Oxygen-Dependent Regulation of Excited-State Deactivation Process of Rational Photosensitizer for Smart Phototherapy

A promising strategy for synergistic cancer therapy by integrating a photosensitizer into a hypoxia-activated prodrug

Herein, we fabricate a multifunctional molecular prodrug BAC where the chemotherapeutical agent camptothecin (CPT) is linked with a boron dipyrromethene (BODIPY)-based photosensitizer by an azobenzene chain which is sensitive to over-expressed azoreductase in hypoxic tumor cells. This prodrug was further loaded into biodegradable monomethoxy poly(ethylene glycol)-b-poly(caprolactone) (mPEG-b-PCL) to improve its solubility and tumor accumulation. The formed BAC nanoparticles (BAC NPs) can destroy aerobic tumor cells with relatively short distance from blood vessels by photodynamic therapy (PDT) under illumination. The PDT action inevitably leads to consumption of O2, and subsequently acute hypoxia which can induce cleavage of azobenzene linkage to boost release of CPT killing the other hypoxic interior tumor cells survived from PDT. Both in vitro and in vivo studies have verified that BAC NPs possess remarkable antitumor activity by a synergistic action of PDT and chemotherapy.

Attachment of −tBu groups to aza-BODIPY core at 3,5-sites with ultra-large Stokes shift to enhance photothermal therapy through apoptosis mechanism

By the introduction of the −tBu groups into aza-BODIPY core, di-tert-butyl-substituted aza-BODIPYs at 3,5-sites (tBuazaBDPs) were prepared for the first time. Based on the X-ray analysis of CN-tBuazaBDP, this molecular structure is twisted. Near-infrared dye SMe-tBuazaBDP has the ultra-large Stokes shift (152 ​nm) in aza-BODIPY system, combining with the twisted intramolecular charge transfer and the free rotation of the −tBu groups at 3,5-sites. Although the barrier-free rotors of the distal −tBu groups in SMe-tBuazaBDP result in low fluorescence quantum yield, the photothermal conversion efficiency is markedly enhanced. SMe-tBuazaBDP nanoparticles with low power laser irradiation were proven to block cancer cell cycle, inhibit cancer cell proliferation, and induce cancer cell apoptosis in photothermal therapy (PTT). The strategy of “direct attachment of −tBu groups to aza-BODIPY core” gives a new design platform for a photothermal therapy agent.

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Di-tert-butyl-substituted aza-BODIPYs at 3,5-sites (tBuazaBDPs) were prepared for the first time.

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Near-infrared dye SMe-tBuazaBDP has the ultra-large Stokes shift (152 ​nm) in aza-BODIPY system.

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SMe-tBuazaBDP ​nanoparticles can photothermally induce apoptosis as a potential photothermal therapy agent.

Three Birds with One Stone: Acceptor Engineering of Hemicyanine Dye with NIR-II Emission for Synergistic Photodynamic and Photothermal Anticancer Therapy

It is challenging to develop a near-infrared (NIR) small molecular photosensitizer for synergistic phototherapy in deep tissues. Herein, first, a heavy-atom-free NIR hemicyanine photosensitizer (BHcy) for 808 nm light-mediated synergistic photodynamic therapy/photothermal therapy (PDT/PTT) anticancer therapy by leveraging the acceptor engineering strategy is reported. This strategy endows BHcy with a more planar and larger π-conjugated structure, resulting in long NIR absorption/emission at 770/915-1200 nm as well as enhanced singlet oxygen (¹ O₂ ) generation ability and photothermal effect, which is ascribed to the reduced energy levels of excited singlet/triplet states and the promoted intersystem crossing process. Notably, BHcy-based nanoparticles (BHcy-NPs) exhibit efficient ¹ O₂ yield (12.9%) and high photothermal conversion efficiency (55.1%). More importantly, BHcy-NPs are able to significantly kill cancer cells by destroying main organelles and inhibit tumor growth in vivo after a single irradiation. Overall, this study provides a strategy to design new heavy-atom-free PDT/PTT agents for potential clinical applications.

PD-1 engineered cytomembrane cloaked molybdenum nitride for synergistic photothermal and enhanced immunotherapy of breast cancer

Incomplete tumor ablation and subsequent tumor metastasis usually occur during photothermal anti-tumor processes. The combination of photothermal and immunotherapy has proven to be a promising method to conquer technical challenges. Inhibiting the programmed death ligand-1 (PD-L1)/programmed cell death protein 1 (PD-1) immune pathway represents one of the most successful immunotherapy strategies. Whereas, the PD-L1 expression level significantly differs, leading to a relatively low response rate to the immune checkpoint blockade (ICB) approaches. Therefore, improving the expression level of PD-L1 becomes one potential method to enhance the response rate. Herein, NIH 3T3 cells were educated to steadily express PD-1 protein. Furthermore, the synthesized molybdenum nitride was then coated with PD-1 protein-modified cytomembrane, which endows it with immune checkpoint blocking capability. Moreover, under the irradiation of near-infrared light, the local mild heat released from the molybdenum nitride causes the apoptosis of tumor cells. More importantly, the elevated temperature simultaneously helps elevate the expression level of PD-L1, further enhancing the response rate of ICB. Finally, the PD-1 cytomembrane coatings interact with the upregulated PD-L1, leading to the activation of the immune system. In summary, we confirmed that the PD-1 protein-coated molybdenum nitride could synergistically ablate tumors and avoid metastasis.

From Low to No O2-Dependent Hypoxia Photodynamic Therapy (hPDT): A New Perspective

The advent of photochemical techniques has revolutionized the landscape of biology and medical sciences. Especially appealing in this context is photodynamic therapy (PDT), which is a photon-initiated treatment modality that uses cytotoxic reactive oxygen species (ROS) to kill malignant cells. In the past decade, PDT has risen to the forefront of cancer therapy. Its optical control enables noninvasive and spatiotemporal manipulation of the treatment process, and its photoactive nature allows unique patterns to avoid drug resistance to conventional chemotherapeutics. However, despite the impressive advances in this field, achieving widespread clinical adoption of PDT remains difficult. A major concern is that in the hostile tumor microenvironment, tumor cells are hypoxic, which hinders ROS generation during PDT action. To overcome this "Achilles' heel", current strategies focus primarily on the improvement of the intratumoral O₂ perfusion, while clinical trials suggest that O₂ enrichment may promote cancer cell proliferation and metastasis, thereby making FDA approval and clinical transformation of these paradigms challenging.In an effort to improve hypoxia photodynamic therapy (hPDT) in the clinic, we have explored "low to no O₂-dependent" photochemical approaches over the years to combat hypoxia-induced resistance. In this Account, we present our contributions to this theme during the past 5 years, beginning with low O₂-dependent approaches (e.g., type I superoxide radical (O₂^•-) generator, photodynamic O₂-economizer, mitochondrial respiration inhibition, cellular self-protective pathway modulation, etc.) and progressing to O₂-independent strategies (e.g., autoadaptive PDT/PTT complementary therapy, O₂-independent artificial photoredox catalysis in cells). These studies have attracted tremendous attention. Particularly in the pioneering work of 2018, we presented the first demonstration that the O₂^•--mediated partial O₂-recyclability mechanism can overcome PDT resistance ( J. Am. Chem. Soc. 2018, 140, 14851-14859). This launched an era of renewed interest in type I PDT, resulting in a plethora of new O₂^•- photogenerators developed by many groups around the world. Moreover, with the discovery of O₂-independent photoredox reactions in living cells, artificial photoredox catalysis has emerged as a new field connecting photochemistry and biomedicine, stimulating the development of next-generation phototherapeutic tools ( J. Am. Chem. Soc. 2022, 144, 163-173). Our recent work also disclosed that "photoredox catalysis in cells" might be a general mechanism of action of PDT ( Proc. Natl. Acad. Sci. U.S.A. 2022, 119, e2210504119). These emergent concepts, molecular designs, photochemical mechanisms, and applications in cancer diagnosis and therapeutics, as well as pros and cons, are discussed in depth in this Account. It is expected that our contributions to date will be of general use to researchers and inspire future efforts to identify more promising hPDT approaches that better meet the clinical needs of cancer therapy.

A J-aggregated nanoporphyrin overcoming phototoxic side effects in superior phototherapy with two-pronged effects

Phototherapy has been a promising therapeutic modality for pathological tissue due to its spatiotemporal selectivity and non-invasive characteristics. However, as a core component of phototherapy, a single photosensitizer (PS) nanoplatform integrating excellent therapeutic efficiency and minimal side effects remains an urgent but unmet need. Here, we construct a J-aggregated nano-porphyrin termed MTE based on the self-assembly of methyl-pheophorbide a derivative MPa-TEG (MT) and natural polyphenolic compound epigallocatechin gallate (EGCG). Due to the synergistic interaction between similar large π-conjugated structural EGCG and MT, MTE with small and uniform size is obtained by effectively hindering Ostwald ripening of MT. Noteworthily, MTE not only effectively avoids the inadvertent side effects of phototoxicity during transport thank to the ability of reactive oxygen species (ROS) scavenging, but also achieves two-pathway augmented superior phototherapy: (1) enhancing photodynamic therapy (PDT) via inhibiting the expression of anti-apoptosis protein surviving; (2) achieving adjuvant mild-temperature laser interstitial thermal therapy (LITT) via reducing the tumor thermoresistance on account that MTE inhibits the overexpression of HSP 70 and HSP 90. This research not only offers a facile strategy to construct multicomponent nanoplatforms but also provides a new pathway for efficient and low-toxicity phototherapy, which is beneficial to the future clinical application.

An Approach to Developing Cyanines with Upconverted Photosensitive Efficiency Enhancement for Highly Efficient NIR Tumor Phototheranostics

Upconverted reactive oxygen species (ROS) photosensitization with one-photon excitation mode is a promising tactic to elongate the excitation wavelengths of photosensitive dyes to near-infrared (NIR) light region without the requirement of coherent high-intensity light sources. However, the photosensitization efficiencies are still finite by the unilateral improvement of excited-state intersystem crossing (ISC) via heavy-atom-effect, since the upconverted efficiency also plays a decisive role in upconverted photosensitization. Herein, a NIR light initiated one-photon upconversion heavy-atom-free small molecule system is reported. The meso-rotatable anthracene in pentamethine cyanine (Cy5) is demonstrated to enrich the populations in high vibrational-rotational energy levels and subsequently improve the hot-band absorption (HBA) efficiency. Moreover, the spin-orbit charge transfer intersystem crossing (SOCT-ISC) caused by electron donated anthracene can further amplify the triplet yield. Benefiting from the above two aspects, the ¹ O₂ generation significantly increases with over 2-fold improved performance compared with heavy-atom-modified method under upconverted light excitation, which obtains efficient in vivo phototheranostic results and provides new opportunities for other applications such as photocatalysis and fine chemical synthesis.

Activatable self-assembled organic nanotheranostics: Aspartyl aminopeptidase triggered NIR fluorescence imaging-guided photothermal/photodynamic synergistic therapy

Phototherapy has developed as a powerful method for remedial modalities. The conventional photosensitizers are "always on" state and lack tumor targeting, which contributed to poor therapeutic effect and high toxicity. Therefore, we developed an aspartyl aminopeptidase (DNPEP) activated self-assembled organic nanoparticles (NRh-Asp NPs) with sensitive external irradiation-induced photothermal therapy and photodynamic therapy (PTT/PDT). NRh-Asp NPs can be activated to NRh-NH₂ NPs by DNPEP, demonstrating strong near-infrared (NIR) fluorescence, and efficiently generating heat and singlet oxygen under the near-infrared laser. NRh-Asp NPs was successfully used for visualizing DNPEP in vitro and in vivo in NIR region, and demonstrated good synergistic anti-cancer efficacy of PDT and PTT. These results suggest that DNPEP-mediated NRh-Asp NPs are promising candidates for image-guided phototherapeutic of tumor.

Monitoring mitochondrial nitroreductase activity in tumors and a hind-limb model of ischemia in mice using a novel activatable NIR fluorescent probe

We report a mitochondria-targeted nitroreductase (NTR)-activated near-infrared fluorescent probe: CS-NO2. Overexpressed NTR in mitochondria was measured with high sensitivity. More importantly, the probe CS-NO2 successfully monitored NTR activity in solid tumors and a hind-limb model of ischemia in mice. This novel finding indicates the promising function of our probe for the diagnosis of solid tumors and hypoxia-associated diseases.

Nitroreductase-induced bioorthogonal ligation for prodrug activation: A traceless strategy for cancer-specific imaging and therapy

Prodrug development is of great interest in cancer therapy. From bio-friendly standpoints, traceless prodrug activation would be an ideal approach for cancer treatment owning to the avoidance of byproduct which might induce side effects in living system. Here, we report a fully traceless strategy for cancer imaging and therapy via a metal-free bioorthogonal ligation triggered by nitroreductase (NTR) overexpressed in solid tumors. The reduction of nitro substrates to amines by NTR and further condensation of amines with aldehydes can be seamlessly combined to yield imine-based resveratrol (RSV) with water as the only byproduct. In comparison with RSV, this precursor exhibited not only the same level of anticancer efficiency both in vitro and in vivo under hypoxia, but also a high sensitivity to hypoxia and much lower perturbation towards normal cells, which holds a great potential of theranostic prodrug for cancer therapy.

A Hypoxia-Activated Prodrug Conjugated with a BODIPY-Based Photothermal Agent for Imaging-Guided Chemo-Photothermal Combination Therapy

Hypoxia-activated prodrugs (HAPs) have drawn increasing attention for improving the antitumor effects while minimizing side effects. However, the heterogeneous distribution of the hypoxic region in tumors severely impedes the curative effect of HAPs. Additionally, most HAPs are not amenable to optical imaging, and it is difficult to precisely trace them in tissues. Herein, we carefully designed and synthesized a multifunctional therapeutic BAC prodrug by connecting the chemotherapeutic drug camptothecin (CPT) and the fluorescent photothermal agent boron dipyrromethene (BODIPY) via hypoxia-responsive azobenzene linkers. To enhance the solubility and tumor accumulation, the prepared BAC was further encapsulated into a human serum albumin (HSA)-based drug delivery system to form HSA@BAC nanoparticles. Since the CPT was caged by a BODIPY-based molecule at the active site, the BAC exhibited excellent biosafety. Importantly, the activated CPT could be quickly released from BAC and could perform chemotherapy in hypoxic cancer cells, which was ascribed to the cleavage of the azobenzene linker by overexpressed azoreductase. After irradiation with a 730 nm laser, HSA@BAC can efficiently generate hyperthermia to achieve irreversible cancer cell death by oxygen-independent photothermal therapy. Under fluorescence imaging-guided local irradiation, both in vitro and in vivo studies demonstrated that HSA@BAC exhibited superior antitumor effects with minimal side effects.

Recent Progress on NIR Fluorescent Probes for Enzymes

The majority of diseases’ biomarkers are enzymes, and the regulation of enzymes is fundamental but crucial. Biological system disorders and diseases can result from abnormal enzymatic activity. Given the biological significance of enzymes, researchers have devised a plethora of tools to map the activity of particular enzymes in order to gain insight regarding their function and distribution. Near-infrared (NIR) fluorescence imaging studies on enzymes may help to better understand their roles in living systems due to their natural imaging advantages. We review the NIR fluorescent probe design strategies that have been attempted by researchers to develop NIR fluorescent sensors of enzymes, and these works have provided deep and intuitive insights into the study of enzymes in biological systems. The recent enzyme-activated NIR fluorescent probes and their applications in imaging are summarized, and the prospects and challenges of developing enzyme-activated NIR fluorescent probes are discussed.

Activation of pyroptosis by specific organelle-targeting photodynamic therapy to amplify immunogenic cell death for anti-tumor immunotherapy

Pyroptosis, a unique lytic programmed cell death, inspired tempting implications as potent anti-tumor strategy in pertinent to its potentials in stimulating anti-tumor immunity for eradication of primary tumors and metastasis. Nonetheless, rare therapeutics have been reported to successfully stimulate pyroptosis. In view of the intimate participation of reactive oxygen species (ROS) in stimulating pyroptosis, we attempted to devise a spectrum of well-defined subcellular organelle (including mitochondria, lysosomes and endoplasmic reticulum)-targeting photosensitizers with the aim of precisely localizing ROS (produced from photosensitizers) at the subcellular compartments and explore their potentials in urging pyroptosis and immunogenic cell death (ICD). The subsequent investigations revealed varied degrees of pyroptosis upon photodynamic therapy (PDT) towards cancerous cells, as supported by not only observation of the distinctive morphological and mechanistic characteristics of pyroptosis, but for the first-time explicit validation from comprehensive RNA-Seq analysis. Furthermore, in vivo anti-tumor PDT could exert eradication of the primary tumors, more importantly suppressed the distant tumor and metastatic tumor growth through an abscopal effect, approving the acquirement of specific anti-tumor immunity as a consequence of pyroptosis. Hence, pyroptosis was concluded unprecedently by our proposed organelles-targeting PDT strategy and explicitly delineated with molecular insights into its occurrence and the consequent ICD.

Aggregation-induced emission photosensitizer-based photodynamic therapy in cancer: from chemical to clinical

Cancer remains a serious threat to human health owing to the lack of effective treatments. Photodynamic therapy (PDT) has emerged as a promising non-invasive cancer treatment that consists of three main elements: photosensitizers (PSs), light and oxygen. However, some traditional PSs are prone to aggregation-caused quenching (ACQ), leading to reduced reactive oxygen species (ROS) generation capacity. Aggregation-induced emission (AIE)-PSs, due to their distorted structure, suppress the strong molecular interactions, making them more photosensitive in the aggregated state instead. Activated by light, they can efficiently produce ROS and induce cell death. PS is one of the core factors of efficient PDT, so proceeding from the design and preparation of AIE-PSs, including how to manipulate the electron donor (D) and receptor (A) in the PSs configuration, introduce heavy atoms or metal complexes, design of Type I AIE-PSs, polymerization-enhanced photosensitization and nano-engineering approaches. Then, the preclinical experiments of AIE-PSs in treating different types of tumors, such as ovarian cancer, cervical cancer, lung cancer, breast cancer, and its great potential clinical applications are discussed. In addition, some perspectives on the further development of AIE-PSs are presented. This review hopes to stimulate the interest of researchers in different fields such as chemistry, materials science, biology, and medicine, and promote the clinical translation of AIE-PSs.

A cyclometallated iridium(III) complex with multi-photon absorption properties as an imaging-guided photosensitizer

Conventional photosensitizers (PSs) often have shorter excitation wavelengths and poor cancer cell targeting, resulting in a limited tissue penetration depth and increased biotoxicity, which are significant barriers to ensuring effective photodynamic therapy (PDT) in vivo. In this work, a cyclometallated iridium(III) complex (Ir-Biotin) with a long excitation wavelength and effective cancer cell targeting was designed and synthesized. The initial in vitro assessment indicated that Ir-Biotin shows excellent PDT activity with a high singlet-oxygen (¹O₂) generation yield (0.19) due to the facilitated intersystem crossing process. Further study shows that Ir-Biotin shows good biocompatibility, has specific selectivity for cancer cells, and can induce apoptosis under laser irradiation. Furthermore, Ir-Biotin can be applied for imaging-guided PDT using an in vivo imaging system, and showed significant anti-tumour effects (tumour growth inhibition value: 87.66%). These results reveal the importance of long excitation wavelengths of photosensitizers for efficient PDT and suggest a promising strategy for developing effective photosensitizers.

Rational design of a small organic photosensitizer for NIR-I imaging-guided synergistic photodynamic and photothermal therapy

Developing a small molecular photosensitizer to achieve multimodal phototherapy has recently garnered attention as a promising strategy for efficient cancer treatment. However, synthesis of a multifunctional small molecular photosensitizer has remained challenging. Here we report an aggregation-induced-emission (AIE)-featured luminogen (AIEgen) TPA-BTZ decorated with long and branched alkyl chains. TPA-BTZ shows long-wavelength emission at ca. 800 nm in the NIR-I region. Moreover, upon laser irradiation, TPA-BTZ could produce O₂˙^- and ¹O₂via both type I and type II mechanisms for enhanced photodynamic therapy (PDT). The propeller-like structure triphenylamine (TPA) rotators not only endow TPA-BTZ with AIE characteristics but also facilitate heat generation by intramolecular rotation for photothermal therapy (PTT). More importantly, long and branched alkyl chains can create intermolecular spatial isolation in the fabricated TPA-BTZ@PEG2000 nanoparticles (NPs) to allow sufficient intramolecular motion for photothermal conversion. Due to these unique features, in vitro and in vivo evaluations demonstrate that the TPA-BTZ@PEG2000 NPs exhibited long-term NIR-imaging ability, superior tumoricidal activity, and suppressed tumor growth. This research provides new insights for developing new AIEgens for NIR imaging-guided multimodal phototherapy.

A novel active mitochondrion-selective fluorescent probe for the NIR fluorescence imaging and targeted photodynamic therapy of gastric cancer

The annual morbidity and mortality due to gastric cancer are still high across the world, posing a serious threat to public health. Improving the diagnosis rate of gastric cancer and exploring new treatments are urgent issues in the clinical field. In recent years, photosensitizer (PS)-based photodynamic therapy (PDT) has proven to be an effective cancer treatment strategy and can be used to treat a variety of cancers. Developing PSs with tumor-targeting ability and high singlet oxygen yield (Φ(¹O₂)) is the key to improving the PDT effect. Herein, we developed a novel diagnosis and treatment system (Cy₁₃₉₅-NPs). Our active thio-photosensitizer is based on the sulfur substitution strategy as it can reduce the S1-T1 energy gap, which can promote the process of intersystem crossing (ISC), thus resulting in high ROS generation efficiency. Cy₁₃₉₅-NPs exhibited stable spectral characteristics, satisfactory biocompatibility and high ¹O₂ yield under laser irradiation due to the introduction of the sulfur atom. In cellular studies, Cy₁₃₉₅-NPs could specifically target MKN45 cells via integrin α_vβ₃-mediated cRGD endocytosis and selectively aggregate in the mitochondria. Cy₁₃₉₅-NPs had no obvious cytotoxicity for MKN45 cells and exerted obvious phototoxicity due to the production of ¹O₂ under laser irradiation. The in vivo results showed that the fluorescence signal from the tumor site was obviously enhanced in 16-48 h, and Cy₁₃₉₅-NPs could selectively target solid tumors with a retention time of about 32 h. Under laser irradiation, Cy₁₃₉₅-NPs significantly inhibited tumor growth and led to significant tumor suppression and apoptosis. In summary, the developed Cy₁₃₉₅-NPs could actively target tumors and exert mitochondrial selectivity, showing an excellent fluorescence imaging effect. Under the irradiation of an 808 nm laser, Cy₁₃₉₅-NPs achieved good inhibition of gastric cancer cells both in vitro and in vivo, thus displaying the functions of tumor targeting, mitochondrial selectivity, fluorescence imaging and tumor inhibition. Our strategy provides a new diagnostic and treatment method for gastric cancers in clinical settings.

Dynamic Effects of Endo-Exogenous Stimulations on Enzyme-Activatable Polymeric Nanosystems with Photo-Sono-Chemo Synergy

Activatable polymeric nanosystems have attracted great interest, and their interactions with endo-exogenous stimulations are highly vital for therapeutic efficacy, which urgently needs systematic study. Herein we focus on systematically investigating these interactions on an enzyme-nanosystem model, the tumor-overexpressed hyaluronidase (HAase) and the doxorubicin-loaded hyaluronic-acid-porphyrin nanoassemblies (DOX@HPNAs), to augment photo-sono-chemo therapies. The HAase degrades the HPNAs in acidic solution at a higher rate than that in neutral solution, which leads to structure disassembly at the nano level, chain cleavage at the molecular level, and strong radiative recovery at the energy level. Upon excitation with light and ultrasound, the enzymatically degraded sample produces ∼2.5 times more singlet oxygen than the HPNAs because of the absence of aggregation-induced quenching and ¹O₂ migration limitation. The nanosystem can be activated by trimodal stimulations (acidity, ultrasound, and HAase), exerting the controllable release behavior and high release content. Moreover, the nanosystem exhibits synergistic effects among efficient photodynamic therapy, high tissue-penetrating sonodynamic therapy, and lasting chemotherapy, which induces significant necrosis and apoptosis of cancer cells. With high compatibility, tumor-targeting ability, and fluorescent-imaging-guided capability, the nanosystem achieves the highest inhibition rate of malignant tumors than the single or dual-modal therapies. Thus, the enzyme-activatable nanosystem enables the therapeutic synergy and also provides insights to develop other polymeric nanosystems.

A Dual-Locked Activatable Phototheranostic Probe for Biomarker-Regulated Photodynamic and Photothermal Cancer Therapy

Activatable phototheranostics holds promise for precision cancer treatment owing to the "turn-on" signals and therapeutic effects. However, most activatable phototheranostic probes only possess photodynamic therapy (PDT) or photothermal therapy (PTT), which suffer from poor therapeutic efficacy due to deficient cellular oxygen and complex tumor microenvironment. We herein report a dual-locked activatable phototheranostic probe that activates near-infrared fluorescence (NIRF) signals in tumor, triggers PDT in response to a tumor-periphery biomarker, and switches from PDT to PTT upon detecting a tumor-core-hypoxia biomarker. This PDT-PTT auto-regulated probe exhibits complete tumor ablation under the photoirradiation of a single laser source by producing cytotoxic singlet oxygen at the tumor periphery and generating hyperthermia at tumor-core where is too hypoxic for PDT. This dual-locked probe represents a promising molecular design approach toward precise cancer phototheranostics.

Acid-Responsive Nanoporphyrin Evolution for Near-Infrared Fluorescence-Guided Photo-Ablation of Biofilm

Combating biofilm infections remains a challenge due to the shield and acidic conditions. Herein, an acid-responsive nanoporphyrin (PN3-NP) based on the self-assembly of a water-soluble porphyrin derivative (PN3) is constructed. Additional kinetic control sites formed by the conjugation of the spermine molecules to a porphyrin macrocycle make PN3 self-assemble into stable nanoparticles (PN3-NP) in the physiological environment. Noteworthily, near-infrared (NIR) fluorescence monitoring and synergistic photodynamic therapy (PDT) and photothermal therapy (PTT) effects of PN3-NP can be triggered by the acidity in biofilms, accompanied by intelligent transformation into dot-like nanospheres. Thus, damage to normal tissue is effectively avoided and accurate diagnosis and treatment of biofilms is achieved successfully. The good results of fluorescence imaging-guided photo-ablation of antibiotic-resistant strains methicillin-resistant Staphylococcus aureus (MRSA) biofilms verify that PN3-NP is a promising alternative to antibiotics. Meanwhile, this strategy also opens new horizons to engineer smart nano-photosensitizer for accurate diagnosis and treatment of biofilms.

Magnetically Boosted Generation of Intracellular Reactive Oxygen Species toward Magneto-Photodynamic Therapy

The generation of reactive oxygen species (ROS) in photodynamic therapy (PDT) involves excited-state intermediates with both singlet and triplet spin configurations, which provides possibilities to modulate the ROS production in PDT under an external magnetic field. Here, we present that magnetically modulated ROS production can promote PDT efficacy and develop a magnetic-field-assisted PDT (magneto-PDT) method for effectively and selectively killing cancer cells. The photosensitization reaction between excited-state riboflavin and oxygen molecules is influenced by the applied field, and the overall magnetic field effect (MFE) shows a moderate increase at a low field (<1000 G) and then a boost up to the saturation ∼100% at a high field (>1000 G). It is found that the spin precession occurring in radical ion pairs (electron transfer from riboflavin to oxygen) facilitates the O₂^•- generation at the low field. In comparison, the spin splitting in an encounter complex (energy transfer from riboflavin to oxygen) benefits the production of ¹O₂ species at the high field. The field modulation on the two types of ROS in PDT, i.e., O₂^•- and ¹O₂, is also demonstrated in living cells. The magneto-PDT strategy shows the capability to inhibit the proliferation of cancer cells (e.g., HeLa, RBL-2H3, and MCF-7) effectively and selectively, which reveals the potential of using the MFE on chemical reactions in biological applications.

Activity-based NIR fluorescent probes based on the versatile hemicyanine scaffold: design strategy, biomedical applications, and outlook

The discovery of a near-infrared (NIR, 650-900 nm) fluorescent chromophore hemicyanine dye with high structural tailorability is of great significance in the field of detection, bioimaging, and medical therapeutic applications. It exhibits many outstanding advantages including absorption and emission in the NIR region, tunable spectral properties, high photostability as well as a large Stokes shift. These properties are superior to those of conventional fluorogens, such as coumarin, fluorescein, naphthalimides, rhodamine, and cyanine. Researchers have made remarkable progress in developing activity-based multifunctional fluorescent probes based on hemicyanine skeletons for monitoring vital biomolecules in living systems through the output of fluorescence/photoacoustic signals, and integration of diagnosis and treatment of diseases using chemotherapy or photothermal/photodynamic therapy or combination therapy. These achievements prompted researchers to develop more smart fluorescent probes using a hemicyanine fluorogen as a template. In this review, we begin by describing the brief history of the discovery of hemicyanine dyes, synthetic approaches, and design strategies for activity-based functional fluorescent probes. Then, many selected hemicyanine-based probes that can detect ions, small biomolecules, overexpressed enzymes and diagnostic reagents for diseases are systematically highlighted. Finally, potential drawbacks and the outlook for future investigation and clinical medicine transformation of hemicyanine-based activatable functional probes are also discussed.

Strong π-π Stacking Stabilized Nanophotosensitizers: Improving Tumor Retention for Enhanced Therapy for Large Tumors in Mice

Conventional photosensitizers (PSs) often show poor tumor retention and are rapidly cleared from the bloodstream, which is one of the key hindrances to guarantee precise and efficient photodynamic therapy (PDT) in vivo. In this work, a photosensitizer assembly nanosystem that sharply enhances tumor retention up to ≈10 days is present. The PSs are synthesized by meso-substituting anthracene onto a BODIPY scaffold (AN-BDP), which then self-assembles into stable nanoparticles (AN-BDP NPs) with amphiphilic block copolymers due to the strong intermolecular π-π interaction of the anthracene. Additionally, the incorporated anthracene excites the PSs, producing singlet oxygen under red-light irradiation. Although AN-BDP NPs can completely suppress regular test size tumors (≈100 mm³ ) by one-time radiation, only 12% tumor growth inhibition rate is observed in the case of large-size tumors (≈350 mm³ ) under the same conditions. Due to the long-time tumor retention, AN-BDP NPs allow single-dose injection and three-time light treatments, resulting in an inhibition rate over 90%, much more efficient than single-time radiation of conventional clinically used PSs including chlorin-e6 (Ce6) and porphyrin with poor tumor retention. The results reveal the importance of long tumor retention time of PSs for efficient PDT, which can accelerate the clinical development of nanophotosensitizers.

Sequence-Activated Fluorescent Nanotheranostics for Real-Time Profiling Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC), as one of the most malignant tumors with dense desmoplastic stroma, forms a specific matrix barrier to hinder effective diagnosis and therapy. To date, a paramount challenge is in the search for intelligent nanotheranostics for such hypopermeable tumors, especially in breaking the PDAC-specific physical barrier. The unpredictable in vivo behaviors of nanotheranostics, that is, real-time tracking where, when, and how they cross the physical barriers and are taken up by tumor cells, are the major bottleneck. Herein, we elaborately design sequence-activated nanotheranostic TCM-U11&Cy@P with dual-channel near-infrared fluorescence outputs for monitoring in vivo behaviors in a sequential fashion. This nanotheranostic with a programmable targeting capability effectively breaks through the PDAC barriers. Ultimately, the released aggregation-induced emission (AIE) particle TCM-U11 directly interacts with PDAC cells and penetrates into the deep tissue. Impressively, this fluorescent nanotheranostic intraoperatively can map human clinical PDAC specimens with high resolution. We believe that this unique sequence-activated fluorescent strategy expands the repertoire of nanotheranostics in the treatment of hypopermeable tumors.

NIR-II Aggregation-Induced Emission Luminogens for Tumor Phototheranostics

As an emerging and powerful material, aggregation-induced emission luminogens (AIEgens), which could simultaneously provide a precise diagnosis and efficient therapeutics, have exhibited significant superiorities in the field of phototheranostics. Of particular interest is phototheranostics based on AIEgens with the emission in the range of second near-infrared (NIR-II) range (1000-1700 nm), which has promoted the feasibility of their clinical applications by virtue of numerous preponderances benefiting from the extremely long wavelength. In this minireview, we summarize the latest advances in the field of phototheranostics based on NIR-II AIEgens during the past 3 years, including the strategies of constructing NIR-II AIEgens and their applications in different theranostic modalities (FLI-guided PTT, PAI-guided PTT, and multimodal imaging-guided PDT-PTT synergistic therapy); in addition, a brief conclusion of perspectives and challenges in the field of phototheranostics is given at the end.

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.

Activatable photothermal reagents were designed for cancer therapy. Dual-site-activatable probe showed a large spectral redshift of 150 nm in the presence of H₂O₂, achieving truly selective activation to minimize side effects during PTT of cancers.

A Tumor-Targeting Near-Infrared Heptamethine Cyanine Photosensitizer with Twisted Molecular Structure for Enhanced Imaging-Guided Cancer Phototherapy

In recent years, cancer phototherapy has been extensively studied as noninvasive cancer treatment. To present efficient recognition toward cancer cells, most photosensitizers (PSs) are required to couple with tumor-targeted ligands. Interestingly, the heptamethine cyanine IR780 displays an intrinsic tumor-targeted feature even without modification. However, the photothermal efficacy and photostability of IR780 are not sufficient enough for clinical use. Herein, we involve a twisted structure of tetraphenylethene (TPE) between two molecules of IR780 to improve the photothermal conversion efficiency (PCE). The obtained molecule T780T shows strong near-infrared (NIR) fluorescence and improved PCE (38.5%) in the dispersed state. Also, the photothermal stability and ROS generation capability of T780T at the NIR range (808 nm) are both promoted. In the aqueous phase, the T780T was formulated into uniform nanoaggregates (∼200 nm) with extremely low fluorescence and PTT response, which would reduce in vivo imaging background and side effect of PTT response in normal tissues. After intravenous injection into tumor-bearing mice, the T780T nanoaggregates display high tumor accumulation and thus remarkably inhibit the tumor growth. Moreover, the enhanced photostability of the T780T allows for twice irradiation after one injection and leads to more significant tumor inhibition. In summary, our study presents a tumor-targeted small-molecule PS for efficient cancer therapy and brings a new design of heptamethine cyanine PS for potential clinical applications.

Optimizing Comprehensive Performance of Aggregation-Induced Emission Nanoparticles through Molecular Packing Modulation for Multimodal Image-Guided Synergistic Phototherapy

Fluorescent nanoparticles (NPs) with aggregation-induced emission (AIE) characteristics hold remarkable potential for image-guided phototherapy. The molecular packing is the key point for optimizing the performance of AIE luminogens (AIEgens) in the aggregated or solid state. However, so far, the packing mode of AIEgens in NPs is still vague, causing some challenges for understanding the relationship between the photophysical property and packing mode, as well as further optimizing the performance of NPs for biomedical applications. In this contribution, by simply controlling the length of alkoxy chains in the donor-acceptor conjugated OPTPA, a packing balance between the twisted molecular structure and effective π-conjugation is actualized. Subsequently, the possibility of amorphous-crystalline transition of AIEgens in the polymer-encapsulated NPs is presented for the first time, and the comprehensive performance of NPs is further optimized. Both in vitro and in vivo experiments indicate that crystalline AIEgen-based NPs are remarkably effective in trimodal imaging-guided synergistic phototherapy.

PEG-Polymer Encapsulated Aggregation-Induced Emission Nanoparticles for Tumor Theranostics

In the field of tumor imaging and therapy, the aggregation-caused quenching (ACQ) effect of fluorescent dyes at high concentration is a great challenge. In this regard, the aggregation-induced emission luminogens (AIEgens) show great potential, since AIEgens effectively overcome the ACQ effect and have better fluorescence quantum yield, photobleaching resistance, and photosensitivity. Polyethylene glycol (PEG)-polymer is the most commonly used carrier to prepare nanoparticles (NPs). The advantage of PEGylation is that it can greatly prolong the metabolic half-life and reduce immunogenicity and toxicity. Considering that the hydrophobicity of most AIEgens hinders their application in organisms, the use of PEG-polymer encapsulation is an effective strategy to overcome this obstacle. Importantly, bioactive functional groups can be modified on PEG-polymers to enhance the biological effect of NPs. The combination of powerful AIEgens and PEG-polymers provides a new strategy for tumor imaging and therapy, which is promising for clinical application.

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.

A metformin-based nanoreactor alleviates hypoxia and reduces ATP for cancer synergistic therapy

Severe hypoxia in solid tumors limits the efficacy of oxygen (O₂)-dependent photodynamic therapy (PDT). The overexpressed heat shock proteins (HSPs) in tumor cells hamper the effect of photothermal therapy (PTT). Herein, a tumor oxygenation-enhanced and ATP-reduced gelatin nanoreactor (MCGPD ∼ RGD NPs) for PDT/PTT-augmented combination cancer therapy is reported. In this nanosystem, the Arg-Gly-Asp (RGD) peptides of MCGPD ∼ RGD NPs can ensure accurate recognition and sufficient accumulation in the tumor site. After accumulation, doxorubicin (DOX) can be released from MCGPD ∼ RGD NPs in a mild acidic tumor microenvironment (TME) for highly efficient chemotherapy. Upon 808 nm laser irradiation, the overexpressed matrix metalloproteinase-2 (MMP-2) in the TME and the heat produced from the PDA coating trigger Gel NP degradation to expose chlorin e6 (Ce6) and Met from the cavity of MCGPD ∼ RGD NPs. The exposed Met elevates the O₂ content and reduces ATP production in tumor sites to spur the successful O₂-dependent PDT and HSP-mediated PTT. The heat generated by the PDA coating directly kills the tumor cells to ensure PTT and amplifies the chemotherapeutic effect. In vitro and in vivo assays indicate that MCGPD ∼ RGD NPs have excellent ability to promote cell apoptosis and to inhibit tumor growth. Overall, this smart responsive hydrogel nanosystem with hypoxia-relieving capacity and ATP-decreasing performance provides a promising strategy against cancer.

Emerging Design Principle of Near-Infrared Upconversion Sensitizer Based on Mitochondria-Targeted Organic Dye for Enhanced Photodynamic Therapy

Upconversion luminescent (UCL) triggered photodynamic therapy (PDT) affords superior outcome for cancer treatment. However, conventional UCL materials which all work by a multiphoton absorption (MPA) process inevitably need extremely high power density far over the maximum permissible exposure (MPE) to laser. Here, a one-photon absorption molecular upconversion sensitizer Cy5.5-Br based on frequency upconversion luminescent (FUCL) is designed for PDT. The unusual super heavy atom effect (SHAE) in Cy5.5-Br strongly enhances its spin-orbit coupling (0.23 cm^-1 ), triplet quantum yield (11.1 %) and triplet state lifetime (18.8 μs) while the potential hot-band absorption of Cy5.5-Br is well maintained. Importantly, Cy5.5-Br can efficiently target the tumour site and kill cancer cells by destroying mitochondria under a biosafety MPE to 808 nm laser. The photostability and antitumor results are obviously superior to that of a Stokes process. This work provides a design criterion for FUCL dyes to realize effective PDT upon a biosafety optical density, possibly bringing more clinical benefits than conventional MPA materials.

Molecular Design of Monochromophore-Based Bifunctional Photosensitizers for Simultaneous Ratiometric Oxygen Reporting and Photodynamic Cancer Therapy

Monitoring the tumor oxygen level when implementing photodynamic therapy (PDT) on malignant cancer has vital significance but remains challenging yet. Herein, by structurally manipulating a 2,4-dimethylpyrrole-engineered asymmetric BODIPY scaffold with different kinds, numbers, and positions of halogen atoms, we rationally designed several monochromophore-based bifunctional photosensitizers, named BDPs (BDP-I, BDP-II, and BDP-III), with self-sensitized photooxidation characteristics for accurate oxygen reporting and photodynamic tumor ablation. We show that different ways of halogen regulation allow available tuning of BDPs' oxygen-dependent ratiometric fluorescence turn-ons upon light irradiation as well as type-II PDT efficiencies before and after self-sensitized photooxidation. Encouragingly, measuring the specific ratiometric signals of the most promising BDP-II enabled the direct observation of initial oxygen concentration in both living 4T1 cells and a tumor-bearing mice model, affording an alternative way for evaluating oxygen supplementation strategies. Meanwhile, the "always on" PDT effect of BDP-II ensured efficient tumor ablation via apoptosis. Our research was thus believed to be of instructive significance for future application of oxygen-related auxiliary strategies and the design of unimolecular multifunctional PDT agents for cancer precision therapy.

A pre-screening strategy to assess resected tumor margins by imaging cytoplasmic viscosity and hypoxia

To assure complete tumor removal, frozen section analysis is the most common procedure for intraoperative pathological assessment of resected tumor margins. However, during one operation, multiple biopsies may be sent for examination, but only few of them are made into cryosections because of the complex preparation protocols and time-consuming pathological analysis, which potentially increases the risk of overlooking tumor involvement. Here, we propose a fluorescence-based pre-screening strategy that allows high-throughput, convenient, and fast gross assessment of resected tumor margins. A dual-activatable cationic fluorescent molecular rotor was developed to specifically illuminate live tumor cells’ cytoplasm by emitting two different fluorescence signals in response to elevations in hypoxia-induced nitroreductase (a biochemical marker) and cytoplasmic viscosity (a biophysical marker), two characteristics of cancer cells. The ability of the fluorescent molecular rotor in detecting tumor cells was evaluated in mouse and human specimens of multiple tissues by comparing with hematoxylin and eosin staining. Importantly, the fluorescent molecular rotor achieved 100 % specificity in discriminating lung and liver cancers from normal tissue, allowing pre-screening of the tumor-free surgical margins and promoting clinical decision. Altogether, this type of fluorescent molecular rotor and the proposed strategy may serve as a new option to facilitate intraoperative assessment of resected tumor margins.

Photodynamic Therapy with Tumor Cell Discrimination through RNA-Targeting Ability of Photosensitizer

Photodynamic therapy (PDT) represents an effective treatment to cure cancer. The targeting ability of the photosensitizer is of utmost importance. Photosensitizers that discriminate cancer cells can avoid the killing of normal cells and improve PDT efficacy. However, the design and synthesis of photosensitizers conjugated with a recognition unit of cancer cell markers is complex and may not effectively target cancer. Considering that the total RNA content in cancer cells is commonly higher than in normal cells, this study has developed the photosensitizer QICY with RNA-targeting abilities for the discrimination of cancer cells. QICY was specifically located in cancer cells rather than normal cells due to their stronger electrostatic interactions with RNA, thereby further improving the PDT effects on the cancer cells. After intravenous injection into mice bearing a xenograft tumor, QICY accumulated into the tumor location through the enhanced permeability and retention effect, automatically targeted cancer cells under the control of RNA, and inhibited tumor growth under 630 nm laser irradiation without obvious side effects. This intelligent photosensitizer with RNA-targeting ability not only simplifies the design and synthesis of cancer-cell-targeting photosensitizers but also paves the way for the further development of highly efficient PDTs.

Sensitive imaging of tumors using a nitroreductase-activated fluorescence probe in the NIR-II window

A nitroreductase (NTR)-activated NIR-II fluorescence probe for tumor imaging is reported. The probe can emit fluorescence in the range of 900-1300 nm, and target hypoxic tumors with NTR overexpression, thus allowing for accurate delineation of tumor margins through deep penetration.

Leveraging BODIPY nanomaterials for enhanced tumor photothermal therapy

In the past ten years, photothermal therapy (PTT) has attracted widespread attention in tumor treatment due to its non-invasiveness and little side effects. PTT utilizes heat produced by photothermal agents under the irradiation of near-infrared light to kill tumor cells. Boron-dipyrromethene (BODIPY), an organic phototherapy agent, has been widely used in tumor phototherapy due to its higher molar extinction coefficient, robust photostability and good phototherapy effect. However, there are some issues in the application of BODIPY for tumor PTT, such as low photothermal conversion efficiency and short absorption wavelength. In this review, we focus on the latest development of BODIPY nanomaterials for overcoming the above problems and enhancing the PTT effect.