A Paclitaxel Prodrug Activatable by Irradiation in a Hypoxic Microenvironment

Self-Sacrificially Degradable Pseudo-Semiconducting Polymer Nanoparticles that Integrate NIR-II Fluorescence Bioimaging, Photodynamic Immunotherapy, and Photo-Activated Chemotherapy

Semiconducting polymers (SP) hold great promise for cancer phototherapy due to their excellent optical properties; however, their clinical application is still hampered by their poor biodegradability. Herein, a self-sacrificially biodegradable pseudo-semiconducting polymer (PSP) for NIR-II fluorescence bioimaging, photodynamic immunotherapy, and photoactivated chemotherapy (PACT) is reported. The PSP can further co-assemble with an amphiphilic polyester with pendant doxorubicin (DOX) in its side chains via reactive oxygen species (ROS)-responsive thioketal linkages (PE^DOX ), which are denoted as NP@PE^DOX /PSP. The NP@PE^DOX /PSP can accumulate at tumor sites and generate ROS for photodynamic immunotherapy as well as near-infrared-II fluorescence (NIR-II) for bioimaging upon irradition at 808 nm. The ROS could break up thioketal linkages in PE^DOX , resulting in rapid doxorubicin (DOX) release for PACT. Finally, both PE^DOX and PSP are degraded sacrificially by intracellular glutathione (GSH), resulting in the dissociation of NP@PE^DOX /PSP. This work highlights the application of self-sacrificially degradable PSP for NIR-II fluorescence bioimaging, photodynamic immunotherapy, and PACT in cancer therapy.

A prodrug hydrogel with tumor microenvironment and near-infrared light dual-responsive action for synergistic cancer immunotherapy

Immunotherapy has been used for cancer treatment, while it faces the common dilemmas of low therapeutic efficacy and serious immunotoxicity. In this study, we report the construction of a tumor microenvironment and near-infrared (NIR) light dual-responsive prodrug hydrogel for cancer synergistic immunotherapy in a more effective and safe manner. Such prodrug hydrogels were in-situ formed via calcium-induced gelation of alginate solution containing protoporphyrin IX (PpIX)-modified iron oxide (Fe₃O₄) nanoparticles and programmed death ligand 1 antibody (aPD-L1) prodrug nanoparticles crosslinked by reactive oxygen species (ROS)-responsive linkers. PpIX served as a photosensitizer to produce singlet oxygen (¹O₂) under NIR laser irradiation for photodynamic therapy (PDT), and Fe₃O₄ nanoparticles mediated chemodynamic therapy (CDT) to generate hydroxyl radical (·OH) via Fenton reaction in the tumor microenvironment. In view of the cumulative actions of PDT and CDT, amplified ROS was generated to not only induce immunogenic cell death (ICD), but also destroy ROS-responsive linkers to achieve on-demand release of aPD-L1 from prodrug nanoparticles. Boosted antitumor immunity was elicited in tumor-bearing mice due to the aPD-L1-mediated immune checkpoint blocking. As a result, the prodrug hydrogel-based synergistic immunotherapy could almost treat bilateral tumors and prevent lung and liver metastasis using 4T1 tumor mouse models. This study thus offers a dual-responsive prodrug hydrogel platform for precision cancer immunotherapy. STATEMENT OF SIGNIFICANCE: Via calcium-induced gelation of alginate, we constructed a prodrug hydrogel with tumor microenvironment and near-infrared light dual-responsive action for synergistic cancer immunotherapy. Such hydrogels can achieve on-demand release of aPD-L1 upon photoactivation in the tumor microenvironment. Through mediating photodynamic and chemodynamic therapy, the prodrug hydrogels can induce enhanced immunogenic cell death and synergistically improve the efficacy of aPD-L1-mediated immune checkpoint blocking. The prodrug hydrogel-based synergistic therapy almost deracinates the primary and distant tumors, and prevents lung and liver metastasis in tumor mouse models.

Taxanes prodrug-based nanomedicines for cancer therapy

Malignant tumor remains a huge threat to human health and chemotherapy still occupies an important place in clinical tumor treatment. As a kind of potent antimitotic agent, taxanes act as the first-line broad-spectrum cancer drug in clinical use. However, disadvantages such as prominent hydrophobicity, severe off-target toxicity or multidrug resistance lead to unsatisfactory therapeutic effects, which restricts its wider usage. The efficient delivery of taxanes is still quite a challenge despite the rapid developments in biomaterials and nanotechnology. Great progress has been made in prodrug-based nanomedicines (PNS) for cancer therapy due to their outstanding advantages such as high drug loading efficiency, low carrier induced immunogenicity, tumor stimuli-responsive drug release, combinational therapy and so on. Based on the numerous developments in this filed, this review summarized latest updates of taxanes prodrugs-based nanomedicines (TPNS), focusing on polymer-drug conjugate-based nanoformulations, small molecular prodrug-based self-assembled nanoparticles and prodrug-encapsulated nanosystems. In addition, the new trends of tumor stimuli-responsive TPNS were also discussed. Moreover, the future challenges of TPNS for clinical translation were highlighted. We here expect this review will inspire researchers to explore more practical taxanes prodrug-based nano-delivery systems for clinical use.

Cyclodextrin-Activated Porphyrin Photosensitization for Boosting Self-Cleavable Drug Release

Supramolecular prodrugs that combine the merits of stimuli-responsiveness and targeting ability in a controllable manner have shown appealing prospects in disease diagnostics and therapeutics. Herein, we report that a new theranostic agent with the host-guest-binding-activated photosensitization has been fabricated by a binary supramolecular assembly consisting of the permethyl-β-cyclodextrin-grafted hyaluronic acid and a combretastatin A-4-appended porphyrin derivative. Illuminated by a red-light source, the production efficiency of singlet oxygen (¹O₂) pronouncedly increases by ∼60-fold once the porphyrin core is encapsulated by cyclodextrins. Consequently, the cell-selective fluorescence emission is dramatically enhanced, the microtubule-targeted drug is rapidly and completely released, and the ¹O₂-involved combinational treatment is simultaneously achieved both in vitro and in vivo. To be envisaged, this complexation-boosted light-activatable photosensitizing prodrug delivery system with improved photophysical performance and remarkable phototheranostic outcomes will make a significant contribution to the creation of more advanced stimulus-based biomaterials.

Sequential Ultrasound-Triggered and Hypoxia-Sensitive Nanoprodrug for Cascade Amplification of Sonochemotherapy

Hypoxia, the typical and conspicuous characteristic of most solid tumors, worsens the tumor invasiveness and metastasis. Here, we engineered a sequential ultrasound (US)/hypoxia-sensitive sonochemotherapeutic nanoprodrug by initially synthesizing the hypoxia-activated azo bond-containing camptothecin (CPT) prodrug (CPT₂-Azo) and then immobilizing it into the mesopores of sonosensitizer-integrated metal organic frameworks (MOF NPs). Upon entering the hypoxic tumor microenvironment (TME), the structure of CPT₂-Azo immobilized MOFs (denoted as MCA) was ruptured and the loaded nontoxic CPT₂-Azo prodrug was released from the MOF NPs. Under US actuation, this sonochemotherapeutic nanoprodrug not only promoted sonosensitizer-mediated sonodynamic therapy (SDT) via the conversion of oxygen into cytotoxic reactive oxygen species (ROS) but also aggravated hypoxia in the TME by elevating oxygen consumption. The exacerbated hypoxia in turn served as a positive amplifier to boost the activation of CPT₂-Azo, and the controllable release of toxic chemotherapeutic drug (CPT), and compensated the insufficient treatment efficacy of SDT. In vitro and in vivo evaluations confirmed that sequential SDT and tumor hypoxia-activated sonochemotherapy promoted the utmost of tumor hypoxia and thereby contributed to the augmented antitumor efficacy, resulting in conspicuous apoptotic cell death and noteworthy tumor suppression in vivo. Our work provides a distinctive insight into the exploitation of the hypoxia-activated sonochemotherapeutic nanoprodrug that utilizes the hypoxic condition in TME, a side effect of SDT, to initiate chemotherapy, thus causing a significantly augmented treatment outcome compared to conventional SDT.

Dissecting extracellular and intracellular distribution of nanoparticles and their contribution to therapeutic response by monochromatic ratiometric imaging

Efficient delivery of payload to intracellular targets has been identified as the central principle for nanomedicine development, while the extracellular targets are equally important for cancer treatment. Notably, the contribution of extracellularly distributed nanoparticles to therapeutic outcome is far from being understood. Herein, we develop a pH/light dual-responsive monochromatic ratiometric imaging nanoparticle (MRIN), which functions through sequentially lighting up the intracellular and extracellular fluorescence signals by acidic endocytic pH and near-infrared light. Enabled by MRIN nanotechnology, we accurately quantify the extracellular and intracellular distribution of nanoparticles in several tumor models, which account for 65-80% and 20-35% of total tumor exposure, respectively. Given that the majority of nanoparticles are trapped in extracellular regions, we successfully dissect the contribution of extracellularly distributed nanophotosensitizer to therapeutic efficacy, thereby maximize the treatment outcome. Our study provides key strategies to precisely quantify nanocarrier microdistribtion and engineer multifunctional nanomedicines for efficient theranostics.

Stimulus-responsive self-assembled prodrugs in cancer therapy

Small-molecule prodrugs have become the main toolbox to improve the unfavorable physicochemical properties of potential therapeutic compounds in contemporary anti-cancer drug development. Many approved small-molecule prodrugs, however, still face key challenges in their pharmacokinetic (PK) and pharmacodynamic (PD) properties, thus severely restricting their further clinical applications. Self-assembled prodrugs thus emerged as they could take advantage of key benefits in both prodrug design and nanomedicine, so as to maximize drug loading, reduce premature leakage, and improve PK/PD parameters and targeting ability. Notably, temporally and spatially controlled release of drugs at cancerous sites could be achieved by encoding various activable linkers that are sensitive to chemical or biological stimuli in the tumor microenvironment (TME). In this review, we have comprehensively summarized the recent progress made in the development of single/multiple-stimulus-responsive self-assembled prodrugs for mono- and combinatorial therapy. A special focus was placed on various prodrug conjugation strategies (polymer-drug conjugates, drug-drug conjugates, etc.) that facilitated the engineering of self-assembled prodrugs, and various linker chemistries that enabled selective controlled release of active drugs at tumor sites. Furthermore, some polymeric nano-prodrugs that entered clinical trials have also been elaborated here. Finally, we have discussed the bottlenecks in the field of prodrug nanoassembly and offered potential solutions to overcome them. We believe that this review will provide a comprehensive reference for the rational design of effective prodrug nanoassemblies that have clinic translation potential.

Recent Advances in Stimuli-Sensitive Amphiphilic Polymer-Paclitaxel Prodrugs

Paclitaxel (PTX) is a broad-spectrum chemotherapy drug employed in the treatment of a variety of tumors. However, the clinical applications of PTX are limited by its poor water solubility. Adjuvants are widely used to overcome this issue. However, these adjuvants often have side effects and poor biodistribution. The smart drug delivery system is a promising strategy for the improvement of solubility, permeability, and stability of drugs, and can promote sustained controlled release, increasing therapeutic efficacy and reducing side effects. Polymeric prodrugs show great advantages for drug delivery due to their high drug loading and stability. There has been some groundbreaking work in the development of PTX-based stimulus-sensitive polymeric prodrug micelles, which is summarized in this study. We consider these in terms of the four main types of stimulus (pH, reduction, enzyme, and reactive oxygen species (ROS)). The design, synthesis, and biomedical applications of stimulus-responsive polymeric prodrugs of PTX are reviewed, and the current research results and future directions of the field are summarized.

Facile synthesis of near-infrared responsive on-demand oxygen releasing nanoplatform for precise MRI-guided theranostics of hypoxia-induced tumor chemoresistance and metastasis in triple negative breast cancer

BACKGROUND

Hypoxia is an important factor that contributes to chemoresistance and metastasis in triple negative breast cancer (TNBC), and alleviating hypoxia microenvironment can enhance the anti-tumor efficacy and also inhibit tumor invasion.

METHODS

A near-infrared (NIR) responsive on-demand oxygen releasing nanoplatform (O2-PPSiI) was successfully synthesized by a two-stage self-assembly process to overcome the hypoxia-induced tumor chemoresistance and metastasis. We embedded drug-loaded poly (lactic-co-glycolic acid) cores into an ultrathin silica shell attached with paramagnetic Gd-DTPA to develop a Magnetic Resonance Imaging (MRI)-guided NIR-responsive on-demand drug releasing nanosystem, where indocyanine green was used as a photothermal converter to trigger the oxygen and drug release under NIR irradiation.

RESULTS

The near-infrared responsive on-demand oxygen releasing nanoplatform O2-PPSiI was chemically synthesized in this study by a two-stage self-assembly process, which could deliver oxygen and release it under NIR irradiation to relieve hypoxia, improving the therapeutic effect of chemotherapy and suppressed tumor metastasis. This smart design achieves the following advantages: (i) the O2 in this nanosystem can be precisely released by an NIR-responsive silica shell rupture; (ii) the dynamic biodistribution process of O2-PPSiI was monitored in real-time and quantitatively analyzed via sensitive MR imaging of the tumor; (iii) O2-PPSiI could alleviate tumor hypoxia by releasing O2 within the tumor upon NIR laser excitation; (iv) The migration and invasion abilities of the TNBC tumor were weakened by inhibiting the process of EMT as a result of the synergistic therapy of NIR-triggered O2-PPSiI.

CONCLUSIONS

Our work proposes a smart tactic guided by MRI and presents a valid approach for the reasonable design of NIR-responsive on-demand drug-releasing nanomedicine systems for precise theranostics in TNBC.

A dual-responsive nanoplatform with feedback amplification improves antitumor efficacy of photodynamic therapy

A good photosensitizer (PS) delivery system could enhance the efficiency and reduce the side effects of anti-tumor photodynamic therapy (PDT) by improving accumulation in the tumor, uptake by tumor cells, and intracellular release of the PS. Thus, we rationally developed a multi-stimulus-responsive PS nanocarrier with a double-layered core-shell structure: mPEG-azo-hyaluronic acid-sulfide-Ce6 (PaHAsC). In PaHAsC, the mPEG coat provides protection before entering the hypoxic tumor microenvironment, where mPEG leaves to expose the HA layer. HA then targets overexpressed CD44 on tumor cells for enhanced internalization. Finally, GSH-mediated intracellular release of Ce6 augments ROS generation and O₂ consumption under light stimulation. This also aggravates hypoxia in tumor sites to accelerate mPEG removal, forming a positive feedback loop. Data show that PaHAsC dramatically improved the PDT efficacy of Ce6, eliminating most tumors and 80% of tumor-bearing mice survived. With a safe profile, PaHAsC has potential for further development and is a useful example of a PS delivery system.

Biodegradable Ru-Containing Polycarbonate Micelles for Photoinduced Anticancer Multitherapeutic Agent Delivery and Phototherapy Enhancement

The lack of selectivity between tumor and healthy cells, along with inefficient reactive oxygen species production in solid tumors, are two major impediments to the development of anticancer Ru complexes. The development of photoinduced combination therapy based on biodegradable polymers that can be light activated in the "therapeutic window" would be beneficial for enhancing the therapeutic efficacy of Ru complexes. Herein, a biodegradable Ru-containing polymer (poly(DCARu)) is developed, in which two different therapeutics (the drug and the Ru complex) are rationally integrated and then conjugated to a diblock copolymer (MPEG-b-PMCC) containing hydrophilic poly(ethylene glycol) and cyano-functionalized polycarbonate with good degradability and biocompatibility. The polymer self-assembles into micelles with high drug loading capacity, which can be efficiently internalized into tumor cells. Red light induces the generation of singlet oxygen and the release of anticancer drug-Ru complex conjugates from poly(DCARu) micelles, hence inhibiting tumor cell growth. Furthermore, the phototherapy of polymer micelles demonstrates remarkable inhibition of tumor growth in vivo. Meanwhile, polymer micelles exhibit good biocompatibility with blood and healthy tissues, which opens up opportunities for multitherapeutic agent delivery and enhanced phototherapy.

Polyprodrug Nanomedicines: An Emerging Paradigm for Cancer Therapy

Nanomedicines have the potential to provide advanced therapeutic strategies in combating tumors. Polymer-prodrug-based nanomedicines are particularly attractive in cancer therapies owing to the maximum drug loading, prolonged blood circulation, and reduced premature leakage and side effects in comparison with conventional nanomaterials. However, the difficulty in precisely tuning the composition and drug loading of polymer-drug conjugates leads to batch-to-batch variations of the prodrugs, thus significantly restricting their clinical translation. Polyprodrug nanomedicines inherit the numerous intrinsic advantages of polymer-drug conjugates and exhibit well-controlled composition and drug loading via direct polymerization of therapeutic monomers, representing a promising nanomedicine for clinical tumor therapies. In this review, recent advances in the development of polyprodrug nanomedicines are summarized for tumor elimination. Various types of polyprodrug nanomedicines and the corresponding properties are first summarized. The unique advantages of polyprodrug nanomedicines and their key roles in various tumor therapies are further highlighted. Finally, current challenges and the perspectives on future research of polyprodrug nanomedicines are discussed.

Achieving Efficient NIR-II Type-I Photosensitizers for Photodynamic/Photothermal Therapy upon Regulating Chalcogen Elements

Second near-infrared (NIR-II) window type-I photosensitizers have intrinsic advantages in photodynamic/photothermal therapy (PDT/PTT) of some malignant tumors with deep infiltration, large size, complicated location, and low possibility of surgery/radiotherapy. Herein, three chalcogen-element-based donor-acceptor-type semiconducting polymers (poly[2,2″-((E)-4,4″-bis(2-octyldodecyl)-[6,6″-bithieno[3,2-b]pyrrolylidene]-5,5″(4H,4″H)-dione)-alt-2,5-(thiophene)] (PTS), poly[2,2″-((E)-4,4″-bis(2-octyldodecyl)-[6,6″-bithieno[3,2-b]pyrrolylidene]-5,5″(4H,4″H)-dione)-alt-2,5-(selenophene)] (PTSe), and poly[2,2″-((E)-4,4″-bis(2-octyldodecyl)-[6,6″-bithieno[3,2-b]pyrrolylidene]-5,5″(4H,4'H)-dione)-alt-2,5-(tellurophene)] (PTTe)) are synthesized and fully characterized, demonstrating strong absorption in the NIR-II region. Upon adjusting the chalcogen elements, the intramolecular charge-transfer characteristics and the heavy-atom effect are tuned to enhance the intersystem crossing rate, improving the photodynamic effect. Moreover, the energy levels and Gibbs free energies are tuned to facilitate the type-I photodynamic process. As a result, PTTe nanoparticles (NPs) produce superoxide anion radicals (O₂ ^•- ) more efficiently and demonstrate higher photothermal conversion efficiency than PTS and PTSe NPs upon NIR-II (1064 nm) laser irradiation, exhibiting unprecedented NIR-II type-I PDT/PTT performance in vitro and in vivo. This work provides ideas for achieving high-performance NIR-II type-I PDT/PTT semiconducting polymers for hypoxic oncotherapy.

Binary dimeric prodrug nanoparticles for self-boosted drug release and synergistic chemo-photodynamic therapy

Chemotherapy is the major strategy for cancer therapy, but its limited therapeutic efficiency and serious toxicity to normal tissues greatly restrict its clinical performance. Herein, we develop carrier-free self-activated prodrug nanoparticles combining chemotherapy and photodynamic therapy to enhance the antitumor efficiency. Reactive oxygen species (ROS)-responsive paclitaxel and porphyrin prodrugs are synthesized and co-assembled into nanoparticles without the addition of any adjuvants, which improves the drug content and reduces carrier-associated toxicity. After entering cancer cells, the obtained co-assembled nanoparticles can generate sufficient ROS upon light irradiation not only for photodynamic therapy, but also triggering on-demand drug release for chemotherapy, thus realizing self-enhanced prodrug activation and synergistic chemo-photodynamic therapy. This simple and effective carrier-free prodrug nanoplatform unifies the distinct traits of on-demand drug release and combination therapy, thus possessing great potential in advancing cancer treatment.

Emerging nanomedicines of paclitaxel for cancer treatment

Malignant tumor is still a leading threat to human health. Despite the rapid development of targeted therapeutic strategies, any treatment specifically acting on single target would inevitably suffer from tumor resistance, largely due to the genetic instability and variability of tumor cells. Thus, traditional therapies such as broad-spectrum chemotherapy would certainly occupy an important position in clinical cancer therapy. Nevertheless, most chemotherapeutic drugs have long been criticized for unsatisfactory therapeutic efficacy with severe off-target toxicity. Although several chemotherapeutic nanomedicines with improved therapeutic safety have been applied in clinics, the therapeutic outcomes still do not fulfill expectation. To address this challenge, enormous efforts have been devoted to developing novel nano-formulations for efficient delivery of chemotherapeutic drugs. Herein, we aim to outline the latest progression in the emerging nanomedicines of paclitaxel (PTX), with special attention to the functional nanocarriers, self-delivering prodrug-nanoassemblies and combination nanotherapeutics of PTX. Finally, the challenges and opportunities of these functional PTX nanomedicines in clinical translation are spotlighted.

Nanomedicine in Clinical Photodynamic Therapy for the Treatment of Brain Tumors

The current treatment for malignant brain tumors includes surgical resection, radiotherapy, and chemotherapy. Nevertheless, the survival rate for patients with glioblastoma multiforme (GBM) with a high grade of malignancy is less than one year. From a clinical point of view, effective treatment of GBM is limited by several challenges. First, the anatomical complexity of the brain influences the extent of resection because a fine balance must be struck between maximal removal of malignant tissue and minimal surgical risk. Second, the central nervous system has a distinct microenvironment that is protected by the blood–brain barrier, restricting systemically delivered drugs from accessing the brain. Additionally, GBM is characterized by high intra-tumor and inter-tumor heterogeneity at cellular and histological levels. This peculiarity of GBM-constituent tissues induces different responses to therapeutic agents, leading to failure of targeted therapies. Unlike surgical resection and radiotherapy, photodynamic therapy (PDT) can treat micro-invasive areas while protecting sensitive brain regions. PDT involves photoactivation of photosensitizers (PSs) that are selectively incorporated into tumor cells. Photo-irradiation activates the PS by transfer of energy, resulting in production of reactive oxygen species to induce cell death. Clinical outcomes of PDT-treated GBM can be advanced in terms of nanomedicine. This review discusses clinical PDT applications of nanomedicine for the treatment of GBM.

Intracellular Enzyme-Responsive Profluorophore and Prodrug Nanoparticles for Tumor-Specific Imaging and Precise Chemotherapy

Responsive drug delivery systems possess great potential in disease diagnosis and treatment. Herein, we develop an activatable prodrug and fluorescence imaging material by engineering the endogenous NAD(P)H:quinone oxidoreductase-1 (NQO1) responsive linker. The as-prepared nanomaterials possess the NQO1-switched drug release and fluorescence enablement, which realizes the tumor-specific chemotherapy and imaging in living mice. The enzyme-sensitive prodrug nanoparticles exhibit selectively potent anticancer performance to NQO1-positive cancer and ignorable off-target toxicity. This work provides an alternative strategy for constructing smart prodrug nanoplatforms with precision, selectivity, and practicability for advanced cancer imaging and therapy.

A redox-responsive dihydroartemisinin dimeric nanoprodrug for enhanced antitumor activity

Redox-responsive drug delivery system emerges as a hopeful platform for tumor treatment. Dihydroartemisinin (DHA) has been investigated as an innovative tumor therapeutic agent. Herein, a DHA dimeric prodrug bridged with disulfide bond as linker (DHA2-SS) has been designed and synthesized. The prepared prodrugs could self-assemble into nanoparticles (SS NPs) with high DHA content (> 90%) and robust stability. These SS NPs display sensitive redox responsive capability and can release DHA under the tumor heterogeneity microenvironment. SS NPs possess preferable antitumor therapeutic activity in contrast with free DHA. Moreover, the possible anti-cancer mechanism of SS NPs was investigated through RNA-seq analysis, bioinformatics and molecular biological method. SS NPs could induce apoptosis via mitochondrial apoptosis pathway, as well as glycolysis inhibition associate with the regulation of PI3K/AKT/HIF-1α signal path, which may offer an underlying therapeutic target for liver cancer. Our study highlights the potential of using redox responsive prodrug nanoparticles to treat cancer, meanwhile provides insights into the anti-cancer mechanism of DHA prodrug.

An Alkynyl-Dangling Ru(II) Polypyridine Complex for Targeted Antimicrobial Photodynamic Therapy

To realize clinical application of antibacterial photodynamic therapy (aPDT), one of the most arduous challenges is how to render aPDT agents high selectivity against bacterial pathogens. In light of the fact that amino group-containing lipids are rich on the outer surfaces of Gram-positive bacteria, we herein constructed an alkynyl-dangling ruthenium(II) polypyridine complex (Ru2) to preferentially label Staphylococcus aureus (S. aureus) and methicillin-resistant Staphylococcus aureus (MRSA) over mammalian cells via the amino-yne bio-orthogonal click reaction. Thanks to the strong singlet oxygen generation ability, Ru2 could photo-inactivate S. aureus and MRSA effectively and specifically. Phosphatidylethanolamine (PE) molecules also exist in mammalian cells but are not accessible for Ru2, leading to its poor binding/uptake and negligible cytotoxicity in the dark and upon irradiation towards mammalian cells as well as low hemolysis, all favorable for aPDT application.

A Sequential Dual-Model Strategy Based on Photoactivatable Metallopolymer for On-Demand Release of Photosensitizers and Anticancer Drugs

The synergistic combination of chemotherapy and photodynamic therapy has attracted considerable attention for its enhanced antitumoral effects; however, it remains challenging to successfully delivery photosensitizers and anticancer drugs while minimizing drug leakage at off-target sites. A red-light-activatable metallopolymer, Poly(Ru/PTX), is synthesized for combined chemo-photodynamic therapy. The polymer has a biodegradable backbone that contains a photosensitizer Ru complex and the anticancer drug paclitaxel (PTX) via a singlet oxygen (1 O2 ) cleavable linker. The polymer self-assembles into nanoparticles, which can efficiently accumulate at the tumor sites during blood circulation. The distribution of the therapeutic agents is synchronized because the Ru complex and PTX are covalently conjugate to the polymer, and off-target toxicity during circulation is also mostly avoided. Red light irradiation at the tumor directly cleaves the Ru complex and produces 1 O2 for photodynamic therapy. Sequentially, the generated 1 O2 triggers the breakage of the linker to release the PTX for chemotherapy. Therefore, this novel sequential dual-model release strategy creates a synergistic chemo-photodynamic therapy while minimizing drug leakage. This study offers a new platform to develop smart delivery systems for the on-demand release of therapeutic agents in vivo.

ROS-responsive dimeric prodrug-based nanomedicine targeted therapy for gastric cancer

Gastric cancer (GC) remains a major public health problem. Ursolic acid (UA) is reported to be effective in inhibiting GC; however, its low solubility and poor biocompatibility have greatly hindered its clinical application. Herein, an innovative reactive oxygen species (ROS)-sensitive UA dimeric prodrug is developed by coupling two UA molecules via a ROS-cleavable linkage, which can self-assemble into stable nanoparticles in the presence of surfactant. This new UA-based delivery system comprises the following major components: (I) dimeric prodrug inner core that can achieve high drug-loading (55%, w/w) and undergo rapid and selective conversion into intact drug molecules in response to ROS; (II) a polyethylene glycol (PEG) shell to improve colloid stability and extend blood circulation, and (III) surface-modified internalizing RGD (iRGD) to increase tumor targeting. Enhancement of the antitumor effect of this delivery system was demonstrated against GC tumors in vitro and in vivo. This novel approach offers the potential for clinical applications of UA.

Photoactivatable nanogenerators of reactive species for cancer therapy

In recent years, reactive species-based cancer therapies have attracted tremendous attention due to their simplicity, controllability, and effectiveness. Herein, we overviewed the state-of-art advance for photo-controlled generation of highly reactive radical species with nanomaterials for cancer therapy. First, we summarized the most widely explored reactive species, such as singlet oxygen, superoxide radical anion (O2 ●-), nitric oxide (●NO), carbon monoxide, alkyl radicals, and their corresponding secondary reactive species generated by interaction with other biological molecules. Then, we discussed the generating mechanisms of these highly reactive species stimulated by light irradiation, followed by their anticancer effect, and the synergetic principles with other therapeutic modalities. This review might unveil the advantages of reactive species-based therapeutic methodology and encourage the pre-clinical exploration of reactive species-mediated cancer treatments.

Hypoxia-responsive block copolymer polyprodrugs for complementary photodynamic-chemotherapy

The inherent hypoxic microenvironment of solid tumors has an important influence on tumor growth, distant metastasis, and invasiveness. The heterogeneous distribution of hypoxic regions inside tumors limits the therapeutic efficacy of O₂-assisted therapeutic strategy (e.g. photodynamic therapy (PDT)). On the other hand, the hypoxia-activable prodrugs cannot work effectively in the regions with enough O₂ concentration. To address the issues, we prepare a block copolymer polyprodrug consisting of polyethylene glycol (PEG) and copolymerized segments of nitroimidazole-linked camptothecin (CPT) methacrylate and 5,10,15,20-tetraphenylporphyrin (TPP)-containing methacrylate monomers for complementary photodynamic-chemotherapy. The polyprodrug can self-assemble into polymeric micelles in aqueous solution with suitable size and high stability. After intravenous injection, the polyprodrug micelles show tumor accumulation. Followed by light irradiation (650 nm) at tumor sites, TPP moieties induce singlet oxygen (¹O₂) production in the oxygen-rich area to exert PDT and cause transformation of the oxygen-rich areas into hypoxia. Simultaneously, in the hypoxic areas, the hypoxia-responsive polyprodrugs can be activated to release free CPT due to the cleavage of nitroimidazole linkages. The polyprodrug micelles with the segments for PDT and hypoxia-activable CPT efficiently suppress the growth of HeLa tumors. The well-defined polyprodrug amphiphiles offer an effective strategy to overcome the disadvantages of single treatment of PDT or hypoxia-responsive prodrugs for complementary photodynamic-chemotherapy of cancers.

Hypoxia-Responsive Polyprodrug Nanocarriers for Near-Infrared Light-Boosted Photodynamic Chemotherapy

The hypoxia environment inside tumors is tightly associated with tumor growth, metastasis, and drug resistance. However, the heterogonous distribution of hypoxic areas limits the efficacy of hypoxia-activatable drug delivery systems. Herein, we report the hypoxia-activable block copolymer polyprodrugs, which are composed of poly(ethylene glycol) (PEG) and copolymerized segments of ortho-nitrobenzyl-linked camptothecin (CPT) methacrylate and 2-(piperidin-1-yl)ethyl methacrylate (PEMA) monomers. After self-assembly in aqueous solution, indocyanine green (ICG) photosensitizers were encapsulated to formulate ICG-loaded micellar nanoparticles (ICG@CPTNB) for near-infrared (NIR) light-boosted photodynamic therapy (PDT), tumor hypoxia aggravation, and responsive drug activation. Through intravenous injection and prolonged blood circulation, the nanoparticles can accumulate into tumor efficiently. Tumor acidity-triggered charge transition of PEMA units remarkably promotes cellular internalization of the nanoparticles. Upon exposure to NIR laser irradiation, ICG inside the nanoparticles produced reactive oxygen species (ROS) along with local hypothermia. Simultaneously, the oxygen consumption during ROS production aggravated the intratumoral hypoxia, which amplified hypoxia-responsive self-immolative CPT release from the nanoparticles. The combined photodynamic chemotherapy using hypoxia-responsive polyprodrug nanoparticles, ICG@CPTNB, overcomes the limitations of single therapy of hypoxia-activable prodrugs or PDT, which remarkably improves the efficiency of tumor growth suppression.

Engineering Paclitaxel Prodrug Nanoparticles via Redox-Activatable Linkage and Effective Carriers for Enhanced Chemotherapy

The current clinical performance of chemotherapy is far from satisfactory, greatly limited by insufficient delivery efficacy and serious systemic side effects. Dimeric prodrug systems are emerging as valuable strategies for boosting the antitumor outcome. Here, dimeric paclitaxel prodrugs were synthesized with different bridged linkers, and the formed prodrug nanoparticles possessed excellent colloidal stability and ultrahigh drug content. The diselenide bond containing paclitaxel prodrugs could respond to a redox-heterogeneous intracellular microenvironment for on-demand drug release and subsequently show a selective cytotoxicity toward tumor cells against normal cells. Furthermore, the optimal carrier materials were screened out according to their contribution on stability, endocytosis, cytotoxicity, biodistribution, and antitumor efficacy. Compared with DSPE-PEG, human serum albumin, and Fe-tannic acid-based complex, F127 anchored dimeric paclitaxel nanoformulations exhibited preferential tumor accumulation and potent anticancer effect. Our present work provides deep insight into the development of advanced nanoformulations with comprehensive advantages for enhancing cancer therapy.

Time-programmed activation of dual polyprodrugs for synergistic cascade oxidation-chemotherapy

Combination therapy using multiple drugs with time-programmed administration is promising for enhanced cancer treatment. However, it is still challenging to achieve time-programmed drug release from a single nanocarrier. Here, dual polyprodrugs of hemicyanine dye (CyNH₂) and doxorubicin (DOX) are developed to achieve time-programmed prodrug activation for synergistic cascade oxidation therapy and chemotherapy. The polyprodrug NP_DOX/Cy, composed of CyNH₂, is modified with a glutathione (GSH)-responsive disulfate group, while DOX is modified with a reactive oxygen species (ROS)-response thioketal (TK) group. Upon uptake by cancer cells overexpressing GSH, CyNH₂ can be activated quickly and accumulate in the mitochondria to induce mitochondrial damage and ROS upregulation, thus achieving subsequent burst activation of DOX through the ROS-triggered cleavage of the TK linker. The early activation of CyNH₂ makes the cancer cells more sensitive to subsequent DOX treatment for a synergistic effect of from oxidation therapy and chemotherapy. Therefore, the polyprodrug with time-programmed drug activation developed in this work provides a promising strategy for synergistic cancer therapy.

Singlet Oxygen Generation in Dark-Hypoxia by Catalytic Microenvironment-Tailored Nanoreactors for NIR-II Fluorescence-Monitored Chemodynamic Therapy

Singlet oxygen (¹ O₂ ) has a potent anticancer effect, but photosensitized generation of ¹ O₂ is inhibited by tumor hypoxia and limited light penetration depth. Despite the potential of chemodynamic therapy (CDT) to circumvent these issues by exploration of ¹ O₂ -producing catalysts, engineering efficient CDT agents is still a formidable challenge since most catalysts require specific pH to function and become inactivated upon chelation by glutathione (GSH). Herein, we present a catalytic microenvironment-tailored nanoreactor (CMTN), constructed by encapsulating MoO₄ ^2- catalyst and alkaline sodium carbonate within liposomes, which offers a favorable pH condition for MoO₄ ^2- -catalyzed generation of ¹ O₂ from H₂ O₂ and protects MoO₄ ^2- from GSH chelation owing to the impermeability of liposomal lipid membrane to ions and GSH. H₂ O₂ and ¹ O₂ can freely cross the liposomal membrane, allowing CMTN with a built-in NIR-II ratiometric fluorescent ¹ O₂ sensor to achieve monitored tumor CDT.

Nanoscale Covalent Organic Frameworks with Donor-Acceptor Structure for Enhanced Photothermal Ablation of Tumors

Covalent organic frameworks (COFs) have shown great potential in catalysis and the biomedical fields, but monodisperse COFs with tunable sizes are hard to obtain. Herein, we successfully developed a series of COFs based on electron donor-acceptor strategy in mild conditions. The synthetic COFs exhibit excellent colloidal stability with uniform spherical morphology. The sizes can be flexibly adjusted by the amount of catalyst, and the absorption spectra also vary with the sizes. By changing the electron-donating ability of the monomers, the corresponding COFs possess a wide range of absorption spectra, which can be even extended to the second near-infrared biowindow. The obtained COFs possess potent photothermal activity under laser irradiation, and could inhibit the growth of tumors effectively. This work provides a strategy for the synthesis of monodisperse COFs with variable absorption for their potential applications.

Stimulus-cleavable chemistry in the field of controlled drug delivery

Stimulus-cleavable nanoscale drug delivery systems are receiving significant attention owing to their capability of achieving exquisite control over drug release via the exposure to specific stimuli. Central to the construction of such systems is the integration of cleavable linkers showing susceptibility to one stimulus or several stimuli with drugs, prodrugs or fluorogenic probes on the one hand, and nanocarriers on the other hand. This review summarises recent advances in stimulus-cleavable linkers from various research areas and the corresponding mechanisms of linker cleavage and biological applications. The feasibility of extending their applications to the majority of nanoscale drug carriers including nanomaterials, polymers and antibodies are further highlighted and discussed. This review also provides general design guidelines to incorporate stimulus-cleavable linkers into nanocarrier-based drug delivery systems, which will hopefully spark new ideas and applications.

Emerging nanotaxanes for cancer therapy

The clinical application of taxane (including paclitaxel, docetaxel, and cabazitaxel)-based formulations is significantly impeded by their off-target distribution, unsatisfactory release, and acquired resistance/metastasis. Recent decades have witnessed a dramatic progress in the development of high-efficiency, low-toxicity nanotaxanes via the use of novel biomaterials and nanoparticulate drug delivery systems (nano-DDSs). Thus, in this review, the achievements of nanotaxanes-targeted delivery and stimuli-responsive nano-DDSs-in preclinical or clinical trials have been outlined. Then, emerging nanotherapeutics against tumor resistance and metastasis have been overviewed, with a particular emphasis on synergistic therapy strategies (e.g., combination with surgery, chemotherapy, radiotherapy, biotherapy, immunotherapy, gas therapy, phototherapy, and multitherapy). Finally, the latest oral nanotaxanes have been briefly discussed.

Recent progress in nanoformulations of cabazitaxel

The antitumor efficacy of various paclitaxel (PTX) and docetaxel (DTX) formulations in clinical applications is seriously affected by drug resistance. Cabazitaxel, a second-generation taxane, exhibits greater anticancer activity than paclitaxel and docetaxel and has low affinity for the P-glycoprotein (P-gp) efflux pump because of its structure. Therefore, cabazitaxel has the potential to overcome taxane resistance. However, owing to the high systemic toxicity and hydrophobicity of cabazitaxel and the instability of its commercial preparation, Jevtana®, the clinical use of cabazitaxel is restricted to patients with metastatic castration-resistant prostate cancer (mCRPC) who show progression after docetaxel-based chemotherapy. Nanomedicine is expected to overcome the limitations associated with cabazitaxel application and surmount taxane resistance. This review outlines the drug delivery systems of cabazitaxel published in recent years, summarizes the challenges faced in the development of cabazitaxel nanoformulations, and proposes strategies to overcome these challenges.

Current Limitations and Recent Progress in Nanomedicine for Clinically Available Photodynamic Therapy

Photodynamic therapy (PDT) using oxygen, light, and photosensitizers has been receiving great attention, because it has potential for making up for the weakness of the existing therapies such as surgery, radiation therapy, and chemotherapy. It has been mainly used to treat cancer, and clinical tests for second-generation photosensitizers with improved physicochemical properties, pharmacokinetic profiles, or singlet oxygen quantum yield have been conducted. Progress is also being made in cancer theranostics by using fluorescent signals generated by photosensitizers. In order to obtain the effective cytotoxic effects on the target cells and prevent off-target side effects, photosensitizers need to be localized to the target tissue. The use of nanocarriers combined with photosensitizers can enhance accumulation of photosensitizers in the tumor site, owing to preferential extravasation of nanoparticles into the tumor vasculature by the enhanced permeability and retention effect. Self-assembly of amphiphilic polymers provide good loading efficiency and sustained release of hydrophobic photosensitizers. In addition, prodrug nanomedicines for PDT can be activated by stimuli in the tumor site. In this review, we introduce current limitations and recent progress in nanomedicine for PDT and discuss the expected future direction of research.

An activatable fluorescent prodrug of paclitaxel and BODIPY

A redox-activated paclitaxel prodrug (PTX-S-BDP) was synthesized. PTX-S-BDP NPs were fabricated by the coassembly of PTX-S-BDP with F-127, which can release PTX under redox conditions and exhibit superior cellular imaging and selectivity to cancer cells.