Synthesis and Characterization of Biodegradable and Thermosensitive Polymeric Micelles with Covalently Bound Doxorubicin-Glucuronide Prodrug via Click Chemistry

Versatile Click Linker Enabling Native Peptide Release from Nanocarriers upon Redox Trigger

Nanocarriers have shown their ability to extend the circulation time of drugs, enhance tumor uptake, and tune drug release. Therapeutic peptides are a class of drug compounds in which nanocarrier-mediated delivery can potentially improve their therapeutic index. To this end, there is an urgent need for orthogonal covalent linker chemistry facilitating the straightforward on-the-resin peptide generation, nanocarrier conjugation, as well as the triggered release of the peptide in its native state. Here, we present a copper-free clickable ring-strained alkyne linker conjugated to the N-terminus of oncolytic peptide LTX-315 via standard solid-phase peptide synthesis (SPPS). The linker contains (1) a recently developed seven-membered ring-strained alkyne, 3,3,6,6-tetramethylthiacycloheptyne sulfoximine (TMTHSI), (2) a disulfide bond, which is sensitive to the reducing cytosolic and tumor environment, and (3) a thiobenzyl carbamate spacer enabling release of the native peptide upon cleavage of the disulfide via 1,6-elimination. We demonstrate convenient "clicking" of the hydrophilic linker-peptide conjugate to preformed pegylated core-cross-linked polymeric micelles (CCPMs) of 50 nm containing azides in the hydrophobic core under aqueous conditions at room temperature resulting in a loading capacity of 8 mass % of peptide to polymer (56% loading efficiency). This entrapment of hydrophilic cargo into/to a cross-linked hydrophobic core is a new and counterintuitive approach for this class of nanocarriers. The release of LTX-315 from the CCPMs was investigated in vitro and rapid release upon exposure to glutathione (within minutes) followed by slower 1,6-elimination (within an hour) resulted in the formation of the native peptide. Finally, cytotoxicity of LTX CCPMs as well as uptake of sulfocyanine 5-loaded CCPMs was investigated by cell culture, demonstrating successful tumor cell killing at concentrations similar to that of the free peptide treatment.

Glycosidase activated prodrugs for targeted cancer therapy

In this review glycosidase activated prodrugs that target cancer cells are discussed.

Amphiphilic Triblock Copolymer Prodrug for Tumor-Specific pH/Reduction Dual-Triggered Drug Delivery: Effect of Self-Assembly Behaviors

Diblock copolymer-based prodrugs have been widely designed for tumor treatment after self-assembly; however, premature drug leakage could not be ignored because their hydrophobic prodrug cores were directly exposed to the media. Here, an amphiphilic triblock copolymer prodrug with a hydrophilic PEG block, a pH-sensitive poly(2-(diisopropylamino)ethyl methacrylate) (PDPA) block, and a hydrophobic reduction-cleavable prodrug block was synthesized for tumor-specific pH/reduction dual-triggered drug delivery, via the successive RAFT polymerization of DPA and a DOX-based monomer (MAL-DOX) with a PEG-based macro-CTA. The core-shell and core-shell-corona nanoparticles could be obtained by one-step and two-step self-assembly. With the pH-sensitive gatekeeper formed by the PDPA block, the core-shell-corona nanoparticles possessed a smaller diameter with narrow distribution and better drug release with lower drug leakage. MTT assays demonstrated the selective cytotoxicity of the core-shell-corona nanoparticles to the cancer cells was dose-dependent because of the reduction-cleavable prodrug. The negligible drug leakage and selective cytotoxicity to cancer cells endow the proposed core-shell-corona prodrug nanoparticles with promising potential for tumor treatment without toxic side effects on the normal cells.

Recent Advancements in Stimuli Responsive Drug Delivery Platforms for Active and Passive Cancer Targeting

The tumor-specific targeting of chemotherapeutic agents for specific necrosis of cancer cells without affecting the normal cells poses a great challenge for researchers and scientists. Though extensive research has been carried out to investigate chemotherapy-based targeted drug delivery, the identification of the most promising strategy capable of bypassing non-specific cytotoxicity is still a major concern. Recent advancements in the arena of onco-targeted therapies have enabled safe and effective tumor-specific localization through stimuli-responsive drug delivery systems. Owing to their promising characteristic features, stimuli-responsive drug delivery platforms have revolutionized the chemotherapy-based treatments with added benefits of enhanced bioavailability and selective cytotoxicity of cancer cells compared to the conventional modalities. The insensitivity of stimuli-responsive drug delivery platforms when exposed to normal cells prevents the release of cytotoxic drugs into the normal cells and therefore alleviates the off-target events associated with chemotherapy. Contrastingly, they showed amplified sensitivity and triggered release of chemotherapeutic payload when internalized into the tumor microenvironment causing maximum cytotoxic responses and the induction of cancer cell necrosis. This review focuses on the physical stimuli-responsive drug delivery systems and chemical stimuli-responsive drug delivery systems for triggered cancer chemotherapy through active and/or passive targeting. Moreover, the review also provided a brief insight into the molecular dynamic simulations associated with stimuli-based tumor targeting.

Stimulus-cleavable chemistry in the field of controlled drug delivery

This review comprehensively summarises stimulus-cleavable linkers from various research areas and their cleavage mechanisms, thus provides an insightful guideline to extend their potential applications to controlled drug release from nanomaterials.

A Doxorubicin-Glucuronide Prodrug Released from Nanogels Activated by High-Intensity Focused Ultrasound Liberated β-Glucuronidase

The poor pharmacokinetics and selectivity of low-molecular-weight anticancer drugs contribute to the relatively low effectiveness of chemotherapy treatments. To improve the pharmacokinetics and selectivity of these treatments, the combination of a doxorubicin-glucuronide prodrug (DOX-propGA3) nanogel formulation and the liberation of endogenous β-glucuronidase from cells exposed to high-intensity focused ultrasound (HIFU) were investigated in vitro. First, a DOX-propGA3-polymer was synthesized. Subsequently, DOX-propGA3-nanogels were formed from this polymer dissolved in water using inverse mini-emulsion photopolymerization. In the presence of bovine β-glucuronidase, the DOX-propGA3 in the nanogels was quantitatively converted into the chemotherapeutic drug doxorubicin. Exposure of cells to HIFU efficiently induced liberation of endogenous β-glucuronidase, which in turn converted the prodrug released from the DOX-propGA3-nanogels into doxorubicin. β-glucuronidase liberated from cells exposed to HIFU increased the cytotoxicity of DOX-propGA3-nanogels to a similar extend as bovine β-glucuronidase, whereas in the absence of either bovine β-glucuronidase or β-glucuronidase liberated from cells exposed to HIFU, the DOX-propGA3-nanogels hardly showed cytotoxicity. Overall, DOX-propGA3-nanogels systems might help to further improve the outcome of HIFU-related anticancer therapy.

Targeted delivery of hyaluronic acid nanomicelles to hepatic stellate cells in hepatic fibrosis rats

Hepatic fibrosis is one kind of liver diseases with a high mortality rate and incidence. The activation and proliferation of hepatic stellate cells (HSCs) is the most fundamental reason of hepatic fibrosis. There are no specific and effective drug delivery carriers for the treatment of hepatic fibrosis at present. We found that when hepatic fibrosis occurs, the expression of CD44 receptors on the surface of HSCs is significantly increased. Based on this finding, we designed silibinin-loaded hyaluronic acid (SLB-HA) micelles to achieve the treatment of hepatic fibrosis. Meanwhile, we constructed liver fibrosis rat model using Sprague–Dawley rats. We demonstrated that HA micelles had specific uptake to HSCs in vitro while avoiding the distribution in normal liver cells and the phagocytosis of macrophages. Importantly, HA micelles showed a significant liver targeting effect in vivo, especially in fibrotic liver which highly expressed CD44 receptors. In addition, SLB-HA micelles could selectively kill activated HSCs, having an excellent anti-hepatic fibrosis effect in vivo and a significant sustained release effect, and also had a good biological safety and biocompatibility. Overall, HA micelles represented a novel nanomicelle system which showed great potentiality in anti-hepatic fibrosis drugs delivery.

Heterofunctional Poly(ethylene glycol) (PEG) Macroinitiator Enabling Controlled Synthesis of ABC Triblock Copolymers

ABC triblock copolymers with a poly(ethylene glycol) (PEG) midblock have attractive properties for biomedical applications because of PEG's favorable properties regarding biocompatibility and hydrophilicity. However, easy strategies to synthesize polymers containing a PEG midblock are limited. In this study, the successful synthesis of a heterofunctional PEG macroinitiator containing both an azoinitiator and an atom transfer radical polymerization (ATRP) initiator is demonstrated. This novel PEG macroinitiator allows the development of elegant synthesis routes for PEG midblock-containing ABC copolymers that does not require protection of initiating sites or polymer end-group postmodification. Polymers with outer blocks composed of different monomers were synthesized to illustrate the versatility of this macroinitiator. N-Isopropylacrylamide (NIPAM) was included to obtain thermosensitive polymers, 2-(dimethylamino)ethyl methacrylate (DMAEMA) provided pH-sensitive properties, and 2-hydroxyethyl acrylate (HEA) functioned as a noncharged hydrophilic block that also allows for postmodifications reactions. This synthesis approach can further contribute to the design of high-precision polymers with tailorable block compositions and polymer topologies, which is highly attractive for applications in nanotechnology.

Overcoming multidrug resistance using folate receptor-targeted and pH-responsive polymeric nanogels containing covalently entrapped doxorubicin

Multidrug resistance (MDR) contributes to failure of chemotherapy. We here show that biodegradable polymeric nanogels are able to overcome MDR via folic acid targeting. The nanogels are based on hydroxyethyl methacrylamide-oligoglycolates-derivatized poly(hydroxyethyl methacrylamide-co-N-(2-azidoethyl)methacrylamide) (p(HEMAm-co-AzEMAm)-Gly-HEMAm), covalently loaded with the chemotherapeutic drug doxorubicin (DOX) and subsequently decorated with a folic acid-PEG conjugate via copper-free click chemistry. pH-Responsive drug release is achieved via the acid-labile hydrazone bond between DOX and the methacrylamide polymeric network. Cellular uptake and cytotoxicity analyses in folate receptor-positive B16F10 melanoma versus folate receptor-negative A549 lung carcinoma cells confirmed specific uptake of the targeted nanogels. Confocal microscopy demonstrated efficient internalization, lysosomal trafficking, drug release and nuclear localization of DOX. We also show that DOX resistance in 4T1 breast cancer cells results in upregulation of the folate receptor, and that folic acid targeted nanogels can be employed to bypass drug efflux pumps, resulting in highly efficient killing of resistant cancer cells. In conclusion, folic acid functionalized nanogels with pH-controlled drug release seem to hold significant potential for treating multidrug resistant malignancies.

Mechanistic applications of click chemistry for pharmaceutical drug discovery and drug delivery

The concept of click chemistry (CC), first introduced by K.B. Sharpless, has been widely adopted for use in drug discovery, novel drug delivery systems (DDS), polymer chemistry, and material sciences. In this review, we outline novel aspects of CC related to drug discovery and drug delivery, with a brief overview of molecular mechanisms underlying each click reaction commonly used by researchers, and the main patents that paved the way for further diverse medicinal applications. We also describe recent progress in drug discovery and polymeric and carbon material-based drug delivery for potential pharmaceutical applications and advancements based on the CC approach, and discuss some intrinsic limitations of this popular conjugation reaction. The use of CC is likely to significantly advance drug discovery and bioconjugation development.

Well-defined podophyllotoxin polyprodrug brushes: preparation via RAFT polymerization and evaluation as drug carriers

Novel poly(triethylene glycol methacrylate)-b-poly(podophyllotoxin methacrylate) copolymers (PTP) with a well-defined structure were designed and synthesized by direct RAFT polymerization with the hydrophobic monomer derivative from the anticancer drug podophyllotoxin.

Polymer-Based Prodrugs: Improving Tumor Targeting and the Solubility of Small Molecule Drugs in Cancer Therapy

The majority of anticancer drugs have poor aqueous solubility, produce adverse effects in healthy tissue, and thus impose major limitations on both clinical efficacy and therapeutic safety of cancer chemotherapy. To help circumvent problems associated with solubility, most cancer drugs are now formulated with co-solubilizers. However, these agents often also introduce severe side effects, thereby restricting effective treatment and patient quality of life. A promising approach to addressing problems in anticancer drug solubility and selectivity is their conjugation with polymeric carriers to form polymer-based prodrugs. These polymer-based prodrugs are macromolecular carriers, designed to increase the aqueous solubility of antitumor drugs, can enhance bioavailability. Additionally, polymer-based prodrugs approach exploits unique features of tumor physiology to passively facilitate intratumoral accumulation, and so improve chemodrug pharmacokinetics and pharmacological properties. This review introduces basic concepts of polymer-based prodrugs, provides an overview of currently emerging synthetic, natural, and genetically engineered polymers that now deliver anticancer drugs in preclinical or clinical trials, and highlights their major anticipated applications in anticancer therapies.

Multiscale approach to investigate self-assembly of telodendrimer based nanocarriers for anticancer drug delivery

Delivery of poorly soluble anticancer drugs can be achieved by employing polymeric drug delivery systems, capable of forming stable self-assembled nanocarriers with drug encapsulated within their hydrophobic cores. Computational investigations can aid the design of efficient drug-delivery platforms; however, simulations of nanocarrier self-assembly process are challenging due to high computational cost associated with the large system sizes (millions of atoms) and long time scales required for equilibration. In this work, we overcome this challenge by employing a multiscale computational approach in conjunction with experiments to analyze the role of the individual building blocks in the self-assembly of a highly tunable linear poly(ethylene glycol)-b-dendritic oligo(cholic acid) block copolymer called telodendrimer. The multiscale approach involved developing a coarse grained description of the telodendrimer, performing simulations over several microseconds to capture the self-assembly process, followed by reverse mapping of the coarse grained system to atomistic representation for structural analysis. Overcoming the computational bottleneck allowed us to run multiple self-assembly simulations and determine average size, drug-telodendrimer micellar stoichiometry, optimal drug loading capacity, and atomistic details such hydrogen-bonding and solvent accessible area of the nanocarrier. Computed results are in agreement with the experimental data, highlighting the success of the multiscale approach applied here.

Trigger responsive polymeric nanocarriers for cancer therapy

Conventional chemotherapy for the treatment of cancer has limited specificity when administered systemically and is often associated with toxicity issues. Enhanced accumulation of polymeric nanocarriers at a tumor site may be achieved by passive and active targeting. Incorporation of trigger responsiveness into these polymeric nanocarriers improves the anticancer efficacy of such systems by modulating the release of the drug according to the tumor environment. Triggers used for tumor targeting include internal triggers such as pH, redox and enzymes and external triggers such as temperature, magnetic field, ultrasound and light. While internal triggers are specific cues of the tumor microenvironment, external triggers are those which are applied externally to control the release. This review highlights the various strategies employed for the preparation of such trigger responsive polymeric nanocarriers for cancer therapy and provides an overview of the state of the art in this field.

Synthesis of an acid-labile polymeric prodrug DOX-acetal-PEG-acetal-DOX with high drug loading content for pH-triggered intracellular drug release

PEGylated doxorubicin (DOX) prodrugs with high drug loading content have been prepared via a combination of CuAAC “click” reaction and ammonolysis reaction, which can be used for pH-triggered delivery of doxorubicin.

Polymer-protein conjugation via a ‘grafting to’ approach – a comparative study of the performance of protein-reactive RAFT chain transfer agents

The performances of various protein-reactive RAFT CTAs to afford polymer-protein conjugation via a grafting-to approach were compared.

A facile strategy to prepare redox-responsive amphiphilic PEGylated prodrug with high drug loading content and low critical micelle concentration

A redox-responsive amphiphilic polymeric prodrug was synthesized in a facile way by polycondensation of oligo(ethylene glycol) with dicarboxylic acids including malic acid and 3,3'-dithiodipropionic acid , followed by esterification with ibuprofen, which was used as a model drug. Because of its amphiphilic nature and relatively high molecular weight, this polymeric prodrug can form stable micelles in aqueous media with a low critical micellar concentration (CMC). Free ibuprofen molecules can be steadily incorporated into the core of these micelles with a surprisingly high loading content (38.9 wt%), owing to hydrophobic interaction and π-π stacking with the ibuprofen moieties in the copolymer. The in vitro release results indicate that there was a relatively slow and sustained release of the conjugated ibuprofen moieties, while encapsulated ibuprofen molecules showed a rapid release. Furthermore, for both the conjugated ibuprofen and the encapsulated ibuprofen there was an accelerated release in the presence of 10 mM dl-dithiothreitol due to cleavage of the disulfide bonds, which lead to disassociation of the micelles. Notably, this prodrug was revealed to have excellent cell compatibilities via a cell counting kit-8 (CCK-8) assay. Confocal laser scanning microscope observations indicated that the micelles based on the polymeric prodrug can be taken up quickly by cells and present a redox-responsive drug release in cytoplasm. This kind of polymeric nanocarrier with a high drug loading content, low CMC, excellent biocompatibility and rapid response to a reductive environment may have tremendous scope in the area of controlled drug delivery.

PEG-pHPMAm-based polymeric micelles loaded with doxorubicin-prodrugs in combination antitumor therapy with oncolytic vaccinia viruses

An enzymatically activatable prodrug of doxorubicin was covalently coupled, using click-chemistry, to the hydrophobic core of poly(ethylene glycol)-b-poly[N-(2-hydroxypropyl)-methacrylamide-lactate] micelles. The release and cytotoxic activity of the prodrug was evaluated in vitro in A549 non-small-cell lung cancer cells after adding β-glucuronidase, an enzyme which is present intracellularly in lysosomes and extracellularly in necrotic areas of tumor lesions. The prodrug-containing micelles alone and in combination with standard and β-glucuronidase-producing oncolytic vaccinia viruses were also evaluated in vivo, in mice bearing A549 xenograft tumors. When combined with the oncolytic viruses, the micelles completely blocked tumor growth. Moreover, a significantly better antitumor efficacy as compared to virus treatment alone was observed when β-glucuronidase virus treated tumor-bearing mice received the prodrug-containing micelles. These findings show that combining tumor-targeted drug delivery systems with oncolytic vaccinia viruses holds potential for improving anticancer therapy.

Self-assembling doxorubicin-tocopherol succinate prodrug as a new drug delivery system: synthesis, characterization, and in vitro and in vivo anticancer activity

Self-assembled prodrugs forming nanoaggregates are a promising approach to enhance the antitumor efficacy and to reduce the toxicity of anticancer drugs. To achieve this goal, doxorubicin was chemically conjugated to d-α-tocopherol succinate through an amide bond to form N-doxorubicin-α-d-tocopherol succinate (N-DOX-TOS). The prodrug self-assembled in water into 250 nm nanostructures when stabilized with d-α-tocopherol poly(ethylene glycol) 2000 succinate. Cryo-TEM analysis revealed the formation of nanoparticles with a highly ordered lamellar inner structure. NMR spectra of the N-DOX-TOS nanoparticles indicated that N-DOX-TOS is located in the core of the nanoparticles while PEG chains and part of the tocopherol are in the corona. High drug loading (34% w/w) and low in vitro drug release were achieved. In vitro biological assessment showed significant anticancer activity and temperature-dependent cellular uptake of N-DOX-TOS nanoparticles. In vivo, these nanoparticles showed a greater antitumor efficacy than free DOX. N-DOX-TOS nanoparticles might have the potential to improve DOX-based chemotherapy.

Facile preparation of cobaltocenium-containing polyelectrolyte via click chemistry and RAFT polymerization

A facile method to prepare cationic cobaltocenium-containing polyelectrolyte is reported. Cobaltocenium monomer with methacrylate is synthesized by copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction between 2-azidoethyl methacrylate and ethynylcobaltocenium hexafluorophosphate. Further controlled polymerization is achieved by reversible addition-fragmentation chain transfer polymerization (RAFT) by using cumyl dithiobenzoate (CDB) as a chain transfer agent. Kinetic study demonstrates the controlled/living process of polymerization. The obtained side-chain cobaltocenium-containing polymer is a metal-containing polyelectrolyte that shows characteristic redox behavior of cobaltocenium.

Decreased reticuloendothelial system clearance and increased blood half-life and immune cell labeling for nano- and micron-sized superparamagnetic iron-oxide particles upon pre-treatment with Intralipid

BACKGROUND

Superparamagnetic iron-oxide nanoparticles are useful as contrast agents for anatomical, functional and cellular MRI, drug delivery agents, and diagnostic biosensors. Nanoparticles are generally cleared by the reticuloendothelial system (RES), in particular taken up by Kupffer cells in the liver, limiting particle bioavailability and in-vivo applications. Strategies that decrease the RES clearance and prolong the circulation residence time of particles can improve the in-vivo targeting efficiency.

METHODS

Intralipid 20.0%, an FDA approved nutritional supplement, was intravenously administered in rats at the clinical dose (2g/kg) 1h before intravenous injection of ultra-small superparamagnetic iron-oxide (USPIO) or micron-sized paramagnetic iron-oxide (MPIO) particles. Blood half-life, monocyte labeling efficiency, and particle biodistribution were assessed by magnetic resonance relaxometry, flow cytometry, inductively-coupled plasma MS, and histology.

RESULTS

Pre-treatment with Intralipid resulted in a 3.1-fold increase in USPIO blood half-life and a 2-fold increase in USPIO-labeled monocytes. A 2.5-fold increase in MPIO blood half-life and a 5-fold increase in MPIO-labeled monocytes were observed following Intralipid pre-treatment, with a 3.2-fold increase in mean iron content up to 2.60pg Fe/monocyte. With Intralipid, there was a 49.2% and 45.1% reduction in liver uptake vs. untreated controls at 48h for USPIO and MPIO, respectively.

CONCLUSIONS

Intralipid pre-treatment significantly decreases initial RES uptake and increases in-vivo circulation and blood monocyte labeling efficiency for nano- and micron-sized superparamagnetic iron-oxide particles.

GENERAL SIGNIFICANCE

Our findings can have broad applications for imaging and drug delivery applications, increasing the bioavailability of nano- and micron-sized particles for target sites other than the liver.

Surface-initiated Polymerization of Azidopropyl Methacrylate and its Film Elaboration via Click Chemistry

Azidopropyl methacrylate (AzPMA), a functional monomer with a pendent azido group, polymerizes from surfaces and provides polymer brushes amenable to subsequent elaboration via click chemistry. In DMF at 50 °C, click reactions between poly(AzPMA) brushes and an alkynylated dye proceed with >90% conversion in a few minutes. However, in aqueous solutions, reaction with an alkyne-containing poly(ethylene glycol) methyl ether (mPEG, Mn=5000) gives <10% conversion after a 12-h reaction at room temperature. Formation of copolymers with AzPMA and polyethylene glycol methyl ether methacrylate (mPEGMA) enables control over the hydrophilicity and functional group density in the copolymer to increase the yield of aqueous click reactions. The copolymers show reaction efficiencies as high as 60%. These studies suggest that for aqueous applications such as bioconjugation via click chemistry, control over brush hydrophilicity is vital.