Far-red light activatable, multifunctional prodrug for fluorescence optical imaging and combinational treatment

Recent clinical trials and optical control as a potential strategy to develop microtubule-targeting drugs in colorectal cancer management

Colorectal cancer (CRC) has remained the second and the third leading cause of cancer-related death worldwide and in the United States, respectively. Although significant improvement in overall survival has been achieved, death in adult populations under the age of 55 appears to have increased in the past decades. Although new classes of therapeutic strategies such as immunotherapy have emerged, their application is very limited in CRC so far. Microtubule (MT) inhibitors such as taxanes, are not generally successful in CRC. There may be some way to make MT inhibitors work effectively in CRC. One potential advantage that we can take to treat CRC may be the combination of optical techniques coupled to an endoscope or other fiber optics-based devices. A combination of optical devices and photo-activatable drugs may allow us to locally target advanced CRC cells with highly potent MT-targeting drugs. In this Editorial review, we would like to discuss the potential of optogenetic approaches in CRC management.

Enhancing Precision in Photodynamic Therapy: Innovations in Light-Driven and Bioorthogonal Activation

Over the past few decades, photodynamic therapy (PDT) has evolved as a minimally invasive treatment modality offering precise control over cancer and various other diseases. To address inherent challenges associated with PDT, researchers have been exploring two promising avenues: the development of intelligent photosensitizers activated through light-induced energy transfers, charges, or electron transfers, and the disruption of photosensitive bonds. Moreover, there is a growing emphasis on the bioorthogonal delivery or activation of photosensitizers within tumors, enabling targeted deployment and activation of these intelligent photosensitive systems in specific tissues, thus achieving highly precise PDT. This concise review highlights advancements made over the last decade in the realm of light-activated or bioorthogonal photosensitizers, comparing their efficacy and shaping future directions in the advancement of photodynamic therapy.

Cleavage and Noncleavage Chemistry in Reactive Oxygen Species (ROS)-Responsive Materials for Smart Drug Delivery

The design and development of advanced drug delivery systems targeting reactive oxygen species (ROS) have gained significant interest in recent years for treating various diseases, including cancer, psychiatric diseases, cardiovascular diseases, neurological diseases, metabolic diseases, and chronic inflammations. Integrating specific chemical bonds capable of effectively responding to ROS and triggering drug release into the delivery system is crucial. In this Review, we discuss commonly used conjugation linkers (chemical bonds) and categorize them into two groups: cleavable linkers and noncleavable linkers. Our goal is to clarify their unique drug release mechanisms from a chemical perspective and provide practical organic synthesis approaches for their efficient production. We showcase numerous significant examples to demonstrate their synthesis routes and diverse applications. Ultimately, we strive to present a comprehensive overview of cleavage and noncleavage chemistry, offering insights into the development of smart drug delivery systems that respond to ROS.

Bioorthogonal chemistry for prodrug activation in vivo

Prodrugs have emerged as a major strategy for addressing clinical challenges by improving drug pharmacokinetics, reducing toxicity, and enhancing treatment efficacy. The emergence of new bioorthogonal chemistry has greatly facilitated the development of prodrug strategies, enabling their activation through chemical and physical stimuli. This "on-demand" activation using bioorthogonal chemistry has revolutionized the research and development of prodrugs. Consequently, prodrug activation has garnered significant attention and emerged as an exciting field of translational research. This review summarizes the latest advancements in prodrug activation by utilizing bioorthogonal chemistry and mainly focuses on the activation of small-molecule prodrugs and antibody-drug conjugates. In addition, this review also discusses the opportunities and challenges of translating these advancements into clinical practice.

Visible light-activated prodrug system with a novel heavy-atom-free photosensitizer

The use of light to activate prodrugs offers a promising method for the precise control of drug release, reducing drug-related side effects, and enhancing therapeutic effectiveness. We have created a novel prodrug system that utilizes a unique, heavy-atom-free photosensitizer to produce singlet oxygen, which then triggers the conversion of the prodrug into its active form. This system has been successfully demonstrated through the creation of "photo-unclick" prodrugs of paclitaxel (PTX), combretastatin A-4 (CA-4), and 10-hydroxy-7-ethylcamptothecin (SN-38). These prodrugs show decreased toxicity in the absence of light, but exhibit increased toxicity when exposed to red light.

Activatable dual-functional molecular agents for imaging-guided cancer therapy

Theranostics has attracted great attention due to its ability to combine the real-time diagnosis of cancers with efficient treatment modalities. Activatable dual-functional molecular agents could be synthesized by covalently conjugating imaging agents, therapeutic agents, stimuli-responsive linkers and/or targeting molecules together. They could be selectively activated by overexpressed physiological stimuli or external triggers at the tumor sites to release imaging agents and cytotoxic drugs, thus offering many advantages for tumor imaging and therapy, such as a high signal-to-noise ratio, low systemic toxicity, and improved therapeutic effects. This review summarizes the recent advances of dual-functional molecular agents that respond to various physiological or external stimuli for cancer theranostics. The molecular designs, synthetic strategies, activatable mechanisms, and biomedical applications of these molecular agents are elaborated, followed by a brief discussion of the challenges and opportunities in this field.

Spatiotemporal Control of Biology: Synthetic Photochemistry Toolbox with Far-Red and Near-Infrared Light

The complex network of naturally occurring biological pathways motivates the development of new synthetic molecules to perturb and/or detect these processes for fundamental research and clinical applications. In this context, photochemical tools have emerged as an approach to control the activity of drug or probe molecules at high temporal and spatial resolutions. Traditional photochemical tools, particularly photolabile protecting groups (photocages) and photoswitches, rely on high-energy UV light that is only applicable to cells or transparent model animals. More recently, such designs have evolved into the visible and near-infrared regions with deeper tissue penetration, enabling photocontrol to study biology in tissue and model animal contexts. This Review highlights recent developments in synthetic far-red and near-infrared photocages and photoswitches and their current and potential applications at the interface of chemistry and biology.

Recent advances in combretastatin A-4 codrugs for cancer therapy

CA4 is a potent microtubule polymerization inhibitor and vascular disrupting agent. However, the in vivo efficiency of CA4 is limited owing to its poor pharmacokinetics resulting from its high lipophilicity and low water solubility. To improve the water solubility, CA4 phosphate (CA4P) has been developed and shows potent antivascular and antitumor effects. CA4P had been evaluated as a vascular disrupting agent in previousc linical trials. However, it had been discontinued due to the lack of a meaningful improvement in progression-free survival and unfavorable partial response data. Codrug is a drug design approach to chemically bind two or more drugs to improve therapeutic efficiency or decrease adverse effects. This review describes the progress made over the last twenty years in developing CA4-based codrugs to improve the therapeutic profile and achieve targeted delivery to cancer tissues. It also discusses the existing problems and the developmental prospects of CA4 codrugs.

Targeted cancer phototherapy using phthalocyanine-anticancer drug conjugates

Phototherapy, the use of light to selectively ablate cancerous tissue, is a compelling prospect. Phototherapy is divided into two major domains: photodynamic and photothermal, whereby photosensitizer irradiation generates reactive oxygen species or heat, respectively, to disrupt the cancer microenvironment. Phthalocyanines (Pcs) are prominent phototherapeutics due to their desirable optical properties and structural versatility. Targeting of Pc photosensitizers historically relied on the enhanced permeation and retention effect, but the weak specificity engendered by this approach has hindered bench-to-clinic translation. To improve specificity, antibody and peptide active-targeting groups have been employed to some effect. An alternative targeting method exploits the binding of anticancer drugs to direct the photosensitizer close to essential cellular components, allowing for precise, synergistic phototherapy. This Perspective explores the use of Pc-drug conjugates as targeted anticancer phototherapeutic systems with examples of Pc-platin, Pc-kinase, and Pc-anthracycline conjugates discussed in detail.

Photosensitive EGFR-Targeted Nanocarriers for Combined Photodynamic and Local Chemotherapy

The major limitation of any cancer therapy lies in the difficulty of precisely controlling the localization of the drug in the tumor cells. To improve this drawback, our study explores the use of actively-targeted chemo-photo-nanocarriers that recognize and bind to epidermal growth factor receptor-overexpressing cells and promote the local on-demand release of the chemotherapeutic agent doxorubicin triggered by light. Our results show that the attachment of high concentrations of doxorubicin to cetuximab-IRDye700DX-mesoporous silica nanoparticles yields efficient and selective photokilling of EGFR-expressing cells mainly through singlet oxygen-induced release of the doxorubicin from the nanocarrier and without any dark toxicity. Therefore, this novel triply functionalized nanosystem is an effective and safe nanodevice for light-triggered on-demand doxorubicin release.

Recent Trends in Tubulin-Binding Combretastatin A-4 Analogs for Anticancer Drug Development

Although significant progress over several decades has been evidenced in cancer therapy, there still remains a need for the development of novel and effective therapeutic strategies to treat several relapsed and intractable cancers. In this regard, tubulin protein has become one of the efficient and major targets for anticancer drug discovery. Considering the antimitotic ability, several tubulin inhibitors have been developed to act against various cancers. Among various tubulin inhibitors available, combretastatin-A4 (CA-4), a naturally occurring lead molecule, offers exceptional cytotoxicity (including the drug-resistant cell lines) and antivascular effects. Although CA-4 offers exceptional therapeutic efficacy, several new advancements have been proposed, such as the structural modification via A and B rings, as well as cis-olefinic bridging, which provide highly efficient analogs with improved tubulin-binding efficiency to meet the anticancer drug development requirements. This review systematically emphasizes the recent trends and latest developments in the anticancer drug design & discovery, using CA-4 analogs as the tubulin inhibiting agents, highlighting their structure-activity relationships (SAR) and resultant pharmacological efficacies.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

Singlet Oxygen-Responsive Polymeric Nanomedicine for Light-Controlled Drug Release and Image-Guided Photodynamic-Chemo Combination Therapy

Coencapsulation of chemotherapeutic agents and photosensitizers into nanocarriers can help to achieve a combination of chemotherapy and photodynamic therapy for superior antitumor effects. However, precise on-demand drug release remains a major challenge. In addition, the loaded photosensitizers usually tend to aggregate, which can significantly weaken their fluorescent signals and photodynamic activities. To address these issues, herein, a smart nanocarrier termed as singlet oxygen-responsive nanoparticle (SOR-NP) was constructed by introducing singlet oxygen (¹O₂)-sensitive aminoacrylate linkers into amphiphilic mPEG-b-PCL copolymers. Boron dipyrromethene (BDP) and paclitaxel (PTX) as model therapeutic agents were coloaded into an ¹O₂-responsive nanocarrier for realizing light-controlled drug release and combination cancer treatment. This polymeric nanocarrier could substantially relieve the aggregation of encapsulated BDP due to the presence of a long hydrophobic chain. Therefore, the formed SOR-NP_BDP/PTX nanodrug could generate bright fluorescent signals and high levels of ¹O₂, which could mediate cell death via PDT and rupture aminoacrylate linker simultaneously, leading to collapse of SOR-NP_BDP/PTX and subsequent PTX release. The light-triggered drug release and combined anticancer effects of SOR-NP_BDP/PTX were validated in HepG2 and MCF-7 cancer cells and H22 tumor-bearing mice. This study provides a promising strategy for tumor-specific drug release and selective photodynamic-chemo combination treatment.

Red light triggered photodynamic-chemo combination therapy using a prodrug caged by photosensitizer

Development of the drug with high therapeutic efficacy and low toxicity is crucial to cancer ablation. In this study, we have demonstrated a red light-responsive prodrug BDP-TK-CPT by connecting the chemotherapeutic agent camptothecin with a boron dipyrromethene (BDP)-based photosensitizer via a reactive oxygen species (ROS)-labile thioketal chain. Since camptothecin is modified by a BDP-based macrocycle at the active site, the formed prodrug displays an extremely low toxicity in dark. However, upon illumination by red light, it can efficiently generate ROS leading to cell death by photodynamic therapy. Meanwhile, the ROS generated can destroy thioketal group to release free camptothecin which further results in local cell death by chemotherapy. The combined antitumor effects of the prodrug have been verified in HepG2, EC109, and HeLa cancer cells and mice bearing H22 tumors. This study may provide an alternative strategy for stimuli-responsive combination treatment of tumors by conjugation of ROS-activatable prodrugs with photosensitizing agents.

Recent progress in photosensitizers for overcoming the challenges of photodynamic therapy: from molecular design to application

Photodynamic therapy (PDT), a therapeutic mode involving light triggering, has been recognized as an attractive oncotherapy treatment. However, nonnegligible challenges remain for its further clinical use, including finite tumor suppression, poor tumor targeting, and limited therapeutic depth. The photosensitizer (PS), being the most important element of PDT, plays a decisive role in PDT treatment. This review summarizes recent progress made in the development of PSs for overcoming the above challenges. This progress has included PSs developed to display enhanced tolerance of the tumor microenvironment, improved tumor-specific selectivity, and feasibility of use in deep tissue. Based on their molecular photophysical properties and design directions, the PSs are classified by parent structures, which are discussed in detail from the molecular design to application. Finally, a brief summary of current strategies for designing PSs and future perspectives are also presented. We expect the information provided in this review to spur the further design of PSs and the clinical development of PDT-mediated cancer treatments.

Silicon phthalocyanines: synthesis and resurgent applications

Their unique axial bonds and NIR optical properties have made silicon phthalocyanines (SiPcs) valuable compounds. Herein, we present key synthetic strategies and emerging applications of SiPcs over the past decade.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

A near-infrared light-mediated cleavable linker strategy using the heptamethine cyanine chromophore

Optical methods offer the potential to manipulate living biological systems with exceptional spatial and temporal control. Caging bioactive molecules with photocleavable functional groups is an important strategy that could be applied to a range of problems, including the targeted delivery of otherwise toxic therapeutics. However existing approaches that require UV or blue light are difficult to apply in organismal settings due to issues of tissue penetration and light toxicity. Photocaging groups built on the heptamethine cyanine scaffold enable the targeted delivery of bioactive molecules using near-IR light (up to 780nm) in live animal settings. Here we provide a detailed procedure demonstrating the utility of the heptamethine cyanine caging group to create a light-cleavable linker between an antibody, panitumumab, and a therapeutic small molecule in the duocarmycin class of natural products. Descriptions of the design and synthesis of the small molecule component, assembly of the antibody conjugate, in vitro analysis of uncaging, in vivo imaging, and impact on tumor progression are provided.

Local and Systemic Antitumor Effects of Photo-activatable Paclitaxel Prodrug on Rat Breast Tumor Models

We demonstrated that a large primary and a small untreated distant breast cancer could be controlled by local treatment with our light-activatable paclitaxel (PTX) prodrug. We hypothesized that the treated tumor would be damaged by the combinational effects of photodynamic therapy (PDT) and locally released PTX and that the distant tumor would be suppressed by systemic antitumor effects. Syngeneic rat breast cancer models (single- and two-tumor models) were established on Fischer 344 rats by subcutaneous injection of MAT B III cells. The rats were injected with PTX prodrug (dose: 1 umole kg-1 , i.v.), and tumors were treated with illumination using a 690-nm laser (75 or 140 mW cm-1 for 30 min, cylindrical light diffuser, drug-light interval [DLI] 9 h). Larger tumors (~16 mm) were effectively ablated (100%) without recurrence for >90 days. All cured rats rejected rechallenged tumor for up to 12 months. In the two-tumor model, the treatment of the local large tumor (~16 mm) also cured the untreated tumor (4-6 mm) through adaptive immune activation. This is our first demonstration that local treatment with our PTX prodrug produces systemic antitumor effects. Further investigations are warranted to understand mechanisms and optimal conditions to achieve clinically translatable systemic antitumor effects.

Development of Prodrugs for PDT-Based Combination Therapy Using a Singlet-Oxygen-Sensitive Linker and Quantitative Systems Pharmacology

Photodynamic therapy (PDT) has become an effective treatment for certain types of solid tumors. The combination of PDT with other therapies has been extensively investigated in recent years to improve its effectiveness and expand its applications. This focused review summarizes the development of a prodrug system in which anticancer drugs are activated locally at tumor sites during PDT treatment. The development of a singlet-oxygen-sensitive linker that can be conveniently conjugated to various drugs and efficiently cleaved to release intact drugs is recapitulated. The initial design of prodrugs, preliminary efficacy evaluation, pharmacokinetics study, and optimization using quantitative systems pharmacology is discussed. Current treatment optimization in animal models using physiologically based a pharmacokinetic (PBPK) modeling approach is also explored.

Prodrug strategies for targeted therapy triggered by reactive oxygen species

Increased levels of reactive oxygen species (ROS) have been associated with numerous pathophysiological conditions including cancer and inflammation and the ROS stimulus constitutes a potential trigger for drug delivery strategies. Over the past decade, a number of ROS-sensitive functionalities have been identified with the purpose of introducing disease-targeting properties into small molecule drugs - a prodrug strategy that offers a promising approach for increasing the selectivity and efficacy of treatments. This review will provide an overview of the ROS-responsive prodrugs developed to date. A discussion on the current progress and limitations is provided along with a reflection on the unanswered questions that need to be addressed in order to advance this novel approach to the clinic.

PBPK modeling-based optimization of site-specific chemo-photodynamic therapy with far-red light-activatable paclitaxel prodrug

Photodynamic therapy (PDT) is a clinically approved therapeutic modality to treat certain types of cancers. However, incomplete ablation of tumor is a challenge. Visible and near IR-activatable prodrug, exhibiting the combined effects of PDT and local chemotherapy, showed better efficacy than PDT alone, without systemic side effects. Site-specifically released chemotherapeutic drugs killed cancer cells surviving from rapid PDT damage via bystander effects. Recently, we developed such a paclitaxel (PTX) prodrug that targets folate receptors. The goals of this study were to determine the optimal treatment conditions, based on modeling, for maximum antitumor efficacy in terms of drug-light interval (DLI), and to investigate the impact of rapid PDT effects on the pharmacokinetic (PK) profiles of the released PTX. PK profiles of the prodrug were determined in key organs and a quantitative systems pharmacology (QSP) model was established to simulate PK profiles of the prodrug and the released PTX. Three illumination time points (DLI = 0.5, 9, or 48 h) were selected for the treatment based on the plasma/tumor ratio of the prodrug to achieve V-PDT (vascular targeted-PDT, 0.5 h), C-PDT (cellular targeted-PDT, 48 h), or both V- and C-PDT (9 h). The anti-tumor efficacy of the PTX prodrug was greatly influenced by the DLI. The 9 h DLI group, when both tumor and plasma concentrations of the prodrug were sufficient, showed the best antitumor effect. The clearance of the released PTX from tumor seemed to be largely impacted by blood circulation. Here, QSP modeling was an invaluable tool for rational optimization of the treatment conditions and for a deeper mechanistic understanding of the positive physiological effect of the combination therapy.

Tunable fabrication of new theranostic Fe3O4-black TiO2 nanocomposites: dual wavelength stimulated synergistic imaging-guided phototherapy in cancer

The development of a simplified theranostic system with high-efficiency for multifunctional imaging-guided photodynamic therapy/photothermal therapy (PDT/PTT) is a great challenge.

The good, the bad, and the ugly - controlling singlet oxygen through design of photosensitizers and delivery systems for photodynamic therapy

Singlet oxygen, although integral to photodynamic therapy, is notoriously uncontrollable, suffers from poor selectivity and has fast decomposition rates in biological media. Across the scientific community, there is a conscious effort to refine singlet oxygen interactions and initiate selective and controlled release to produce a consistent and reproducible therapeutic effect in target tissue. This perspective aims to provide an insight into the contemporary design principles behind photosensitizers and drug delivery systems that depend on a singlet oxygen response or controlled release. The discussion will be accompanied by in vitro and in vivo examples, in an attempt to highlight advancements in the field and future prospects for the more widespread application of photodynamic therapy.

Light-activatable cannabinoid prodrug for combined and target-specific photodynamic and cannabinoid therapy

Cannabinoids are emerging as promising antitumor drugs. However, complete tumor eradication solely by cannabinoid therapy remains challenging. In this study, we developed a far-red light activatable cannabinoid prodrug, which allows for tumor-specific and combinatory cannabinoid and photodynamic therapy. This prodrug consists of a phthalocyanine photosensitizer (PS), reactive oxygen species (ROS)-sensitive linker, and cannabinoid. It targets the type-2 cannabinoid receptor (CB2R) overexpressed in various types of cancers. Upon the 690-nm light irradiation, the PS produces cytotoxic ROS, which simultaneously cleaves the ROS-sensitive linker and subsequently releases the cannabinoid drug. We found that this unique multifunctional prodrug design offered dramatically improved therapeutic efficacy, and therefore provided a new strategy for targeted, controlled, and effective antitumor cannabinoid therapy.

S,S-Chiral Linker Induced U Shape with a Syn-facial Sensitizer and Photocleavable Ethene Group

There is a major need for light-activated materials for the release of sensitizers and drugs. Considering the success of chiral columns for the separation of enantiomer drugs, we synthesized an S,S-chiral linker system covalently attached to silica with a sensitizer ethene near the silica surface. First, the silica surface was modified to be aromatic rich, by replacing 70% of the surface groups with (3-phenoxypropyl)silane. We then synthesized a 3-component conjugate [chlorin sensitizer, S,S-chiral cyclohexane and ethene building blocks] in 5 steps with a 13% yield, and covalently bound the conjugate to the (3-phenoxypropyl)silane-coated silica surface. We hypothesized that the chiral linker would increase exposure of the ethene site for enhanced ¹ O₂ -based sensitizer release. However, the chiral linker caused the sensitizer conjugate to adopt a U shape due to favored 1,2-diaxial substituent orientation; resulting in a reduced efficiency of surface loading. Further accentuating the U shape was π-π stacking between the (3-phenoxypropyl)silane and sensitizer. Semiempirical calculations and singlet oxygen luminescence data provided deeper insight into the sensitizer's orientation and release. This study has lead to insight on modifications of surfaces for drug photorelease and can help lead to the development of miniaturized photodynamic devices.

Hypoxia-Activated Anticancer Prodrug for Bioimaging, Tracking Drug Release, and Anticancer Application

A novel anticancer theranostic prodrug, FDU-DB-NO₂, specifically activated by hypoxia for selective two-photon imaging hypoxia status, real-time tracking drug release, and solid tumor therapy was designed. The devised prodrug consists of an anticancer drug floxuridine (FDU), a fluorescence dye precursor 4'-(diethylamino)-1,1'-biphenyl-2-carboxylate (DB), and a hypoxic trigger 4-nitrobenzyl group. In normal cells, FDU-DB-NO₂ is "locked". Whereas in tumor cells, the prodrug is "unlocked" by hypoxia and results in fluorescent dye 7-(diethylamino)coumarin (CM) generation along with FDU release. The amounts and rates of CM formation and FDU release were controlled by hypoxic status and increased with the decreasing of the O₂ concentration. The hypoxic status, distribution of oxygen, and amount of FDU release in tumor cells, spheroids, and tumor tissue could be visualized by fluorescence. FDU-DB-NO₂ showed high cytotoxicity against hypoxic MCF-7 and MCG-803 cell lines and no cytotoxicity against normoxic BRL-3A cells and exhibited effective inhibition on tumor growth of MCF-7-cell-inoculated xenograft nude mice. This strategy may provide a promising platform for selective two-photon imaging hypoxia status, real-time tracking drug release, and personalized solid tumor treatment.

Defining the conditional basis of silicon phthalocyanine near-IR ligand exchange

Bond cleavage reactions initiated by long-wavelength light are needed to extend the scope of the caged-uncaged paradigm into complex physiological settings. Axially unsymmetrical silicon phthalocyanines (SiPcs) undergo efficient release of phenol ligands in a reaction contingent on three factors - near-IR light (690 nm), hypoxia, and a thiol reductant. These studies detail efforts to define the mechanistic basis for this unique conditionally-dependent bond cleavage reaction. Spectroscopic studies provide evidence for the formation of a key phthalocyanine radical anion intermediate formed from the triplet state in a reductant-dependent manner. Computational chemistry studies indicate that phenol ligand solvolysis proceeds through a heptacoordinate silicon transition state and that this solvolytic process is favored following SiPc radical anion formation. These results provide insight regarding the central role that radical anion intermediates formed through photoinduced electron transfer with biological reductants can play in long-wavelength uncaging reactions.

ROS-responsive drug delivery systems for biomedical applications

In the field of biomedicine, stimuli-responsive drug delivery systems (DDSs) have become increasingly popular due to their site-specific release ability in response to a certain physiological stimulus, which may result in both enhanced treatment outcome and reduced side effects. Reactive oxygen species (ROS) are the unavoidable consequence of cell oxidative metabolism. ROS play a crucial part in regulating biological and physiological processes, whereas excessive intracellular ROS usually lead to the oxidation stress which has implications in several typical diseases such as cancer, inflammation and atherosclerosis. Therefore, ROS-responsive DDSs have elicited widespread popularity for their promising applications in a series of biomedical research because the payload is only released in targeted cells or tissues that overproduce ROS. According to the design of ROS-responsive DDSs, the main release mechanisms of therapeutic agents can be ascribed to ROS-induced carrier solubility change, ROS-induced carrier cleavage or ROS-induced prodrug linker cleavage. This review summarized the latest development and novel design of ROS-responsive DDSs and discussed their design concepts and the applications in the biomedical field.

An updated overview on the development of new photosensitizers for anticancer photodynamic therapy

Photodynamic therapy (PDT), based on the photoactivation of photosensitizers (PSs), has become a well-studied therapy for cancer. Photofrin®, belonging to the first generation of PS, is still widely used for the treatment of different kinds of cancers; however, it has several drawbacks that significantly limit its general clinical use. Consequently, there has been extensive research on the design of PS molecules with optimized pharmaceutical properties, with aiming of overcoming the disadvantages of traditional PS, such as poor chemical purity, long half-life, excessive accumulation into the skin, and low attenuation coefficients. The rational design of novel PS with desirable properties has attracted considerable research in the pharmaceutical field. This review presents an overview on the classical photosensitizers and the most significant recent advances in the development of PS with regard to their potential application in oncology.

Folate-PEG Conjugates of a Far-Red Light-Activatable Paclitaxel Prodrug to Improve Selectivity toward Folate Receptor-Positive Cancer Cells

We recently demonstrated the far-red light-activatable prodrug of paclitaxel (PTX), Pc-(L-PTX)2. Upon illumination with a 690 nm laser, Pc-(L-PTX)2 showed combinational cell killing from rapid photodynamic therapy damage by singlet oxygen, followed by sustained chemotherapy effects from locally released PTX. However, its high lipophilicity (log D7.4 > 3.1) caused aggregation in aqueous solutions and has nonselectivity toward cancer cells. To solve these important problems, we prepared folic acid (FA)-conjugated and photoactivatable prodrugs of PTX with a polyethylene glycol (PEG) spacer of various chain lengths: FA-PEG n -Pc-L-PTX [n = 0 (0k, 5), ∼23 (1k, 7a), ∼45 (2k, 7b), ∼80 (3.5k, 7c), or ∼114 (5k, 7d)]. The PEGylated prodrugs 7a-d had a much improved hydrophilicity compared with the non-PEGylated prodrug, Pc-(L-PTX)2. As the PEG length increased, the hydrophilicity of the prodrug increased (log D7.4 values: 1.28, 0.09, -0.24, and -0.59 for 1k, 2k, 3.5k, and 5k PEG prodrugs, respectively). Fluorescence spectral data suggested that the PEGylated prodrugs had good solubility in the culture medium at lower concentrations (<1-2 μM), but showed fluorescence quenching due to limited solubility at higher concentrations (>2 μM). Dynamic light scattering indicated that all of the prodrugs formed nanosized particles in both phosphate-buffered saline and culture medium at a concentration of 5 μM. The PEG length affected both nonspecific and folate receptor (FR)-mediated uptake of the prodrugs. The enhanced cellular uptake was observed for the prodrugs with medium-sized PEGs (1k, 2k, or 3.5k) in FR-positive SKOV-3 cells, but not for the prodrugs with no PEG or with the longest PEG (5k), which suggests the optimal range of PEG length around 1k-3.5k for effective uptake of our prodrug system. Consistent with the cellular uptake pattern, medium-sized PEGylated prodrugs showed more potent phototoxic activity (IC50s, ∼130 nM) than prodrugs with no PEG or the longest PEG (IC50, ∼400 nM). In conclusion, we have developed far-red light-activatable prodrugs with improved water solubility and FR-targeting properties compared with the nontargeted prodrug.

Quantitative modeling of the dynamics and intracellular trafficking of far-red light-activatable prodrugs: implications in stimuli-responsive drug delivery system

The combination of photodynamic therapy (PDT) with anti-tumor agents is a complimentary strategy to treat local cancers. We developed a unique photosensitizer (PS)-conjugated paclitaxel (PTX) prodrug in which a PS is excited by near-infrared wavelength light to site-specifically release PTX while generating singlet oxygen (SO) to effectively kill cancer cells with both PTX and SO. The aim of the present study was to identify the determinants influencing the combined efficacy of this light-activatable prodrug, especially the bystander killing effects from released PTX. Using PS-conjugated PTX as a model system, we developed a quantitative mathematical model describing the intracellular trafficking. Dynamics of the prodrug and the model predictions were verified with experimental data using human cancer cells in vitro. The sensitivity analysis suggested that parameters related to extracellular concentration of released PTX, prodrug uptake, target engagement, and target abundance are critical in determining the combined killing efficacy of the prodrug. We found that released PTX cytotoxicity was most sensitive to the retention time of the drug in extracellular space. Modulating drug internalization and conjugating the agents targeted to abundant receptors may provide a new strategy for maximizing the killing capacity of the far-red light-activatable prodrug system. These results provide guidance for the design of the PDT combination study in vivo and have implications for other stimuli-responsive drug delivery systems.

A Molecular Combination of Zinc(II) Phthalocyanine and Tamoxifen Derivative for Dual Targeting Photodynamic Therapy and Hormone Therapy

The combination of photodynamic therapy and other cancer treatment modalities is a promising strategy to enhance therapeutic efficacy and reduce side effects. In this study, a tamoxifen-zinc(II) phthalocyanine conjugate linked by a triethylene glycol chain has been synthesized and characterized. Having tamoxifen as the targeting moiety, the conjugate shows high specific affinity to MCF-7 breast cancer cells overexpressed estrogen receptors (ERs) and tumor tissues, therefore leading to a cytotoxic effect in the dark due to the cytostatic tamoxifen moiety, and a high photocytotoxicity due to the photosensitizing phthalocyanine unit against the MCF-7 cancer cells. The high photodynamic activity of the conjugate can be attributed to its high cellular uptake and efficiency in generating intracellular reactive oxygen species. Upon addition of exogenous 17β-estradiol as an ER inhibitor, the cellular uptake and photocytotoxicity of the conjugate are reduced significantly. As shown by confocal microscopy, the conjugate is preferentially localized in the lysosomes of the MCF-7 cells.

In Vivo Activation of Duocarmycin-Antibody Conjugates by Near-Infrared Light

Near-IR photocaging groups based on the heptamethine cyanine scaffold present the opportunity to visualize and then treat diseased tissue with potent bioactive molecules. Here we describe fundamental chemical studies that enable biological validation of this approach. Guided by rational design, including computational analysis, we characterize the impact of structural alterations on the cyanine uncaging reaction. A modest change to the ethylenediamine linker (N,N'-dimethyl to N,N'-diethyl) leads to a bathochromic shift in the absorbance maxima, while decreasing background hydrolysis. Building on these structure-function relationship studies, we prepare antibody conjugates that uncage a derivative of duocarmycin, a potent cytotoxic natural product. The optimal conjugate, CyEt-Pan-Duo, undergoes small molecule release with 780 nm light, exhibits activity in the picomolar range, and demonstrates excellent light-to-dark selectivity. Mouse xenograft studies illustrate that the construct can be imaged in vivo prior to uncaging with an external laser source. Significant reduction in tumor burden is observed following a single dose of conjugate and near-IR light. These studies define key chemical principles that enable the identification of cyanine-based photocages with enhanced properties for in vivo drug delivery.