Physico-Chemical Strategies to Enhance Stability and Drug Retention of Polymeric Micelles for Tumor-Targeted Drug Delivery

Active Transportation of Liposome Enhances Tumor Accumulation, Penetration, and Therapeutic Efficacy

Liposomes are the first and mostly explored nanocarriers for cancer drug delivery, which have shown great promise in clinical applications, but their limited accumulation and penetration into the tumor interstitial space, significantly reduce the therapeutic efficacy. Here, a γ-glutamyltranspeptidase (GGT)-triggered charge-switchable approach is reported that can trigger the fast endocytosis and transcytosis of the liposome in tumor microenvironments to overcome the harsh biological barriers in tumor tissues. The active transporting liposomal nanocarrier (GCSDL) is prepared by surface modification with a glutathione (GSH) moiety and encapsulated with doxorubicin (DOX). When the GCSDL contacts with tumor vascular endothelial cells, the overexpressed GGT enzyme on cytomembrane catalyzes the hydrolysis of GSH to generate cationic primary amines. The cationic GCSDL triggers fast caveolae-mediated endocytosis and vesicle-mediated transcytosis, resulting in sequential transcytosis to augment its tumor accumulation and penetration. Along with continual intercellular transportation, GCSDL can release DOX throughout the tumor to induce cancer cell apoptosis, resulting in complete eradication of hepatocellular carcinoma and cessation of pancreatic ductal adenocarcinoma's progression. This study develops an efficient strategy to realize high tumor accumulation and deep penetration for the liposomal drug delivery system via active transcytosis.

Robust and smart polypeptide-based nanomedicines for targeted tumor therapy

Nanomedicines based on synthetic polypeptides are among the most versatile and advanced platforms for tumor therapy. Notably, several polypeptide-based nanodrugs are currently under human clinical assessments. The previous (pre)clinical studies clearly show that dynamic stability (i.e. stable in circulation while destabilized in tumor) of nanomedicines plays a vital role in their anti-tumor performance. Various strategies have recently been developed to design dynamically stabilized polypeptide-based nanomedicines by e.g. crosslinking the nanovehicles with acid, reactive oxygen species (ROS), glutathione (GSH), or photo-sensitive linkers, inter-crosslinking between vehicles and drugs, introducing π-π stacking or lipid-lipid interactions in the nanovehicles, chemically conjugating drugs to vehicles, and forming unimolecular micelles. Interestingly, these robust and smart nanodrugs have demonstrated improved tumor targetability, anti-tumor efficacy, as well as safety profiles in different tumor models. In this review, representative strategies to robust and smart polypeptide-based nanomedicines for targeted treatment of varying malignancies are highlighted. The exciting development of dynamic nanomedicines will foresee further increasing clinical translation in the future.

Correlation between in vitro stability and pharmacokinetics of poly(ε-caprolactone)-based micelles loaded with a photosensitizer

Polymeric micelles are extensively investigated as drug delivery systems for hydrophobic drugs including photosensitizers (PSs). In order to benefit from micelles as targeted delivery systems for PS, rather than only solubilizers, the stability and cargo retention of the (PS-loaded) micelles should be properly assessed in biologically relevant media to get insight into the essential parameters predicting their in vivo performance (i.e., pharmacokinetics). In the present study, asymmetric flow field-flow fractionation (AF4) was used to investigate the in vitro stability in human plasma of empty and meta-tetra(hydroxyphenyl)chlorin (mTHPC)-loaded dithiolane-crosslinked micelles based on poly(ɛ-caprolactone)-co-poly(1,2-dithiolane‑carbonate)-b-poly(ethylene glycol) (p(CL-co-DTC)-PEG) and non (covalently)-crosslinked micelles composed of poly(ε-caprolactone)-b-poly(ethylene glycol) (pCL-PEG). AF4 allows separation of the micelles from plasma proteins, which showed that small non (covalently)-crosslinked pCL₉-PEG (17 nm) and pCL₁₅-PEG (22 nm) micelles had lower stability in plasma than pCL₂₃-PEG micelles with larger size (43 nm) and higher degree of crystallinity of pCL, and had also lower stability than covalently crosslinked p(CL₉-DTC_3.9)-PEG and p(CL₁₈-DTC_7.5)-PEG micelles with similar small sizes (~20 nm). In addition, PS (re)distribution to specific plasma proteins was observed by AF4, giving strong indications for the (in)stability of PS-loaded micelles in plasma. Nevertheless, fluorescence spectroscopy in human plasma showed that the retention of mTHPC in non (covalently)-crosslinked but semi-crystalline pCL₂₃-PEG micelles (>8 h) was much longer than that in covalently crosslinked p(CL₁₈-DTC_7.5)-PEG micelles (~4 h). In line with this, in vivo circulation kinetics showed that pCL₂₃-PEG micelles loaded with mTHPC had significantly longer half-life values (t½-β of micelles and mTHPC was 14 and 18 h, respectively) than covalently crosslinked p(CL₁₈-DTC_7.5)-PEG micelles (t½-β of both micelles and mTHPC was ~2 h). As a consequence, long circulating pCL₂₃-PEG micelles resulted in significantly higher tumor accumulation of both the micelles and loaded mTHPC as compared to short circulating p(CL₁₈-DTC_7.5)-PEG micelles. These in vivo data were in good agreement with the in vitro stability studies. In conclusion, the present study points out that AF4 and fluorescence spectroscopy are excellent tools to evaluate the (in)stability of nanoparticles in biological media and thus predict the (in)stability of drug loaded nanoparticles after i.v. administration, which is favorable to screen promising delivery systems with reduced experimental time and costs and without excessive use of animals.

Multifunctional peptides for tumor therapy

Controlled nano-systems for drug delivery are designed to deliver therapeutical drugs to desirable sites on demand. Due to the diverse physiological functions of peptides, it is reasonable to introduce peptides into anti-tumor nano-system. The integration of peptides into nanomaterials has complementary advantages, which not only avoids the rapid degradation of peptides in vivo, but also improves the intelligence and functionality of the nano-system. We summarized the functional peptides with targeting and stimulus-responsive properties, and the present review outlined the most relevant and recent developed peptide-based multifunctional nanomaterials for tumor therapy.

Nanoparticles in the Biological Context: Surface Morphology and Protein Corona Formation

A recent paper demonstrated that the formation of a protein corona is not a general property of all types of nanosized objects. In fact, it varies between a massive aggregation of plasma proteins onto the nanoparticle down to traces (e.g., a few proteins per 10 nanoparticles), which can only be determined by mass spectrometry in comparison to appropriate negative controls and background subtraction. Here, differences between various types of nanosized objects are discussed in order to determine general structure-property-relations from a physico-chemical viewpoint. It is highlighted that "not all nanoparticles are alike" and shown that their internal morphology, especially the difference between a strongly hydrated/swollen shell versus a sharp "hard" surface and its accessibility, is most relevant for biomedical applications.

Polymeric micelles for anticancer drug delivery

Polymeric micelles have gained interest as novel drug delivery systems for the treatment and diagnosis of cancer, as they offer several advantages over conventional drug therapies. This includes drug targeting to tumor tissue, in vivo biocompatibility and biodegradability, prolonged circulation time, enhanced accumulation, retention of the drug loaded micelle in the tumor and decreased side effects. This article provides an overview on the current state of micellar formulations as nanocarriers for anticancer drugs and their effectiveness in cancer therapeutics, including their clinical status. The type of copolymers used, their physicochemical properties and characterization as well as recent developments in the design of functional polymeric micelles are highlighted. The article also presents the design and outcomes of various types of stimuli-responsive polymeric micelles.

Polymeric micelles for the delivery of poorly soluble drugs: From nanoformulation to clinical approval

Over the last three decades, polymeric micelles have emerged as a highly promising drug delivery platform for therapeutic compounds. Particularly, poorly soluble small molecules with high potency and significant toxicity were encapsulated in polymeric micelles. Polymeric micelles have shown improved pharmacokinetic profiles in preclinical animal models and enhanced efficacy with a superior safety profile for therapeutic drugs. Several polymeric micelle formulations have reached the clinical stage and are either in clinical trials or are approved for human use. This furthers interest in this field and underscores the need for additional learning of how to best design and apply these micellar carriers to improve the clinical outcomes of many drugs. In this review, we provide detailed information on polymeric micelles for the solubilization of poorly soluble small molecules in topics such as the design of block copolymers, experimental and theoretical analysis of drug encapsulation in polymeric micelles, pharmacokinetics of drugs in polymeric micelles, regulatory approval pathways of nanomedicines, and current outcomes from micelle formulations in clinical trials. We aim to describe the latest information on advanced analytical approaches for elucidating molecular interactions within the core of polymeric micelles for effective solubilization as well as for analyzing nanomedicine's pharmacokinetic profiles. Taking into account the considerations described within, academic and industrial researchers can continue to elucidate novel interactions in polymeric micelles and capitalize on their potential as drug delivery vehicles to help improve therapeutic outcomes in systemic delivery.

Polymeric micelles for anticancer drug delivery

Polymeric micelles have gained interest as novel drug delivery systems for the treatment and diagnosis of cancer, as they offer several advantages over conventional drug therapies. This includes drug targeting to tumor tissue, in vivo biocompatibility and biodegradability, prolonged circulation time, enhanced accumulation, retention of the drug loaded micelle in the tumor and decreased side effects. This article provides an overview on the current state of micellar formulations as nanocarriers for anticancer drugs and their effectiveness in cancer therapeutics, including their clinical status. The type of copolymers used, their physicochemical properties and characterization as well as recent developments in the design of functional polymeric micelles are highlighted. The article also presents the design and outcomes of various types of stimuli-responsive polymeric micelles.

Biotin-decorated all-HPMA polymeric micelles for paclitaxel delivery

To avoid poly(ethylene glycol)-related issues of nanomedicines such as accelerated blood clearance, fully N-2-hydroxypropyl methacrylamide (HPMAm)-based polymeric micelles decorated with biotin for drug delivery were designed. To this end, a biotin-functionalized chain transfer agent (CTA), 4-cyano-4-[(dodecylsulfanylthiocarbonyl)-sulfanyl]pentanoic acid (biotin-CDTPA), was synthesized for reversible addition-fragmentation chain-transfer (RAFT) polymerization. Amphiphilic poly(N-2-hydroxypropyl methacrylamide)-block-poly(N-2-benzoyloxypropyl methacrylamide) (p(HPMAm)-b-p(HPMAm-Bz)) with molecular weights ranging from 8 to 24 kDa were synthesized using CDTPA or biotin-CDTPA as CTA and 2,2'-azobis(2-methylpropionitrile) as initiator. The copolymers self-assembled in aqueous media into micelles with sizes of 40-90 nm which positively correlated to the chain length of the hydrophobic block in the polymers, whereas the critical micelle concentrations decreased with increasing hydrophobic block length. The polymer with a molecular weight of 22.1 kDa was used to prepare paclitaxel-loaded micelles which had sizes between 61 and 70 nm, and a maximum loading capacity of around 10 wt%. A549 lung cancer cells overexpressing the biotin receptor, internalized the biotin-decorated micelles more efficiently than non-targeted micelles, while very low internalization of both types of micelles by HEK293 human embryonic kidney cells lacking the biotin receptor was observed. As a consequence, the paclitaxel-loaded micelles with biotin decoration exhibited stronger cytotoxicity in A549 cells than non-targeted micelles. Overall, a synthetic pathway to obtain actively targeted poly(ethylene glycol)-free micelles fully based on a poly(HPMAm) backbone was established. These polymeric micelles are promising systems for the delivery of hydrophobic anticancer drugs.

Systematic evaluation of design features enables efficient selection of Π electron-stabilized polymeric micelles

Polymeric micelles (PM) based on poly(ethylene glycol)-b-poly(N-2-benzoyloxypropyl methacrylamide) (mPEG-b-p(HPMA-Bz)) loaded with paclitaxel (PTX-PM) have shown promising results in overcoming the suboptimal efficacy/toxicity profile of paclitaxel. To get insight into the stability of PTX-PM formulations upon storage and to optimize their in vivo tumor-targeted drug delivery properties, we set out to identify a lead PTX-PM formulation with the optimal polymer composition. To this end, PM based on four different mPEG_5k-b-p(HPMA-Bz) block copolymers with varying molecular weight of the hydrophobic block (17-3 kDa) were loaded with different amounts of PTX. The hydrodynamic diameter was 52 ± 1 nm for PM prepared using polymers with longer hydrophobic blocks (mPEG_5k-b-p(HPMA-Bz)_17k and mPEG_5k-b-p(HPMA-Bz)_10k) and 39 ± 1 nm for PM composed of polymers with shorter hydrophobic blocks (mPEG_5k-b-p(HPMA-Bz)_5k and mPEG_5k-b-p(HPMA-Bz)_3k). The best storage stability and the slowest PTX release was observed for PM with larger hydrophobic blocks. On the other hand, smaller sized PM of shorter mPEG_5k-b-p(HPMA-Bz)_5k showed a better tumor penetration in 3D spheroids. Considering better drug retention capacity of the mPEG_5k-b-p(HPMA-Bz)_17k and smaller size of the mPEG_5k-b-p(HPMA-Bz)_5k as two desirable design features, we argue that PM based on these two polymers are the lead candidates for further in vivo studies.

π-π-Stacked Poly(ε-caprolactone)-b-poly(ethylene glycol) Micelles Loaded with a Photosensitizer for Photodynamic Therapy

To improve the in vivo stability of poly(ε-caprolactone)-b-poly(ethylene glycol) (PCL-PEG)-based micelles and cargo retention by π-π stacking interactions, pendant aromatic rings were introduced by copolymerization of ε-caprolactone with benzyl 5-methyl-2-oxo-1,3-dioxane-5-carboxylate (TMC-Bz). It was shown that the incorporation of aromatic rings yielded smaller micelles (18–30 nm) with better colloidal stability in PBS than micelles without aromatic groups. The circulation time of i.v. injected micelles containing multiple pendant aromatic groups was longer (t½-α: ~0.7 h; t½-β: 2.9 h) than that of micelles with a single terminal aromatic group (t½ < 0.3 h). In addition, the in vitro partitioning of the encapsulated photosensitizer (meta-tetra(hydroxyphenyl)chlorin, mTHPC) between micelles and human plasma was favored towards micelles for those that contained the pendant aromatic groups. However, this was not sufficient to fully retain mTHPC in the micelles in vivo, as indicated by similar biodistribution patterns of micellar mTHPC compared to free mTHPC, and unequal biodistribution patterns of mTHPC and the host micelles. Our study points out that more detailed in vitro methods are necessary to more reliably predict in vivo outcomes. Furthermore, additional measures beyond π-π stacking are needed to stably incorporate mTHPC in micelles in order to benefit from the use of micelles as targeted delivery systems.

Folate decorated polymeric micelles for targeted delivery of the kinase inhibitor dactolisib to cancer cells

One of the main challenges in clinical translation of polymeric micelles is retention of the drug in the nanocarrier system upon its systemic administration. Core crosslinking and coupling of the drug to the micellar backbone are common strategies to overcome these issues. In the present study, polymeric micelles were prepared for tumor cell targeting of the kinase inhibitor dactolisib which inhibits both the mammalian Target of Rapamycin (mTOR) kinase and phosphatidylinositol-3-kinase (PI3K). We employed platinum(II)-based linker chemistry to couple dactolisib to the core of poly(ethylene glycol)-b-poly(acrylic acid) (PEG-b-PAA) polymeric micelles. The formed dactolisib-PEG-PAA unimers are amphiphilic and self-assemble in an aqueous milieu into core-shell polymeric micelles. Folate was conjugated onto the surface of the micelles to yield folate-decorated polymeric micelles which can target folate receptor over-expressing tumor cells. Fluorescently labeled polymeric micelles were prepared using a lissamine-platinum complex linked in a similar manner as dactolisib. Dactolisib polymeric micelles showed good colloidal stability in water and released the coupled drug in buffers containing chloride or glutathione. Folate decorated micelles were avidly internalized by folate-receptor-positive KB cells and displayed targeted cellular cytotoxicity at 50-75 nM IC₅₀. In conclusion, we have prepared a novel type of folate-receptor targeted polymeric micelles in which platinum(II) linker chemistry modulates drug retention and sustained release of the coupled inhibitor dactolisib.

α3β1 Integrin-Targeting Polymersomal Docetaxel as an Advanced Nanotherapeutic for Nonsmall Cell Lung Cancer Treatment

Docetaxel (DTX) widely used for treating nonsmall cell lung cancer (NSCLC) patients is associated with dose-limiting side effects, especially neurotoxicity and myelosuppression. Here, we have developed cyclic cNGQGEQc peptide-directed polymersomal docetaxel (cNGQ-PS-DTX) as a targeted and multifunctional formulation for NSCLC. cNGQ-PS-DTX carrying 8.1 wt % DTX had a size of 93 nm, neutral surface charge, high stability, and glutathione-triggered DTX release behavior. Cytotoxicity studies demonstrated a clearly better antitumor activity of cNGQ-PS-DTX in α3β1 integrin overexpressing A549 human lung cancer cells than free DTX and nontargeted PS-DTX. cNGQ-PS-DTX showed a remarkably high tolerability (over 8 times better than free DTX) and slow elimination in mice. Importantly, cNGQ-PS-DTX exhibited greatly improved tumor accumulation and higher suppression of subcutaneous and orthotopic A549 xenografts as compared to PS-DTX and free DTX controls. α3β1 integrin-targeting polymersomal docetaxel emerges as an advanced nanotherapeutic for NSCLC treatment.

Dual-responsive TPGS crosslinked nanocarriers to overcome multidrug resistance

Efficient delivery of chemotherapeutic agents into tumor cells and reversal of chemoresistance are crucially important to enhance cancer therapy.

Functional DNA-based hydrogel intelligent materials for biomedical applications

Multifunctional intelligent DNA hydrogels have been reviewed for many biomedical applications.

Redox-responsive hollow mesoporous silica nanoparticles constructed via host-guest interactions for controllable drug release

A novel redox-responsive hollow mesoporous silica (HMS) was constructed by host-guest interaction between β-cyclodextrin modified hollow mesoporus silica nanoparticles (HMS@β-CD) and the ferrocene-containing amphiphilic block copolymer PEG-b-PMAFc (PPFc), the prepared HMS@β-CD@PPFc system was used to control drug delivery in targeted cancer therapy through redox stimulus. The self-assembled morphology was investigated by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Intracellular localization of DOX-loaded HMS@β-CD@PPFc in A549 cells was further investigated by confocal laser scanning microscopy (CLSM), and the results indicated that DOX-loaded HMS@β-CD@PPFc was ingested by A549 cells effectively. Furthermore, the redox agent H₂O₂ was used to trigger the release of DOX. The cytotoxicity evaluated by MTT method indicated that HMS@β-CD@PPFc had good biocompatibility and was promising as the drug carrier.

Poly(ethylene glycol)-poly(ε-caprolactone)-based micelles for solubilization and tumor-targeted delivery of silibinin

Introduction: Silibinin is a naturally occurring compound with known positive impacts on prevention and treatment of many types of human illnesses in general and cancer in particular. Silibinin is poorly water soluble which results in its insufficient bioavailability and lack of therapeutic efficacy in cancer. Here, we proposed to examine the potential of micelles composed of poly(ethylene glycol) (PEG) as the hydrophilic block and poly(ε-caprolactone) (PCL), poly(α-benzylcarboxylate-ε-caprolactone) (PBCL), or poly(lactide)-(PBCL) (PLA-PBCL) as hydrophobic blocks for enhancing the water solubility of silibinin and its targeted delivery to tumor.

Methods: Co-solvent evaporation method was used to incorporate silibinin into PEG-PCL based micelles. Drug release profiles were assessed using dialysis bag method. MTT assay also was used to analyze functional activity of drug delivery in B16 melanoma cells.

Results: Silibinin encapsulated micelles were shown to be less than 60 nm in size. Among different structures under study, the one with PEG-PBCL could incorporate silibinin with the highest encapsulation efficiency being 95.5%, on average. PEG-PBCL micelles could solubilize 1 mg silibinin in 1 mL water while the soluble amount of silibinin was found to be 0.092 mg/mL in the absence of polymeric micelles. PEG-PBCL micelles provided the sustained release of silibinin indicated with less than 30% release of silibinin within 24 hours. Silibinin encapsulated in PEG-PBCL micelles resulted in growth inhibitory effect in B16 cancer cells which was significantly higher than what observed with free drug.

Conclusion: Our findings showed that PEG-PBCL micellar nanocarriers can be a useful vehicle for solubilization and targeted delivery of silibinin.

Reduction-Triggered Paclitaxel Release Nano-Hybrid System Based on Core-Crosslinked Polymer Dots with a pH-Responsive Shell-Cleavable Colorimetric Biosensor

Herein, we describe the fabrication and characterization of carbonized disulfide core-crosslinked polymer dots with pH-cleavable colorimetric nanosensors, based on diol dye-conjugated fluorescent polymer dots (L-PD), for reduction-triggered paclitaxel (PTX) release during fluorescence imaging-guided chemotherapy of tumors. L-PD were loaded with PTX (PTX loaded L-PD), via π-π stackings or hydrophobic interactions, for selective theragnosis by enhanced release of PTX after the cleavage of disulfide bonds by high concentration of glutathione (GSH) in a tumor. The nano-hybrid system showed fluorescence quenching behavior with less than 2% of PTX released under physiological conditions. However, in a tumor microenvironment, the fluorescence recovered at an acidic-pH, and PTX (approximately 100% of the drug release) was released efficiently out of the matrix by reduction caused by the GSH level in the tumor cells, which improved the effectiveness of the cancer treatment. Therefore, the colorimetric nanosensor showed promising potential in distinguishing between normal and cancerous tissues depending on the surrounding pH and GSH concentrations so that PTX can be selectively delivered into cancer cells for improved cancer diagnosis and chemotherapy.

Transcytosis - An effective targeting strategy that is complementary to "EPR effect" for pancreatic cancer nano drug delivery

Numerous nano drug delivery systems have been developed for preclinical cancer research in the past 15 years with the hope for a fundamental change in oncology. The robust nanotherapeutic research has yielded early-stage clinical products as exemplified by the FDA-approved nano formulations (Abraxane® for paclitaxel and Onyvide® for irinotecan) for the treatment of solid tumors, including pancreatic ductal adenocarcinoma (PDAC). It is generally believed that enhanced permeability and retention (EPR) plays a key role in nanocarriers' accumulation in preclinical tumor models and is a clinically relevant phenomenon in certain cancer types. However, use of EPR effect as an across-the-board explanation for nanoparticle tumor access is likely over-simplified, particularly in the stroma rich solid tumors such as PDAC. Recently, ample evidences including our own data showed that it is possible to use transcytosis as a major mechanism for PDAC drug delivery. In this mini-review, we summarize the key studies that discuss how transcytosis can be employed to enhance EPR effect in PDAC, and potentially, other cancer malignancies. We also mentioned other vasculature engineering approaches that work beyond the classic EPR effect.

CD44-Targeted Multifunctional Nanomedicines Based on a Single-Component Hyaluronic Acid Conjugate with All-Natural Precursors: Construction and Treatment of Metastatic Breast Tumors in Vivo

Metastasis is responsible for >90% of the deaths of breast cancer patients in the clinic. Here, we report on cross-linked multifunctional hyaluronic acid nanoparticles carrying docetaxel (DTX-CMHN) for enhanced suppression of highly metastatic 4T1 breast tumors in vivo. DTX-CMHN was formed from a single and all-natural hyaluronic acid-g-polytyrosine-lipoic acid conjugate (HA-g-PTyr-LA; HA, 20 kDa; PTyr, 2.2 kDa), and the size of DTX-CMHN increased from 69 to 78 to 96 nm as the increasing degree of substitution (DS) of PTyr increased from 4 to 11 to 15, respectively. Robust encapsulation of DTX was obtained when DS ≥ 11. DTX-CMHN while steady in a nonreducing environment was destabilized under 10 mM glutathione releasing ∼90% of the DTX within 24 h. It is noteworthy that DTX-CMHN exhibited better antitumor, antimigration, and anti-invasion activity in CD44-overexpressed 4T1-Luc breast cancer cells than free DTX. Interestingly, DTX-CMHN displayed a long elimination half-life of 5.75 h, in contrast to half-lives of 2.11 and 0.75 h for its non-cross-linked counterpart (DTX-MHN) and free DTX, respectively. In vivo therapeutic studies showed significantly better inhibition of primary 4T1-Luc tumor growth and lung metastasis and lower toxicity of DTX-CMHN compared with that of free DTX. These multifunctional nanoformulations based on a single and all-natural hyaluronic acid conjugate emerge as a potential nanoplatform for targeted treatment of CD44-positive metastatic tumors.

Nanocarriers and Their Loading Strategies

Nanocarriers are of paramount significance for drug delivery and nanomedicine technology. Given the imperfect systems and nonideal therapeutic effects, there are works to be done in synthesis as much as in biological studies, if not more so. Building the foundation of synthesis would offer more tools and deeper insights for exploring the biological systems with extreme complexity. This review aims at a broad-scope summary and classification of nanocarriers for drug delivery, with focus on the synthetic strategy and structural implications. The nanocarriers are divided into four categories according to the loading principle: molecular-level loading, surface loading, matrix loading, and cavity loading systems. Making comparisons across diverse nanocarrier systems would make it easier to see the fundamental characteristics, from where the weakness can be addressed and the strengths combined. The systematic comparisons may also inspire new ideas and methods.

Intracellular Dynamic Disentangling of Doxorubicin Release from Luminescent Nanogold Carriers by Fluorescence Lifetime Imaging Microscopy (FLIM) under Two-Photon Excitation

There is still a lack of available techniques to follow noninvasively the intracellular processes as well to track or disentangle various signals from the therapeutic agents at the site of action in the target cells. We present here the assessment of the intracellular kinetics of doxorubicin (DOX) and gold nanoparticle (AuNP) carriers by mapping simultaneously fluorescence and photoluminescence signals by fluorescence lifetime imaging microscopy under two-photon excitation (TPE-FLIM). The new nano-chemotherapeutic system AuNPs@gelatin-hyd-DOX has been fabricated by DOX loading onto the surface of gelatin-biosynthesized AuNPs (AuNPs@gelatin) through a pH-sensitive hydrazone bond. The successful loading of DOX onto the AuNPs was studied by spectroscopic methods and steady-state fluorescence, and the nanosystem pH-responsive character was validated under simulated biological conditions at different pH values (i.e., pH 4.6, 5.3, and 7.4). Considering that the fluorescence lifetime of DOX molecules at a specific point in the cell is a reliable indicator of the discrimination of the different states of the drug in the internalization path, i.e., released versus loaded, the kinetics of AuNPs@gelatin-hyd-DOX cellular uptake and DOX release was compared to that of free DOX, resulting in two different drug internalization pathways. Finally, cell viability tests were conducted against NIH:OVCAR-3 cell line to prove the efficiency of our chemotherapeutic nanosystem. TPE-FLIM technique could be considered promising for noninvasive, high-resolution imaging of cells with improved capabilities over current one-photon-excited FLIM.

The synthesis and co-micellization of PCL-P(HEMA/HEMA-LA) and PCL-P(HEMA/HEMA-FA) as shell cross-linked drug carriers with target/redox properties

In order to obtain target/redox shell cross-linked micelles (TCM), copolymers poly(ε-caprolactone)-poly(2-hydroxyethyl methacrylate/methacrylate-alpha lipoic acid) and poly(ε-caprolactone)-poly(2-hydroxyethyl methacrylate/methacrylate-folate, PCL-P(HEMA/HEMA-LA) and PCL-P(HEMA/HEMA-FA) were designed and synthesized. The copolymers PCL-P(HEMA/HEMA-LA) could form reduction-sensitive cross-linked micelles (CM) by using a catalytic amount of DTT. The micelles maintained high stability against dilution but were destroyed in 10 mM dithiothreitol (DTT). The drug loaded content (DLC) of CM was 8.9%, which was almost twice as much as non-cross-linked micelle (NCM). In vitro drug release at pH 7.4 showed that the cumulative release rate of CM in 36 h was less than 30%, while it was about 50% for NCM. When PCL-P(HEMA/HEMA-LA) and PCL-P(HEMA/HEMA-FA) (FA 1%, 3% and 5%) formed target/redox micelles, IC50 of TCM with FA 3% was the lowest (1.4 µg/mL) to Hela cells with excessive expression folate receptors. The cell uptake of TCM by Hela cells is higher than target non-cross-linked micelles (TNCM), while there was not much difference between both micelles uptaken by A549 cells, which are lack of folate receptors. Therefore, the drug carriers of TCM have potential to be explored as shell cross-linked target/redox drug carriers to the cancer cells on the surface with excessive folate receptors.

Improved Pharmacokinetics of Icariin (ICA) within Formulation of PEG-PLLA/PDLA-PNIPAM Polymeric Micelles

Icariin (ICA) is a major flavonoid that contains the active compound Epimedii Folium. However, ICA’s pharmacokinetic characteristics remain unsatisfactory due to its low bioavailability, and hence limited drugability. In order to improve its pharmacokinetics and achieve prolonged blood circulation time, a novel polymeric micelle, made of the self-assembled micelle between poly (ethylene glycol)-poly (_L-lactic acid) (PEG-PLLA) and poly (_D-lactic acid)-poly(N-isopropylacrylamide) (PDLA-PNIPAM), was designed to encapsulate ICA. Our experimental results showed that this polymeric micelle formulation of ICA exhibited uniform nano-size distribution and high stability within 48 h. The new formulation also allowed sustained ICA release in an in vitro drug release study. Furthermore, in vivo experiments revealed that ICA bioavailability in the PEG-PLLA/PDLA-PNIPAM polymeric micelle formulation was significantly higher compared to ICA alone, or ICA in the traditional Pluronic F127 micelle formulation. Finally, we show that metabolite analysis confirmed that ICA within the PEG-PLLA/PDLA-PNIPAM polymeric micelle formulation provided better drug protection, reduced drug metabolites production, and decreased undesired first-pass effects. Overall, these data show that ICA within PEG-PLLA/PDLA-PNIPAM polymeric micelle formulation exhibit advantages, in terms of improved physicochemical properties, sustained release of ICA in vitro, and improved bioavailability of ICA in vivo, which represent a feasible approach for improving the drugability of pharmaceutical small molecules with low bioavailability or poor stability.

Biomolecules Turn Self-Assembling Amphiphilic Block Co-polymer Platforms Into Biomimetic Interfaces

Biological membranes constitute an interface between cells and their surroundings and form distinct compartments within the cell. They also host a variety of biomolecules that carry out vital functions including selective transport, signal transduction and cell-cell communication. Due to the vast complexity and versatility of the different membranes, there is a critical need for simplified and specific model membrane platforms to explore the behaviors of individual biomolecules while preserving their intrinsic function. Information obtained from model membrane platforms should make invaluable contributions to current and emerging technologies in biotechnology, nanotechnology and medicine. Amphiphilic block co-polymers are ideal building blocks to create model membrane platforms with enhanced stability and robustness. They form various supramolecular assemblies, ranging from three-dimensional structures (e.g., micelles, nanoparticles, or vesicles) in aqueous solution to planar polymer membranes on solid supports (e.g., polymer cushioned/tethered membranes,) and membrane-like polymer brushes. Furthermore, polymer micelles and polymersomes can also be immobilized on solid supports to take advantage of a wide range of surface sensitive analytical tools. In this review article, we focus on self-assembled amphiphilic block copolymer platforms that are hosting biomolecules. We present different strategies for harnessing polymer platforms with biomolecules either by integrating proteins or peptides into assemblies or by attaching proteins or DNA to their surface. We will discuss how to obtain synthetic structures on solid supports and their characterization using different surface sensitive analytical tools. Finally, we highlight present and future perspectives of polymer micelles and polymersomes for biomedical applications and those of solid-supported polymer membranes for biosensing.

Logical design and application of prodrug platforms

This review summarizes the current state of prodrugs and elaborates the logical design and future development of the prodrug platform.

Synthesis of a SN38 prodrug grafted to amphiphilic phosphorylcholine polymers and their prodrug miceller properties

Polymer prodrug micelles, combining the advantages of prodrugs and polymer micelles, can greatly improve the solubility, permeability and stability of drugs.

Hypoxia- and singlet oxygen-responsive chemo-photodynamic Micelles featured with glutathione depletion and aldehyde production

Triggered drug release from anti-tumor nanomedicine is an efficient approach to address the dilemma of systemic nanocarrier stability and on-demand drug liberation in tumor sites.

Effect of Formulation and Processing Parameters on the Size of mPEG- b-p(HPMA-Bz) Polymeric Micelles

Micelles composed of block copolymers of poly(ethylene glycol)- b-poly( N-2-benzoyloxypropyl methacrylamide) (mPEG- b-p(HPMA-Bz)) have shown great promise as drug-delivery carriers due to their excellent stability and high loading capacity. In the present study, parameters influencing micelle size were investigated to tailor sizes in the range of 25-100 nm. Micelles were prepared by a nanoprecipitation method, and their size was modulated by the block copolymer properties such as molecular weight, their hydrophilic-to-hydrophobic ratio, homopolymer content, as well as formulation and processing parameters. It was shown that the micelles have a core-shell structure using a combination of dynamic light scattering and transmission electron microscopy analysis. By varying the degree of polymerization of the hydrophobic block ( N_B) between 68 and 10, at a fixed hydrophilic block mPEG_5k ( N_A = 114), it was shown that the hydrophobic core of the micelle was collapsed following the power law of ( N_B × N_agg)^1/3. Further, the calculated brush height was similar for all the micelles examined (10 nm), indicating that crew-cut micelles were made. Both addition of homopolymer and preparation of micelles at lower concentrations or lower rates of addition of the organic solvent to the aqueous phase increased the size of micelles due to partitioning of the hydrophobic homopolymer chains to the core of the micelles and lower nucleation rates, respectively. Furthermore, it was shown that by using different solvents, the size of the micelles substantially changed. The use of acetone, acetonitrile, ethanol, tetrahydrofuran, and dioxane resulted in micelles in the size range of 45-60 nm after removal of the organic solvents. The use of dimethylformamide and dimethylsulfoxide led to markedly larger sizes of 75 and 180 nm, respectively. In conclusion, the results show that by modulating polymer properties and processing conditions, micelles with tailorable sizes can be obtained.

Reversibly core-crosslinked PEG-P(HPMA) micelles: Platinum coordination chemistry for competitive-ligand-regulated drug delivery

HYPOTHESIS

The presence of pendant thioether groups on poly(ethylene glycol)-poly(N(2-hydroxypropyl) methacrylamide) (PEG-P(HPMA)) block copolymers allows for platinum-mediated coordinative micellar core-crosslinking, resulting in enhanced micellar stability and stimulus-responsive drug delivery.

EXPERIMENTS

A new PEG-P(HPMA) based block copolymer with pendant 4-(methylthio)benzoyl (MTB) groups along the P(HPMA) block was synthesized by free radical polymerization of a novel HPMA-MTB monomer using a PEG based macro-initiator. As crosslinker the metal-organic linker [ethylenediamineplatinum(II)]²⁺ was used, herein called Lx, which is a coordinative linker molecule that has been used for the conjugation of drug molecules to a number of synthetic or natural carrier systems such as hyperbranched polymers and antibodies.

FINDINGS

The introduction of Lx in the micellar core results in a smaller size, a lower critical micelle concentration and a better retention of the hydrophobic drug curcumin thanks to coordination bonds between the central platinum atom of Lx and thioether groups on different polymer chains. The drug release from Lx crosslinked micelles is significantly accelerated under conditions mimicking the intracellular environment due to competitive coordination and subsequent micellar de-crosslinking. Because of their straightforward preparation and favorable drug release characteristics, core-crosslinked Lx PEG-P(HPMA) micelles hold promise as a versatile nanomedicine platform.

Cyclic RGD-Peptide-Functionalized Polylipopeptide Micelles for Enhanced Loading and Targeted Delivery of Monomethyl Auristatin E

Monomethyl auristatin E (MMAE) is an extremely potent peptide drug that is currently used in the form of antibody drug conjugates (ADCs) for treating different cancers. ADCs are, however, associated with low drug conjugation, immunogenicity, small scale production, and high costs. Here, cRGD-functionalized polylipopeptide micelles (cRGD-Lipep-Ms) were explored for enhanced loading and targeted delivery of MMAE to HCT-116 colorectal tumor xenografts. Interestingly, cRGD-Lipep-Ms achieved an MMAE loading content of 5.5 wt %, which was 55-fold higher than that of poly(ethylene glycol)- b-poly(d,l-lactide) micelles. MMAE-loaded cRGD-Lipep-Ms (MMAE-cRGD-Lipep-Ms) showed a small hydrodynamic size of 59 nm, minimal drug leakage in 10% FBS, and efficient uptake and superb antiproliferative activity in α_vβ₅-overexpressing HCT-116 tumor cells. Remarkably, MMAE-cRGD-Lipep-Ms displayed over 10-fold better toleration than free MMAE in mice and completely suppressed growth of HCT-116 colorectal tumor xenografts. These polylipopeptide micelles have appeared to be an attractive alternative to ADCs for targeted delivery of potent peptide drugs.

Micelles Structure Development as a Strategy to Improve Smart Cancer Therapy

Micelles as colloidal suspension have attracted considerable attention due to their potential use for both cancer diagnosis and therapy. These structures have proven their ability to deliver poorly water-soluble anticancer drugs, improve drug stability, and have good penetration and site-specificity, leading to enhance therapeutic efficacy. Micelles are composed of hydrophobic and hydrophilic components assembled into nanosized spherical, ellipsoid, cylindrical, or unilamellar structures. For their simple formation, they are widely studied, either by using opposite polymers attachment consisting of two or more block copolymers, or by using fatty acid molecules that can modify themselves in a rounded shape. Recently, hybrid and responsive stimuli nanomicelles are formed either by integration with metal nanoparticles such as silver, gold, iron oxide nanoparticles inside micelles or by a combination of lipids and polymers into single composite. Herein, through this special issue, an updated overview of micelles development and their application for cancer therapy will be discussed.

Highly Augmented Drug Loading and Stability of Micellar Nanocomplexes Composed of Doxorubicin and Poly(ethylene glycol)-Green Tea Catechin Conjugate for Cancer Therapy

Low drug loading and instability in blood circulation are two key challenges that impede the successful clinical translation of nanomedicine, as they result in only marginal therapeutic efficacy and toxic side effects associated with premature drug leakage, respectively. Herein, highly stable and ultrahigh drug loading micellar nanocomplexes (MNCs) based on the self-assembly of the anticancer drug doxorubicin (DOX) and a poly(ethylene glycol)-epigallocatechin-3-O-gallate (EGCG) conjugate are developed. The formation of these MNCs is facilitated by strong favorable intermolecular interactions between the structurally similar aromatic EGCG and DOX molecules, which impart exceptionally high drug-loading capability of up to 88% and excellent thermodynamic and kinetic stability. Unlike two clinical formulations of DOX-free DOX and liposomal DOX, which are not effective below their lethal dosages, these DOX-loaded MNCs demonstrate significant tumor growth inhibition in vivo on a human liver cancer xenograft mouse model with minimal unwanted toxicity. Overall, these MNCs can represent a safe and effective strategy to deliver DOX for cancer therapy.

Integrated Multifunctional Micelles Co-Self-Assembled from Polypeptides Conjugated with Natural Ferulic Acid and Lipoic Acid for Doxorubicin Delivery

The development of safe, easily accessible, and multifunctional nanocarriers is a big topic in nanomedicine research. Here, integrated multifunctional micelles (IMM) were developed by co-self-assembly of poly(ethylene glycol)-b-poly(l-lysine) derivatives with natural ferulic acid (FA) or lipoic acid (LA). FA confers IMM with intrinsic antitumor activity, improved loading of doxorubicin (DOX) through π-π stacking, and reduced DOX cardiotoxicity. LA provides IMM with reversible crosslinking property, which leads to a high colloidal stability with inhibited drug leakage and triggered intracellular DOX release. Notably, our results showed that cRGD-decorated IMM (cRGD-IMM) had a small size (≈56 nm) and superior loading of DOX (27.1 wt. %). Blank cRGD-IMM, though nontoxic to normal cells, exhibited obvious antiproliferative activity against cancer cells including B16F10 and HCT-116 cells at 150 μg FA equiv. mL^-1 . DOX-loaded cRGD-IMM displayed enhanced growth inhibition of α_v β₃ -positive B16F10 and HCT-116 cells, a long elimination half-life of 3.85 h, and a high maximum-tolerated dose of over 100 mg DOX equiv. kg^-1 . Histological analysis revealed that DOX-loaded cRGD-IMM at 100 mg DOX equiv. kg^-1 caused negligible cardiotoxicity, which is a major issue for the clinical use of DOX. These integrated multifunctional micelles with excellent safety and accessibility have emerged as a new platform for targeted cancer chemotherapy.

Light- and pH-dually responsive dendrimer-star copolymer containing spiropyran groups: synthesis, self-assembly and controlled drug release

Dendrimer-star copolymer containing spiropyran groups could self-assemble into micelles and presented light- and pH-dually responsive properties.

Self-Assembly of Thermoresponsive Recombinant Silk-Elastinlike Nanogels

Recombinant silk-elastinlike protein polymers (SELPs) combine the biocompatibility and thermoresponsiveness of human tropoelastin with the strength of silk. Direct control over structure of these monodisperse polymers allows for precise correlation of structure with function. This work describes the fabrication of the first SELP nanogels and evaluation of their physicochemical properties and thermoresponsiveness. Self-assembly of dilute concentrations of SELPs results in nanogels with enhanced stability over micelles due to physically crosslinked beta-sheet silk segments. The nanogels respond to thermal stimuli via size changes and aggregation. Modifying the ratio and sequence of silk to elastin in the polymer backbone results in alterations in critical gel formation concentration, stability, aggregation, size contraction temperature, and thermal reversibility. The nanogels sequester hydrophobic compounds and show promise in delivery of bioactive agents.

Antioxidant nanomaterials in advanced diagnoses and treatments of ischemia reperfusion injuries

Organ ischemia with inadequate oxygen supply followed by reperfusion (which initiates a complex of inflammatory responses and oxidative stress) occurs in different clinical conditions and surgical procedures including stroke, myocardial infarction, limb ischemia, renal failure, organ transplantation, free-tissue-transfer, cardiopulmonary bypass, and vascular surgery. Even though pharmacological treatments protect against experimental ischemia reperfusion (I/R) injury, there has not been enough success in their application for patient benefits. The main hurdles in the treatment of I/R injury are the lack of diagnosis tools for understanding the complicated chains of I/R-induced signaling events, especially in the acute phase after ischemia, determining the affected regions of the tissue over time, and then, targeting and safe delivery of antioxidants, drugs, peptides, genes and cells to the areas requiring treatment. Besides the innate antioxidant and free radical scavenging properties, some nanoparticles also show higher flexibility in drug delivery and imaging. This review highlights three main approaches in nanoparticle-mediated targeting of I/R injury: nanoparticles (1) as antioxidants for reducing tissue oxidative stress, (2) for targeted delivery of therapeutic agents to the ischemic regions or cells, and (3) for imaging I/R injury at the molecular, cellular or tissue level and monitoring its evolution using contrasts induced by nanoparticles. These approaches can also be combined to realize so called theranostics for providing simultaneous diagnosis of ischemic regions and treatments by targeted delivery.

Micellar Stability in Biological Media Dictates Internalization in Living Cells

The dynamic nature of polymeric assemblies makes their stability in biological media a crucial parameter for their potential use as drug delivery systems in vivo. Therefore, it is essential to study and understand the behavior of self-assembled nanocarriers under conditions that will be encountered in vivo such as extreme dilutions and interactions with blood proteins and cells. Herein, using a combination of fluorescence spectroscopy and microscopy, we studied four amphiphilic PEG-dendron hybrids and their self-assembled micelles in order to determine their structure-stability relations. The high molecular precision of the dendritic block enabled us to systematically tune the hydrophobicity and stability of the assembled micelles. Using micelles that change their fluorescent properties upon disassembly, we observed that serum proteins bind to and interact with the polymeric amphiphiles in both their assembled and monomeric states. These interactions strongly affected the stability and enzymatic degradation of the micelles. Finally, using spectrally resolved confocal imaging, we determined the relations between the stability of the polymeric assemblies in biological media and their cell entry. Our results highlight the important interplay between molecular structure, micellar stability, and cell internalization pathways, pinpointing the high sensitivity of stability-activity relations to minor structural changes and the crucial role that these relations play in designing effective polymeric nanostructures for biomedical applications.

Small molecule delivery to solid tumors with chitosan-coated PLGA particles: A lesson learned from comparative imaging

For polymeric nanoparticles (NPs) to deliver more drugs to tumors than free drug solution, it is critical that the NPs establish interactions with tumor cells and avoid removal from the tumors. Since traditional polyethylene glycol (PEG) surface layer interferes with the cell-NP interaction in tumors, we used a water-soluble and blood-compatible chitosan derivative called zwitterionic chitosan (ZWC) as an alternative surface coating for poly(lactic-co-glycolic acid) (PLGA) NPs. The ZWC-coated PLGA NPs showed pH-dependent surface charge profiles and differential cellular interactions according to the pH of the medium. The in vivo delivery of ZWC-coated NPs was evaluated in mice bearing LS174T-xenografts using magnetic resonance (MR) imaging and fluorescence whole body imaging, which respectively tracked iron oxide particles and indocyanine green (ICG) encapsulated in the NPs as tracers. MR imaging showed that ZWC-coated NPs were more persistent in tumors than PEG-coated NPs, in agreement with the in vitro results. However, the fluorescence imaging indicated that the increased NP retention in tumors by the ZWC coating did not significantly affect the ICG distribution in tumors due to the rapid release of the dye. This study shows that stable drug retention in NPs during circulation is a critical prerequisite to successful translation of the potential benefits of surface-engineered NPs.