Activatable near-infrared emission-guided on-demand administration of photodynamic anticancer therapy with a theranostic nanoprobe

Smart molecular designs and applications of activatable organic photosensitizers

Photodynamic therapy (PDT) - which combines light, oxygen and photosensitizers (PS) to generate reactive oxygen species - has emerged as an effective approach for targeted ablation of pathogenic cells with reduced risk of inducing resistance. Some organic PS are now being applied for PDT in the clinic or undergoing evaluation in clinical trials. A limitation of the first-generation organic PS was their potential off-target toxicity. This shortcoming prompted the design of constructs that can be activated by the presence of specific biomolecules - from small biomolecules to large enzymes - in the target cells. Here, we review advances in the design and synthesis of activatable organic PS and their contribution to PDT in the past decade. Important areas of research include novel synthetic methodologies to engineer smart PS with tuneable singlet oxygen generation, their integration into larger constructs such as bioconjugates, and finally, representative examples of their translational potential as antimicrobial and anticancer therapies.

Dual-Locked Fluorescent Probes Activated by Aminopeptidase N and the Tumor Redox Environment for High-Precision Imaging of Tumor Boundaries

Clear delineation of tumor margins is essential for accurate resection and decreased recurrence rate in the clinic. Fluorescence imaging is emerging as a promising alternative to traditional visual inspection by surgeons for intraoperative imaging. However, traditional probes lack accuracy in tumor diagnosis, making it difficult to depict tumor boundaries accurately. Herein, we proposed an offensive and defensive integration (ODI) strategy based on the "attack systems (invasive peptidase) and defense systems (reductive microenvironment)" of multi-dimensional tumor characteristics to design activatable fluorescent probes for imaging tumor boundaries precisely. Screened out from a series of ODI strategy-based probes, ANQ performed better than traditional probes based on tumor unilateral correlation by distinguishing between tumor cells and normal cells and minimizing false-positive signals from living metabolic organs. To further improve the signal-to-background ratio in vivo, derivatized FANQ, was prepared and successfully applied to distinguish orthotopic hepatocellular carcinoma tissues from adjacent tissues in mice models and clinical samples. This work highlights an innovative strategy to develop activatable probes for rapid diagnosis of tumors and high-precision imaging of tumor boundaries, providing more efficient tools for future clinical applications in intraoperative assisted resection.

RSS and ROS Sequentially Activated Carbon Monoxide Release for Boosting NIR Imaging-Guided On-Demand Photodynamic Therapy

Carbon monoxide shows great therapeutic potential in anti-cancer. In particular, the construction of multifunctional CO delivery systems can promote the precise delivery of CO and achieve ideal therapeutic effects, but there are still great challenges in design. In this work, a RSS and ROS sequentially activated CO delivery system is developed for boosting NIR imaging-guided on-demand photodynamic therapy. This designed system is composed of a CO releaser (BOD-CO) and a photosensitizer (BOD-I). BOD-CO can be specifically activated by hydrogen sulfide with simultaneous release of CO donor and NIR fluorescence that can identify H2S-rich tumors and guide light therapy, also depleting H2S in the process. Moreover, BOD-I generates 1O2 under long-wavelength light irradiation, enabling both PDT and precise local release of CO via a photooxidation mechanism. Such sequential activation of CO release by RSS and ROS ensured the safety and controllability of CO delivery, and effectively avoided leakage during delivery. Importantly, cytotoxicity and in vivo studies reveal that the release of CO combined with the depletion of endogenous H2S amplified PDT, achieving ideal anticancer results. It is believed that such theranostic nanoplatform can provide a novel strategy for the precise CO delivery and combined therapy involved in gas therapy and PDT.

Charge Transfer-Promoted Excited State of a Heavy-Atom-Free Photosensitizer for Efficient Application of Mitochondria-Targeted Fluorescence Imaging and Hypoxia Photodynamic Therapy

Conventional photosensitizers (PSs) used in photodynamic therapy (PDT) have shown preliminary success; however, they are often associated with several limitations including potential dark toxicity in healthy tissues, limited efficacy under acidic and hypoxic conditions, suboptimal fluorescence imaging capabilities, and nonspecific targeting during treatment. In response to these challenges, we developed a heavy-atom-free PS, denoted as Cz-SB, by incorporating ethyl carbazole into a thiophene-fused BODIPY core. A comprehensive investigation into the photophysical properties of Cz-SB was conducted through a synergistic approach involving experimental and computational investigations. The enhancement of intersystem crossing (kISC) and fluorescence emission (kfl) rate constants was achieved through a donor-acceptor pair-mediated charge transfer mechanism. Consequently, Cz-SB demonstrated remarkable efficiency in generating reactive oxygen species (ROS) under acidic and low-oxygen conditions, making it particularly effective for hypoxic cancer PDT. Furthermore, Cz-SB exhibited good biocompatibility, fluorescence imaging capabilities, and a high degree of localization within the mitochondria of living cells. We posit that Cz-SB holds substantial prospects as a versatile PS with innovative molecular design, representing a potential "one-for-all" solution in the realm of cancer phototheranostics.

Unsymmetrical Benzothieno-Fused BODIPYs as Efficient NIR Heavy-Atom-Free Photosensitizers

Heavy-atom-free photosensitizers are potentially suitable for use in photodynamic therapy (PDT). In this contribution, a new family of unsymmetrical benzothieno-fused BODIPYs with reactive oxygen efficiency up to 50% in air-saturated toluene was reported. Their efficient intersystem crossing (ISC) resulted in the generation of both 1O2 and O2-• under irradiation. More importantly, the PDT efficacy of a respective 4-methoxystyryl-modified benzothieno-fused BODIPY in living cells exhibited an extremely high phototoxicity with an ultralow IC50 value of 2.78 nM. The results revealed that the incorporation of an electron-donating group at the α-position of the unsymmetrical benzothieno-fused BODIPY platform might be an effective approach for developing long-wavelength absorbing heavy-atom-free photosensitizers for precision cancer therapy.

Specific imaging of intracellular hydrogen sulfide by a positively charged NIR fluorescent probe

The poor water solubility of traditional activatable organic molecular probes usually limits their detection ability in physiological environment. In this work, a positively charged H2S probe was designed, which exhibited a significantly enhanced responsiveness to H2S in the aggregated state due to the increased positive charge density on the aggregate surface. Under physiological conditions, the probe could be activated by H2S with specificity and sensitivity to release near-infrared fluorescence signal. Moreover, endogenous H2S levels in living cells were successfully monitored by using this probe. We expect that this probe can provide a new strategy for the design of activatable probes to break the limitation of poor water solubility of conventional organic molecular probes.

Precise Spatiotemporal Identification of Mitochondrial H2S Fluctuations through Exploiting an On-Demand Photoactivated Probe

Various signal molecules participate in complex biological processes in mitochondria. However, most currently available probes have problems in elucidating the functions of these active species in mitochondria due to the inability to light up these probes exclusively at the desired mitochondrial location, thereby compromising the specificity and accuracy. In this study, we present an on-demand photoactivation approach to the molecular design of optimized probes for precise spatiotemporal identification of mitochondrial H2S fluctuations. The designed probe with native yellow fluorescence can monitor the process into mitochondria but maintains nonfluorescent response to H2S during cellular delivery, providing the accurate timing of accumulation in mitochondria. On-demand photoactivation exclusively at the desired mitochondrial location affords a significant aggregation-enhanced and emissive response to H2S with lighting up red fluorescence at 690 nm, which is the only way to get such an emissive phenomenon and greatly improves the specificity and accuracy of targeting mitochondrial H2S. By using this photocontrolled fluorescence responsiveness to H2S, precise spatiotemporal identification of mitochondrial H2S fluctuations is successfully performed. Our work could facilitate advances toward interrogating the physiological and pathological consequences of mitochondrial H2S in various biological events.

Hydrogen Sulfide Responsive Phototherapy Agents: Design Strategies and Biological Applications

Hydrogen sulfide (H2S) is one of the critical gasotransmitters, which play important roles in regular physiological processes, especially in vital signaling pathways. However, fluctuations in endogenous H2S concentration can be linked to serious health problems, such as neurodegenerative diseases, cancer, diabetes, inflammation, cardiovascular diseases, and hypertension. Thus, it has attracted a great deal of attention in therapeutic applications, specifically in the field of phototherapy. Photodynamic therapy (PDT) and photothermal therapy (PTT) are two subclasses of phototherapy, which utilize either reactive oxygen species (ROS) or local temperature increase upon irradiation of a photosensitizer (PS) to realize the therapeutic action. Phototherapies offer unique advantages compared to conventional methods; thus, they are highly promising and popular. One of the design principles followed in new generation PSs is to build activity-based PSs, which stay inactive before getting activated by disease-associated stimuli. These activatable PSs dramatically improve the selectivity and efficacy of the therapy. In this review, we summarize small molecule and nanomaterial-based PDT and PTT agents that are activated selectively by H2S to initiate their cytotoxic effect. We incorporate single mode PDT and PTT agents along with synergistic and/or multimodal photosensitizers that can combine more than one therapeutic approach. Additionally, H2S-responsive theranostic agents, which offer therapy and imaging at the same time, are highlighted. Design approaches, working principles, and biological applications for each example are discussed in detail.

Implications of hydrogen sulfide in colorectal cancer: Mechanistic insights and diagnostic and therapeutic strategies

Hydrogen sulfide (H₂S) is an important signaling molecule in colorectal cancer (CRC). It is produced in the colon by the catalytic synthesis of the colonocytes' enzymatic systems and the release of intestinal microbes, and is oxidatively metabolized in the colonocytes' mitochondria. Both endogenous H₂S in colonic epithelial cells and exogenous H₂S in intestinal lumen contribute to the onset and progression of CRC. The up-regulation of endogenous synthetases is thought to be the cause of the elevated H₂S levels in CRC cells. Different diagnostic probes and combination therapies, as well as tumor treatment approaches through H₂S modulation, have been developed in recent years and have become active area of investigation for the diagnosis and treatment of CRC. In this review, we focus on the specific mechanisms of H₂S production and oxidative metabolism as well as the function of H₂S in the occurrence, progression, diagnosis, and treatment of CRC. We also discuss the present challenges and provide insights into the future research of this burgeoning field.

Activatable photothermal agents with target-initiated large spectral separation for highly effective reduction of side effects

Photothermal agents (PTAs) with minimized side effects are critical for transforming cancer photothermal therapy (PTT) into clinical applications. However, most currently available PTAs lack true selective activation to reduce side effects because of heavy spectral overlap between photothermal agents and their corresponding products. This study reports the construction of activatable PTAs with target-initiated large spectral separation for highly effective reduction of side effects. Such designed probes involve two H2O2-activatable PTAs, aza-BOD-B1 (single activatable site) and aza-BOD-B2 (multiple activatable site). After interacting with H2O2, aza-BOD-B1 only displays a mild absorption redshift (60 nm) from 750 nm to 810 nm with serious spectral overlap, resulting in a mild photothermal effect on normal tissues upon 808 nm light irradiation. In contrast, aza-BOD-B2 displays a large absorption spectral separation (150 nm) from 660 nm to 810 nm, achieving true selective activation to minimize side effects during PTT of cancer. Besides, in vitro and in vivo investigations demonstrated that aza-BOD-B2 can specifically induce photothermal ablation of cancer cells and tumors while leaving normal sites undamaged, whereas aza-BOD-B1 exhibits undesirable side effects on normal cells. Our study provides a practical solution to the problem of undesired side effects of phototherapy, an advance in precision medicine.

Novel Donor-Acceptor Conjugated Polymer-Based Nanomicelles for Photothermal Therapy in the NIR Window

In this study, a novel donor-acceptor conjugated polymer PDPPDTP was designed and synthesized by D-A polymerization using 2,6-di(trimethyltin)-N-dithieno[3,2-b:20,30-d]pyrrole as the electron-donating (D) unit and 3,6-bis(5-bromothiophen-2-yl)-2,5-dihexadecylpyrrolo[3,4-c]pyrrole-1,4-dione as the electron-accepting (A) unit. The prepared polymer has strong absorption in the near-infrared (NIR) range of 700-900 nm. Moreover, it shows excellent photothermal performance under irradiation at 808 nm. Next, the biodegradable amphiphilic polymer polyethylene glycol-polycaprolactone was used to encapsulate the new conjugated polymer into nanomicelles by the microemulsion method. The obtained PDPPDTP-loaded micelles exhibited a regular spherical structure, and their hydrodynamic diameter was about 78 nm, characterized by transmission electron microscopy and dynamic light scattering. Notably, the micelles exhibited good stability, and the encapsulation efficiency of the conjugated polymer in the micelles was ∼80%. In vitro cell experiments demonstrated that the nanomicelles not only showed good biocompatibility and low toxicity but also could effectively inhibit the proliferation of breast cancer cells 4T1 under the NIR light irradiation of 808 nm. Furthermore, in vivo studies of photothermal therapy (PTT) efficacy showed that the PDPPDTP-loaded micelles exhibited a remarkable tumor growth inhibition in a syngeneic murine tumor model, indicating that the nanomicelles loaded with this novel conjugated polymer could be further explored as a new type of theranostic agent and applied in the PTT of tumors.

An NBD tertiary amine is a fluorescent quencher and/or a weak green-light fluorophore in H2S-specific probes

The thiolysis of NBD piperazinyl amine (NBD-PZ) is highly selective for H₂S over GSH and has been widely used for the development of many H₂S fluorescent probes. Whether the NBD amine in H₂S-specific probes could be a fluorescent quencher should be further clarified, because NBD amines have been used as environment-sensitive fluorophores for many years. Here, we compared the properties of NBD-based secondary and tertiary amines under the same conditions. For example, the emission of NBD-N(Et)2 is much smaller in water and less responsive to changes in polarity than that of NBD-NHEt. The emission of NBD-PZ-TPP is also smaller than that of NBD-NH-TPP both in aqueous buffer and in live cells. In addition, confocal bioimaging signals of NBD-PZ-TPP with excitation at 405 nm and 454 nm are much weaker than that at 488 nm. Based on these results as well as the previous work on NBD-based probes, we discuss and summarize the design strategies and sensing mechanisms for different NBD-based H₂S probes. Moreover, NBD-PZ-TPP may be a useful tool for reaction with and imaging of mitochondrial H₂S in live cells. This work should be useful for clarification of the roles of NBD in H₂S-specific fluorescent probes as well as for facilitating the development of future NBD-based probes.

Synthesis of tetraphenylethene-based D-A conjugated molecules with near-infrared AIE features, and their application in photodynamic therapy

Herein, five aggregation-induced emission (AIE) photosensitizers (PSs) with D-π-A structures are smoothly designed and synthesized through donor and acceptor engineering. The photophysical properties and theoretical calculation results show that the synergistic effect of methoxy substituted tetraphenylethene (MTPE), 3,4-ethylenedioxythiophene can enhance the intramolecular charge transfer effect (ICT), and promote the intersystem crossing (ISC) process of the whole molecule. In these AIE-PSs, the best-performing AIE-PS (MTPE-DT-Py) has bright NIR (740 nm) emission, the highest ¹O₂ generation efficiency (5.9-fold that of Rose Bengal) and efficient mitochondrial targeting ability. Subsequently, PDT anti-cancer and anti-bacterial experiments indicate that MTPE-DT-Py could obviously target mitochondria and kill breast cancer cells (MCF-7), and selectively inactivate S. aureus (G(+)) under white light irradiation. This work mainly proposes a practical design strategy for high effect AIE-PSs and provides more excellent candidates for fluorescence imaging-guided photodynamic therapy.

Antibody guided activatable NIR photosensitizing system for fluorescently monitored photodynamic therapy with reduced side effects

Photodynamic therapy (PDT) utilizing an organic dye (photosensitizer) capable of killing cancer cells in the body upon light irradiation is one of the promising non-invasive treatment modalities for many cancers. A known drawback of PDT is a side-effect caused by existing photosensitizers to organs due to insufficient specificity and accidental light exposure of a patient during the delivery of the photosensitizer in the bloodstream. To overcome this issue, we developed a novel antibody guided, activatable photosensitizing system, Ab-mI₂XCy-Ac, where the trastuzumab (Ab) is linked to the non-active (not phototoxic and not fluorescent) dye, mI₂XCy-Ac, that contains the hydroxyl group protected by acetyl (Ac). This targeting, non-photo-active conjugate was shown to be safely (without detectable side-effects) delivered to the targeted tumor, where it is activated by the esterase-mediated acetyl group cleavage and effectively treats the tumor upon NIR light irradiation. It was demonstrated in the Her2 positive BT-474 tumor mouse model that the treatment efficacy of the activatable photosensitizing system is about the same as for the permanently active photosensitizer, Ab-mI₂XCy, while the side-effects are noticeably reduced. In addition, this activatable system enables fluorescence monitoring of the photosensitizer activation events.

Hydrogen Sulfide: An Emerging Precision Strategy for Gas Therapy

Advances in nanotechnology have enabled the rapid development of stimuli-responsive therapeutic nanomaterials for precision gas therapy. Hydrogen sulfide (H₂ S) is a significant gaseous signaling molecule with intrinsic biochemical properties, which exerts its various physiological effects under both normal and pathological conditions. Various nanomaterials with H₂ S-responsive properties, as new-generation therapeutic agents, are explored to guide therapeutic behaviors in biological milieu. The cross disciplinary of H₂ S is an emerging scientific hotspot that studies the chemical properties, biological mechanisms, and therapeutic effects of H₂ S. This review summarizes the state-of-art research on H₂ S-related nanomedicines. In particular, recent advances in H₂ S therapeutics for cancer, such as H₂ S-mediated gas therapy and H₂ S-related synergistic therapies (combined with chemotherapy, photodynamic therapy, photothermal therapy, and chemodynamic therapy) are highlighted. Versatile imaging techniques for real-time monitoring H₂ S during biological diagnosis are reviewed. Finally, the biosafety issues, current challenges, and potential possibilities in the evolution of H₂ S-based therapy that facilitate clinical translation to patients are discussed.

Cell-Trappable BODIPY-NBD Dyad for Imaging of Basal and Stress-Induced H2S in Live Biosystems

H₂S is a gaseous signaling molecule that is involved in many physiological and pathological processes. In general, the level of intracellular H₂S (<1 μM) is much lower than that of GSH (∼1-10 mM), leading to the remaining challenge of selective detection and differentiation of endogenous H₂S in live biosystems. To this end, we quantitatively demonstrate that the thiolysis of NBD amine has much higher selectivity for H₂S over GSH than that of the reduction of aryl azide. Subsequently, we developed the first NBD-based cell-trappable probe 1 (AM-BODIPY-NBD) for highly selective and ultrasensitive imaging of intracellular H₂S. Probe 1 demonstrates a 207-fold fluorescence enhancement at 520 nm after reaction with H₂S/esterase to produce a bright BODIPY (quantum yield 0.42) and a detection limit of 15.7 nM. Probe 1 is water-soluble, cell-trappable, and not cytotoxic. Based on this excellent chemical tool, relative levels of basal H₂S in different cell lines were measured to reveal a positive correlation between endogenous H₂S and the metastatic potential of colon and breast cancer cells. In addition, H₂S biogenesis in vivo was also validated by probe 1 both in tobacco leaves under viral infection and in zebrafish after tail amputation. It is anticipated that probe 1 will have widespread applications in imaging and for investigating different H₂S-related biological processes and diseases.

A phase-transfer catalyst-based nanoreactor for accelerated hydrogen sulfide bio-imaging

Hydrogen sulfide (H₂S) is an important signaling molecule in various biological processes; however, its real-time monitoring in living cells is hampered by long detection time for current fluorescent probes. To overcome this challenge, we designed a phase-transfer catalyst (PTC) approach to accelerate the reaction between the probe and the analyte by conjugating common fluorescent probes - mostly hydrophobic small molecules - with an amphiphilic PEG-PPG-PEG polymer, enabling the controllable assembly of H₂S nanoprobes in an aqueous solution. The PEG block helps to establish a PTC microenvironment that endows the assembled nanoprobes with a significantly reduced detection time (3-10 min; versus 20-60 min for small-molecule probes). Based on this approach, we synthesised two nanoprobes of different wavelengths, DS-Blue-nano and DN-Green-nano, which can sensitively detect H₂S in living macrophage cells with bright fluorescence starting at as early as 7 min and reaching stability at 15 min. These data suggest PTC-based nanoprobes as a new and generic approach for constructing sensitive fluorescent probes for the real-time imaging of H₂S, and perhaps other molecules in future, under biological conditions.

Orthogonally aligned cyclic BODIPY arrays with long-lived triplet excited states as efficient heavy-atom-free photosensitizers

In photosensitizers, long triplet excited state lifetimes are key to their efficient electron transfer or energy transfer processes. Herein, we report a novel class of cyclic trimeric BODIPY arrays which were efficiently generated from easily accessible meso-mesityldipyrrinone and arylboronic acids in one pot. Arylboronic acid, for the first time, was used to provide a boron source for BODIPY derivatives. Due to the well-defined and orthogonally aligned BODIPY cores as verified by X-ray crystallography, these BODIPY arrays show strong exciton coupling effects and efficient intersystem crossings, and are novel heavy-atom-free photosensitizers with a long-lived triplet excited state (lifetime up to 257.5 μs) and good reactive oxygen species generation efficiency (up to 0.72) contributed by both 1O2 and O2 -˙ under light irradiation.

BODIPY-Based Photodynamic Agents for Exclusively Generating Superoxide Radical over Singlet Oxygen

Developing Type-I photosensitizers is considered as an efficient approach to overcome the deficiency of traditional photodynamic therapy (PDT) for hypoxic tumors. However, it remains a challenge to design photosensitizers for generating reactive oxygen species by the Type-I process. Herein, we report a series of α,β-linked BODIPY dimers and a trimer that exclusively generate superoxide radical (O₂ ^-. ) by the Type-I process upon light irradiation. The triplet formation originates from an effective excited-state relaxation from the initially populated singlet (S₁ ) to triplet (T₁ ) states via an intermediate triplet (T₂ ) state. The low reduction potential and ultralong lifetime of the T₁ state facilitate the efficient generation of O₂ ^-. by inter-molecular charge transfer to molecular oxygen. The energy gap of T₁ -S₀ is smaller than that between ³ O₂ and ¹ O₂ thereby precluding the generation of singlet oxygen by the Type-II process. The trimer exhibits superior PDT performance under the hypoxic environment.

A light-activatable photosensitizer for photodynamic therapy based on a diarylethene derivative

Herein, a light-activatable photosensitizer based on a diarylethene derivative, DAE-TPE, was developed for photodynamic therapy. Upon UV exposure, the "opened" form (OF) of DAE-TPE NPs was converted to the "closed" form (CF), and photosensitization was activated. The CF of DAE-TPE NPs exhibited sufficient photodynamic therapy effects upon HeLa cells.

A Novel Photosensitizer for Lipid Droplet-Location Photodynamic Therapy

Lipid droplets (LDs), an extremely important cellular organelle, are responsible for the storage of neutral lipids in multiple biological processes, which could be a potential target site for photodynamic therapy (PDT) of cancer. Herein, a lipid droplet–targeted photosensitizer (BODSeI) is developed, allowing for fluorescence imaging–guided PDT. Owing to the location of lipid droplets, BODSeI demonstrates enhanced PDT efficiency with an extremely low IC50 value (around 125 nM). Besides, BODSeI shows good biocompatibility and high photostability. Therefore, BODSeI is promising for droplet-location PDT, which may trigger wide interest for exploring the pathway of lipid droplet–location PDT.

Responsive optical probes for deep-tissue imaging: Photoacoustics and second near-infrared fluorescence

Optical imaging has played a vital role in development of biomedicine and image-guided theragnostic. Nevertheless, the clinical translation of optical molecular imaging for deep-tissue visualization is still limited by poor signal-to-background ratio and low penetration depth owing to light scattering and tissue autofluorescence. Hence, to facilitate precise diagnosis and accurate surgery excision in clinical practices, the responsive optical probes (ROPs) are broadly designed for specific reaction with biological analytes or disease biomarkers via chemical/physical interactions for photoacoustic and second near-infrared fluorescence (NIR-II, 900-1700 nm) fluorescence imaging. Herein, the recent advances in the development of ROPs including molecular design principles, activated mechanisms and treatment responses for photoacoustic and NIR-II fluorescence imaging are reviewed. Furthermore, the present challenges and future perspectives of ROPs for deep-tissue imaging are also discussed.

Recent Advances in Activatable Organic Photosensitizers for Specific Photodynamic Therapy

Photodynamic therapy is an alternative modality for the therapy of diseases such as cancer in a minimally invasive manner. The essential photosensitizer, which acts as a catalyst when absorbing light, converts oxygen into cytotoxic reactive oxygen species that ablate malignant cells through apoptosis and/or necrosis, destroy tumor microvasculature, and stimulate immunity. An activatable photosensitizer whose photoactivity could be turned on by a specific disease biomarker is capable of distinguishing healthy cells from diseased cells, thereby reducing off-target photodamage. In this Minireview, we highlight progress in activatable organic photosensitizers over the past five years, including: (i) biorthogonal activatable BODIPYs; (ii) activatable Se-rhodamine with single-cell resolution; (iii) silicon phthalocyanine targeting oxygen tension; (iv) general D-π-A scaffolds; and (v) AIEgens. The potential challenges and opportunities for developing new types of activatable organic photosensitizers to overcome the hypoxia dilemmas of photodynamic therapy are discussed.

Red-Light-Responsive Ru Complex Photosensitizer for Lysosome Localization Photodynamic Therapy

Photoresponsive ruthenium (Ru) complexes have been extensively studied in the photodynamic therapy (PDT) of cancer. The metal-to-ligand charge transfer (MLCT) absorption maximum of most Ru complexes is located in the short-wavelength visible region, which is well suited for superficial tumors but shows inefficient therapeutic effects for more deep-seated ones. Moreover, Ru complexes are primarily located in the mitochondria or nucleus, always resulting in high levels of dark toxicity and DNA mutation. Herein, we reported a new ruthenium complex (Ru-I) for red-light-triggered PDT. The activation wavelength of Ru-I is successfully extended to 660 nm. Importantly, the complex photosensitizer can be quickly taken up by cancer cells and selectively accumulated in the lysosome, an ideal localization for PDT purposes. Intratumoral injection of Ru-I into tumor-bearing mice achieved excellent therapeutic effects and thus holds great promise for applications in lysosome localization photodynamic therapy.

Precise imaging of mitochondria in cancer cells by real-time monitoring of nitroreductase activity with a targetable and activatable fluorescent probe

An activatable and mitochondrial-targetable fluorescent probe was developed. This designed probe showed ratiometric fluorescence and light-up near-infrared emission responsiveness to nitroreductase, achieving precise imaging of mitochondria in cancer cells by real-time monitoring of nitroreductase activity.

Recent advances in phase change material based nanoplatforms for cancer therapy

Cancer has become a severe threat to human life due to its high mortality and metastatic rate. Effective inhibition and killing of cancer cells using chemotherapeutic drugs have been a promising means in clinical cancer therapy. However, the low selectivity, drug-resistance, uncontrollability and serious side effects of chemotherapy significantly limit its further development. There is an urgent need for new treatment strategies to compensate for deficiencies inherent in chemotherapy alone. A growing body of research shows that combined treatment strategies have the potential to overcome this dilemma by achieving significantly enhanced synergistic effects and reduced side effects. Emerging phase change materials (PCMs) create an ideal nanoplatform for cancer combination therapy due to their universal loading properties, stable and temperature-responsive phase transition capability, and excellent natural biocompatibility/biodegradability. The release of therapeutic agents can be precisely controlled through external, non-intrusive stimuli (such as NIR light and ultrasound), avoiding systemic toxicity associated with conventional chemotherapy. Herein, the construction methods and design principles of PCM-based nanoplatforms serving as strict gatekeeper and smart payload delivery systems are discussed in detail. Moreover, the advantages and disadvantages of these nanoplatforms are provided. A suitable discussion and perspective of the remaining challenges and future opportunities for PCM-based nanoplatforms in cancer treatment are also given in conclusion.

Mitochondrion-targeting near-infrared fluorescent probe for detecting intracellular nanomolar level hydrogen sulfide with high recognition rate

Hydrogen sulfide (H₂S) typically plays biphasic biological roles in living organisms with subnormal H₂S exerting cytoprotective effects such as participating in cardioprotective signaling pathways while H₂S with higher-than-normal concentrations in localized tissues acting the opposite way such as inhibiting mitochondrial respiration. Such concentration-dependent biological and pathological roles of H₂S with the wide involvement of mitochondria and the elusive feature of H₂S definitely highlight the vital significance of fast and precise estimation of the physiological level of H₂S in specific microenvironments, particularly within cellular mitochondria. In this work, we developed a new type of fluorescent probe (QcyCHO) featured with H₂S-triggered off-to-on near-infrared (NIR) fluorescence conversion within ~ 10 min, limit of detection (LOD) down to 8.3 nM, and high recognition specificity over other similarly interfering species. The ideal mitochondrion-targeting ability, high recognition specificity over typical interfering substances and other physiologically relevant species, and the ability for mapping intracellular H₂S in living cells of QcyCHO probe were also unequivocally confirmed, which imply its potential for shedding light on the biology of H₂S and therapeutic development in H₂S-associated diseases by identifying the specific physiological stimuli inducing H₂S production and determining the levels of H₂S at the location and time of stimulation.

Hydrogen sulfide-activatable prodrug-backboned block copolymer micelles for delivery of chemotherapeutics

A novel hydrogen sulfide-activatable block copolymer prodrug with high tumor specificity was developed for cancer chemotherapy.

Research advances in integrated theranostic probes for tumor fluorescence visualization and treatment

At present, cancer is obviously a major threat to human health worldwide. Accurate diagnosis and treatment are in great demand and have become an effective method to alleviate the development of cancer and improve the survival rate of patients. A large number of theranostic probes that combine diagnosis and treatment methods have been developed as promising tools for tumor precision medicine. Among them, fluorescent theranostic probes have developed rapidly in the frontier research field of precision medicine with their real time, low toxicity, and high-resolution merit. Therefore, this review focuses on recent advances in the development of fluorescent theranostic probes, as well as their applications for cancer diagnosis and treatment. Initially, small-molecule fluorescent theranostic probes mainly including tumor microenvironment-responsive fluorescent prodrugs and phototherapeutic probes were introduced. Subsequently, nanocomposite probes are expounded based on four types of nano-fluorescent particles combining different therapies (chemotherapy, photothermal therapy, photodynamic therapy, gene therapy, etc.). Then, the capsule-type "all in one" probes, which occupy an important position in theranostic probes, are summarized according to the surface carrier type. This review aims to present a comprehensive guide for researchers in the field of tumor-related theranostic probe design and development.

Thermosensitive Polymer Dot Nanocomposites for Trimodal Computed Tomography/Photoacoustic/Fluorescence Imaging-Guided Synergistic Chemo-Photothermal Therapy

Precision delivery of theranostic agents to the tumor site is essential to improve their diagnostic and therapeutic efficacy and concurrently minimize adverse effects during treatment. In this study, a novel concept of near-infrared (NIR) light activation of conjugated polymer dots (Pdots) at thermosensitive hydrogel nanostructures is introduced for multimodal imaging-guided synergistic chemo-photothermal therapy. Interestingly, owing to the attractive photothermal conversion efficiency of Pdots, the Pdots@hydrogel as theranostic agents is able to undergo a controllable softening or melting state under the irradiation of NIR laser, resulting in light-triggered drug release in a controlled way and concurrently hydrogel degradation. Besides, the novel Pdots@hydrogel nanoplatform can serve as the theranostic agent for enhanced trimodal photoacoustic (PA)/computed tomography (CT)/fluorescence (FL) imaging-guided synergistic chemo-photothermal therapy of tumors. More importantly, the constructed intelligent nanocomposite Pdots@hydrogel exhibits excellent biodegradability, strong NIR absorption, bright PA/CT/FL signals, and superior tumor ablation effect. Therefore, the concept of a light-controlled multifunctional Pdots@hydrogel that integrates multiple diagnostic/therapeutic modalities into one nanoplatform can potentially be applied as a smart nanotheranostic agent to various perspectives of personalized nanomedicine.

Endogenous H2S-Activable Liposomal Nanoplatform for Synergistic Colorectal Tumor Ablation at Mild Apparent Temperature

Photoinduced hyperthermia possesses great potential in photothermal therapy and thermal-responsive chemotherapy of tumors. However, traditional thermal-triggered drug release requires high temperature, which results in unpleasant activation of thermal-induced cellular self-protection. In this work, a Cu-complex modified and drug-loaded liposomal nanoplatform was constructed for endogenous H₂S-activated synergistic ablation of colorectal tumors. In response to H₂S, the incorporated Cu-complex contributed to the formation of semiconductor CuS on the surface of the as-designed liposomal nanoplatform, which led to local heating under near-infrared (NIR) laser irradiation to achieve simultaneous photothermal therapy and drug release. It is noteworthy that although the drug release occurred at a mild apparent temperature, it was actually triggered by the high eigen temperature on the surface of the liposomal nanoplatform. Therefore, efficient and synergistic photothermal and chemotherapy was achieved under mild apparent temperatures. This work provides insights into achieving selective and bioactivated photothermal therapy and therefore thermal-controlled drug release without using excessive hyperthermia.

In Vivo Imaging of Senescent Vascular Cells in Atherosclerotic Mice Using a β-Galactosidase-Activatable Nanoprobe

Senescence-associated diseases have severely diminished the quality of life and health of patients. However, a sensitive assay of these diseases remains limited due to a lack of straightforward methods. Considering that senescence-associated β-galactosidase (SA-β-Gal) is overexpressed in senescent cells, the detection of SA-β-Gal in senescent cells and tissues might be a feasible strategy for the early diagnosis of SA diseases. In this study, a β-galactosidase-activatable nanoprobe BOD-L-βGal-NPs was developed for the imaging of senescent cells and vasculature in atherosclerotic mice via real-time monitoring of β-Gal. BOD-L-βGal-NPs was fabricated by encapsulating a newly designed NIR ratiometric probe BOD-L-βGal within a poly(lactic-co-glycolic) acid (PLGA) core. Nanoprobe BOD-L-βGal-NPs showed good accumulation in arteries, thus successfully visualizing senescent cells and vasculature in atherosclerotic mice by tail vein injection. Our findings indicated that nanoprobe BOD-L-βGal-NPs holds great potential for the early diagnosis and therapy of atherosclerosis and other aging-associated diseases.

Rational design of a "dual lock-and-key" supramolecular photosensitizer based on aromatic nucleophilic substitution for specific and enhanced photodynamic therapy

Photosensitizing agents are essential for precise and efficient photodynamic therapy (PDT). However, most of the conventional photosensitizers still suffer from limitations such as aggregation-caused quenching (ACQ) in physiological environments and toxic side-effects on normal tissues during treatment, leading to reduced therapeutic efficacy. Thus, integrating excellent photophysical properties and accurate carcinoma selectivity in a photosensitizer system remains highly desired. Herein, a “dual lock-and-key” supramolecular photosensitizer BIBCl–PAE NPs for specific and enhanced cancer therapy is reported. BIBCl–PAE NPs are constructed by encapsulating a rationally designed glutathione (GSH)-activatable photosensitizer BIBCl in a pH-responsive diblock copolymer. In normal tissues, BIBCl is “locked” in the hydrophobic core of the polymeric micelles due to ACQ. Under the “dual key” activation of low pH and high levels of GSH in a tumor microenvironment, the disassembly of micelles facilitates the reaction of BIBCl with GSH to release water-soluble BIBSG with ideal biocompatibility, enabling the highly efficient PDT. Moreover, benefiting from the Förster resonance energy transfer effect of BIBSG, improved light harvesting ability and ¹O₂ production are achieved. In vitro and vivo experiments have demonstrated that BIBCl–PAE NPs are effective in targeting and inhibiting carcinoma. BIBCl–PAE NPs show superior anticancer efficiency relative to non-activatable controls.

The “dual lock-and-key” supramolecular photosensitizers enable specific and enhanced photodynamic therapy (PDT).

Aggregation Enhanced Responsiveness of Rationally Designed Probes to Hydrogen Sulfide for Targeted Cancer Imaging

Activatable molecular probes hold great promise for targeted cancer imaging. However, the hydrophobic nature of most conventional probes makes them generate precipitated agglomerate in aqueous media, thereby annihilating their responsiveness to analytes and precluding their practical applications for bioimaging. This study reports the development of two small molecular probes with unprecedented aggregation enhanced responsiveness to H₂S for in vivo imaging of H₂S-rich cancers. The subtle modulation of the equilibrium between hydrophilicity and lipophilicity by N-methylpyridinium endows these designed probes with the capability of spontaneously self-assembling into nanoprobes under physiological conditions. Such probes in an aggregated state, rather than a molecular dissolved state, show NIR fluorescence light up and photoacoustic signals turn on upon H₂S specific activation, allowing in vivo visualization and differentiation of cancers based on differences in H₂S content. Thus, our study presents an effective design strategy which should pave the way to molecular design of optimized probes for precision cancer diagnostics.

Highly efficient H2S scavengers via thiolysis of positively-charged NBD amines

H2S is a well-known toxic gas and also a gaseous signaling molecule involved in many biological processes. Advanced chemical tools that can regulate H2S levels in vivo are useful for understanding H2S biology as well as its potential therapeutic effects. To this end, we have developed a series of 7-nitro-1,2,3-benzoxadiazole (NBD) amines as potential H2S scavengers. The kinetic studies of thiolysis reactions revealed that incorporation of positively-charged groups onto the NBD amines greatly increased the rate of the H2S-specific thiolysis reaction. We demonstrate that these reactions proceed effectively, with second order rate constants (k 2) of >116 M-1 s-1 at 37 °C for NBD-S8. Additionally, we demonstrate that NBD-S8 can effectively scavenge enzymatically-produced and endogenous H2S in live cells. Furthering the biological significance, we demonstrate NBD-S8 mediates scavenging of H2S in mice.

Rational design of water-dispersible and biocompatible nanoprobes with H2S-triggered NIR emission for cancer cell imaging

We present an approach for constructing a H₂S-specific nanoprobe by the entrapment of a small molecule probe within the hydrophobic interior of surface cross-linked micelles (SCMs), endowing the designed nanoprobes with good water solubility and biocompatibility. Importantly, the obtained nanoprobes displayed good responsiveness to H₂S in both ratiometric fluorescence and light-up NIR emission modes, thus enabling accurate identification of H₂S-rich colorectal cancer cells.

Single-wavelength phototheranostics for colon cancer via the thiolytic reaction

It's a huge challenge to develop effective nanosystems that combine the capabilities of diagnoses and therapies together for colon cancer in the clinic. Herein, we constructed a far-red absorbing phototheranostic nanosystem (FR-H2S) based on the thiolytic reaction of a dinitrophenyl modified phototheranostic prodrug and over-expressed H2S in colon cancer sites for precise imaging-guided phototherapy. FR-H2S with a BODIPY core not only could work as an imaging probe for diagnosis but also act as a phototherapeutic agent for cancer treatment under a single FR laser source (650 nm). FR-H2S exhibited a gradually enhanced fluorescence emission for precise diagnosis of H2S-rich colon tumor sites. After entering tumor cells, FR-H2S could generate abundant 1O2 and heat for phototherapies timely by using the same laser source (650 nm). We believe that this precise imaging-guided phototheranostic nanosystem could provide a promising approach to colon cancer with minimal damage.

Optimized Fluorescent Probe for Specific Imaging of Glutathione S-Transferases in Living Cells and Mice

GSTP1 has been considered to be a marker for malignancy in many tissues. However, the existing GST fluorescent probes are unfavorable for in vivo imaging because of the limited emission wavelength or insufficient fluorescence enhancement (six-fold). The limited fluorescence enhancement of GST fluorescent probes is mainly ascribed to the high background signals resulting from the spontaneous reaction between GSH and the probes. In this work, a highly specific GST probe with NIR emission has been successfully developed through optimization of the essential unit of the probe to repress the spontaneous reaction. The novel GST probe exhibits over 100-fold fluorescence enhancement upon incubation with GSTP1/GSH and high selectivity over other potential interference. In addition, the probe has been proved to be capable of tracking endogenous GST in A549 cells. Finally, the in vivo imaging results demonstrate that the probe can be used for effective imaging of endogenous GST activity in subcutaneous tumor mouse with high contrast.

Activatable Photodynamic Therapy with Therapeutic Effect Prediction Based on a Self-correction Upconversion Nanoprobe

Though emerging as a promising therapeutic approach for cancers, the crucial challenge for photodynamic therapy (PDT) is activatable phototoxicity for selective cancer cell destruction with low "off-target" damage and simultaneous therapeutic effect prediction. Here, we design an upconversion nanoprobe for intracellular cathepsin B (CaB)-responsive PDT with in situ self-corrected therapeutic effect prediction. The upconversion nanoprobe is composed of multishelled upconversion nanoparticles (UCNPs) NaYF₄:Gd@NaYF₄:Er,Yb@NaYF₄:Nd,Yb, which covalently modified with an antenna molecule 800CW for UCNPs luminance enhancement under NIR irradiation, photosensitizer Rose Bengal (RB) for PDT, Cy3 for therapeutic effect prediction, and CaB substrate peptide labeled with a QSY7 quencher. The energy of UCNPs emission at 540 nm is transferred to Cy3/RB and eventually quenched by QSY7 via two continuous luminance resonance energy transfer processes from interior UCNPs to its surface-extended QSY7. The intracellular CaB specifically cleaves peptide to release QSY7, which correspondingly activates RB with reactive oxygen species (ROS) generation for PDT and recovers Cy3 luminance for CaB imaging. UCNPs emission at 540 nm remains unchanged during the peptide cleavage process, which is served as an internal standard for Cy3 luminance correction, and the fluorescence intensity ratio of Cy3 over UCNPs (FI583/FI540) is measured for self-corrected therapeutic effect prediction. The proposed self-corrected upconversion nanoprobe implies significant potential in precise tumor therapy.

Fine-tuning the electronic structure of heavy-atom-free photosensitizers for fluorescence imaging and mitochondria-targeted photodynamic therapy

Theranostics that combines both diagnosis and therapy into a single platform has recently emerged as a promising biomedical approach for cancer treatment; however, the development of efficient theranostic agents with excellent optical properties remains a challenge. Here, we report novel mitochondria-targeting BODIPY photosensitizers (R-BODs) that possess considerable singlet oxygen generation capabilities and good fluorescence properties for imaging-guided photodynamic therapy (PDT). The incorporation of sulfur atoms into the π-conjugated skeleton of BODIPY along with the introduction of different functional groups at the meso-position of the BODIPY core is essential for tuning the photophysical and photosensitizing properties. Notably, the MeOPh-substituted thiophene-fused BODIPY (MeO-BOD, R = p-methoxyphenyl) displayed the highest singlet oxygen generation capability (Φ Δ ≈ 0.85 in air-saturated acetonitrile) and a moderate fluorescence quantum yield (Φ f = 17.11). Furthermore, MeO-BOD showed good biocompatibility, low dark toxicity and superior fluorescence imaging properties in living cells. More importantly, the PDT efficacy of mitochondria-specific anchoring of MeO-BOD was remarkably amplified with an extremely low half-maximal inhibitory concentration (IC50) value of 95 nM. We believe that the incorporation of an electron-donating group at the meso-position of the thiophene-fused BODIPY platform may be an effective approach for developing theranostic agents for precision cancer therapy.

Tailoring nanoparticles based on boron dipyrromethene for cancer imaging and therapy

Boron dipyrromethene (BODIPY), as a traditional fluorescent dye, has drawn increasing attention because of its excellent photophysical properties like adjustable spectra and outstanding photostability. BODIPY dyes could be assembled into nanoparticles for cancer imaging and therapy via rational design. In this review, the bio-applications of BODIPY-containing nanoparticles are introduced in detail, such as cellular imaging, near-infrared fluorescence imaging, computed tomography imaging, photoacoustic imaging, phototherapy, and theranostics. The construction strategies of BODIPY-containing nanoparticles are emphasized so the review has three sections-self-assembly of small molecules, chemical conjugation with hydrophilic compounds, and physical encapsulation. This review not only summarizes various and colorific bio-applications of BODIPY-containing nanoparticles, but also provides reasonable design methods of BODIPY-containing nanoparticles for cancer theranostics. This article is categorized under: Diagnostic Tools > in vivo Nanodiagnostics and Imaging.

An electron-deficiency-based framework for NIR-II fluorescence probes

Increasing the electron withdrawing ability of substituents in monochlorinated BODIPY could vary the emission from the NIR-I to NIR-II region together with enhanced response rate, indicative of a promising approach for activatable NIR-II probes.

A Förster Resonance Energy Transfer Switchable Fluorescent Probe With H2S-Activated Second Near-Infrared Emission for Bioimaging

Real-time and accurate detection of endogenous hydrogen sulfide is of great biomedical significance. Here, a FRET-based fluorescent probe for ratiometric detection of H₂S was designed to comprise an AIE luminophore TPE as an energy donor and a monochlorinated BODIPY dye as an energy acceptor and H₂S-responsive site. Such a designed probe showed H₂S-dependent ratiometric and light-up NIR-II emission, enabling accurate imaging of H₂S-rich cancer cells and identification of H₂S-rich tumors with high resolution.

Tumor microenvironment-activated nanosystems with selenophenol substituted BODIPYs as photosensitizers for photodynamic therapy

NIR-light-absorbing photosensitizers with the capability of selective localization and activation in tumor regions are of great importance for practical photodynamic therapy (PDT). Here, selenophenol substituted BODIPYs were designed and synthesized as new photosensitizers for PDT. One of these obtained BODIPYs, IBSeOV, possesses an intense and low energy absorption with a high singlet oxygen quantum yield (Φ_Δ = 60%). Considering manganese dioxide (MnO₂) nanosheets as versatile nanocarriers in cancer theranostics, nanosystem IBSeOV/MnO₂ was then fabricated to furnish tumor environment selective activation. Such designed nanoplatform allowed for GSH-controllable ¹O₂ production and exhibited low cytotoxicity in dark but good photocytotoxicity to cancer cells. The in vivo antitumor outcome suggested the high treatment efficiency of IBSeOV/MnO₂ for tumor therapy.