Drug delivery to tumours: recent strategies

Despite several advancements in chemotherapy, the real therapy of cancer still remains a challenge. The development of new anti-cancer drugs for the treatment of cancer has not kept pace with the progress in cancer therapy, because of the nonspecific drug distribution resulting in low tumour concentrations and systemic toxicity. The main hindrance for the distribution of anti-cancer agents to the tumour site is the highly disorganized tumour vasculature, high blood viscosity in the tumour, and high interstitial pressure within the tumour tissue. Recently, several approaches such as drug modifications and development of new carrier systems for anti-cancer agents have been attempted to enhance their tumour reach. Approaches such as drug delivery through enhanced permeability and retention (EPR) effect have resulted in a significant improvement in concentration in tumours, while approaches such as drug-carrier implants and microparticles have resulted in improvement in local chemotherapy of cancer. This review discusses different strategies employed for the delivery of anti-cancer agents to tumours, such as through EPR effect, local chemotherapeutic approaches using drug delivery systems, and special strategies such as receptor-mediated delivery, pH-based carriers, application of ultrasound and delivery to resistant tumour cells and brain using nanoparticles.     

A review of mechanistic insight and application of pH-sensitive liposomes in drug delivery

Abstract

The pH-sensitive liposomes have been extensively used as an alternative to conventional liposomes in effective intracellular delivery of therapeutics/antigen/DNA/diagnostics to various compartments of the target cell. Such liposomes are destabilized under acidic conditions of the endocytotic pathway as they usually contain pH-sensitive lipid components. Therefore, the encapsulated content is delivered into the intracellular bio-environment through destabilization or its fusion with the endosomal membrane. The therapeutic efficacy of pH-sensitive liposomes enables them as biomaterial with commercial utility especially in cancer treatment. In addition, targeting ligands including antibodies can be anchored on the surface of pH-sensitive liposomes to target specific cell surface receptors/antigen present on tumor cells. These vesicles have also been widely explored for antigen delivery and serve as immunological adjuvant to enhance the immune response to antigens. The present review deals with recent research updates on application of pH-sensitive liposomes in chemotherapy/diagnostics/antigen/gene delivery etc.     

Liposomal encapsulated anti-cancer drugs

Among several drug delivery systems, liposomal encapsulated anti-cancer agents represent an advanced and versatile technology. Several formulations of liposomal anthracyclines are approved, e.g. for the treatment of metastatic breast cancer (pegylated and non-pegylated liposomal doxorubicin) or AIDS-related Kaposi's sarcoma (pegylated liposomal doxorubicin and liposomal daunorubicin). Meanwhile, virtually all anti-cancer drugs have been encapsulated in liposomes using different technologies. This review will summarize preclinical and clinical data of approved and exemplary emerging liposomal anti-cancer agents.     

Pharmacotherapy by intracellular delivery of drugs using fusogenic liposomes: application to vaccine development.

We prepared fusogenic liposomes by fusing conventional liposomes with an ultra-violet inactivated Sendai virus. Fusogenic liposomes can deliver encapsulated contents into the cytoplasm directly in a Sendai virus fusion-dependent manner. Based on the high delivery rates into the cytoplasm, we originally planned to apply the fusogenic liposomes to cancer chemotherapy and gene therapy. We have recently also examined the use of fusogenic liposomes as an antigen delivery vehicle. In terms of vaccine development, cytoplasmic delivery is crucial for the induction of the cytotoxic T lymphocyte (CTL) responses that play a pivotal role against infectious diseases and cancer. In this context, our recent studies suggested that fusogenic liposomes could deliver encapsulated antigens into the cytoplasm and induce MHC class I-restricted, antigen-specific CTL responses. In addition, fusogenic liposomes are also effective as a mucosal vaccine carrier. In this review, we present the feasibility of fusogenic liposomes as a versatile and effective antigen delivery system.     

天然药物靶向给药系统的研究

采用新型药物载体使天然药物具有靶向作用是近年来药剂学的研究热点之一。综述脂质体、纳米粒、微球、微乳、药质体等新型载体在天然药物靶向给药系统研究中的应用,并介绍膜融合脂质体、纳米脂质载体、药脂结合物纳米粒以及分泌颗粒类似物等几种新型靶向给药系统的药物载体。     

pH-Sensitive PEGylated liposomes functionalized with a fibronectin-mimetic peptide show enhanced intracellular delivery to colon cancer cell

pH-sensitive liposomes undergo rapid destabilization under mildly acidic conditions such as those found in endocytotic vesicles. Though this makes them promising drug carriers, their application is limited due to their rapid clearance from circulation by the reticulo-endothelial system. Researchers have therefore used pH-sensitive liposomes that are sterically stabilized by polyethylene glycol (PEG) molecules (stealth liposomes) on the liposome surface. The goal of this study is to bring bio-functionality to pH-sensitive PEGylated liposomes in order to facilitate their potential use as a targeted drug delivery agent. To improve the selectivity of these nanoparticles, we included a targeting moiety, PR_b which specifically recognizes and binds to integrin α(5)β(1) expressing cells. PR_b (KSSPHSRN(SG)(5)RGDSP) is a novel fibronectin-mimetic peptide sequence that mimics the cell adhesion domain of fibronectin. Integrin α(5)β(1) is expressed on several types of cancer cells, including colon cancer, and plays an important role in tumor growth and metastasis. We have thoroughly studied the release of calcein from pH-sensitive PEGylated liposomes by varying the lipid composition of the liposomes in the absence and presence of the targeting peptide, PR_b, and accounting for the first time for the effect of both pH and time (photo-bleaching effect) on the fluorescence signal of calcein. We have demonstrated that we can design PR_b-targeted pH-sensitive PEGylated liposomes, which can undergo destabilization under mildly acidic conditions and have shown that incorporating the PR_b peptide does not significantly affect the pH-sensitivity of the liposomes. PR_b-targeted pH-sensitive PEGylated liposomes bind to CT26.WT colon carcinoma cells that express integrin α(5)β(1), undergo cellular internalization, and release their load intracellularly in a short period of time as compared to other formulations. Our studies demonstrate that PR_b-functionalized pH-sensitive targeted delivery systems have the potential to deliver a payload directly to cancer cells in an efficient and specific manner.     

脂质体作为抗肿瘤药物载体的应用研究

目前，应用脂质体作为抗肿瘤药物载体已成为趋势。脂质体可明显提高抗肿瘤药物的靶向性，延长药物的作用时间，降低药物毒性。作者对国内外普通脂质体和修饰脂质体（包括长循环脂质体、免疫脂质体、温度敏感脂质体和pH敏感脂质体）的相关文献进行了综述。结果表明，脂质体是抗肿瘤药物的理想载体，在肿瘤治疗中有着广阔的应用前景。     

pH- and sugar-sensitive layer-by-layer films and microcapsules for drug delivery

The present review provides an overview on the recent progress in the development of pH- and sugar-sensitive layer-by-layer (LbL) thin films and microcapsules in relation to their potential applications in drug delivery. pH-sensitive LbL films and microcapsules have been studied for the development of peptide and protein drug delivery systems to the gastrointestinal tract, anti-cancer drugs to tumor cells, anti-inflammatory drugs to inflamed tissues, and the intracellular delivery of DNA, where pH is shifted from neutral to acidic. pH-induced decomposition or permeability changes of LbL films and microcapsules form the basis for the pH-sensitive release of drugs. Sugar-sensitive LbL films and microcapsules have been studied mainly for the development of an artificial pancreas that can release insulin in response to the presence of glucose. Therefore, glucose oxidase, lectin, and phenylboronic acid have been used for the construction of glucose-sensitive LbL films and microcapsules. LbL film-coated islet cells are also candidates for an artificial pancreas. An artificial pancreas would make a significant contribution to improving the quality of life of diabetic patients by replacing repeated subcutaneous insulin injections.     

Fusogenic pH sensitive liposomal formulation for rapamycin: Improvement of antiproliferative effect

Context: Liposomes are increasingly employed to deliver chemotherapeutic agents, antisense oligonucleotides, and genes to various therapeutic targets.

Objective: The present investigation evaluates the ability of fusogenic pH-sensitive liposomes of rapamycin in increasing its antiproliferative effect on human breast adenocarcinoma (MCF-7) cell line.

Materials and methods: Cholesterol (Chol) and dipalmitoylphosphatidylcholine (DPPC) (DPPC:Chol, 7:3) were used to prepare conventional rapamycin liposomes by a modified ethanol injection method. Dioleoylphosphatidylethanolamine (DOPE) was used to produce fusogenic and pH-sensitive properties in liposomes simultaneously (DPPC:Chol:DOPE, 7:3:4.2). The prepared liposomes were characterized by their size, zeta potential, encapsulation efficiency percent (EE%), and chemical stability during 6 months. The antiproliferative effects of both types of rapamycin liposomes (10, 25, and 50?nmol/L) with optimized formulations were assessed on MCF-7 cells, as cancerous cells, and human umbilical vein endothelial cells (HUVEC), as healthy cells, employing the diphenyltetrazolium bromide (MTT) assay for 72?h.

Results and discussion: The particle size, zeta potential, and EE% of the liposomes were 165?±?12.3 and 178?±?15.4?nm, ?39.6?±?1.3, and ?41.2?±?2.1?mV as well as 76.9?±?2.6 and 76.9?±?2.6% in conventional and fusogenic pH-sensitive liposomes, respectively. Physicochemical stability results indicated that both liposome types were relatively stable at 4?°C than 25?°C. In vitro antiproliferative evaluation showed that fusogenic pH-sensitive liposomes had better antiproliferative effects on MCF-7 cells compared to the conventional liposomes. Conversely, fusogenic pH-sensitive liposomes had less cytotoxicity on HUVEC cell line.     

Estrogen-anchored pH-sensitive liposomes as nanomodule designed for site-specific delivery of doxorubicin in breast cancer therapy

The present investigation reports the development of nanoengineered estrogen receptor (ER) targeted pH-sensitive liposome for the site-specific intracellular delivery of doxorubicin (DOX) for breast cancer therapy. Estrone, a bioligand, was anchored on the surface of pH-sensitive liposome for drug targeting to ERs. The estrone-anchored pH-sensitive liposomes (ES-pH-sensitive-SL) showed fusogenic potential at acidic pH (5.5). In vitro cytotoxicity studies carried out on ER-positive MCF-7 breast carcinoma cells revealed that ES-pH-sensitive-SL formulation was more cytotoxic than non-pH-sensitive targeted liposomes (ES-SL). The flow cytometry analysis confirmed significant enhanced uptake (p < 0.05) of ES-pH-sensitive-SL by MCF-7 cells. Intracellular delivery and nuclear localization of the DOX was confirmed by fluorescence microscopy. The mechanism for higher cytotoxicity shown by estrone-anchored pH-sensitive liposomal-DOX was elucidated using reactive oxygen species (ROS) determination. The in vivo biodistribution studies and antitumor activities of formulations were evaluated on tumor bearing female Balb/c mice followed by intravenous administration. The ES-pH-sensitive-SL efficiently suppressed the breast tumor growth in comparison to both ES-SL and free DOX. Serum enzyme activities such as LDH and CPK levels were assayed for the evaluation of DOX induced cardiotoxicity. The ES-pH-sensitive-SL accelerated the intracellular trafficking of encapsulated DOX, thus increasing the therapeutic efficacy. The findings support that estrone-anchored pH-sensitive liposomes could be one of the promising nanocarriers for the targeted intracellular delivery of anticancer agents to breast cancer with reduced systemic side effects.     

新型脂质体的研究进展

作为药物传递系统的载体,几种新型脂质体如膜融合脂质体、柔性脂质体、表面修饰脂质体等的研究己取得显著进展.本文归纳和分析了近期有关脂质体稳定性、靶向性及修饰材料的文献.     

Mechanisms of drug release in pH-sensitive micelles for tumour targeted drug delivery system: A review

During the past decades, chemotherapy has been regarded as the most effective method for tumor therapy, but still faces significant challenges, such as poor tumor selectivity and multidrug resistance. The development of targeted drug delivery systems brings certain dramatic advantages for reducing the side effects and improving the therapeutic efficacy. Coupling a specific stimuli-triggered drug release mechanism with these delivery systems is one of the most prevalent approaches for targeted therapy. Among these approaches, pH-sensitive micelles are regarded as the most general strategy with advantages of increasing solubility of water-insoluble drugs, pH-sensitive release, high drug loading, etc.This review will focus on the potential of pH-sensitive micelles in tumor therapy, analyze four types of drug-loaded micelles and mechanisms of drug release and give an exhaustive collection of recent investigations. Sufficient understanding of these mechanisms will help us to design more efficient pH-sensitive drug delivery system to address the challenges encountered in targeted drug delivery systems for tumor therapy.     

pH-sensitive liposomes: mechanism of triggered release to drug and gene delivery prospects

A growing amount of literature describes the development and applications of novel targeting and/or release triggering schemes to improve the therapeutic index of drugs encapsulated within liposomes. The composition of liposomal systems can be modified to facilitate site specific release in response to environmental conditions, namely at the gross anatomical level, at the cellular or sub-cellular level. They are designed in order to release their contents in response to acidic pH within the endosomal system while remaining stable in plasma thus improving the cytoplasmic delivery of various polar materials and macromolecules such as anti-tumor drugs, toxins, proteins and DNA. This review covers various aspect of research on pH-sensitive liposomes.     

Nanotech revolution for the anti-cancer drug delivery through blood- brain-barrier

Nanotechnology-based drug delivery was born as a chance for pharmaceutical weapons to be delivered in the body sites where drug action is required. Specifically, the incorporation of anti-cancer agents in nanodevices of 100-300 nm allows their delivery in tissues that have a fenestrated vasculature and a reduced lymphatic drainage. These two features are typical of neoplastic tissues and, therefore, allow the accumulation of nanostructured devices in tumours. An important issue of anti-cancer pharmacological strategies is the overcoming of anatomical barriers such as the bloodbrain- barrier (BBB) that protects brain from toxicological injuries but, at the same time, makes impossible for most of the pharmacological agents with anti-cancer activity to reach tumour cells placed in the brain and derived from either primary tumours or metastases. In fact, only highly lipophilic molecules can passively diffuse through BBB to reach central nervous system (CNS). Another possibility is to use nanotechnological approaches as powerful tools to across BBB, by both prolonging the plasma half-life of the drugs and crossing fenestrations of BBB damaged by brain metastases. Moreover, modifications of nanocarrier surface with specific endogenous or exogenous ligands can promote the crossing of intact BBB as in the case of primary brain tumours. This aim can be achieved through the binding of the nanodevices to carriers or receptors expressed by the endothelial cells of BBB and that can favour the internalization of the nanostructured devices delivering anti-cancer drugs. This review summarizes the most meaningful advances in the field of nanotechnologies for brain delivery of drugs.     

Tumour-specific uptake of anti-cancer drugs: the future is here

A challenge of anti-cancer treatment is the specific delivery of the drugs in order to avoid deleterious effects on normal cells. In fact, anti-cancer drugs have potent effects also on normal cells due to the strong similarity of the mechanisms of growth regulation of normal cells if compared to their transformed counterparts. The recent developments in nanotechnology allow the old Ehrlich's dream to deliver anti-cancer drugs in tumour tissue through their encapsulation in drug delivery systems (DDS). In the present review we analyze the different reasons to encapsulate an anti-tumour drug in DDS including eventual damages induced by their extravasation or by eccipients used to their solubilisation, the rapid break-down of the drug in vivo and the specific bio-distribution of the drug in tumour tissues. The delivery strategies of anti-cancer drugs are based upon the particular structure of tumour neo-angiogenic vessels that allow the passive targeting or enhanced permeability and retention (EPR). In order to avoid the entrapping of DDS in reticulo-endothelial system the nanoparticles can be modified with the addition on their surface of inert polyetilenglicole (PEG) molecules that inhibit the opsonisation of DDS by macrophages. The addition of targeting moieties, antibodies or Fab fragments or small peptides and aptamers, on the surface of DDS can allow the active targeting of DDS to tumour cells. In conclusion, a new avenue in anti-cancer treatment has been disclosed with the use of DDS.     

pH-sensitive, serum-stable and long-circulating liposomes as a new drug delivery system

The lack of stability in blood and the short blood circulation time of pH-sensitive liposomes are major drawbacks for their application in-vivo. To develop pH-sensitive, serum-stable and long-circulating liposomes as drug delivery systems, the impact of polyethylene glycol-derived phosphatidylethanolamine (DSPE-PEG) on the properties of pH-sensitive liposomes was investigated. pH-sensitive liposomes were prepared with dioleoylphosphatidylethanolamine (DOPE) and oleic acid (DOPE/oleic acid liposome) or DOPE and 1,2-dipalmitoylsuccinylglycerol (DOPE/DPSG liposome). The inclusion of DSPE-PEG enhanced the serum stability of both DOPE/oleic acid and DOPE/DPSG liposomes, but also shifted the pH-response curve of pH-sensitive liposomes to more acidic regions and reduced the maximum leakage percentage. The impact of DSPE-PEG, however, was much lower in the DOPE/DPSG liposomes than in the DOPE/oleic acid liposomes. In tumour tissue homogenates, where the pH is lower than normal healthy tissues, the pH-sensitive DOPE/DPSG liposomes released the entrapped markers rapidly, in comparison with pH-insensitive dipalmitoylphosphatidylcholine/cholesterol/DSPE-PEG liposomes. Moreover, the release rate was not affected by the content of DSPE-PEG. The blood circulation time of methotrexate incorporated in DOPE/UDPSG liposomes was significantly prolonged with increasing content of DSPE-PEG. Taken together, the liposomes composed of DOPE, DPSG and DSPE-PEG (up to 5%) were pH sensitive, plasma stable and had a long circulation time in the blood. The complete destabilization of the liposomes at tumour tissues suggests that the liposomes might be useful for the targeted delivery of drugs such as anticancer agents.     

pH值敏感型脂质体表面修饰技术的研究进展

pH 值敏感型脂质体表面修饰已成为脂质体制剂的研究热点,经过表面修饰的pH值敏感型脂质体进入人体后,在此特异性片段的介导下,被靶细胞识别,达到在靶细胞有针对性地释放药物和减少对机体正常组织损害的目的.对pH值敏感型脂质体进行表面修饰的主要目的有3个:延长体循环时间、增强pH值敏感性和提高靶向性.主要从这3方面综述近十年来pH值敏感型脂质体表面修饰的研究进展,为pH值敏感型脂质体表面修饰的进一步研究提供参考.     

Liposome technology for drug delivery against mycobacterial infections

Mycobacteria are intracellular pathogens that invade and reside inside macrophages. There has been a rapid resurgence in infections caused by the genus mycobacteria. Chemotherapy of mycobacterial infections is prolonged, hepatotoxic and very often inadequate in achieving optimal drug concentrations inside the cells. Recent advances in controlled delivery systems for drugs such as liposomes have sparked a renewed interest in their potential application for the treatment of mycobacterial infections. The versatility of liposomes in incorporation of hydrophilic/hydrophobic components, non-toxic nature, biodegradability, biocompatibility and property of sustained release makes them attractive candidates for the delivery of antitubercular drugs. Liposome research in the area of mycobacterial diseases has evolved and matured through several phases; from the laboratory to the clinics. This review, thus focuses on the use of liposomes for the treatment of various types of mycobacterial diseases.     

pH-sensitive immunoliposomes specific to the CD33 cell surface antigen of leukemic cells

A promising avenue in cancer therapy using liposomal formulations is the combination of site-specific delivery with triggered drug release. The use of trigger mechanisms in liposomes could be relevant for drugs susceptible to lysosomal hydrolytic/enzymatic degradation. Here, we propose a polymeric pH-sensitive liposome system that is designed to release its content inside the endosomes through a polymer structural change following receptor-mediated internalization. Specifically, pH-sensitive immunoliposomes (ILs) were obtained by including a terminally alkylated copolymer of N-isopropylacrylamide (NIPAM) in the liposome bilayer and by coupling the anti-CD33 monoclonal antibody to target leukemic cells. In vitro release of encapsulated fluorescent probes and cytosine arabinoside (ara-C) revealed that pH-sensitivity of the vector was retained in the presence of the antibody upon incubation in plasma. Flow cytometry and confocal microscopy analyses demonstrated that the pH-sensitive ILs were efficiently internalized by various CD33+ leukemic cell lines while limited interaction was found for liposomes decorated with an isotype-matched control antibody. Finally, the pH-sensitive ILs-CD33 formulation exhibited the highest cytotoxicity against HL60 cells, confirming the role of the NIPAM copolymer in promoting the escape of intact ara-C in the endosomes. These results suggest that this pH-sensitive liposomal formulation could be beneficial in the treatment of acute myeloid leukemia.     

类脂囊泡在不同给药途径中的研究进展

纳米载体具有靶向递送药物至特定部位的潜力,因此纳米载体的开发一直受到广泛关注。近10年里,制备基于表面活性剂的纳米囊泡以改善药物递送,引起了研究者们的兴趣。类脂囊泡由非离子表面活性剂在水性介质中自组装形成,具有既能包裹亲水性化合物又能包裹亲脂性化合物的囊泡结构,广泛应用于各种给药途径。与脂质体相比,类脂囊泡不仅具有与其相似的优点,如既能装载亲水性药物又能装载亲脂性药物,而且类脂囊泡的制备方法更简单,制备原料价格更便宜,稳定性更好,克服了脂质体的主要缺点。本文综述了类脂囊泡各制备方法的优缺点及其在不同给药途径中的研究进展。