Development of anti-p185HER2 immunoliposomes for cancer therapy

Antibody-conjugated pH-sensitive liposomes for HER-2 positive breast cancer: development, characterization, in vitro and in vivo assessment

The object of the current study was to develop and evaluate trastuzumab-conjugated Paclitaxel (PTX) and Elacridar (ELA)-loaded PEGylated pH-sensitive liposomes (TPPLs) for site-specific delivery of an anticancer drug. In this study, paclitaxel is used as an anticancer drug which promotes microtubules polymerization and arrest cell cycle progression at mitosis and subsequently leading to cell death. The single use of PTX causes multiple drug resistance (MDR) and results failure of the therapy. Hence, the combination of PTX and P-glycoprotein inhibitor (ELA) are used to achieve maximum therapeutic effects of PTX. Moreover, monoclonal antibody (trastuzumab) is used as ligand for the targeting the drug bearing carriers to BC. Thus, trastuzumab anchored pH-sensitive liposomes bearing PTX and ELA were developed using thin film hydration method and Box-Behnken Design (BBD) for optimizing various formulation variables. The optimized liposomes undergo characterization such as vesicle size, PDI, and zeta potential, which were observed to be 122 ± 2.14 nm, 0.224, and -15.5 mV for PEGylated pH-sensitive liposomes (PEG-Ls) and 134 ± 1.88 nm, 0.238, and -13.98 mV for TPPLs, respectively. The results of the in vitro drug release study of both formulations (PEG-Ls and TPPLs) showed enhanced percentage drug release at an acidic pH 5 as compared to drug release at a physiological pH 7.4. Further, the in vitro cytotoxicity studies were performed in the SK-BR-3 and MDA-MB-231 cell lines. The cellular uptake study of FITC-loaded TPPLs in SK-BR-3 cells showed greater uptake than FITC-loaded PEG-Ls, while in MDA-MB-231 cells there was no significant difference in cell uptake between FITC-loaded TPPLs and FITC-loaded PEG-Ls. Hence, it can be concluded that the HER-2 overexpressing cancer cell line (SK-BR-3) was showed better cytotoxicity and cell uptake of TPPLs than the cells that expressed low levels of HER2 (MDA-MB-231). The in vivo tumor regression study, TPPLs showed significantly more tumor burden reduction i.e. up ∼74% as compared to other liposomes after 28 days. Furthermore, the in vivo studies of TPPLs showed a minimal toxicity profile, minimal hemolysis, higher tumor tissue distribution, and superior antitumor efficacy as compared to other formulations. These studies confirmed that TPPLs are a safe and efficacious treatment for breast cancer.

Regulatory T cells in the bone marrow microenvironment in patients with prostate cancer

Human prostate cancer frequently metastasizes to bone marrow. What defines the cellular and molecular predilection for prostate cancer to metastasize to bone marrow is not well understood. CD4⁺CD25⁺ regulatory T (Treg) cells contribute to self-tolerance and tumor immune pathology. We now show that functional Treg cells are increased in the bone marrow microenvironment in prostate cancer patients with bone metastasis, and that CXCR4/CXCL12 signaling pathway contributes to Treg cell bone marrow trafficking. Treg cells exhibit active cell cycling in the bone marrow, and bone marrow dendritic cells express high levels of receptor activator of NFκB (RANK), and promote Treg cell expansion through RANK and its ligand (RANKL) signals. Furthermore, Treg cells suppress osteoclast differentiation induced by activated T cells and M-CSF, adoptive transferred Treg cells migrate to bone marrow, and increase bone mineral intensity in the xenograft mouse models with human prostate cancer bone marrow inoculation. In vivo Treg cell depletion results in reduced bone density in tumor bearing mice. The data indicates that bone marrow Treg cells may form an immunosuppressive niche to facilitate cancer bone metastasis and contribute to bone deposition, the major bone pathology in prostate cancer patients with bone metastasis. These findings mechanistically explain why Treg cells accumulate in the bone marrow, and demonstrate a previously unappreciated role for Treg cells in patients with prostate cancer. Thus, targeting Treg cells may not only improve anti-tumor immunity, but also ameliorate bone pathology in prostate cancer patients with bone metastasis.

Liposomal doxorubicin as targeted delivery platform: Current trends in surface functionalization

Liposomal delivery systems have significantly enhanced the efficacy and safety of chemotherapeutic agents compared to free (non-liposomal) formulations. Liposomes are vesicles made up of lipophilic bilayer and a hydrophilic core which provides perfect opportunity for their application as transport vehicle for various therapeutic and diagnostic agents. Doxorubicin is the most exploited chemotherapeutic agent for evaluation of different liposomal applications, as its physicochemical properties permit high drug entrapment and easy remote loading in pre-formulated liposomes. Pegylated liposomal doxorubicin clinically approved and, on the market, Doxil®, exemplifies the benefits offered upon the surface modification of liposome with polyethylene glycol. This unique formulation prolonged the drug residence time in the circulation and increased accumulation of doxorubicin in tumor tissue via passive targeting (enhanced permeability and retention effect). However, there is ample scope for further improvement in the efficiency of targeting tumors by coupling biological active ligands onto the liposome surface to generate intelligent drug delivery systems. Small biomolecules such as peptides, fraction of antibodies and carbohydrates have the potential to target receptors present on the surface of the malignant cells. Hence, active targeting of malignant cells using functionalised nanocarrier (liposomes encapsulated with doxorubicin) have been attempted which is reviewed in this article.

Liposomes in Active, Passive and Acoustically-Triggered Drug Delivery

Cancer has become one of the most deadly noncommunicable diseases globally. Several modalities used to treat cancer patients exist today yet many have failed to prove high efficacy with low side effects. The most common example of such modalities is the use of chemotherapeutic drugs to treat cancerous cells and deter their uncontrolled proliferation. In addition to the destruction of cancerous tissues, chemotherapy destroys healthy tissues as it lacks the specificity to annihilate cancerous cells only and preferentially, which result in adverse side effects including nausea, hair fall and myocardial infarction. To prevent the side effects of non-selective chemotherapy, cancer therapy research has been focused on the implementation of nanocarrier systems that act as vehicles to encapsulate drugs and selectively transport their agent to the tumor site. In this paper, we shed light on liposomes along with three anticancer drug delivery approaches: passive, active and ultrasound-triggered drug delivery.

Mesoporous Silica as a Versatile Platform for Cancer Immunotherapy

Immunotherapy has been recognized for decades as a promising therapeutic method for cancer treatment. To enhance host immune responses against cancer, antigen-presenting cells (APCs; e.g., dendritic cells) or T cells are educated using immunomodulatory agents including tumor-associated antigens and adjuvants, and manipulated to induce a cascading adaptive immune response targeting tumor cells. Mesoporous silica materials are promising candidates to improve cancer immunotherapy based on their attractive properties that include high porosity, high biocompatibility, facile surface modification, and self-adjuvanticity. Here, the recent progress on mesoporous-silica-based immunotherapies based on two material forms is summarized: 1) mesoporous silica nanoparticles (MSNs), which can be internalized into APCs, and 2) micrometer-sized mesoporous silica rods (MSRs) that can form a 3D space to recruit APCs. Subcutaneously injected MSN-based cancer vaccines can be taken up by peripheral APCs or by APCs in lymphoid organs to educate the immune system against cancer cells. MSR cancer vaccines can recruit immune cells into the MSR scaffold to induce cancer-specific immunity. Both vaccine systems successfully stimulate the adaptive immune response to eradicate cancer in vivo. Thus, mesoporous silica has potential value as a material platform for the treatment of cancer or infectious diseases.

Cytotoxicity of Sodium Arsenite-loaded Anti-HER2 Immunoliposomes Against HER2-expressing Human Breast Cancer Cell Lines

Background:

Chemotherapy is a routine approach in treatment of patients with cancer, while side effects of chemotherapeutic drugs are inevitable. To minimize side effects, specific targeting of neoplastic cells is a promising strategy in cancer therapy. Sodium arsenite is a metalloid toxin with anti-neoplastic properties, but low selectivity and carcinogenic activity have limited its clinical usage.

Methods:

Targeting of HER2-overexpressing (SK-BR-3) and HER2-low expressing (MCF-7) cancerous breast cell lines by two different liposomal forms of sodium arsenite (bare liposome and trastuzumab-conjugated liposome) was investigated in the current study. Levels of HER2 expression in the above mentioned cell lines were confirmed by western blotting. Size and morphology of the constructed liposomes were characterized by atomic force microscopy (AFM) and dynamic light scattering (DLS). Viability of the cells after treatment was assessed using MTT assay.

Results:

Sodium arsenite in the free and liposomal forms showed growth inhibitory effects against both SK-BR-3 and MCF-7 cell lines in an examined concentration range of 1-20 µM, although this effect was more significant in SK-BR-3 cell line. Loading of sodium arsenite in anti-HER2 immunoliposomes significantly enhanced its cytotoxicity while the specificity was also improved. By encapsulation of sodium arsenite in anti-HER2 immunoliposomes, its efficacy in ablation of SKBR- 3 cells was increased about 1.4-fold compared to the free or liposomal forms.

Conclusion:

In conclusion, targeted delivery of sodium arsenite using anti-HER2 immunoliposomes can be considered as an alternative strategy for specific treatment of HER2-positive breast cancers.

Size-Dependency of the Surface Ligand Density of Liposomes Prepared by Post-insertion

In the active targeting of a drug delivery system (DDS), the density of the ligand on the functionalized liposome determines its affinity for binding to the target. To evaluate these densities on the surface of different sized liposomes, 4 liposomes with various diameters (188, 137, 70, 40 nm) were prepared and their surfaces were modified with fluorescently labeled ligand-lipid conjugates by the post-insertion method. Each liposomal mixture was fractionated into a series of fractions using size exclusion chromatography (SEC), and the resulting liposome fractions were precisely analyzed and the surface ligand densities calculated. The data collected using this methodology indicate that the density of the ligand on a particle is greatly dependent on the size of the liposome. This, in turn, indicates that smaller liposomes (75-40 nm) tend to possess higher densities. For developing active targeting systems, size and the density of the ligands are two important and independent factors that can affect the efficiency of a system as it relates to medical use.

Internalization of CD239 highly expressed in breast cancer cells: a potential antigen for antibody-drug conjugates

Antibody-drug conjugates (ADCs) are attractive in cancer therapy because they can directly bind to cancer cells and provide anticancer activity. To kill cancer cells with ADCs, the target antigens are required not only to be highly and/or selectively expressed on cancer cells but also internalized by the cells. CD239, also known as the Lutheran blood group glycoprotein (Lu) or basal cell adhesion molecule (B-CAM), is a specific receptor for laminin α5, a major component of basement membranes. Here, we show that CD239 is strongly expressed in a subset of breast cancer cells and internalized into the cells. We also produced a human single-chain variable fragment (scFv) specific to CD239 fused with human IgG₁ Fc, called C7-Fc. The binding affinity of the C7-Fc antibody is similar to that of mouse monoclonal antibodies. Although the C7-Fc antibody alone does not influence cellular functions, when conjugated with a fragment of diphtheria toxin lacking the receptor-binding domain (fDT), it can selectively kill breast cancer cells. Interestingly, fDT-bound C7-Fc shows anticancer activity in CD239-highly positive SKBR3 cells, but not in weakly positive cells. Our results show that CD239 is a promising antigen for ADC-based breast cancer therapy.

Particle Targeting in Complex Biological Media

Over the past few decades, nanoengineered particles have gained increasing interest for applications in the biomedical realm, including diagnosis, imaging, and therapy. When functionalized with targeting ligands, these particles have the potential to interact with specific cells and tissues, and accumulate at desired target sites, reducing side effects and improve overall efficacy in applications such as vaccination and drug delivery. However, when targeted particles enter a complex biological environment, the adsorption of biomolecules and the formation of a surface coating (e.g., a protein corona) changes the properties of the carriers and can render their behavior unpredictable. For this reason, it is of importance to consider the potential challenges imposed by the biological environment at the early stages of particle design. This review describes parameters that affect the targeting ability of particulate drug carriers, with an emphasis on the effect of the protein corona. We highlight strategies for exploiting the protein corona to improve the targeting ability of particles. Finally, we provide suggestions for complementing current in vitro assays used for the evaluation of targeting and carrier efficacy with new and emerging techniques (e.g., 3D models and flow-based technologies) to advance fundamental understanding in bio-nano science and to accelerate the development of targeted particles for biomedical applications.

Topology of Surface Ligands on Liposomes: Characterization Based on the Terms, Incorporation Ratio, Surface Anchor Density, and Reaction Yield

The surface topology of ligands on liposomes is an important factor in active targeting in drug delivery systems. Accurately evaluating the density of anchors and bioactive functional ligands on a liposomal surface is critical for ensuring the efficient delivery of liposomes. For evaluating surface ligand density, it is necessary to clarify that on the ligand-modified liposomal surfaces, some anchors are attached to ligands but some are not. To distinguish between these situations, a key parameter, surface anchor density, was introduced to specify amount of total anchors on the liposomal surface. Second, the parameter reaction yield was introduced to identify the amount of ligand-attached anchors among total anchors, since the conjugation efficiency is not always the same nor 100%. Combining these independent parameters, we derived: incorporation ratio=surface anchor density×reaction yield. The term incorporation ratio defines the surface ligand density. Since the surface anchor density represents the density of polyethylene glycol (PEG) on the surfaces in most cases, it also determines liposomal function. It is possible to accurately characterize various PEG and ligand densities and to define the surface topologies. In conclusion, this quantitative methodology can standardize the liposome preparation process and qualify the modified liposomal surfaces.

Ligand-targeted theranostic nanomedicines against cancer

Nanomedicines have significant potential for cancer treatment. Although the majority of nanomedicines currently tested in clinical trials utilize simple, biocompatible liposome-based nanocarriers, their widespread use is limited by non-specificity and low target site concentration and thus, do not provide a substantial clinical advantage over conventional, systemic chemotherapy. In the past 20years, we have identified specific receptors expressed on the surfaces of tumor endothelial and perivascular cells, tumor cells, the extracellular matrix and stromal cells using combinatorial peptide libraries displayed on bacteriophage. These studies corroborate the notion that unique receptor proteins such as IL-11Rα, GRP78, EphA5, among others, are differentially overexpressed in tumors and present opportunities to deliver tumor-specific therapeutic drugs. By using peptides that bind to tumor-specific cell-surface receptors, therapeutic agents such as apoptotic peptides, suicide genes, imaging dyes or chemotherapeutics can be precisely and systemically delivered to reduce tumor growth in vivo, without harming healthy cells. Given the clinical applicability of peptide-based therapeutics, targeted delivery of nanocarriers loaded with therapeutic cargos seems plausible. We propose a modular design of a functionalized protocell in which a tumor-targeting moiety, such as a peptide or recombinant human antibody single chain variable fragment (scFv), is conjugated to a lipid bilayer surrounding a silica-based nanocarrier core containing a protected therapeutic cargo. The functionalized protocell can be tailored to a specific cancer subtype and treatment regimen by exchanging the tumor-targeting moiety and/or therapeutic cargo or used in combination to create unique, theranostic agents. In this review, we summarize the identification of tumor-specific receptors through combinatorial phage display technology and the use of antibody display selection to identify recombinant human scFvs against these tumor-specific receptors. We compare the characteristics of different types of simple and complex nanocarriers, and discuss potential types of therapeutic cargos and conjugation strategies. The modular design of functionalized protocells may improve the efficacy and safety of nanomedicines for future cancer therapy.

Improved therapeutic activity of HER2 Affibody-targeted cisplatin liposomes in HER2-expressing breast tumor models

OBJECTIVES

The purpose of this study was to investigate whether the conjugation of anti-HER2-Affibody to cisplatin PEGylated liposome can efficiently enhance the therapeutic effectiveness of the targeted liposome.

METHODS

First, Affibody molecules were incubated with Mal-PEG2000-DSPE micelle to afford formation of a maleimide-mediated thioether coupling to the COOH-terminal cysteine of Affibody. Cisplatin-loaded liposomes composed of hydrogenated soy phosphatidylcholine/ cholesterol/mPEG2000-DSPE (56.5:38.5:5 molar ratio) (150 mM) were prepared and characterized by their physicochemical properties. Affibody-conjugated micelles were then transferred into preformed liposomes by means of post insertion. The cytotoxicity and cellular uptake of Affibody-targeted (affisome) and nontargeted liposomes were tested in HER2(+) SK-BR-3, and the in vivo therapeutic activity was evaluated in TUBO breast cancer models.

RESULTS

Anti-HER2 affisome demonstrated a higher amount of platinum intracellularly, and affected HER2(+)-SK-BR-3 cell death was at lower concentrations compared with its liposome counterparts. Further, cisplatin-affisome showed greater therapeutic efficiency than nontargeted liposome in HER2(+)-TUBO models. Equally promising, the affisome-treated mice did extend the survival of animals by several days and even left one tumor-free survivor.

CONCLUSIONS

Affibody-targeting endowed cisplatin liposomes with significantly enhanced, albeit modest, therapeutic activity in HER2-overexpressing tumor model; however, further values are yet to be determined to advance clinical translation of these targeted nanoparticulates.

Drug Delivery Approaches in Addressing Clinical Pharmacology-Related Issues: Opportunities and Challenges

Various drug delivery approaches can be used to maximize therapeutic efficacy and minimize side effects, by impacting absorption, distribution, metabolism, and elimination (ADME) of a drug compound. For those drugs with poor water solubility or low permeability, techniques such as amorphous solid dispersion, liposomes, and complexations have been used to improve their oral bioavailability. Modified release (MR) formulations have been widely used to improve patient compliance, as well as to reduce side effects, especially for those drugs with short half-lives or narrow therapeutic windows. More than ten drugs using sterile long-acting release (LAR) formulations with clear clinical benefit have been successfully marketed. Furthermore, drug delivery systems have been used in delaying drug clearance processes. Additionally, modifying the in vivo drug distribution using targeted delivery systems has significantly improved oncology treatments. All the drug delivery approaches have their advantages and limitations. For both brand and generic drugs, the achievement of consistent quality and therapeutic performance using drug delivery systems can also pose serious challenges in developing a drug for the market, which requires close collaboration among industry, academia, and regulatory agencies. With the advent of personalized medicines, there will be great opportunities and challenges in utilizing drug delivery systems to provide better products and services for patients.

Monoclonal antibody-targeted fluorescein-5-isothiocyanate-labeled biomimetic nanoapatites: a promising fluorescent probe for imaging applications

Multifunctional biomimetic nanoparticles (NPs) are acquiring increasing interest as carriers in medicine and basic research since they can efficiently combine labels for subsequent tracking, moieties for specific cell targeting, and bioactive molecules, e.g., drugs. In particular, because of their easy synthesis, low cost, good biocompatibility, high resorbability, easy surface functionalization, and pH-dependent solubility, nanocrystalline apatites are promising candidates as nanocarriers. This work describes the synthesis and characterization of bioinspired apatite nanoparticles to be used as fluorescent nanocarriers targeted against the Met/hepatocyte growth factor receptor, which is considered a tumor associated cell surface marker of many cancers. To this aim the nanoparticles have been labeled with Fluorescein-5-isothiocyanate (FITC) by simple isothermal adsorption, in the absence of organic, possibly toxic, molecules, and then functionalized with a monoclonal antibody (mAb) directed against such a receptor. Direct labeling of the nanoparticles allowed tracking the moieties with spatiotemporal resolution and thus following their interaction with cells, expressing or not the targeted receptor, as well as their fate in vitro. Cytofluorometry and confocal microscopy experiments showed that the functionalized nanocarriers, which emitted a strong fluorescent signal, were rapidly and specifically internalized in cells expressing the receptor. Indeed, we found that, once inside the cells expressing the receptor, mAb-functionalized FITC nanoparticles partially dissociated in their two components, with some mAbs being recycled to the cell surface and the FITC-labeled nanoparticles remaining in the cytosol. This work thus shows that FITC-labeled nanoapatites are very promising probes for targeted cell imaging applications.

A novel anti-VEGF165 monoclonal antibody-conjugated liposomal nanocarrier system: physical characterization and cellular uptake evaluation in vitro and in vivo

Vascular endothelial growth factor (VEGF) is an important target for cancer therapy. In the present study, we conjugated the novel fully-human anti-VEGF165 monoclonal antibody, mAb165, with a PEGylated liposome (lip) to produce a monoclonal antibody-conjugated PEGylated liposome (mAb-lip). Physical characterization of mAb-lips showed an average particle size of 108nm. Using a bicinchoninic acid (BCA) assay, the coupling efficiency of mAb165 conjugated to the liposome was 69.8±0.5μg mAb/μmol phospholipid. In addition, we confirmed that conjugation between mAb165 and the liposome did not affect the structure and VEGF binding affinity of the antibody. Cell uptake of mAb-lips was assessed in four cell lines: MCF-7, HepG-2, SGC-7901, and L02 cells. Confocal microscopy and flow cytometry demonstrated that there was no significant difference in cell uptake between mAb-lips and mAb-free liposome either in VEGF-expressing tumor cells or normal cells. Moreover, the cytotoxicity of paclitaxel encapsulated in mAb-lips was not increased in the four cell lines. However, in BALB/c nude mice bearing MCF-7 xenografts, mAb-lips showed superior targeting activity to tumor tissues when compared with the unmodified liposome, which was demonstrated by the fact that rhodamine-labeled mAb-lips exhibited higher fluorescence intensity in tumor tissues than the unmodified liposome. Thus, our study indicated that mAb-lips may have the potential to enhance the therapeutic index of anticancer agents through targeted delivery to tumor cells in vivo.

Anti-EGFR antibody conjugated organic-inorganic hybrid lipid nanovesicles selectively target tumor cells

Chemical conjugation of anti-epidermal growth factor receptor monoclonal antibodies (anti-EGFR mAbs) to organic-inorganic hybrid liposomal immunocerasomes via maleimide-thiol coupling chemistry is explored as a mechanism for selectively targeting cancer cells. The cellular uptake and internalization of immunocerasomes are investigated in A431 cells that express an abnormally high level of EGFR, DU145 cells that overexpress EGFR, and HL-60 cells that are used as a negative control. The internalization study reveals a strong correlation between the receptor-mediated endocytosis of immunocerasomes and the membrane expression of EGFR. Further, free anti-EGFR mAbs and immunocerasomes conjugated with anti-EGFR mAbs at nanomolar doses display similar anti-proliferative effects on A431 cells. Additionally, serum proteins greatly reduce the cellular uptake of cerasomes that is mediated by non-specific receptors, but have no adverse effects on the specific EGFR-mediated delivery of immunocerasomes to A431 cells.

Gene therapy for heart disease: molecular targets, vectors and modes of delivery to myocardium

Despite the numerous hurdles that gene therapy has encountered along the way, clinical trials over the last few years are showing promising results in many fields of medicine, including cardiology, where many targets are moving toward clinical development. In this review, the authors discuss the current state of the art in terms of clinical and preclinical development. They also examine vector technology and available vector-delivery strategies.

Application of pharmacokinetic and pharmacodynamic analysis to the development of liposomal formulations for oncology

Liposomal formulations of anticancer agents have been developed to prolong drug circulating lifetime, enhance anti-tumor efficacy by increasing tumor drug deposition, and reduce drug toxicity by avoiding critical normal tissues. Despite the clinical approval of numerous liposome-based chemotherapeutics, challenges remain in the development and clinical deployment of micro- and nano-particulate formulations, as well as combining these novel agents with conventional drugs and standard-of-care therapies. Factors requiring optimization include control of drug biodistribution, release rates of the encapsulated drug, and uptake by target cells. Quantitative mathematical modeling of formulation performance can provide an important tool for understanding drug transport, uptake, and disposition processes, as well as their role in therapeutic outcomes. This review identifies several relevant pharmacokinetic/pharmacodynamic models that incorporate key physical, biochemical, and physiological processes involved in delivery of oncology drugs by liposomal formulations. They capture observed data, lend insight into factors determining overall antitumor response, and in some cases, predict conditions for optimizing chemotherapy combinations that include nanoparticulate drug carriers.

Liposomal siRNA nanocarriers for cancer therapy

Small interfering RNAs (siRNA) have recently emerged as a new class of therapeutics with a great potential to revolutionize the treatment of cancer and other diseases. A specifically designed siRNA binds and induces post-transcriptional silencing of target genes (mRNA). Clinical applications of siRNA-based therapeutics have been limited by their rapid degradation, poor cellular uptake, and rapid renal clearance following systemic administration. A variety of synthetic and natural nanoparticles composed of lipids, polymers, and metals have been developed for siRNA delivery, with different efficacy and safety profiles. Liposomal nanoparticles have proven effective in delivering siRNA into tumor tissues by improving stability and bioavailability. While providing high transfection efficiency and a capacity to form complexes with negatively charged siRNA, cationic lipids/liposomes are highly toxic. Negatively charged liposomes, on the other hand, are rapidly cleared from circulation. To overcome these problems we developed highly safe and effective neutral lipid-based nanoliposomes that provide robust gene silencing in tumors following systemic (intravenous) administration. This delivery system demonstrated remarkable antitumor efficacy in various orthotopic human cancer models in animals. Here, we briefly overview this and other lipid-based approaches with preclinical applications in different tumor models for cancer therapy and potential applications as siRNA-nanotherapeutics in human cancers.

Albumin-fatty acid interactions at monolayer interface

The fluid mosaic model of Singer and Nicolson in 1972 shows how proteins are embedded in membranes. To elucidate the interactions between proteins and the surrounding lipids, stearic acid (SA) and bovine serum albumin (BSA) were used as lipid-protein components to mimic the normal membrane bilayer environment using the Langmuir-Blodgett technique. Surface pressure (π)-molecular area (A) isotherms were recorded for the SA monolayer in the presence of BSA on water. The mixed monolayer was successfully transferred onto an oxidized silicon wafer and imaged by tapping mode atomic force microscopy (AFM). Miscibility, compressibility and thermodynamic stability of the mixed system were examined. A large negative deviation of A ex, together with the minimum value of ΔG ex, was observed when the mole fraction of BSA (X BSA) was 0.8, indicating this to be the most stable mixture. In a compressibility analysis, X BSA was observed at below 50 mN m(-1), denoting a liquid-expanded phase and showing the occurrence of a strong interaction of SA with BSA molecules in this phase. AFM observations supported the quantitative data indicating that BSA was strongly attracted onto the membrane surface as predicted.

NANOMATERIALS FOR PROTEIN MEDIATED THERAPY AND DELIVERY

There has been a significant amount of research done on liposomes and nanoparticles as drug carriers for protein drugs. Proteins and enzymes have been used both as targeting moieties and for their therapeutic potential. High specificity and rapid reaction rates make proteins and enzymes excellent candidates for therapeutic treatment, but some limitations exist. Many of these limitations can be addressed by a well studied nanotechnology based delivery system. Such a system can provide a medium for delivery, stabilization of the drugs, and enable site specific accumulation of drugs. Nanomedicines such as these have great potential to revolutionize the pharmaceutical industry and improve healthcare worldwide.

Targeted thermosensitive liposomes: an attractive novel approach for increased drug delivery to solid tumors

INTRODUCTION

Currently available chemotherapy is hampered by a lack in tumor specificity and resulting toxicity. Small and long-circulating liposomes can preferentially deliver chemotherapeutic drugs to tumors upon extravasation from tumor vasculature. Although clinically used liposomal formulations demonstrated significant reduction in toxicity, enhancement of therapeutic activity has not fully met expectations.

AREAS COVERED

Low drug bioavailability from liposomal formulations and limited tumor accumulation remain major challenges to further improve therapeutic activity of liposomal chemotherapy. The aim of this review is to highlight strategies addressing these challenges. A first strategy uses hyperthermia and thermosensitive liposomes to improve tumor accumulation and trigger liposomal drug bioavailability. Image-guidance can aid online monitoring of heat and drug delivery and further personalize the treatment. A second strategy involves tumor-specific targeting to enhance drug delivery specificity and drug internalization. In addition, we review the potential of combinations of the two in one targeted thermosensitive-triggered drug delivery system.

EXPERT OPINION

Heat-triggered drug delivery using thermosensitive liposomes as well as the use of tumor vasculature or tumor cell-targeted liposomes are both promising strategies to improve liposomal chemotherapy. Preclinical evidence has been encouraging and both strategies are currently undergoing clinical evaluation. A combination of both strategies rendering targeted thermosensitive liposomes (TTSL) may appear as a new and attractive approach promoting tumor drug delivery.

Ligand-targeted particulate nanomedicines undergoing clinical evaluation: current status

Since the introduction of Doxil® on the market nearly 20years 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.

Click conjugated polymeric immuno-nanoparticles for targeted siRNA and antisense oligonucleotide delivery

Efficient and targeted cellular delivery of small interfering RNAs (siRNAs) and antisense oligonucleotides (AONs) is a major challenge facing oligonucleotide-based therapeutics. The majority of current delivery strategies employ either conjugated ligands or oligonucleotide encapsulation within delivery vehicles to facilitate cellular uptake. Chemical modification of the oligonucleotides (ONs) can improve potency and duration of activity, usually as a result of improved nuclease resistance. Here we take advantage of innovations in both polymeric delivery vehicles and ON stabilization to achieve receptor-mediated targeted delivery of siRNAs or AONs for gene silencing. Polymeric nanoparticles comprised of poly(lactide-co-2-methyl, 2-carboxytrimethylene carbonate)-g-polyethylene glycol-furan/azide are click-modified with both anti-HER2 antibodies and nucleic acids on the exterior PEG corona. Phosphorothioate (PS), 2'F-ANA, and 2'F-RNA backbone chemical modifications improve siRNA and AON potency and duration of activity. Importantly, delivery of these nucleic acids on the exterior of the polymeric immuno-nanoparticles are as efficient in gene silencing as lipofectamine transfection without the associated potential toxicity of the latter.

Impact of tumor HER2/ERBB2 expression level on HER2-targeted liposomal doxorubicin-mediated drug delivery: multiple low-affinity interactions lead to a threshold effect

Numerous targeted nanotherapeutics have been described for potential treatment of solid tumors. Although attention has focused on antigen selection and molecular design of these systems, there has been comparatively little study of how cellular heterogeneity influences interaction of targeted nanoparticles with tumor cells. Antigens, such as HER2/ERBB2, are heterogeneously expressed across different indications, across patients, and within individual tumors. Furthermore, antigen expression in nontarget tissues necessitates optimization of the therapeutic window. Understanding the performance of a given nanoparticle under different regimens of antigen expression has the ability to inform patient selection and clinical development decisions. In this work, HER2-targeted liposomal doxorubicin was used as a model-targeted nanoparticle to quantitatively investigate the effect of HER2 expression levels on delivery of doxorubicin to the nucleus. We find quantitatively greater nuclear doxorubicin delivery with increasing HER2 expression, exhibiting a threshold effect at approximately 2 × 10(5) HER2 receptors/cell. Kinetic modeling indicated that the threshold effect arises from multiple low-affinity interactions between the targeted liposome and HER2. These results support previous data showing little or no uptake into human cardiomyocytes, which express levels of HER2 below the threshold. Finally, these results suggest that HER2-targeted liposomal doxorubicin may effectively target tumors that fall below traditional definitions of HER2-positive tumors, thereby expanding the potential population of patients that might benefit from this agent.

Second generation liposomal cancer therapeutics: transition from laboratory to clinic

Recent innovations and developments in nanotechnology have revolutionized cancer therapeutics. Engineered nanomaterials are the current workhorses in the emerging field of cancer nano-therapeutics. Lipid vesicles bearing anti-tumor drugs have turned out to be a clinically feasible and promising nano-therapeutic approach to treat cancer. Efficient entrapment of therapeutics, biocompatibility, biodegradability, low systemic toxicity, low immunogenicity and ability to bypass multidrug resistance mechanisms has made liposomes a versatile drug/gene delivery system in cancer chemotherapy. The present review attempts to explore the recent key advances in liposomal research and the vast arsenal of liposomal formulations currently being utilized in treatment and diagnosis of cancer.

Heavy Chain Only Antibodies: A New Paradigm in Personalized HER2+ Breast Cancer Therapy

Unlike conventional antibodies, heavy chain only antibodies derived from camel contain a single variable domain (VHH) and two constant domains (CH2 and CH3). Cloned and isolated VHHs possess unique properties that enable them to excel conventional therapeutic antibodies and their smaller antigen-binding fragments in cancer targeting and therapy. VHHs express low immunogenicity, are highly robust and easy to manufacture and have the ability to recognize hidden or uncommon epitopes. We highlight the utility of VHH in design of new molecular, multifunctional particulate and immune cell-based systems for combating HER2+ breast cancer.

Immunoconjugates and long circulating systems: origins, current state of the art and future directions

Significant progress has been made recently in the area of immunoconjugated drugs and drug delivery systems (DDS). The immuno-modification of either the drug or DDS has proven to be a very promising approach that has significantly improved the targeted accumulation in pathological sites while decreasing its undesirable side effects in healthy tissues. The arrangement for both prolonged life in the circulation and specific target recognition represents another potent strategy in the development of immuno-targeted systems. The longevity of immuno-targeted DDS such as immunoliposomes and immunomicelles improves their targetability even in the presence of the additional passive accumulation in areas with a compromised vasculature. The added use of the immuno-targeted systems takes advantage of the specific microenvironment of pathological sites including lowered pH, increased temperature, and variation in the enzymatic activity. "Smart" stimulus-responsive systems combine different valuable functionalities including PEG-protection, targeting antibody, cell-penetration, and stimulus-sensitive functions. In this review we examined the evolution, current status and future directions in the area of therapeutical immunoconjugates and long-circulating immuno-targeted DDS.

Nuclisome--targeting the tumor cell nucleus

The Nuclisome concept builds on a novel two-step targeting strategy with the aim to deliver short-range Auger-electron-emitting radionuclides to nuclear DNA of tumor cells. The concept is based on the use of Nuclisome-particles, i.e., tumor-targeted PEG-stabilized liposomes loaded with a unique DNA-intercalating compound that enables specific and effective delivery of radionuclides to DNA. The specific and potent two-step targeting leads to eradication of tumor cells while toxicity to normal organs is reduced to a minimum. Results of in vitro and in vivo studies point towards the Nuclisome concept as a promising strategy for the treatment of small tumor masses and, in particular, for the elimination of spread single cells and micrometastases.

Challenges in development of targeted liposomal therapeutics

Liposomes, phospholipid vesicles with a bilayered membrane structure, have been widely used as pharmaceutical carriers for drugs and genes, in particular for treatment of cancer. To enhance the efficacy of the liposomal drugs, drug-loaded liposomes are targeted to the tumors by means of passive (enhanced permeability and retention mediated) targeting, based on the longevity of liposomes in blood and its accumulation in pathological sites with compromised vasculature, and active targeting, based on the attachment of specific ligands to the liposomal surface to bind certain antigens on the target cells. Antibody-targeted liposomes loaded with anticancer drugs demonstrate high potential for clinical applications. This review highlights evolution of liposomes for both passive and active targeting and challenges in development of targeted liposomal therapeutics specifically antibody-targeted liposomes.

In vivo CEST MR imaging of U87 mice brain tumor angiogenesis using targeted LipoCEST contrast agent at 7 T

LipoCEST are liposome-encapsulating paramagnetic contrast agents (CA) based on chemical exchange saturation transfer with applications in biomolecular MRI. Their attractive features include biocompatibility, subnanomolar sensitivity, and amenability to functionalization for targeting biomarkers. We demonstrate MR imaging using a targeted lipoCEST, injected intravenously. A lipoCEST carrying Tm(III)-complexes was conjugated to RGD tripeptide (RGD-lipoCEST), to target integrin α(ν)β(3) receptors involved in tumor angiogenesis and was compared with an unconjugated lipoCEST. Brain tumors were induced in athymic nude mice by intracerebral injection of U87MG cells and were imaged at 7 T after intravenous injection of either of the two contrast agents (n = 12 for each group). Chemical exchange saturation transfer-MSME sequence was applied over 2 h with an average acquisition time interval of 13.5 min. The chemical exchange saturation transfer signal was ∼1% in the tumor and controlateral regions, and decreased to ∼0.3% after 2 h; while RGD-lipoCEST signal was ∼1.4% in the tumor region and persisted for up to 2 h. Immunohistochemical staining revealed a persistent colocalization of RGD-lipoCEST with α(ν)β(3) receptors in the tumor region. These results constitute an encouraging step toward in vivo MRI imaging of tumor angiogenesis using intravenously injected lipoCEST.

Heterogeneous liposome membranes with pH-triggered permeability enhance the in vitro antitumor activity of folate-receptor targeted liposomal doxorubicin

The killing efficacy of doxorubicin from liposome-based delivery carriers has been shown to correlate strongly with its intracellular trafficking and, in particular, its fast and extensive release from the delivery carrier. However, previously explored pH-triggered mechanisms that were designed to become activated during liposome endocytosis have also been shown to interfere with the liposome stability in vivo. We have designed pH-triggered gel-phase liposomes with heterogeneous membranes for the delivery of doxorubicin. These liposomes are triggered to form "leaky" interfacial boundaries between gel-gel phase separated domains on the membrane bilayer with lowering pH. The pH-triggered mechanism does not compromise liposome stability in vivo and results in superior in vitro killing efficacy of delivered doxorubicin when liposomes are endocytosed by a clathrin-mediated pathway. In the present work, we evaluate the general applicability of these liposomes when targeted to the folate receptor (FR) of KB cancer cells in vitro and become endocytosed by a less acidic pathway: the caveolae pathway. FR-targeting liposomes exhibit almost 50% decrease in cell association for increase in liposome size from 120 to 280 nm in diameter after relatively short incubation times (up to 4 h). The fraction of internalized vesicles, however, is approximately 60% of the cell associated vesicles independent of their size. Our findings demonstrate that, for the same doxorubicin uptake per cancer cell, the killing effect of doxorubicin delivered by pH-triggered lipid vesicles is greater (IC(50) = 0.032 mM for a 6 h incubation) than when delivered by a conventional non-pH-responsive composition (IC(50) = 0.194 mM). These findings suggest higher bioexposure of cells to the therapeutic agent possibly via faster and more extensive release from the carrier. Animal studies of FR-targeting non-pH-responsive liposomal doxorubicin report stronger therapeutic potential for the targeted approach relative to nontargeted liposomes and to free doxorubicin. The findings of the present study suggest that the targeted pH-triggered liposomes could potentially further enhance the therapeutic outcomes of doxorubicin in vivo.

Particulate vaccines: on the quest for optimal delivery and immune response

Subunit vaccines offer a safer alternative to traditional organism-based vaccines, but their immunogenicity is impaired. This hurdle might be overcome by the use of micro- and nanodelivery systems carrying the antigen(s). This review discusses the rationale for the use of particulate vaccines and provides an overview of antigen-delivery vehicles currently under investigation. It further highlights the cellular uptake, antigen processing and the presentation by antigen-presenting cells because these processes are partially governed by particle characteristics and eventually determine the immunological outcome. Finally, we address the attractive concept of concomitant delivery of antigens and immunopotentiators. The condensed knowledge could be an asset for rationally designing antigen-delivery vehicles to obtain safe and efficacious vaccines.

Effective tumor targeting and enhanced anti-tumor effect of liposomes engrafted with peptides specific for tumor lymphatics and vasculature

The use of liposomes to target drugs to tumors represents an attractive therapeutic strategy, especially when used with convenient targeting moieties such as peptides. Here we explored several peptides for their ability to target liposomes to tumors. The metal chelator lipid 3(nitrilotriacetic acid)-ditetradecylamine (NTA(3)-DTDA) was incorporated into liposomes to enable the engraftment of His-tagged peptides containing targeting motifs specific for tumor vasculature markers VEGFR-1 (p39-Flt-1) and neuropilin-1 (p24-NRP-1), or a motif known to accumulate in hypoxic areas of tumors (p47-LyP-1). Peptide-engrafted liposomes were examined for their biodistribution and anti-tumor effects after i.v. administration. Our results show that radiolabelled liposomes engrafted with either p24-NRP-1 or p47-LyP-1 and then injected into mice bearing subcutaneous B16-F1 tumors, show increased accumulation in the tumor. For p24-NRP-1-liposomes, tumor targeting was significantly increased when the stabilizing lipid phosphatidylethanolamine polyethylene glycol-750 (PE-PEG(750)) was used instead of PE-PEG(2000) in the liposome lipid mixture. Importantly, compared to the controls, p24-NRP-1 liposomes containing 10 mol% PE-PEG(750) and loaded with doxorubicin significantly inhibited the rate of tumor growth in the tumor-bearing mice. Our findings demonstrate that the use of drug-containing liposomes incorporating NTA(3)-DTDA and engrafted with NRP-1 targeting peptide is a convenient strategy to enhance the therapeutic effect of non-targeted doxorubicin.

The effect of internalizing human single chain antibody fragment on liposome targeting to epithelioid and sarcomatoid mesothelioma

Immunoliposomes (ILs) anchored with internalizing human antibodies capable of targeting all subtypes of mesothelioma can be useful for targeted imaging and therapy of this malignant disease. The objectives of this study were to evaluate both the in vitro and in vivo tumor targeted internalization of novel internalizing human single chain antibody (scFv) anchored ILs on both epithelioid (M28) and sarcomatoid (VAMT-1) subtypes of human mesothelioma. ILs were prepared by post-insertion of mesothelioma-targeting human scFv (M1) onto preformed liposomes and radiolabeled with (111)In ((111)In-IL-M1), along with control non-targeted liposomes ((111)In-CL). Incubation of (111)In-IL-M1 with M28, VAMT-1, and a control non-tumorigenic cell line (BPH-1) at 37 °C for 24 h revealed efficient binding and rapid internalization of ILs into both subtypes of tumor cells but not into the BPH-1 cells; internalization accounted for approximately 81-94% of total cell accumulation in mesothelioma cells compared to 37-55% in control cells. In tumor-bearing mice intravenous (i.v.) injection of (111)In-IL-M1 led to remarkable tumor accumulation: 4% and 4.7% injected dose per gram (% ID/g) for M28 and VAMT-1 tumors, respectively, 48 h after injection. Furthermore, tumor uptake of (111)In-IL-M1 in live xenograft animal models was verified by single photon emission computed tomography (SPECT/CT). In contrast, i.v. injection of (111)In-CL in tumor-bearing mice revealed very low uptake in both subtypes of mesothelioma, 48 h after injection. In conclusion, M1 scFv-anchored ILs showed selective tumor targeting and rapid internalization into both epithelioid and sarcomatoid subtypes of human mesothelioma, demonstrating its potential as a promising vector for enhanced tumor drug targeting.

Barriers to non-viral vector-mediated gene delivery in the nervous system

Efficient methods for cell line transfection are well described, but, for primary neurons, a high-yield method different from those relying on viral vectors is lacking. Viral transfection has several drawbacks, such as the complexity of vector preparation, safety concerns, and the generation of immune and inflammatory responses when used in vivo. However, one of the main problems for the use of non-viral gene vectors for neuronal transfection is their low efficiency when compared with viral vectors. Transgene expression, or siRNA delivery mediated by non-viral vectors, is the result of multiple processes related to cellular membrane crossing, intracellular traffic, and/or nuclear delivery of the genetic material cargo. This review will deal with the barriers that different nanoparticles (cationic lipids, polyethyleneimine, dendrimers and carbon nanotubes) must overcome to efficiently deliver their cargo to central nervous system cells, including internalization into the neurons, interaction with intracellular organelles such as lysosomes, and transport across the nuclear membrane of the neuron in the case of DNA transfection. Furthermore, when used in vivo, the nanoparticles should efficiently cross the blood-brain barrier to reach the target cells in the brain.

Tumor-targeted delivery of liposome-encapsulated doxorubicin by use of a peptide that selectively binds to irradiated tumors

Tumor-targeted drug delivery improves anti-tumor efficacy and reduces systemic toxicity by limiting bioavailability of cytotoxic drugs to within tumors. Targeting reagents, such as peptides or antibodies recognizing molecular targets over-expressed within tumors, have been used to improve liposome-encapsulated drug accumulation within tumors and resulted in enhanced tumor growth control. In this report, we expand the scope of targeting reagents by showing that one peptide, HVGGSSV which was isolated from an in vivo screening of phage-displayed peptide library due to its selective binding within irradiated tumors, enabled highly selective tumor-targeted delivery of liposome-encapsulated doxorubicin and resulted in enhanced cytotoxicity within tumors. Targeting liposomes (TL) and non-targeting liposomes (nTL) were labeled with Alexa Fluor 750. Biodistribution of the liposomes within tumor-bearing mice was studied with near infrared (NIR) imaging. In the single dose pharmacokinetic study, the liposomal doxorubicin has an extended circulation half life as compared to the free doxorubicin. Targeting liposomes partitioned to the irradiated tumors and improved drug deposition and retention within tumors. The tumor-targeted delivery of doxorubicin improved tumor growth control as indicated with reduced tumor growth rate and tumor cell proliferation, enhanced tumor blood vessel destruction, and increased treatment-associated apoptosis and necrosis of tumor cells. Collectively, the results demonstrated the remarkable capability of the HVGGSSV peptide in radiation-guided drug delivery to tumors.

Internalizing cancer antibodies from phage libraries selected on tumor cells and yeast-displayed tumor antigens

A number of approaches have been utilized to generate antibodies to cancer cell surface receptors that can be used as potential therapeutics. A number of these therapeutic approaches, including antibody-drug conjugates, immunotoxins, and targeted nucleic acid delivery, require antibodies that not only bind receptor but also undergo internalization into the cell upon binding. We previously reported on the ability to generate cancer cell binding and internalizing antibodies directly from human phage antibody libraries selected for internalization into cancer cell lines. While a number of useful antibodies have been generated using this approach, limitations include the inability to direct the selections to specific antigens and to identify the antigen bound by the antibodies. Here we show that these limitations can be overcome by using yeast-displayed antigens known to be associated with a cell type to select the phage antibody output after several rounds of selection on a mammalian cell line. We used this approach to generate several human phage antibodies to yeast-displayed EphA2 and CD44. The antibodies bound both yeast-displayed and mammalian cell surface antigens, and were endocytosed upon binding to mammalian cells. This approach is generalizable to many mammalian cell surface proteins, results in the generation of functional internalizing antibodies, and does not require antigen expression and purification for antibody generation.

Exchange kinetics of protein-functionalized micelles and liposomes studied by Förster resonance energy transfer

Protein-functionalized micelles and liposomes are attractive delivery systems for applications ranging from targeted drug delivery to molecular imaging. In particular, systems that use pegylated phospholipids have become popular, but little is known about the stability of these lipid-functionalized proteins toward exchange. In this study, Förster resonance energy transfer (FRET) between the fluorescent proteins ECFP and EYFP was used to investigate the lipid exchange behavior of protein-functionalized liposomes and micelles. Native chemical ligation was used as an efficient method to site-specifically couple varying amounts of proteins to pegylated phospholipids. No exchange was observed between protein-functionalized phospholipids in sterically stabilized liposomes. In micelles, however, protein-functionalized lipids were found to exchange with a half-time of exchange ranging from almost 2 h at room temperature to 4 min at 37 degrees C. These pegylated micelles remained intact at lipid concentrations down to 0.15 microM, indicating that they are even more stable than previously assumed. The results obtained in this study provide a useful frame of reference for assessing the potential role of protein exchange in biomedical applications of these lipid-based nanoparticles.

The pH-dependent association with cancer cells of tunable functionalized lipid vesicles with encapsulated doxorubicin for high cell-kill selectivity

To enable selective cell-kill, we designed functionalized lipid vesicles with pH-triggered heterogeneous membranes and encapsulated doxorubicin that exhibit tunable surface topography. These vesicles "hide" (mask) the targeting ligands from their surface during circulation in the blood, and only progressively "expose" these ligands as they gradually penetrate deeper into the tumor interstitium, where after endocytosis they burst release their contents. The stimulus to activate the binding reactivity is the pH gradient between the blood stream (pH 7.4-7.0) and the increasingly acidic pH inside the tumor interstitium (pH 6.7-6.5). Doxorubicin release is activated at the endosomal pH 5.5-5.0. We show that tunable functionalized vesicles exhibit environmentally-dependent (pH-dependent) association with cancer cells resulting in high cell-kill selectivity. When lowering the extracellular pH from 7.4 to 6.5, tunable functionalized vesicles deliver doxorubicin to cancer cells that increases from 41% to 93% of maximum resulting in cancer cell killing that increases from 23 to 71% of maximum, respectively. This proof-of-concept shows the potential of tunable targeted liposomal chemotherapy to selectively kill cancer cells in an environmentally-dependent way.

Cellular endocytosis and gene delivery

Endocytosis is the process by which cells take up macromolecules from the surrounding medium. The best-characterized process is the so-called clathrin-dependent endocytosis, although much is also currently known about clathrin-independent endocytic processes such as those involving caveolae and lipid rafts. An understanding of endocytosis and the cellular trafficking that occurs thereafter has a great deal of relevance to current molecular medicine. Gene therapy, which is presently being investigated for its therapeutic potential in treating immunodeficiency and metabolic diseases, cancer and heart disease, employs a variety of viral and nonviral vectors, which can be delivered to the target cells of the body and are subsequently endocytosed and dissembled. A variety of vectors can be used to deliver genes to organs in vivo or cells ex vivo. Various routes of vector delivery have been investigated. The mechanisms by which vectors such as adenoviruses, adeno-associated viruses, retroviruses and liposomes enter the cell are increasingly being investigated as the effort to increase the efficiency of gene therapy continues. This review focuses on mechanisms of endocytosis and how they relate to the internal trafficking of viral and nonviral vectors in gene therapy.

Passive and active drug targeting: drug delivery to tumors as an example

The paradigm of using nanoparticulate pharmaceutical carriers has been well established over the past decade, both in pharmaceutical research and in the clinical setting. Drug carriers are expected to stay in the blood for long time, accumulate in pathological sites with affected and leaky vasculature (tumors, inflammations, and infarcted areas) via the enhanced permeability and retention (EPR) effect, and facilitate targeted delivery of specific ligand-modified drugs and drug carriers into poorly accessible areas. Among various approaches to specifically target drug-loaded carrier systems to required pathological sites in the body, two seem to be most advanced--passive (EPR effect-mediated) targeting, based on the longevity of the pharmaceutical carrier in the blood and its accumulation in pathological sites with compromised vasculature, and active targeting, based on the attachment of specific ligands to the surface of pharmaceutical carriers to recognize and bind pathological cells. Here, we will consider and discuss these two targeting approaches using tumor targeting as an example.

Nanomedicine based approaches for the delivery of siRNA in cancer

Small interfering RNA (siRNA) technology holds great promise as a therapeutic intervention for targeted gene silencing in cancer and other diseases. However, in vivo systemic delivery of siRNA-based therapeutics to tumour tissues/cells remains a challenge. The major limitations against the use of siRNA as a therapeutic tool are its degradation by serum nucleases, poor cellular uptake and rapid renal clearance following systemic administration. Several siRNA-based loco-regional therapeutics are already in clinical trials. Further development of siRNAs for anti-cancer therapy depends on the development of safe and effective nanocarriers for systemic administration. To overcome these hurdles, nuclease-resistant chemically modified siRNAs and variety of synthetic and natural biodegradable lipids and polymers have been developed to systemically deliver siRNA with different efficacy and safety profiles. Cationic liposomes have emerged as one of the most attractive carriers because of their ability to form complexes with negatively charged siRNA and high in vitro transfection efficiency. However, their effectiveness as potential therapeutic carriers is limited by potential for pulmonary toxicity. Recently, our laboratories described the use of neutral 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine based nanoliposomes in murine tumour models. We found this approach to be safe and 10- and 30-fold more effective than cationic liposomes and naked siRNA, respectively, for systemic delivery of siRNA into tumour tissues. Here, we review potential approaches for systemic delivery of siRNA for cancer therapy.