Styrene-maleic acid-copolymer conjugated zinc protoporphyrin as a candidate drug for tumor-targeted therapy and imaging

Previous studies indicated the potential of zinc protoporphyrin (ZnPP) as an antitumor agent targeting to the tumor survival factor heme oxygenase-1, and/or for photodynamic therapy (PDT). In this study, to achieve tumor-targeted delivery, styrene-maleic acid-copolymer conjugated ZnPP (SMA-ZnPP) was synthesized via amide bond, which showed good water solubility, having ZnPP loading of 15%. More importantly, it forms micelles in aqueous solution with a mean particle size of 111.6?nm, whereas it has an apparent Mw of 65?kDa. This micelle formation was not detracted by serum albumin, suggesting it is stable in circulation. Further SMA-ZnPP conjugate will behave as an albumin complex in blood with much larger size (235?kDa) by virtue of the albumin binding property of SMA. Consequently, SMA-ZnPP conjugate exhibited prolonged circulating retention and preferential tumor accumulation by taking advantage of enhanced permeability and retention (EPR) effect. Clear tumor imaging was thus achieved by detecting the fluorescence of ZnPP. In addition, the cytotoxicity and PDT effect of SMA-ZnPP conjugate was confirmed in human cervical cancer HeLa cells. Light irradiation remarkably increased the cytotoxicity (IC₅₀, from 33 to 5?μM). These findings may provide new options and knowledge for developing ZnPP based anticancer theranostic drugs.     

Enhanced permeability and retention of macromolecular drugs in solid tumors: A royal gate for targeted anticancer nanomedicines

Over the past two decades cancer has ascended the causes of human death to be number one or two in many nations world wide. A major limitation inherent to most conventional anticancer chemotherapeutic agents is their lack of tumor selectivity. One way to achieve selective drug targeting to solid tumors is to exploit abnormalities of tumor vasculature, namely, hypervascularisation; aberrant vascular architecture; extensive production of vascular permeability factors stimulating extravasation within tumor tissues; and lack of lymphatic drainage. Maeda and his colleagues have extensively studied tumor vascular abnormalities in terms of active and selective delivery of anticancer drugs to tumor tissues, notably defining the enhanced permeability and retention effect (EPR effect) of macromolecular drugs in solid tumors. Due to their large molecular size, nanosized macromolecular anticancer drugs administered intravenously (i.v.) escape renal clearance. Often they can not penetrate the tight endothelial junctions of normal blood vessels, but they can extravasate in tumour vasculature and become trapped in the tumor vicinity. With time the tumor concentration will build up reaching several folds higher than that of the plasma due to lack of efficient lymphatic drainage in solid tumor; an ideal application for EPR-based selective anticancer drug delivery. Establishing this principle hastened development of various polymer conjugates and polymeric micelles as well as multifunctional nanoparticles for targeted cancer chemotherapy. Indeed this selective high local concentration of nanosized anticancer drugs in tumor tissues has proven superior in therapeutic effect with minimal side effects in both preclinical and clinical settings.

In this review the mechanisms and factors involved in the EPR effect, as well as the uniqueness of nanoscale drugs for tumor targeting through EPR effect, will be discussed in detail.     

Enhanced permeability and retention of macromolecular drugs in solid tumors: a royal gate for targeted anticancer nanomedicines

Over the past two decades cancer has ascended the causes of human death to be number one or two in many nations world wide. A major limitation inherent to most conventional anticancer chemotherapeutic agents is their lack of tumor selectivity. One way to achieve selective drug targeting to solid tumors is to exploit abnormalities of tumor vasculature, namely, hypervascularisation; aberrant vascular architecture; extensive production of vascular permeability factors stimulating extravasation within tumor tissues; and lack of lymphatic drainage. Maeda and his colleagues have extensively studied tumor vascular abnormalities in terms of active and selective delivery of anticancer drugs to tumor tissues, notably defining the enhanced permeability and retention effect (EPR effect) of macromolecular drugs in solid tumors. Due to their large molecular size, nanosized macromolecular anticancer drugs administered intravenously (i.v.) escape renal clearance. Often they can not penetrate the tight endothelial junctions of normal blood vessels, but they can extravasate in tumour vasculature and become trapped in the tumor vicinity. With time the tumor concentration will build up reaching several folds higher than that of the plasma due to lack of efficient lymphatic drainage in solid tumor; an ideal application for EPR-based selective anticancer drug delivery. Establishing this principle hastened development of various polymer conjugates and polymeric micelles as well as multifunctional nanoparticles for targeted cancer chemotherapy. Indeed this selective high local concentration of nanosized anticancer drugs in tumor tissues has proven superior in therapeutic effect with minimal side effects in both preclinical and clinical settings. In this review the mechanisms and factors involved in the EPR effect, as well as the uniqueness of nanoscale drugs for tumor targeting through EPR effect, will be discussed in detail.     

The EPR effect: Unique features of tumor blood vessels for drug delivery, factors involved, and limitations and augmentation of the effect

The enhanced permeability and retention (EPR) effect is a unique phenomenon of solid tumors related to their anatomical and pathophysiological differences from normal tissues. For example, angiogenesis leads to high vascular density in solid tumors, large gaps exist between endothelial cells in tumor blood vessels, and tumor tissues show selective extravasation and retention of macromolecular drugs. This EPR effect served as a basis for development of macromolecular anticancer therapy. We demonstrated methods to enhance this effect artificially in clinical settings. Of great importance was increasing systolic blood pressure via slow angiotensin II infusion. Another strategy involved utilization of NO-releasing agents such as topical nitroglycerin, which releases nitrite. Nitrite is converted to NO more selectively in the tumor tissues, which leads to a significantly increased EPR effect and enhanced antitumor drug effects as well. This review discusses molecular mechanisms of factors related to the EPR effect, the unique anatomy of tumor vessels, limitations and techniques to avoid such limitations, augmenting tumor drug delivery, and experimental and clinical findings.     

Cancer-targeted polymeric drugs

A major challenge in cancer chemotherapy is the selective delivery of small molecule anti cancer agents to tumor cells. Water-soluble polymer-drug conjugates exhibit good water solubility, increased half-life, and potent anti tumor effects. By localizing the drug at the desired site of action, macromolecular therapeutics have improved efficacy and enhanced safety at lower doses. Since small molecule drugs and macromolecular drugs enter cells by different pathways, multi-drug resistance (MDR) can be minimized. Anti-cancer polymer-drug conjugates can be divided into two targeting modalities: passive and active. Tumor tissues have anatomic characteristics that differ from normal tissues. Macromolecules penetrate and accumulate preferentially in tumors relative to normal tissues, leading to extended pharmacological effects. This "enhanced permeability and retention" (EPR) effect is the principal reason for current successes with macromolecular anti-cancer drugs. Both natural and synthetic polymers have been used as drug carriers, and several bioconjugates have been clinically approved or are in human clinical trials. While clinically useful anti-tumor activity has been achieved using passive macromolecular drug delivery systems, further selectivity is possible by active targeting. Attachment of targeting moieties to the polymer backbone can further exploit differences between cancer and normal cells through selective receptor-mediated endocytosis. This strategy would augment the EPR effect, thereby further improving the therapeutic index of the macromolecular drug. This review discusses the development and therapeutic potential of prototype macromolecular drugs for use in cancer chemotherapy. Specific examples are selected to illustrate the basic design principles for soluble polymeric drug delivery systems.     

Polymeric micelles of zinc protoporphyrin for tumor targeted delivery based on EPR effect and singlet oxygen generation

Polymeric micelles of zinc protoporphyrin (ZnPP) with water soluble biocompatible and amphiphilic polymer, polyethylene glycol (PEG) demonstrated unique characteristics to target tumor tissues selectively based on the enhanced permeability and retention (EPR) effect. The micellar macromolecular drug of ZnPP (SMA–ZnPP and PEG–ZnPP) previously showed notable anticancer activity as a consequence of selective tumor targeting ability and its potent HO-1 inhibitory potential, resulting in suppressed biliverdin/bilirubin production in tumors thereby leading to oxystress induced tumor cell killing. Furthermore, recent findings also showed that ZnPP efficiently generated reactive singlet oxygen under illumination of visible light, laser, or xenon light source, which could augment its oxystress induced cell killing abilities. In the present paper, we report the synergistic effects of light induced photosensitizing capabilities and HO-1 inhibitory potentials of these unique micelles when tested in vitro and in vivo on tumor models under localized, mild illumination conditions using a tungsten–xenon light source. The results indicate that these water soluble polymeric micelles of ZnPP portend to be promising candidates for targeted chemotherapy as well as photodynamic therapy against superficial tumors as well as solid tumors located at light penetrable depths.     

Dual-Targeting Nanoparticles: Codelivery of Curcumin and 5-Fluorouracil for Synergistic Treatment of Hepatocarcinoma

Chemotherapy has been the standard for cancer therapy, but the nonspecific cytotoxicity of chemotherapeutic agents and drug resistance of tumor cells has limited its efficacy. However, multidrug combination therapy and targeting therapy have resulted in enhanced anticancer effects and have become increasingly important strategies in clinical applications. In this study, a biotin-/lactobionic acid–modified poly(ethylene glycol)-poly(lactic-co-glycolic acid)-poly(ethylene glycol) (BLPP) copolymer was synthesized, and curcumin- and 5-fluorouracil-loaded nanoparticles (BLPPNPs/C + F) were prepared to enhance the treatment of hepatocellular carcinoma. Blank BLPPNPs were shown to have great biocompatibility via both in vitro and in vivo studies. Good targeting of tumor cells of BLPPNPs was confirmed by flow cytometry, fluorescence microscopy, and biodistribution. The synergistic anticancer effects of BLPPNPs/C + F were demonstrated by cytotoxicity and animal studies, while western blotting was used to further verify the synergistic effect of curcumin and 5-fluorouracil. The dual-targeting and drug-loaded codelivery nanosystem demonstrated higher cellular uptake and stronger cytotoxicity for tumor cells. Therefore, these dual-targeting NPs are a promising codelivery carrier that could be made available for cellular targeting of anticancer drugs to achieve better intracellular delivery and synergistic anticancer efficacy.     

New generation of liposomal drugs for cancer

This review is focused on liposomes as a delivery system for anticancer agents and more specifically on the advantages of using liposomes as drug nanocarrier in cancer chemotherapy. The main advantages of liposomal drugs over the non-encapsulated drugs include: (1) improved pharmacokinetics and drug release, (2) enhanced intracellular penetration, (3) tumor targeting and preventing adverse side effects and (4) ability to include several active ingredients in one complex liposomal drug delivery system (DDS). The review also includes our recent data on advanced liposomal anticancer drug delivery systems. As a conclusion we propose a novel liposomal DDS which includes inhibitors of pump resistance combined in one liposomal drug delivery system with an inhibitor of antiapoptotic cellular defense, an apoptosis inducer (a traditional anticancer drug) and a targeting moiety. The proposed drug delivery system utilizes a novel three tier approach, simultaneously targeting three molecular targets: (1) extracellular receptors or antigen expressed on the surface of plasma membrane of cancer cells in order to direct the whole system specifically to the tumor, preventing adverse side effects on healthy tissues; (2) drug efflux pumps in order to inhibit them and enhance drug retention by cancer cells, increasing intracellular drug accumulation and thereby limiting the need for prescribed high drug doses that cause adverse drug side effects; and (3) intracellular controlling mechanisms of apoptosis in order to suppress cellular antiapoptotic defense.     

Polymeric micelles with stimuli-triggering systems for advanced cancer drug targeting

Abstract

Since the 1990s, nanoscale drug carriers have played a pivotal role in cancer chemotherapy, acting through passive drug delivery mechanisms and subsequent pharmaceutical action at tumor tissues with reduction of adverse effects. Polymeric micelles, as supramolecular assemblies of amphiphilic polymers, have been considerably developed as promising drug carrier candidates, and a number of clinical studies of anticancer drug-loaded polymeric micelle carriers for cancer chemotherapy applications are now in progress. However, these systems still face several issues; at present, the simultaneous control of target-selective delivery and release of incorporated drugs remains difficult. To resolve these points, the introduction of stimuli-responsive mechanisms to drug carrier systems is believed to be a promising approach to provide better solutions for future tumor drug targeting strategies. As possible trigger signals, biological acidic pH, light, heating/cooling and ultrasound actively play significant roles in signal-triggering drug release and carrier interaction with target cells. This review article summarizes several molecular designs for stimuli-responsive polymeric micelles in response to variation of pH, light and temperature and discusses their potentials as next-generation tumor drug targeting systems.     

Lipid-based drug delivery systems for cancer treatment

It is a fact that chemotherapy agents have little specificity for cancer cells, this leading to low concentrations into the tumor interstititum and severe side effects on healthy tissues. The formulation of lipid-based nanomedicines against cancer has been hypothesized to improve drug localization into the tumor tissue and to increase the anticancer efficacy of concentional drugs, while minimizing their systemic adverse effects. In this review, special attention is devoted to the analysis of the state-of-the-art in the development of lipid-based drug carriers against cancer. Specifically, the most significant in vitro and in vivo results on the use of niosomes, liposomes, and solid lipid nanoparticles are revised. It is concluded that biodistribution profiles of chemotherapy agents can be controlled by their loading to such nanoplatforms. Lipid-based nanomedicines offer an interesting approach to the delivery of anticancer drugs to brain tumors, and to reverse multi-drug resistance of cancer cells. Finally, a deep evaluation of the applicability of drug delivery strategies in the formulation of lipid-based nanoplatforms is carried out. They involve active drug targeting (including ligand-mediated delivery, and stimuli-sensitive carriers), and passive drug targeting (through the enhanced permeability and retention effect) to tumors.     

Structure optimisation to improve the delivery efficiency and cell selectivity of a tumour-targeting cell-penetrating peptide

Cell-penetrating peptide (CPP) is used for the delivery of biomacromolecules across the cell membrane and is limited in cancer therapy due to the lack of cell selectivity. Epidermal growth factor receptor (EGFR) has been widely used in clinical targeted therapy for tumours. Here, we reported a novel tumour targeting cell-penetrating peptide (TCPP), EHB (ELBD-C6H) with 20-fold and 3000-fold greater transmembrane ability and tumour cell selectivity than our previously reported S3-HBD and classic CPP TAT, respectively. In this new TCPP, a specific alpha helix structure was inserted into a repeated amino acid (AA) sequence formed by tandem multiple selected key AA residues of vaccinia growth factor (VGF), and this sequence was then fused to a tailored heparin binding domain sequence (C6H) derived from heparin-binding epidermal growth factor-like growth factor to intensify its targeting delivery ability. EHB could carry anticancer proteins such as MAP30 (Momordica Antiviral Protein 30?kDa) into EGFR-overexpressing cancer cell and inhibit cell growth, but it had a greatly reduced interaction with normal cells. These results indicated that EHB, as a novel efficient TCPP for the selective delivery of drug molecules into cancer cells, would help to improve the efficacy and safety of anti-tumour drugs.     

Advances in polymeric micelles for drug delivery and tumor targeting

A plethora of formulation techniques have been reported in the literature for targeting drugs to specific sites. Polymeric micelles (PMs) can be targeted to tumor sites by passive as well as active mechanisms. Some inherent properties of PMs, including size in the nanorange, stability in plasma, longevity in vivo, and pathological characteristics of tumor allow PMs to be targeted to the tumor site by a passive mechanism called the enhanced permeability and retention effect. PMs formed from an amphiphilic block copolymer are suitable for encapsulation of poorly water-soluble, hydrophobic anticancer drugs. Other characteristics of PMs such as separate functionality at the outer shell are useful for targeting the anticancer drug to tumor by active mechanisms. PMs can be conjugated with many ligands such as antibody fragments, epidermal growth factors, α₂-glycoprotein, transferrin, and folate to target micelles to cancer cells. Application of heat or ultrasound are the alternative methods to enhance drug accumulation in tumoral cells. Targeting using micelles can also be directed toward tumor angiogenesis, which is a potentially promising target for anticancer drugs. PMs have been used for the delivery of many anticancer agents in preclinical and clinical studies. This review summarizes recently available information regarding targeting of anticancer drugs to the tumor site using PMs.From the Clinical EditorThis review summarizes recent developments related to targeted anticancer drug delivery to tumor sites using polymeric micelles via active and passive mechanisms. Polymeric micelles can be conjugated with diverse ligands such as antibodies fragments, epidermal growth factors, α2-glycoprotein, transferrin, folate to target micelles to cancer cells.     

A pharmaceutical study of doxorubicin-loaded PEGylated nanoparticles for magnetic drug targeting

One of the new strategies to improve cancer chemotherapy is based on new drug delivery systems, like the polyethylene glycol-coated superparamagnetic iron oxide nanoparticles (PEG-SPION, thereafter called PS). In this study, PS are loaded with doxorubicin (DOX) anticancer drug, using a pre-formed DOX-Fe²⁺ complex reversible at lower pH of tumour tissues and cancer cells. The DOX loaded PS (DLPS, 3% w/w DOX/iron oxide) present a hydrodynamic size around 60 nm and a zeta potential near zero at physiological pH, both parameters being favourable for increased colloidal stability in biological media and decreased elimination by the immune system. At physiological pH of 7.4, 60% of the loaded drug is gradually released from the DLPS in ∼2 h. The intracellular release and distribution of DOX is followed by means of confocal spectral imaging (CSI) of the drug fluorescence. The in vitro cytotoxicity of the DLPS on MCF-7 breast cancer cells is equivalent to that of a DOX solution. The reversible association of DOX to the SPION surface and the role of polymer coating on the drug loading/release are discussed, both being critical for the design of novel stealth magnetic nanovectors for chemotherapy.     

Overcoming chemotherapy resistance via simultaneous drug-efflux circumvention and mitochondrial targeting

Multidrug resistance (MDR) has been considered as a huge challenge to the effective chemotherapy. Therefore, it is necessary to develop new strategies to effectively overcome MDR. Here, based on the previous research of N-(2-hydroxypropyl)methacrylamide (HPMA) polymer–drug conjugates, we designed an effective system that combined drug-efflux circumvention and mitochondria targeting of anticancer drug doxorubicin (Dox). Briefly, Dox was modified with mitochondrial membrane penetrating peptide (MPP) and then attached to (HPMA) copolymers (P-M-Dox). Our study showed that macromolecular HPMA copolymers successfully bypassed drug efflux pumps and escorted Dox into resistant MCF-7/ADR cells via endocytic pathway. Subsequently, the mitochondria accumulation of drugs was significantly enhanced with 11.6-fold increase by MPP modification. The excellent mitochondria targeting then resulted in significant enhancement of reactive oxygen species (ROS) as well as reduction of adenosine triphosphate (ATP) production, which could further inhibit drug efflux and resistant cancer cell growth. By reversing Dox resistance, P-M-Dox achieved much better suppression in the growth of 3D MCF-7/ADR tumor spheroids compared with free Dox. Hence, our study provides a promising approach to treat drug-resistant cancer through simultaneous drug efflux circumvention and direct mitochondria delivery.     

Drug targeting strategies in cancer treatment: an overview

Classic chemotherapy his little or no specificity for cancer cells, normally resulting in low accumulation at the tumor region (inefficacy), and in severe side effects (toxicity). This challenge has resulted in the development of several deliver strategies for chemotherapy agents to improve their concentration at the tumor site, simultaneously increasing their anticancer efficacy, while reducing the associated adverse systemic effects. In this work, the potential of drug delivery strategies involving the use of nanocarriers for controlling the biodistribution of antitumor drugs is deeply revised: passive targeting (through the enhanced permeability and retention effect, EPR effect) and active targeting (including stimuli-sensitive carriers and ligand-mediated delivery). Special attention will be also focussed on the recent approaches for overcoming multi-drug resistence. Finally, a general view of the problem of "nanotoxicity" in cancer treatment is also given.     

Recent advances in tumor vasculature targeting using liposomal drug delivery systems

Tumor vessels possess unique physiological features that might be exploited for improved drug delivery. The targeting of liposomal anticancer drugs to tumor vasculature is increasingly recognized as an effective strategy to obtain superior therapeutic efficacy with limited host toxicity compared with conventional treatments. This review introduces recent advances in the field of liposomal targeting of tumor vasculature, along with new approaches that can be used in the design and optimization of liposomal delivery systems. In addition, cationic liposome is focused on as a promising carrier for achieving efficient vascular targeting. The clinical implications are discussed of several approaches using a single liposomal anticancer drug formulation: dual targeting, vascular targeting (targeting tumor endothelial cells) and tumor targeting (targeting tumor cells).     

Ligand-targeted particulate nanomedicines undergoing clinical evaluation: Current status

Since the introduction of Doxil® on the market nearly 20 years ago, a number of nanomedicines have become part of treatment regimens in the clinic. With the exception of antibody–drug conjugates, these nanomedicines are all devoid of targeting ligands and rely solely on their physicochemical properties and the (patho)physiological processes in the body for their biodistribution and targeting capability. At the same time, many preclinical studies have reported on nanomedicines exposing targeting ligands, or ligand-targeted nanomedicines, yet none of these have been approved at this moment. In the present review, we provide a concise overview of 13 ligand-targeted particulate nanomedicines (ligand-targeted PNMs) that have progressed into clinical trials. The progress of each ligand-targeted PNM is discussed based on available (pre)clinical data. Main conclusions of these analyses are that (a) ligand-targeted PNMs have proven to be safe and efficacious in preclinical models; (b) the vast majority of ligand-targeted PNMs is generated for the treatment of cancer; (c) contribution of targeting ligands to the PNM efficacy is not unambiguously proven; and (d) targeting ligands do not cause localization of the PNM within the target tissue, but rather provide benefits in terms of target cell internalization and target tissue retention once the PNM has arrived at the target site. Increased understanding of the in vivo fate and interactions of the ligand-targeted PNMs with proteins and cells in the human body is mandatory to rationally advance the clinical translation of ligand-targeted PNMs. Future perspectives for ligand-targeted PNM approaches include the delivery of drugs that are unable or inefficient in passing cellular membranes, treatment of drug resistant tumors, targeting of the tumor blood supply, the generation of targeted vaccines and nanomedicines that are able to cross the blood–brain barrier.     

In search of the Holy Grail: Folate-targeted nanoparticles for cancer therapy

Targeted drug therapy or “smart” drug delivery, potentially combined with simultaneous imaging modalities to monitor the delivery of drugs to specific tissues, is arguably the “holy grail” of pharmacology. Therapeutic approaches that exploit nanoparticles to deliver drugs selectively to cancer cells are currently considered one of the most promising avenues in the area of cancer therapeutics and imaging. The potential to deliver active chemotherapeutic drugs in the vicinity or directly within specific tumors via receptor mediated pathways, and to image tumors through the use of nanoparticles has been conceptually and experimentally shown for several classes of nanoparticles. Nanoparticles functionalized with the vitamin folic acid are of particular interest as a variety of malignant tumors are known to overexpress the folate receptor(s). Indeed, several nanoparticle architectures with improved retention time, administration route, biocompatibility, absorption, and clearance are being proposed and are in late stage clinical development. This commentary highlights some of the most important concepts related to nanoparticles and folate-mediated drug delivery and imaging in cancer research.     

Development of innovative paclitaxel-loaded small PLGA nanoparticles: Study of their antiproliferative activity and their molecular interactions on prostatic cancer cells

Taxanes, including paclitaxel, are anti-cancer drugs approved for the treatment of prostate cancer but which have limited clinical application due to their hydrophobicity, their low therapeutic index and the emergence of chemoresistance. These side effects may be avoided through the use of new drug delivery systems such as nanoparticles, and paclitaxel-loaded PLGA nanoparticles up to 200 nm in size have shown encouraging results. As it is known that size affects the tissular penetration and distribution of tumors via the enhanced permeability and retention effect, so nanoparticles smaller than 100 nm are potentially interesting vehicles for improving paclitaxel delivery and efficacy.