Simultaneous two-photon excitation of photofrin in relation to photodynamic therapy

Near-infrared metal agents assisting precision medicine: from strategic design to bioimaging and therapeutic applications

Metal agents have made incredible strides in preclinical research and clinical applications in recent years, but their short emission/absorption wavelengths continue to be a barrier to their distribution, therapeutic action, visual tracking, and efficacy evaluation. Nowadays, the near-infrared window (NIR, 650-1700 nm) provides a more accurate imaging and treatment option. Thus, there has been ongoing research focusing on developing multifunctional NIR metal agents for imaging and therapy that have deeper tissue penetration. The design, characteristics, bioimaging, and therapy of NIR metal agents are covered in this overview of papers and reports published to date. To start with, we focus on describing the structure, design strategies, and photophysical properties of metal agents from the NIR-I (650-1000 nm) to NIR-II (1000-1700 nm) region, in order of molecular metal complexes (MMCs), metal-organic complexes (MOCs), and metal-organic frameworks (MOFs). Next, the biomedical applications brought by these superior photophysical and chemical properties for more accurate imaging and therapy are discussed. Finally, we explore the challenges and prospects of each type of NIR metal agent for future biomedical research and clinical translation.

Photodynamic therapy in oral cancer: a review of clinical studies

A significant mortality rate is associated with oral cancer, particularly in cases of late-stage diagnosis. Since the last decades, oral cancer survival rates have only gradually improved despite advances in treatment. This poor success rate is mainly due to the development of secondary tumors, local recurrence, and regional failure. Invasive treatments frequently have a negative impact on the aesthetic and functional outcomes of survivors. Novel approaches are thus needed to manage this deadly disease in light of these statistics. In photodynamic therapy (PDT), a light-sensitive medication called a photosensitizer is given first, followed by exposure to light of the proper wavelength that matches the absorbance band of the photosensitizer. The tissue oxygen-induced cytotoxic free radicals kill tumor cells directly, harm the microvascular structure, and cause inflammatory reactions at the targeted sites. In the case of early lesions, PDT can be used as a stand-alone therapy, and in the case of advanced lesions, it can be used as adjuvant therapy. The current review article discussed the uses of PDT in oral cancer therapy based on recent advances in this field.

Photophysical Exploration of Zn(II) Polypyridine Photosensitizers in Two-Photon Photodynamic Therapy: Insights from Theory

The high incidence and difficulties of treatment of cancer have always been a challenge for mankind. Two-photon photodynamic therapy (TP-PDT) as a less invasive technique provides a new perspective for tumor treatment due to its low-energy near-infrared excitation, high targeting, and minor damage. At present, the emerging metal complexes used as the photosensitizers (PSs) in TP-PDT have aroused great interest. However, most metal complexes as PSs in TP-PDT still face some problems, such as slow clearance, unsatisfactory two-photon absorption (TPA) characteristics, high price, low reactivity, and poor solubility. In this work, density functional theory and time-dependent density functional theory were used to characterize the one/two-photon response, solvation free energy, and lipophilicity of a series of novel PSs applied in TP-PDT. The results suggest that based on complex 1, replacing Ru(II) center with Zn(II) (complex 2) can effectively prolong the triplet excited state lifetime while reducing the cost and environmental pollution, and the azetidine heterospirocycles were introduced into the ligand scaffold (complex 3), which effectively reduced the vibration relaxation of the ligand group and improved the water solubility; further, the addition of acetylenyl groups subtly enhanced the light absorption and significantly improved the two-photon response (complex 4). In addition, all complexes met the requirement of a PS and could be used as potential candidates for TP-PDT. In particular, complex 4 has the advantages of high solvation free energy, a large TPA cross-section (1413 GM), a long triplet state lifetime (671 μs), good chemical reactivity, and low cost, and it is easy to be scavenged by organisms. Overall, this contribution may provide an important clue to formulate clear design principles for type I/II PSs and rational design of PSs with high intersystem crossing rates, a long lifetime, and therapeutic excitation wavelengths.

Overview of Nanoparticle-Based Approaches for the Combination of Photodynamic Therapy (PDT) and Chemotherapy at the Preclinical Stage

Simple Summary

The present review represents the outstanding and promising recent literature reports (2017–2022) on nanoparticle-based formulations developed for anticancer therapy with photodynamic therapy (PDT), photosensitizers, and chemotherapeutics. Besides brief descriptions of chemotherapeutics’ classification and of PDT mechanisms and limitations, several examples of nanosystems endowed with different responsiveness (e.g., acidic pH and reactive oxygen species) and peculiarity (e.g., tumor oxygenation capacity, active tumor targeting, and biomimetic features) are described, and for each drug combination, in vitro and in vivo results on preclinical cancer models are reported.

Abstract

The widespread diffusion of photodynamic therapy (PDT) as a clinical treatment for solid tumors is mainly limited by the patient’s adverse reaction (skin photosensivity), insufficient light penetration in deeply seated neoplastic lesions, unfavorable photosensitizers (PSs) biodistribution, and photokilling efficiency due to PS aggregation in biological environments. Despite this, recent preclinical studies reported on successful combinatorial regimes of PSs with chemotherapeutics obtained through the drugs encapsulation in multifunctional nanometric delivery systems. The aim of the present review deals with the punctual description of several nanosystems designed not only with the objective of co-transporting a PS and a chemodrug for combination therapy, but also with the goal of improving the therapeutic efficacy by facing the main critical issues of both therapies (side effects, scarce tumor oxygenation and light penetration, premature drug clearance, unspecific biodistribution, etc.). Therefore, particular attention is paid to the description of bio-responsive drugs and nanoparticles (NPs), targeted nanosystems, biomimetic approaches, and upconverting NPs, including analyzing the therapeutic efficacy of the proposed photo-chemotherapeutic regimens in in vitro and in vivo cancer models.

Development of tumor-specific liposomes containing quantum dots-photosensitizer conjugate used for radiotherapy

This study aims to develop a multifunctional liposomal radiosensitizer to destroy more tumor cells by using lower radiation doses compared to clinically used 6 MV X-ray doses. To achieve this aim, first Chlorine-e6 (Ce6) was covalently bound to functional groups of outer surfaces of quantum dots (QDs) through EDC/NHS reactions. Then, QDs-Ce6 conjugate loaded, nanosized, PEG-coated, and tumor-specific folic acid-modified immunoliposome dispersions were prepared by film method. Enhanced anti-proliferation activity of free and liposomal conjugate against 4T1 (murine breast cancer) cell lines was investigated at different X-ray doses (5, 10, 15, and 20 Gy). As a result, the best radiosensitizer effect was observed at a 5 Gy X-ray dose and it was found that following the X-ray irradiation, immunoliposome dispersions containing QDs-Ce6 conjugate killed 26.8 ± 1.7% more cancer cells than radiation alone.

Recent Advances in BODIPY Compounds: Synthetic Methods, Optical and Nonlinear Optical Properties, and Their Medical Applications

Organoboron compounds are attracting immense research interest due to their wide range of applications. Particularly, low-coordinate organoboron complexes are receiving more attention due to their improbable optical and nonlinear optical properties, which makes them better candidates for medical applications. In this review, we summarize the various synthetic methods including multicomponent reactions, microwave-assisted and traditional pathways of organoboron complexes, and their optical and nonlinear properties. This review also includes the usage of organoboron complexes in various fields including biomedical applications.

Recent Approaches towards the Development of Ru(II) Polypyridyl Complexes for Anticancer Photodynamic Therapy

Photodynamic therapy (PDT) is a remarkable alternative or complementary technique to chemotherapy, radiotherapy or immunotherapy to treat certain forms of cancer. The synergistic effect of light, photosensitizer (PS) and oxygen allows for the treatment of tumours with an extremely high spatio-tumoral control, therefore minimizing the severe side effects usually observed in chemotherapy. The currently employed PDT PSs based on porphyrins have, in some cases, some limitations, which include a low absorbance in the therapeutic window, a low body clearance, photobleaching, among others. In this context, Ru(ii) polypyridyl complexes are interesting alternatives. They have low lying excited energy states and the presence of a heavy metal increases the possibility of spin-orbit coupling. Moreover, their photophysical properties are relatively easy to tune and they have very low photobleaching rates. All of these make them attractive candidates for further development as therapeutically suitable PDT PSs. In this review, after having presented this field of research, we discuss the developments made by our group in this field of research since 2017. We notably describe how we tuned the photophysical properties of our complexes from the visible region to the therapeutically suitable red region. This was accompanied by the preparation of PSs with enhanced phototoxicity and high phototoxicity index. We also discuss the use of two-photon excitation to eradicate tumours in nude mice. Furthermore, we describe our approach for the selective delivery of our complexes using targeting agents. Lastly, we report on our very recent synergistic approach to treat cancer using bimetallic Ru(ii)-Pt(iv) prodrug candidates.

Mitochondria-Specific Agents for Photodynamic Cancer Therapy: A Key Determinant to Boost the Efficacy

Mitochondria-targeted photodynamic therapy (Mt-PDT), which enables the photogenerated cytotoxic oxygen species with fatal oxidative damage to block mitochondrial functions, has been considered as a promising method to enhance the anticancer effectiveness. Aiming at the challenges of PDT, in the past few decades, numerous mitochondria-targeting molecular agents have been developed to boost the PDT efficacy via directly destroying the mitochondria or activating mitochondria-mediated cell death pathways. Herein, a review for recent advances of Mt-PDT is highlighted including: mitochondrial targeting design principles and strategies, therapeutic performance of mitochondria-targeted agents-mediated PDT as well as the agent-free Mt-PDT. In addition, it puts together the achievements of the combinatory mitochondria-anchoring PDT and other anticancer strategies, demonstrating the advantages provided by Mt-PDT. The existing challenges are discussed and future settlements for the development of mitochondria-specific agents are also forecasted.

Theoretical insights on type I/II photoreactions of potential Zn(II) polypyridyl photosensitizers for two-photon photodynamic therapy

Large two-photon absorption cross-sections are vital to photosensitizers (PSs) in TP-PDT, which can be used to develop in-depth treatment for diseased cells and minimize the harm to surrounding cells. Here, we conduct a study about photophysical properties of one Ru(II) polypyridyl complex and two designed Zn(II) polypyridyl complexes by means of DFT and TD-DFT methods. The main results are as follows: firstly, the two-photon absorption spectrum of two designed complexes Zn-OMe and Zn-OCOOCH₃ are all within the phototherapeutic window (550-900 nm). Secondly, large SOC values and small energy gaps ΔE_S-T of these complexes guarantee the efficiency of ISC process. Thirdly, their T₁ energy is greater than that required for generating ¹O₂ (0.98 eV) via Type II photoreaction. In addition, the calculated results of vertical electron affinities (VEA) and vertical ionization potentials (VIP) show that these complexes are able to form superoxide ions O₂^(-) via Type I photoreaction. Specifically, both of two designed Zn-centric complexes have larger TPA cross-sections than that of Ru-centric complex. In a word, we are pleased to report two potential photosensitizers with excellent performance and reasonable price for Type I/II photoreactions. We expect our study will offer some theoretical guidance and help in TP-PDT.

Light-Triggered Cancer Cell Specific Targeting and Liposomal Drug Delivery in a Zebrafish Xenograft Model

Cell-specific drug delivery remains a major unmet challenge for cancer nanomedicines. Here, light-triggered, cell-specific delivery of liposome-encapsulated doxorubicin to xenograft human cancer cells in live zebrafish embryos is demonstrated. This method relies on light-triggered dePEGylation of liposome surfaces to reveal underlying targeting functionality. To demonstrate general applicability of this method, light-triggered, MDA-MB-231 breast cancer cell specific targeting in vivo (embryonic zebrafish) is shown using both clinically relevant, folate-liposomes, as well as an experimental liposome-cell fusion system. In the case of liposome-cell fusion, the delivery of liposomal doxorubicin direct to the cytosol of target cancer cells results in enhanced cytotoxicity, compared to doxorubicin delivery via either folate-liposomes or free doxorubicin, as well as a significant reduction in xenograft cancer cell burden within the embryonic fish.

A Dinuclear Ruthenium(II) Complex Excited by Near-Infrared Light through Two-Photon Absorption Induces Phototoxicity Deep within Hypoxic Regions of Melanoma Cancer Spheroids

The dinuclear photo-oxidizing Ru^II complex [{Ru(TAP₂)}₂(tpphz)]⁴⁺ (TAP = 1,4,5,8- tetraazaphenanthrene, tpphz = tetrapyrido[3,2-a:2′,3′-c:3″,2′′-h:2‴,3′′′-j]phenazine), 1⁴⁺, is readily taken up by live cells localizing in mitochondria and nuclei. In this study, the two-photon absorption cross section of 1⁴⁺ is quantified and its use as a two-photon absorbing phototherapeutic is reported. It was confirmed that the complex is readily photoexcited using near-infrared, NIR, and light through two-photon absorption, TPA. In 2-D cell cultures, irradiation with NIR light at low power results in precisely focused phototoxicity effects in which human melanoma cells were killed after 5 min of light exposure. Similar experiments were then carried out in human cancer spheroids that provide a realistic tumor model for the development of therapeutics and phototherapeutics. Using the characteristic emission of the complex as a probe, its uptake into 280 μm spheroids was investigated and confirmed that the spheroid takes up the complex. Notably TPA excitation results in more intense luminescence being observed throughout the depth of the spheroids, although emission intensity still drops off toward the necrotic core. As 1⁴⁺ can directly photo-oxidize DNA without the mediation of singlet oxygen or other reactive oxygen species, phototoxicity within the deeper, hypoxic layers of the spheroids was also investigated. To quantify the penetration of these phototoxic effects, 1⁴⁺ was photoexcited through TPA at a power of 60 mW, which was progressively focused in 10 μm steps throughout the entire z-axis of individual spheroids. These experiments revealed that, in irradiated spheroids treated with 1⁴⁺, acute and rapid photoinduced cell death was observed throughout their depth, including the hypoxic region.

Synthesis of highly water soluble tetrabenzoporphyrins and their application toward photodynamic therapy

Novel tetraaryl-(pyridinium-4-yl)-tetrabenzoporphyrins have been successfully synthesized via a Heck-based sequence reaction. These tetrabenzoporphyrins were substituted with eight pyridyl groups at the fused benzene rings. Methylation of the pyridyl groups with methyl iodide afforded highly water soluble tetrabenzoporphyrins carrying eight ionic groups. The extended [Formula: see text]-conjugation broadened and red-shifted the absorption band of these porphyrins to 650–750 nm. These cationic tetrabenzoporphyrins showed non-toxicity in the dark up to 100 uM. High phototoxicity with IC[Formula: see text] values lower than 18 [Formula: see text]M were obtained for these tetrabenzoporphyrins.

Polymerization-Enhanced Two-Photon Photosensitization for Precise Photodynamic Therapy

Two-photon excited photodynamic therapy (2PE-PDT) has attracted great attention in recent years due to its great potential for deep-tissue and highly spatiotemporally precise cancer therapy. Photosensitizers (PSs) with high singlet oxygen (¹O₂) generation efficiency and large two-photon absorption (2PA) cross-sections are highly desirable, but the availability of such PSs is limited by challenges in molecular design. In this work, we report that the polymerization of small-molecule PSs with aggregation-induced emission (AIE) could yield conjugated polymer PSs with good brightness, high ¹O₂ generation efficiency, and large 2PA cross-sections. A pair of conjugated polymer PSs were designed and synthesized, and the corresponding AIE PS dots were prepared by nanoprecipitation, which exhibited outstanding 2PE-PDT performance in in vitro cancer cell ablation and in vivo zebrafish liver tumor treatment. Our work highlights a strategy to design highly efficient PSs for 2PE-PDT.

The development of ruthenium(ii) polypyridyl complexes and conjugates for in vitro cellular and in vivo applications

Ruthenium(ii) [Ru(ii)] polypyridyl complexes have been the focus of intense investigations since work began exploring their supramolecular interactions with DNA. In recent years, there have been considerable efforts to translate this solution-based research into a biological environment with the intention of developing new classes of probes, luminescent imaging agents, therapeutics and theranostics. In only 10 years the field has expanded with diverse applications for these complexes as imaging agents and promising candidates for therapeutics. In light of these efforts this review exclusively focuses on the developments of these complexes in biological systems, both in cells and in vivo, and hopes to communicate to readers the diversity of applications within which these complexes have found use, as well as new insights gained along the way and challenges that researchers in this field still face.

Molecular photosensitisers for two-photon photodynamic therapy

Two-photon excitation has attracted the attention of biologists, especially after the development of two-photon excited microscopy in the nineties. Since then, new applications have rapidly emerged such as the release of biologically active molecules and photodynamic therapy (PDT) using two-photon excitation. PDT, which requires a light-activated drug (photosensitiser), is a clinically approved and minimally invasive treatment for cancer and for non-malignant diseases. This feature article focuses on the engineering of molecular two-photon photosensitisers for PDT, which should bring important benefits to the treatment, increase the treatment penetration depth with near-infrared light excitation, improve the spatial selectivity and reduce the photodamage to healthy tissues. After an overview of the two-photon absorption phenomenon and the methods to evaluate two-photon induced phototoxicity on cell cultures, the different classes of photosensitisers described in the literature are discussed. The two-photon PDT performed with historical one-photon sensitisers are briefly presented, followed by specifically engineered cyclic tetrapyrrole photosensitisers, purely organic photosensitisers and transition metal complexes. Finally, targeted two-photon photosensitisers and theranostic agents that should enhance the selectivity and efficiency of the treatment are discussed.

New porphyrin-based dendrimers with alkene linked fluorenyl antennae for optics

The biphotonic (σ₂) singlet oxygen generation capability (Φ_Δ) of new TFP-based photosensitizers peripherally functionalized with various dendrons containing 2-ethynylflorenyl chromophores is reported along with remarkable fluorescence yields (Φ_F) of up to 17%.

Critical Overview of the Use of Ru(II) Polypyridyl Complexes as Photosensitizers in One-Photon and Two-Photon Photodynamic Therapy

Photodynamic Therapy (PDT) is an emerging technique to treat certain types of cancer, bacterial, fungal, and viral infections, and skin diseases. In past years, different research groups developed new ruthenium-containing photosensitizers (PSs) with tuned photophysical and biological properties to better fit the requirements of PDT. In this Account, we report and discuss the latest results in this research area, emphasizing particularly our own research. For example, inspired by the DNA intercalating complex [Ru(bpy)₂(dppz)]²⁺ (bpy = 2,2'-bipyridine; dppz = (dipyrido[3,2-a:2',3'-c]phenazine), a series of ruthenium complexes bearing differently functionalized dppz ligands were synthesized to target DNA. The introduction of the substituents on the dppz ligand did not reduce much the affinity of the complexes to DNA but highly affected their cellular uptake. The most effective complex in this series, [Ru(bpy)₂(dppz-7-OMe)]²⁺, showed IC₅₀ values in the low micromolar range against several types of cancer cells upon light irradiation and, importantly, a high phototoxic index (PI) of >150. This value is comparable to or even better than several PSs used in clinics under comparable experimental conditions. This compound was found to localize in the nucleus and to induce DNA damage in HeLa cells upon light irradiation. Interestingly, cells in the mitotic phase were found to be more affected and to have a different mechanism of cell death (apoptosis) upon light irradiation than those in the interphase (paraptosis). To take advantage of that, the PS was combined with a cell cycle inhibitor to synchronize cells in the mitotic phase, further improving the phototoxicity by a factor of 3.6. In addition, our group recently demonstrated that [Ru(bphen)₂(benzene-1,2-dislufinate)] (bphen = 4,7-diphenyl-1,10-phenanthroline) localizes in mitochondria and has an IC₅₀ value of 0.62 μM with a PI of over 80 in HeLa cells upon light irradiation at 420 nm. Interestingly, this complex was also found to efficiently kill Gram-positive Staphylococcus aureus under light irradiation. Antimicrobial PDT (aPDT) is another field of research where Ru(II) polypyridyl complexes can play an interesting role to fight antibiotics resistance. [Ru(dqpCO₂Me)(ptpy)]²⁺ (dqpCO₂Me = 4-methylcarboxy-2,6-di(quinolin-8-yl)pyridine), ptpy = 4'-phenyl-2,2':6',2″-terpyridine) is additionally efficient against Gram-negative Escherichia coli. The efficacy of positively charged Ru(II) PSs is related to their affinity to the negatively charged membrane of Gram-negative bacteria. A drawback of many Ru(II) polypyridyl PSs is their low absorption in the biological optical window (600-900 nm) where light penetration depth into tissue is the highest. The lowest energy transition in the UV/Vis spectra of Ru(II) polypyridyl complexes is usually a metal-to-ligand charge-transfer band. To shift the absorption into this range, tuning of the ligand system, for example, by extending π-systems, has been described in the literature. Another approach to make excitation in the optical biological window possible is Two-Photon Absorption (2PA). High photon density is needed and usually confocal laser beams are used for excitation. In collaboration with the Chao group, a series of homoleptic Ru(II) complexes bearing tertiary alkyl ammonium substituted bipyridine ligands with two photon cross sections between 185 and 250 GM at around 800 nm was tested in vitro. They showed IC₅₀ values in the micromolar range and PIs between 103 and 313. The highly positive-charged complexes were found to enter the cell via endocytosis and to target lysosomes. Studies on 3D tumor cell spheroids, a model closer to real tumors than commonly used 2D cell monolayers, were also performed. It could be demonstrated that 2P-PDT treatment with 800 nm laser irradiation was significantly more effective than that with 450 nm laser irradiation.

Two-photon photodynamic ablation of tumor cells by mitochondria-targeted iridium(iii) complexes in aggregate states

Two iridium(iii) complexes with a rotary phenyl substituent ligand were used as mitochondria-targeted photosensitizers for two-photon photodynamic therapy in aggregate states.

Singlet oxygen generation properties of an inclusion complex of cyclic free-base porphyrin dimer and fullerene C60

We demonstrate that a cyclic free-base porphyrin dimer and its inclusion complex with fullerene C₆₀ possess the ability to generate singlet oxygen (¹O₂) under visible light irradiation.

Metal Complexes for Two-Photon Photodynamic Therapy: A Cyclometallated Iridium Complex Induces Two-Photon Photosensitization of Cancer Cells under Near-IR Light

Photodynamic therapy (PDT) uses photosensitizers (PS) which only become cytotoxic upon light‐irradiation. Transition‐metal complexes are highly promising PS due to long excited‐state lifetimes, and high photo‐stabilities. However, these complexes usually absorb higher‐energy UV/Vis light, whereas the optimal tissue transparency is in the lower‐energy NIR region. Two‐photon excitation (TPE) can overcome this dichotomy, with simultaneous absorption of two lower‐energy NIR‐photons populating the same PS‐active excited state as one higher‐energy photon. We introduce two low‐molecular weight, long‐lived and photo‐stable iridium complexes of the [Ir(N^C)₂(N^N)]⁺ family with high TP‐absorption, which localise to mitochondria and lysosomal structures in live cells. The compounds are efficient PS under 1‐photon irradiation (405 nm) resulting in apoptotic cell death in diverse cancer cell lines at low light doses (3.6 J cm⁻²), low concentrations, and photo‐indexes greater than 555. Remarkably 1 also displays high PS activity killing cancer cells under NIR two‐photon excitation (760 nm), which along with its photo‐stability indicates potential future clinical application.

Beyond the Barriers of Light Penetration: Strategies, Perspectives and Possibilities for Photodynamic Therapy

Photodynamic therapy (PDT) is a photochemistry based treatment modality that involves the generation of cytotoxic species through the interactions of a photosensitizer molecule with light irradiation of an appropriate wavelength. PDT is an approved therapeutic modality for several cancers globally and in several cases has proved to be effective where traditional treatments have failed. The key parameters that determine PDT efficacy are 1. the photosensitizer (nature of the molecules, selectivity, and macroscopic and microscopic localization etc.), 2. light application (wavelength, fluence, fluence rate, irradiation regimes etc.) and 3. the microenvironment (vascularity, hypoxic regions, stromal tissue density, molecular heterogeneity etc.). Over the years, several groups aimed to monitor and manipulate the components of these critical parameters to improve the effectiveness of PDT treatments. However, PDT is still misconstrued to be a surface treatment primarily due to the limited depths of light penetration. In this review, we present the recent advances, strategies and perspectives in PDT approaches, particularly in cancer treatment, that focus on increasing the 'damage zone' beyond the reach of light in the body. This is enabled by a spectrum of approaches that range from innovative photosensitizer excitation strategies, increased specificity of phototoxicity, and biomodulatory approaches that amplify the biotherapeutic effects induced by photodynamic action. Along with the increasing depth of understanding of the underlying physical, chemical and physiological mechanisms, it is anticipated that with the convergence of these strategies, the clinical utility of PDT will be expanded to a powerful modality in the armamentarium for the management of cancer.

Prospects in the Application of Photodynamic Therapy in Oral Cancer and Premalignant Lesions

Oral cancer is a global health burden with significantly poor survival, especially when the diagnosis is at its late stage. Despite advances in current treatment modalities, there has been minimal improvement in survival rates over the last five decades. The development of local recurrence, regional failure, and the formation of second primary tumors accounts for this poor outcome. For survivors, cosmetic and functional compromises resulting from treatment are often devastating. These statistics underscore the need for novel approaches in the management of this deadly disease. Photodynamic therapy (PDT) is a treatment modality that involves administration of a light-sensitive drug, known as a photosensitizer, followed by light irradiation of an appropriate wavelength that corresponds to an absorbance band of the sensitizer. In the presence of tissue oxygen, cytotoxic free radicals that are produced cause direct tumor cell death, damage to the microvasculature, and induction of inflammatory reactions at the target sites. PDT offers a prospective new approach in controlling this disease at its various stages either as a stand-alone therapy for early lesions or as an adjuvant therapy for advanced cases. In this review, we aim to explore the applications of PDT in oral cancer therapy and to present an overview of the recent advances in PDT that can potentially reposition its utility for oral cancer treatment.

Mitochondria-targeted Triphenylamine Derivatives Activatable by Two-Photon Excitation for Triggering and Imaging Cell Apoptosis

Photodynamic therapy (PDT) leads to cell death by using a combination of a photosensitizer and an external light source for the production of lethal doses of reactive oxygen species (ROS). Since a major limitation of PDT is the poor penetration of UV-visible light in tissues, there is a strong need for organic compounds whose activation is compatible with near-infrared excitation. Triphenylamines (TPAs) are fluorescent compounds, recently shown to efficiently trigger cell death upon visible light irradiation (458 nm), however outside the so-called optical/therapeutic window. Here, we report that TPAs target cytosolic organelles of living cells, mainly mitochondria, triggering a fast apoptosis upon two-photon excitation, thanks to their large two-photon absorption cross-sections in the 760–860 nm range. Direct ROS imaging in the cell context upon multiphoton excitation of TPA and three-color flow cytometric analysis showing phosphatidylserine externalization indicate that TPA photoactivation is primarily related to the mitochondrial apoptotic pathway via ROS production, although significant differences in the time courses of cell death-related events were observed, depending on the compound. TPAs represent a new class of water-soluble organic photosensitizers compatible with direct two-photon excitation, enabling simultaneous multiphoton fluorescence imaging of cell death since a concomitant subcellular TPA re-distribution occurs in apoptotic cells.

Development of a D–π–A pyrazinium photosensitizer possessing singlet oxygen generation

(D–π–)₂A pyrazinium dyes (OEJ-1 and OEJ-2) bearing a counter anion (X⁻ = Br⁻ or I⁻) have been newly developed as a photosensitizer possessing singlet oxygen (¹O₂) generation.

Overcoming the Achilles' heel of photodynamic therapy

This review summarizes the latest progress in deep photodynamic therapy (PDT), which overcomes the Achilles' heel of PDT.

A new medium for triplet–triplet annihilated upconversion and photocatalytic application

The effect of an OH-containing medium on low power upconversion with high efficiency (about 36%) was determined in alcohol solvent containing beta-cyclodextrin.

Effective Two-Photon Excited Photodynamic Therapy of Xenograft Tumors Sensitized by Water-Soluble Bis(arylidene)cycloalkanone Photosensitizers

A series of bis(arylidene)cycloalkanone photosensitizers modified by polyethylene glycol (PEG) have been studied for two-photon excited photodynamic therapy (2PE-PDT). As compared with their prototype compounds, these PEGylated photosensitizers show enhanced water solubilities while their photophysical and photochemical properties, including linear absorption, two-photon absorption, fluorescence, and singlet oxygen quantum yield, remain unaltered. In vitro behaviors (cellular uptake, subcellular localization, photocytotoxicity in both PDT and 2PE-PDT) of these photosensitizers reveal that an optimized lipid-water partition coefficient can be obtained by adjusting the length and position of the PEG chains. Among them, the photosensitizer modified asymmetrically by two tetraethylene glycol chains presents the best performance as a 2PE-PDT candidate. Selective blood-vessel closure and obvious therapeutic effect in inhibiting the growth of tumors are confirmed by in vivo 2PE-PDT after intravenous injection of this photosensitiezer. The survival periods of treated tumor-bearing mice are significantly prolonged. This study demonstrates the feasibility of using a simple molecule to construct a potential candidate for 2PE-PDT.

Functionalized fullerenes in photodynamic therapy

Since the discovery of C60 fullerene in 1985, scientists have been searching for biomedical applications of this most fascinating of molecules. The unique photophysical and photochemical properties of C60 suggested that the molecule would function well as a photosensitizer in photodynamic therapy (PDT). PDT uses the combination of non-toxic dyes and harmless visible light to produce reactive oxygen species that kill unwanted cells. However the extreme insolubility and hydrophobicity of pristine CO60, mandated that the cage be functionalized with chemical groups that provided water solubility and biological targeting ability. It has been found that cationic quaternary ammonium groups provide both these features, and this review covers work on the use of cationic fullerenes to mediate destruction of cancer cells and pathogenic microorganisms in vitro and describes the treatment of tumors and microbial infections in mouse models. The design, synthesis, and use of simple pyrrolidinium salts, more complex decacationic chains, and light-harvesting antennae that can be attached to C60, C70 and C84 cages are covered. In the case of bacterial wound infections mice can be saved from certain death by fullerene-mediated PDT.

Unraveling the Degradation Mechanism of Purine Nucleotides Photosensitized by Pterins: The Role of Charge-Transfer Steps

Photosensitized reactions contribute to the development of skin cancer and are used in many applications. Photosensitizers can act through different mechanisms. It is currently accepted that if the photosensitizer generates singlet molecular oxygen ((1) O2 ) upon irradiation, the target molecule can undergo oxidation by this reactive oxygen species and the reaction needs dissolved O2 to proceed, therefore the reaction is classified as (1) O2 -mediated oxidation (type II mechanism). However, this assumption is not always correct, and as an example, a study on the degradation of 2'-deoxyguanosine 5'-monophosphate photosensitized by pterin is presented. A general mechanism is proposed to explain how the degradation of biological targets, such as nucleotides, photosensitized by pterins, naturally occurring (1) O2 photosensitizers, takes place through an electron-transfer-initiated process (type I mechanism), whereas the contribution of the (1) O2 -mediated oxidation is almost negligible.

Dendritic molecular assemblies for singlet oxygen generation: meso-tetraphenylporphyrin-based biphotonic sensitizers with remarkable luminescence

Thanks to its 2-ethynylflorenyl-containing arms, dendrimer 1 sensitizes molecular oxygen in 70% yield and exhibits a fluorescence yield of 20%.

Quantum chemical insights into the dependence of porphyrin basicity on the meso-aryl substituents: thermodynamics, buckling, reaction sites and molecular flexibility

The chemical and sensing properties of porphyrins are frequently tunedviathe introduction of peripheral substituents. Their interaction with the porphyrin core is investigated.

Novel carbazole-based two-photon photosensitizer for efficient DNA photocleavage in anaerobic condition using near-infrared light

Novel carbazole derivatives are first reported as two-photon photosensitizers for DNA photodamage under near-infrared light exposure.

A cyclometallated fluorenyl Ir(iii) complex as a potential sensitiser for two-photon excited photodynamic therapy (2PE-PDT)

A cyclometallated fluorenyl Ir(iii) complex that exhibits enhanced triplet oxygen sensing properties for two photon photodynamic therapy is reported.