Reactive oxygen species in photodynamic therapy

Photosensitizer-mediated production of reactive oxygen species (ROS) plays a key role in photodynamic therapy (PDT). Reactive oxygen species mainly includes singlet oxygen, hydrogen peroxide, hydroxyl radical, superoxide anion radical and so on, among which singlet oxygen is the most important reactive oxygen species. In this paper, we firstly introduce the basic property of reactive oxygen species, generation and elimination of endogenous reactive oxygen species and generation mechanism of exogenous reactive oxygen species based on light, oxygen and photosensitizer. Next, we focus on the photosensitizers used to generate reactive oxygen species, factors that affect the yield of reactive oxygen species, and methods to increase the reactive oxygen species concentration at the tumor site. There are many factors that can affect the reactive oxygen species concentration at the tumor site, mainly among which are the singlet oxygen yield of the photosensitizer, the targeting of the photosensitizer, the oxygen concentration, and the illumination frequency. To achieve a good PDT effect, many photosensitizers or their nanoparticles with high singlet oxygen generation and good targeting ability are synthesized. Perfluorocarbon is used to deliver oxygen to overcome oxygen deficiency. Upconversion nanoparticles and two-photon absorption are very useful to solve the problem of weak penetration of short-wavelength light. New PDT methods are also introduced in the text, such as PDT combined photothermal therapy or chemotherapy. Then, we summarized various methods and technologies for reactive oxygen species detection, including direct detection of phosphorescence spectrophotometry, fluorescent probes and chemical probes for indirect detection, electron spin resonance, and positron emission tomography. The direct phosphorescence method mainly detects singlet oxygen based on the 1270 nm phosphorescence emission when singlet oxygen returns to the ground state. The first method to enhance the 1270 nm phosphor signal is to use a NIR photomultiplier tube (PMT) or an InGaAs linear array in combination with scanning of the excitation laser beam, and the second one is based on the use of a NIR camera with a filter. Because the lifetime of singlet oxygen is very short, time resolution is especially important. There are basically three main photon counting techniques: gated photon counting (GPC), multichannel scaling (MCS), and time-correlated single photon counting (TCSPC). In addition, singlet oxygen-sensitized delayed fluorescence (SOSDF) can also be used to detect singlet oxygen. Various new types of probes can specifically detect different types of reactive oxygen species, for example, 9, 10-anthracenedipropionic acid and MitoSOX can be used to detect singlet oxygen and superoxide anion in living cells respectively. The fluorescent probe itself does not have fluorescence, but reacts with reactive oxygen species to form an inner oxide, and emits strong fluorescence under the excitation of a certain wavelength of light. The difference of chemical probe is that it does not require light excitation because it can emit light spontaneously after reacted with reactive oxygen species. Electrons spin resonance (ESR) and positron emission tomography (PET) are also available to detect reactive oxygen species by using a probe. Finally, we discussed the mechanism of reactive oxygen species killing tumor cells, mainly in the following three aspects: (1) direct toxicity to tumor cells and induce their necrosis, apoptosis or autophagy; (2) damage on the tumor capillary endothelial cells and destruction of the microvasculature; (3) activation of an acute inflammatory response in host defense mechanisms. The effect of reactive oxygen species on the immune system and the apoptotic pathway of tumor cells from genes, proteins to the cell level were described in detail.