Currently, the limited penetration of nanoparticles remains a major challenge for antitumor nanomedicine to penetrate into the tumor tissues. Herein, we propose a size-shrinkable drug delivery system based on a polysaccharide-modified dendrimer with tumor microenvironment responsiveness for the first time to our knowledge, which was formed by conjugating the terminal glucose of hyaluronic acid (HA) to the superficial amidogen of poly(amidoamine) (PAMAM), using a matrix metalloproteinase-2 (MMP-2)-cleavable peptide (PLGLAG) via click reaction. These nanoparticles had an initial size of ∼200 nm, but once deposited in the presence of MMP-2, they experienced a dramatic and fast size change and dissociated into their dendrimer building blocks (∼10 nm in diameter) because of cleavage of PLGLAG. This rapid size-shrinking characteristic not only promoted nanoparticle extravasation and accumulation in tumors benefited from the enhanced permeability and retention effect but also achieved faster nanoparticle diffusion and penetration. We have further conducted comparative studies of MMP-2-sensitive macromolecules (HA-pep-PAMAM) and MMP-2-insensitive macromolecules (HA-PAMAM) synthesized with a similar particle size, surface charge, and chemical composition and evaluated in both monolayer cells and multicellular spheroids. The results confirmed that the enzyme-responsive size shrink is an implementable strategy to enhance drug penetration and to improve therapeutic efficacy. Meanwhile, macromolecule-based nanoparticles with size-variable characteristics not only promote drug penetration, but they can also be used as gene delivery systems, suggesting great potential as nano-delivery systems.

Multidrug resistance (MDR) is the major obstacle for chemotherapy. In a previous study, we have successfully synthesized a novel doxorubicin (DOX) derivative modified by triphenylphosphonium (TPP) to realize mitochondrial delivery of DOX and showed the potential of this compound to overcome DOX resistance in MDA-MB-435/DOX cells. (1) To introduce specificity for DOX-TPP to cancer cells, here we report on the conjugation of DOX-TPP to hyaluronic acid (HA) by hydrazone bond with adipic acid dihydrazide (ADH) as the acid-responsive linker, producing HA- hydra-DOX-TPP nanoparticles. Hyaluronic acid (HA) is a natural water-soluble linear glycosaminoglycan, which was hypothesized to increase the accumulation of nanoparticles containing DOX-TPP in the mitochondria of tumor cells upon systemic administration, overcoming DOX resistance, in vivo. Our results showed HA- hydra-DOX-TPP to self-assemble to core/shell nanoparticles of good dispersibility and effective release of DOX-TPP from the HA- hydra-DOX-TPP conjugate in cancer cells, which was followed by enhanced DOX mitochondria accumulation. The HA- hydra-DOX-TPP nanoparticles also showed improved anticancer effects, better tumor cell apoptosis, and better safety profile compared to free DOX in MCF-7/ADR bearing mice.

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Self-assembled peptide nanofibers have been widely studied in cancer nanotherapeutics with their excellent biocompatibility and low toxicity of degradation products, showing the significant potential in inhibiting tumor progression. However, poor solubility prevents direct intravenous administration of nanofibers. Although water-soluble peptide precursors have been formed via the method of phosphorylation for intravenous administration, their opportunities for broad in vivo application are limited by the weak capacity of encapsulating drugs. Herein, we designed a novel restructured reduced glutathione (GSH)-responsive drug delivery system encapsulating doxorubicin for systemic administration, which achieved the intracellular restructuration from three-dimensional micelles into one-dimensional nanofibers. After a long blood circulation, micelles endocytosed by tumor cells could degrade in response to high GSH levels, achieving more release and accumulation of doxorubicin at desired sites. Further, the synergistic chemotherapy effects of self-assembled nanofibers were confirmed in both in vitro and in vivo experiments.

Breast cancer is the leading cause of cancer-related death for women, and multidrug resistance (MDR) is the major obstacle faced by chemotherapy for breast cancer. We have previously synthesized a doxorubicin (DOX) derivative by conjugating DOX with triphenylphosphonium (TPP) to achieve mitochondrial delivery, which induced higher cytotoxicity in drug-resistant breast cancer cells than DOX itself. Due to its amphiphilicity, TPP-DOX is difficult to physically entrap in nanocarriers. Thus, we linked it to hyaluronic acid (HA) by a novel ionic bond utilizing the specific bromide ion of TPP to form supra-molecular self-assembled structures (HA-ionic-TPP-DOX). The product was analyzed uisng H-NMR,C-NMR and mass spectrometry. The HA nanocarriers (HA-ionic-TPP-DOX) were shown to self-assemble into spherical nanoparticles, and sensitive to acidic pH in terms of morphology and drug release. Compared with free DOX, HA-ionic-TPP-DOX produced much greater intracellular DOX accumulation and mitochondrial localization, leading to increased ROS production, slightly decreased mitochondrial membrane potential, increased cytotoxicity in MCF-7/ADR cells and enhanced tumor targeting in vivo. In xenotransplant zebrafish model with the MCF-7/ADR cell line, both TPP-DOX and HA-ionic-TPP-DOX inhibited tumor cell proliferation without inducing significant side effects compared with free DOX. In addition, we observed a better anti-tumor effect of HA-ionic-TPP-DOX on MCF-7/ADR cells in zebrafish than that of TPP-DOX treatment. Furthermore, HA-ionic-DOX-TPP exhibited favorable biocompatibility and anti-tumor effects in MCF-7/ADR tumor-bearing nude mice in comparison with the effects of TPP-DOX and DOX, suggesting the potential of HA-ionic-TPP-DOX for the targeted delivery and controlled release of TPP-DOX, which can lead to the sensitization of resistant breast tumors.

Doxorubicin (DOX) is a first-line chemo drug for cancer therapy, yet it fails to treat multi-drug-resistant tumors. Hypoxia is a major causative factor leading to chemotherapy failure. Particularly, hypoxia up-regulates its responsive transcription factorhypoxia-inducible factors (HIF)to induce the overexpression of drug resistant genes. Metformin (MET) is recently found to cooperate with DOX against multiple tumors. As a mitochondrial inhibitor, MET could suppress tumor oxygen consumption, and thereby modulate the hypoxic tumor microenvironment. In this study, we used cationic liposomes to codeliver both DOX and MET for treating multi-drug-resistant breast cancer cellsMCF7/ADR. Faster release of MET enhanced the cytotoxicity of DOX through attenuating hypoxic stress both in vivo and in vitro. MET diminished the cellular oxygen consumption and inhibited HIF1α and P-glycoprotein (Pgp) expression in vitro. In addition, the dual-drug-loaded liposomes increased tumor targeting and intratumoral blood oxygen saturation, which suggested that the tumor reoxygenation effect of MET facilitated the exertion of its synergistic activity with DOX against MCF7/ADR xenografts. In general, our study represents a feasible strategy to boost the therapeutic effect in treating multi-drug-resistant cancer by improving the hypoxic tumor microenvironment.

Nitrogen mustard (NM) is among the earliest drugs used to treat malignant tumors and it kills tumor cells by cross-linking DNA. Unfortunately, because of the short halflife and unfavorable selectivity, NM causes significant damage to normal tissues. As NM can increase the levels of reactive oxygen species (ROS) in tumor cells, a ROS-activated nitrogen mustard prodrug (NM-Pro) was synthesized and mixed with NM at a specific ratio to obtain an "NM-ROS-NM-Pro-NM" positive feedback system, which ultimately achieves a specific lethal effect on hematological neoplasms. The further encapsulation of NM/NM-Pro in liposomes allows the sustained release of the drug and prolongs the residence time in vivo.Here, we prepared stable liposomes with a uniform particle size of 170.6 ± 2.2 nm. The optimal ratio of NM to NM-Pro in this study was 2:1. The active drug NM in the NM/NM-Pro system continuously stimulated ROS production by the cells, which in turn further activated the NM-Pro to continuously generate NM. The positive feedback pathway between the NM and NM-Pro resulted in the specific death of tumor cells. Furthermore, the K562 hematological neoplasm model was utilized to evaluate the therapeutic effect of NM/NM-Pro liposomes in vivo. After encapsulation in liposomes, the targeting of tumor cells was increased approximately two times compared with that of normal cells, and NM/NM-Pro liposomes exhibited reduced toxicity, without an increase in drug activity compared to the NM/NM-Pro combination. The NM/NM-Pro delivery system exerts a positive feedback effect on ROS production in tumor cells and displays good potential for the specific killing of hematoma cells.

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