts: c-myc mRNA, TK1 mRNA, and GalNAc-T mRNA.

A multicolor nanoprobe for detection and imaging of tumor-related mRNAs in living cells.

Photodynamic therapy (PDT) is a well-established modality for cancer therapy, which locally kills cancer cells when light irradiates a photosensitizer. However, conventional PDT is often limited by the extremely short lifespan and severely limited diffusion distance of reactive oxygen species (ROS) generated by photosensitizer, as well as the penetration depth of visible light activation. Here, we develop a near-infrared (NIR) triggered nanophotosensitizer based on mitochondria targeted titanium dioxide-coated upconversion nanoparticles for PDT against cancer. When irradiated by NIR laser, the nanophotosensitizer could produce ROS in mitochondria, which induced the domino effect on ROS burst. The overproduced ROS accumulated in mitochondria, resulting in mitochondrial collapse and irreversible cell apoptosis. Confocal fluorescence imaging indicated that the mitochondrial targeting and real-time imaging of ROS burst could be achieved in living cells. The complete removal of tumor in vivo confirmed the exce...

A Near-Infrared Triggered Nanophotosensitizer Inducing Domino Effect on Mitochondrial Reactive Oxygen Species Burst for Cancer Therapy

The development of highly effective anticancer drugs that cause minimal damage to the surrounding normal tissues is a challenging topic in cancer therapy. Herein, we demonstrate a dual-targeted organic molecule that functions as a photothermal agent by actively targeting tumor tissue and mitochondria to selectively kill cancer cells. The synthesized photothermal agent exhibited high photothermal conversion efficiency, low cytotoxicity, and good biological compatibility. In vivo experiments showed an excellent tumor inhibitory effect of the dual-targeted photothermal agent.

Wei Pan

Papers

Fluorescent probes for organelle-targeted bioactive species imaging

A biomimetic nanoreactor for synergistic chemiexcited photodynamic therapy and starvation therapy against tumor metastasis.

A multicolor nanoprobe for detection and imaging of tumor-related mRNAs in living cells.

A Near-Infrared Triggered Nanophotosensitizer Inducing Domino Effect on Mitochondrial Reactive Oxygen Species Burst for Cancer Therapy

A Dual-Targeted Organic Photothermal Agent for Enhanced Photothermal Therapy