Drug Delivery Systems with a "Tumor-Triggered" Targeting or Intracellular Drug Release Property Based on DePEGylation

Coating nanosized anticancer drug delivery systems (DDSs) with poly(ethylene glycol) (PEG), the so-called PEGylation, has been proven an effective method to enhance hydrophilicity, aqueous dispersivity, and stability of DDSs. What is more, as PEG has the lowest level of protein absorption of any known polymer, PEGylation can reduce the clearance of DDSs by the mononuclear phagocyte system (MPS) and prolong their blood circulation time in vivo. However, the "stealthy" characteristic of PEG also diminishes the uptake of DDSs by cancer cells, which may reduce drug utilization. Therefore, dynamic protection strategies have been widely researched in the past years. Coating DDSs with PEG through dynamic covalent or noncovalent bonds that are stable in blood and normal tissues, but can be broken in the tumor microenvironment (TME), can achieve a DePEGylation-based "tumor-triggered" targeting or intracellular drug release, which can effectively improve the utilization of drugs and reduce their side effects. In this review, the stimuli and methods of "tumor-triggered" targeting or intracellular drug release, based on DePEGylation, are summarized. Additionally, the targeting and intracellular controlled release behaviors of the DDSs are briefly introduced.

Zwitterionic Block Copolymer Prodrug Micelles for pH Responsive Drug Delivery and Hypoxia-Specific Chemotherapy

Tirapazamine (TPZ) and its derivatives (TPZD) have shown their great potential for efficiently killing hypoxic cancer cells. However, unsatisfactory clinical outcomes resulting from the low bioavailability of the low-molecular TPZ and TPZD limited their further applications. Precise delivery and release of these prodrugs via functional nanocarriers can significantly improve the therapeutic effects due to the targeted drug delivery and enhanced permeability and retention (EPR) effect. Herein, zwitterionic block copolymer (BCP) micelles with aldehyde functional groups are prepared from the self-assembly of poly(2-methacryloyloxyethyl phosphorylcholine-b-poly(di(ethylene glycol) methyl ether methacrylate-co-4-formylphenyl methacrylate) [PMPC-b-P(DEGMA-co-FPMA)]. TPZD is then grafted onto PMPC-b-P(DEGMA-co-FPMA) to obtain a polymer-drug conjugate, PMPC-b-P(DEGMA-co-FPMA-g-TPZD) (BCP-TPZ), through the formation of a pH-responsive imine bond, exhibiting a pH-dependent drug release profile owing to the cleavage of the imine bond under acidic conditions. Outstandingly, BCP-TPZ shows around 13.7-fold higher cytotoxicity to hypoxic cancer cells in comparison to normoxic cancer cells evaluated through an in vitro cytotoxicity assay. The pH-responsiveness and hypoxia-specific cytotoxicity confer BCP-TPZ micelles a great potential to achieve precise delivery of TPZD and thus enhance the therapeutic effect toward tumor-hypoxia.

Recent developments in selenium-containing polymeric micelles: prospective stimuli, drug-release behaviors, and intrinsic anticancer activity

Selenium is capable of forming a dynamic covalent bond with itself and other elements and can undergo metathesis and regeneration reactions under optimum conditions. Its dynamic nature endows selenium-containing polymers with striking sensitivity towards some environmental alterations. In the past decade, several selenium-containing polymers were synthesized and used for the preparation of oxidation-, reduction-, and radiation-responsive nanocarriers. Recently, thioredoxin reductase, sonication, and osmotic pressure triggered the cleavage of Se-Se bonds and swelling or disassembly of nanostructures. Moreover, some selenium-containing nanocarriers form oxidation products such as seleninic acids and acrylates with inherent anticancer activities. Thus, selenium-containing polymers hold promise for the fabrication of ultrasensitive and multifunctional nanocarriers of radiotherapeutic, chemotherapeutic, and immunotherapeutic significance. Herein, we discuss the most recent developments in selenium-containing polymeric micelles in light of their architecture, multiple stimuli-responsive properties, emerging immunomodulatory activities, and future perspectives in the delivery and controlled release of anticancer agents.

Integrin αvβ3-targeted liposomal drug delivery system for enhanced lung cancer therapy

Conventional chemotherapy for tumor treatment remains flawed because it fails to limit cytotoxicity to a small set of selectable tissues. Active targeting techniques for the delivery of drugs to specific sites are increasingly used to enhance drug accumulation at tumor sites with the aim of reducing side effects in vivo. Liposomes, modified with different targeting ligands, are considered to be one of the most promising targeted drug carriers. Herein, novel linear and cyclic arginine-glycine-aspartate (RGD) peptide-based lipids were synthesized to develop modified liposomal drug delivery systems with active targeting and pH-sensitivity. The RGD-modified liposomes showed excellent active targeting ability for integrin α_vβ₃ receptors, resulting in improved cellular uptake. The modified liposomes also enhanced intracellular doxorubicin (DOX) release because of their degradation in an acidic environment. Consequently, the RGD-modified, DOX-loaded liposomes exhibited significant antitumor efficacy and low toxicity in vitro and in vivo. In particular, 5% cRGD-lipid modified DOX-loaded liposome showed the greatest inhibition of tumor growth in mice among the tested formulations, and much less toxicity than free DOX. In conclusion, the DOX-loaded pH-sensitive liposome modified with 5% cRGD-lipid developed in the current study provides a potential approach for improved tumor therapy.

Sequential post-polymerization modification of a pentafluorophenyl ester-containing homopolymer: a convenient route to effective pH-responsive nanocarriers for anticancer drugs

Recently, pH-responsive polymeric micelles have gained significant attention as effective carriers for anti-cancer drug delivery. Herein, pH-responsive polymeric micelles were constructed by a simple post-polymerization modification of a single homopolymer, poly(pentafluorophenyl acrylate) (PPFPA). The PPFPA was first subjected to modification with 1-amino-2-propanol yielding the amphiphilic copolymer of poly(pentafluorophenyl acrylate)-ran-poly(N-(2-hydroxypropyl acrylamide)). A series of amphiphilic random copolymers of different compositions could self-assemble into spherical micelles with a unimodal size distribution in aqueous solution. Then, 1-(3-aminopropyl)imidazole (API), a reagent to introduce charge conversional entities, was reacted with the remaining PPFPA segment in the micellar core resulting in API-modified micelles which can encapsulate doxorubicin (DOX), a hydrophobic anti-cancer drug. As monitored by dynamic light scattering, the API-modified micelles underwent disintegration upon pH switching from 7.4 to 5.0, presumably due to imidazolyl group protonation. This pH-responsiveness of the API-modified micelles was responsible for the faster and greater in vitro DOX release in an acidic environment than neutral pH. Cellular uptake studies revealed that the developed carriers were internalized into MDA-MB-231 cells within 30 min via endocytosis and exhibited cytotoxicity in a dose-dependent manner.

CD44 targeted redox-triggered self-assembly with magnetic enhanced EPR effects for effective amplification of gambogic acid to treat triple-negative breast cancer

Gambogic acid (GA) is a natural anti-tumor drug whose application is restricted by its poor aqueous solubility and inefficient bioavailability. Developing nanomaterials with excellent biocompatibility can amplify the therapeutic effects of GA. In this study, a tumor-targeted redox controllable self-assembled nano-system with magnetic enhanced EPR effects (mPEG-HA/CSO-SS-Hex/SPION/GA) was developed to improve the anticancer efficacy of GA. The nano-system is constituted by three layers: the outer layer is mono-aminated poly(ethylene glycol) grafted hyaluronic acid (mPEG-HA), which can target the CD44 receptor in breast cancer cells; the middle layer consists of disulfide linked hexadecanol (Hex) and chitosan oligosaccharide (CSO) to control the drug release by reduction response; the core layer is superparamagnetic iron oxide nanoparticles (SPION), which can enhance the EPR effect by magnetic guidance and contribute to GA entrapment. Different experiments were performed to characterize the complex self-assembly, and the cytotoxicity, pharmacokinetics, and in vivo antitumor activity of the self-assembly were investigated to evaluate its anti-tumor effects. The results revealed that mPEG-HA/CSO-SS-Hex/SPION/GA is an excellent nanosystem with appropriate size and sensitive responsiveness; it can accumulate in tumor sites and achieve excellent therapeutic effects on triple-negative breast cancer (TNBC). In summary, a CD44-targeted redox-triggered self-assembly nanosystem with magnetic enhanced EPR effects was developed for effective amplification of GA; it has potential to act as an effective carrier in drug delivery for chemotherapy of TNBC.

Synthesis of polymeric micelles with dual-functional sheddable PEG stealth for enhanced tumor-targeted drug delivery

We reported the synthesis of polymeric micelles with a dual-functional sheddable mPEG stealth to realize simultaneously tumor-triggered targeting and intracellular micelle destabilization for enhanced anticancer drug delivery.

Hyaluronic Acid-Coated Nanomedicine for Targeted Cancer Therapy

Hyaluronic acid (HA) has been widely investigated in cancer therapy due to its excellent characteristics. HA, which is a linear anionic polymer, has biocompatibility, biodegradability, non-immunogenicity, non-inflammatory, and non-toxicity properties. Various HA nanomedicines (i.e., micelles, nanogels, and nanoparticles) can be prepared easily using assembly and modification of its functional groups such as carboxy, hydroxy and N-acetyl groups. Nanometer-sized HA nanomedicines can selectively deliver drugs or other molecules into tumor sites via their enhanced permeability and retention (EPR) effect. In addition, HA can interact with overexpressed receptors in cancer cells such as cluster determinant 44 (CD44) and receptor for HA-mediated motility (RHAMM) and be degraded by a family of enzymes called hyaluronidase (HAdase) to release drugs or molecules. By interaction with receptors or degradation by enzymes inside cancer cells, HA nanomedicines allow enhanced targeting cancer therapy. In this article, recent studies about HA nanomedicines in drug delivery systems, photothermal therapy, photodynamic therapy, diagnostics (because of the high biocompatibility), colloidal stability, and cancer targeting are reviewed for strategies using micelles, nanogels, and inorganic nanoparticles.

DePEGylation strategies to increase cancer nanomedicine efficacy

To maximize drug targeting to solid tumors, cancer nanomedicines with prolonged circulation times are required. To this end, poly(ethylene glycol) (PEG) has been widely used as a steric shield of nanomedicine surfaces to minimize serum protein absorption (opsonisation) and subsequent recognition and clearance by cells of the mononuclear phagocyte system (MPS). However, PEG also inhibits interactions of nanomedicines with target cancer cells, limiting the effective drug dose that can be reached within the target tumor. To overcome this dilemma, nanomedicines with stimuli-responsive cleavable PEG functionality have been developed. These benefit from both long circulation lifetimes en route to the targeted tumor as well as efficient drug delivery to target cancer cells. In this review, various stimuli-responsive strategies to dePEGylate nanomedicines within the tumor microenvironment will be critically reviewed.

RGD-modified polymer and liposome nanovehicles: Recent research progress for drug delivery in cancer therapeutics

Over the past few decades, as the demand for cancer treatment has increased, more rational treatment options (considering size, mode of administration, biocompatibility, efficacy, etc.) and plenty of specifically active targeted nanovehicles have been developed. Integrin receptors targeting are one of the most frequently used approaches because of its highly expressed in cancer cells. In particular, the arginine-glycine-aspartic acid (RGD) peptide and its derivatives have been widely used as ligands for integrin to increase direct targeting capabilies. Polymers as well as liposomes are commonly used as nanovehicles for drug delivery. A variety of work is focused on the RGD-modified polymer and liposome nanovehicles for cancer therapeutics. The goal of this article is to review the published literature in recent years concerning the RGD-modified liposome and polymer nanovehicles to highlight its successful designs for improving cancer therapy and discuss the current challenges as well as the possible development prospects.

Responsive Hybrid (Poly)peptide-Polymer Conjugates

(Poly)peptide-polymer conjugates continue to garner significant interest in the production of functional materials given their composition of natural and synthetic building blocks that confer select and synergistic properties. Owing to opportunities to design predefined architectures and structures with different morphologies, these hybrid conjugates enable new approaches for producing micro- or nanomaterials. Their modular design enables the incorporation of multiple responsive properties into a single conjugate. This review presents recent advances in (poly)peptide-polymer conjugates for drug-delivery applications, with a specific focus on the utility of the (poly)peptide component in the assembly of particles and nanogels, as well as the role of the peptide in triggered drug release.

Acetal-linked PEGylated paclitaxel prodrugs forming free-paclitaxel-loaded pH-responsive micelles with high drug loading capacity and improved drug delivery

Endosomal pH-responsive micellar nanoparticles were prepared by self-assembly of an amphiphilic poly(ethylene glycol)-acetal-paclitaxel (PEG-acetal-PTX) prodrug, and free PTX could be encapsulated in the hydrophobic core of the nanoparticles. These nanoparticles exhibited excellent storage stability for over 6months under normal conditions, but disassembled quickly in response to faintly acidic environment. Incorporating physical encapsulation and chemical conjugation, the PTX concentration in the nanoparticles solution could reach as high as 3665μg/mL, accompanying with a high drug loading capacity of 60.3%. Additionally, benefitting from the difference in drug release mechanism and rate between encapsulated PTX and conjugated PTX, a programmed drug release behavior was observed, which may result in higher intracellular drug concentration and longer action time. CCK-8 assays showed that the nanoparticles demonstrated superior antitumor activity than free PTX against both HeLa and MDA-MB-231 cells. These prodrug-based nanomedicines have a great potential in developing translational PTX formulations for cancer therapy.

Targeted glioma chemotherapy by cyclic RGD peptide-functionalized reversibly core-crosslinked multifunctional poly(ethylene glycol)-b-poly(ε-caprolactone) micelles

Cyclic RGD peptide-functionalized reversibly core-crosslinked biodegradable poly(ethylene glycol)-b-poly(ε-caprolactone) (PEG-PCL) micelles (cRGD-RCCMs) were designed and developed for highly potent and targeted glioma chemotherapy. To achieve crosslinkable core, dithiolane-functionalized trimethylene carbonate (DTC) was incorporated into PCL block. Interestingly, cRGD-RCCMs displayed a high doxorubicin (DOX) loading content of ∼18wt%, small hydrodynamic size of ∼50nm, and excellent colloidal stability with minimum drug leakage under physiological conditions while fast DOX release under cytoplasmic-mimicking reductive environments. MTT, confocal microscopy and flow cytometry measurement results pointed out that cRGD-RCCMs with 30% cRGD surface density (cRGD30-RCCMs) showed an evident selectivity, efficient cytoplasmic drug release, and superior antitumor activity to clinically used pegylated liposomal doxorubicin (DOX-LPs) in α_vβ₃ integrin overexpressing U87MG glioblastoma cells. Strikingly, DOX-loaded cRGD30-RCCMs demonstrated a prolonged circulation time showing an elimination half-life of ∼4.7h, three times exceeding that of the non-crosslinked counterparts, and a remarkably enhanced tumor accumulation of 7.7%ID/g. Furthermore, in vivo therapeutic studies revealed that DOX-loaded cRGD30-RCCMs effectively suppressed tumor growth, significantly prolonged survival time, and lessened side effects in subcutaneous U87MG glioblastoma-bearing nude mice. These reversibly core-crosslinked multifunctional biodegradable micelles might be developed into advanced and clinically viable targeted anticancer nanomedicines.

STATEMENT OF SIGNIFICANCE

Nanomedicines based on biodegradable micelles and nanoparticles offer a most promising treatment for malignant tumors. The therapeutic outcomes of current nanomedicines are, however, trimmed by their instability, low tumor retention, inefficient tumor cell uptake, and inferior drug release control. We report herein that cRGD-functionalized, rapidly glutathione-responsive, and reversibly core-crosslinked biodegradable micellar doxorubicin based on PEG-PCL block copolymer mediates potent and targeted glioma chemotherapy, affording significantly better treatment efficacy and lower systemic toxicity than the non-crosslinked micellar doxorubicin and clinically used pegylated liposomal doxorubicin controls. These reversibly core-crosslinked multifunctional biodegradable micelles have emerged as a robust, simple, versatile, and safe nanoplatform that might elegantly bridge the gap between the scientific and translational anticancer nanomedicine research.

Enhanced acaricidal activity of ricinine achieved by the construction of nano-formulation using amphiphilic block copolymer

Amphiphilic block copolymer PEO–PCL improves the encapsulation of ricinine and enhances the acaricidal efficiency of the pesticide on V. unguiculata (L.) when compared to the formulations made by surfactants.

A Simple Dual-pH Responsive Prodrug-Based Polymeric Micelles for Drug Delivery

To precisely deliver drug molecules at a targeted site and in a controllable manner, there has been great interest in designing a synergistical drug delivery system that can achieve both surface charge-conversion and controlled release of a drug in response to different stimuli. Here we outline a simple method to construct an intelligent drug carrier, which can respond to two different pH values, therefore achieving charge conversion and chemical-bond-cleavage-induced drug release in a stepwise fashion. This drug carrier comes from the self-assembly of a block copolymer-DOX conjugate synthesized through a Schiff base reaction between poly(2-(diisopropylamino)ethyl methacrylate-b-poly(4-formylphenyl methacrylate-co-polyethylene glycol monomethyl ether methacrylate) (PDPA-b-P(FPMA-co-OEGMA)) and DOX. The surface charge of the BCP-DOX micelles reversed from negative to positive when encountering a weakly acidic environment due to the protonation of PDPA segments. In vitro cellular uptake measurement shows that the cellular uptake and internalization of the BCP-DOX micelles can be significantly enhanced at pH ∼ 6.5. Moreover, this drug carrier exhibits a pH-dependent drug release owing to the cleavage of the imine bond at pH < 5.5. With this dual-pH responsive feature, these micelles may have the ability to precisely deliver DOX to the cancer cells.

Thermo-Controlled in Situ Phase Transition of Polymer-Peptides on Cell Surfaces for High-Performance Proliferative Inhibition

We herein report a thermocontrolled strategy for realizing in situ conformational transition of polymer-peptide conjugates at cell surfaces to manipulate and monitor HER2 receptor clustering, which finally result in effective breast cancer cell proliferation inhibition. Functional paring motifs (HBP) are covalently linked to a synthetic thermoresponsive polymer PNIPAAm to incorporate temperature control properties to HER2 targeting peptide. At 40 °C, the PNIPAAm polymers collapse and act as a "shield" to block the aggregation of HBP. Upon cooling to 35 °C, polymers are in their extended state and HBP are expose in aqueous and aggregate subsequently with enhanced fluorescence, allowing for promoting and in situ monitoring of receptor clustering. Ultimately, HER2 receptor clustering leads to cytoplasmic domain phosphorylation, which further results in effective cancer cell proliferation inhibition. We envision that this useful approach has the potential to be applied for molecule-targeted tumor therapy.

PEG based anti-cancer drug conjugated prodrug micelles for the delivery of anti-cancer agents

Development of polymer prodrug conjugates has evolved recently in the nano-medicine field for cancer diagnosis and treatment. This review focuses on the development of different types of PEG based polymer drug conjugates used for the delivery of anti-cancer agents.

Controlled biodegradation of polymers using nanoparticles and its application

Controlled biodegradation mechanism has been revealed using different nanoparticles which eventually regulate pH of media.