Targeted drug delivery to tumors: myths, reality and possibility

Gold nanoparticles: emerging paradigm for targeted drug delivery system

The application of nanotechnology in medicine, known as nanomedicine, has introduced a plethora of nanoparticles of variable chemistry and design considerations for cancer diagnosis and treatment. One of the most important field is the design and development of pharmaceutical drugs, based on targeted drug delivery system (TDDS). Being inspired by physio-chemical properties of nanoparticles, TDDS are designed to safely reach their targets and specifically release their cargo at the site of disease for enhanced therapeutic effects, thereby increasing the drug tissue bioavailability. Nanoparticles have the advantage of targeting cancer by simply being accumulated and entrapped in cancer cells. However, even after rapid growth of nanotechnology in nanomedicine, designing an effective targeted drug delivery system is still a challenging task. In this review, we reveal the recent advances in drug delivery approach with a particular focus on gold nanoparticles. We seek to expound on how these nanomaterials communicate in the complex environment to reach the target site, and how to design the effective TDDS for complex environments and simultaneously monitor the toxicity on the basis of designing such delivery complexes. Hence, this review will shed light on the research, opportunities and challenges for engineering nanomaterials with cancer biology and medicine to develop effective TDDS for treatment of cancer.

Blood-stable, tumor-adaptable disulfide bonded mPEG-(Cys)4-PDLLA micelles for chemotherapy

Although targeted delivery mediated by ligand modified or tumor microenvironment sensitive nanocarriers has been extensively pursued for cancer chemotherapy, the efficiency is still limited by premature drug release after systemic administration. Herein we report a highly blood-stable, tumor-adaptable drug carrier made of disulfide (DS) bonded mPEG-(Cys)(4)-PDLLA micelles. Intravenously injected disulfide bonded micelles stably retained doxorubicin in the bloodstream and efficiently delivered the drug to a tumor, with a 7-fold increase of the drug in the tumor and 1.9-fold decrease in the heart, as compared with self-assembled (SA), non-crosslinked mPEG-PDLLA micelles. In vivo administration of disulfide bonded micelles led to doxorubicin accumulation in cancer cell nuclei, which was not observed after administration of self-assembled micelles. With a doxorubicin dose as low as 2 mg/kg, disulfide bonded micelles almost completely suppressed tumor growth in mice.

pH-Sensitive polymeric micelle-based pH probe for detecting and imaging acidic biological environments

To overcome the limitations of monomeric pH probes for acidic tumor environments, this study designed a mixed micelle pH probe composed of polyethylene glycol (PEG)-b-poly(L-histidine) (PHis) and PEG-b-poly(L-lactic acid) (PLLA), which is well-known as an effective antitumor drug carrier. Unlike monomeric histidine and PHis derivatives, the mixed micelles can be structurally destabilized by changes in pH, leading to a better pH sensing system in nuclear magnetic resonance (NMR) techniques. The acidic pH-induced transformation of the mixed micelles allowed pH detection and pH mapping of 0.2-0.3 pH unit differences by pH-induced "on/off"-like sensing of NMR and magnetic resonance spectroscopy. The micellar pH probes sensed pH differences in nonbiological phosphate buffer and biological buffers such as cell culture medium and rat whole blood. In addition, the pH-sensing ability of the mixed micelles was not compromised by loaded doxorubicin. In conclusion, PHis-based micelles could have potential as a tool to simultaneously treat and map the pH of solid tumors in vivo.

External magnetic field-induced selective biodistribution of magnetoliposomes in mice

This study looked at the effect of an external magnet on the biodistribution of magnetoliposomes intravenously administrated in mice (8 mg iron/kg) with and without induced acute inflammation. Our results showed that due to enhanced vascular permeability, magnetoliposomes accumulated at the site of inflammation in the absence of an external magnetic field, but the amount of iron present increased under the effect of a magnet located at the inflammation zone. This increase was dependent on the time (20 or 60 min) of exposure of the external magnetic field. It was also observed that the presence of the magnet was associated with lower amounts of iron in the liver, spleen, and plasma than was found in mice in which a magnet had not been applied. The results of this study confirm that it is possible to target drugs encapsulated in magnetic particles by means of an external magnet.

Gold Nanorods Conjugated with Doxorubicin and cRGD for Combined Anticancer Drug Delivery and PET Imaging

A multifunctional gold nanorod (GNR)-based nanoplatform for targeted anticancer drug delivery and positron emission tomography (PET) imaging of tumors was developed and characterized. An anti-cancer drug (i.e., doxorubicin (DOX)) was covalently conjugated onto PEGylated (PEG: polyethylene glycol) GNR nanocarriers via a hydrazone bond to achieve pH-sensitive controlled drug release. Tumor-targeting ligands (i.e., the cyclo(Arg-Gly-Asp-D-Phe-Cys) peptides, cRGD) and ⁶⁴Cu-chelators (i.e., 1,4,7-triazacyclononane-N, N', N''-triacetic acid (NOTA)) were conjugated onto the distal ends of the PEG arms to achieve active tumor-targeting and PET imaging, respectively. Based on flow cytometry analysis, cRGD-conjugated nanocarriers (i.e., GNR-DOX-cRGD) exhibited a higher cellular uptake and cytotoxicity than non-targeted ones (i.e., GNR-DOX) in vitro. However, GNR-DOX-cRGD and GNR-DOX nanocarriers had similar in vivo biodistribution according to in vivo PET imaging and biodistribution studies. Due to the unique optical properties of GNRs, this multifunctional GNR-based nanoplatform can potentially be optimized for combined cancer therapies (chemotherapy and photothermal therapy) and multimodality imaging (PET, optical, X-ray computed tomography (CT), etc.).

Peptide targeted lipid nanoparticles for anticancer drug delivery

Encapsulating anticancer drugs in nanoparticles has proven to be an effective mechanism to alter the pharmacokinetic and pharmacodynamic profiles of the drugs, leading to clinically useful cancer therapeutics like Doxil and DaunoXome. Underdeveloped tumor vasculature and lymphatics allow these first-generation nanoparticles to passively accumulate within the tumor, but work to create the next-generation nanoparticles that actively participate in the tumor targeting process is underway. Lipid nanoparticles functionalized with targeting peptides are among the most often studied. The goal of this article is to review the recently published literature of targeted nanoparticles to highlight successful designs that improved in vivo tumor therapy, and to discuss the current challenges of designing these nanoparticles for effective in vivo performance.

Locoregional drug delivery using image-guided intra-arterial drug eluting bead therapy

Lipiodol-based transarterial chemoembolization (TACE) has been performed for over 3 decades for the treatment of solid tumors and describes the infusion of chemotherapeutic agents followed by embolization with particles. TACE is an effective treatment for inoperable hepatic tumors, especially hypervascular tumors such as hepatocellular carcinoma. Recently, drug eluting beads (DEBs), in which a uniform embolic material is loaded with a drug and delivered in a single image-guided step, have been developed to reduce the variability in a TACE procedure. DEB-TACE results in localization of drug to targeted tumors while minimizing systemic exposure to chemotherapeutics. Once localized in the tissue, drug is eluted from the DEB in a controlled manner and penetrates hundreds of microns of tissue from the DEB surface. Necrosis is evident surrounding a DEB in tissue days to months after therapy; however, the contribution of drug and ischemia is currently unknown. Future advances in DEB technology may include image-ability, DEB size tailored to tumor anatomy and drug combinations.

Drug targeting to tumors: principles, pitfalls and (pre-) clinical progress

Many different systems and strategies have been evaluated for drug targeting to tumors over the years. Routinely used systems include liposomes, polymers, micelles, nanoparticles and antibodies, and examples of strategies are passive drug targeting, active drug targeting to cancer cells, active drug targeting to endothelial cells and triggered drug delivery. Significant progress has been made in this area of research both at the preclinical and at the clinical level, and a number of (primarily passively tumor-targeted) nanomedicine formulations have been approved for clinical use. Significant progress has also been made with regard to better understanding the (patho-) physiological principles of drug targeting to tumors. This has led to the identification of several important pitfalls in tumor-targeted drug delivery, including I) overinterpretation of the EPR effect; II) poor tumor and tissue penetration of nanomedicines; III) misunderstanding of the potential usefulness of active drug targeting; IV) irrational formulation design, based on materials which are too complex and not broadly applicable; V) insufficient incorporation of nanomedicine formulations in clinically relevant combination regimens; VI) negligence of the notion that the highest medical need relates to metastasis, and not to solid tumor treatment; VII) insufficient integration of non-invasive imaging techniques and theranostics, which could be used to personalize nanomedicine-based therapeutic interventions; and VIII) lack of (efficacy analyses in) proper animal models, which are physiologically more relevant and more predictive for the clinical situation. These insights strongly suggest that besides making ever more nanomedicine formulations, future efforts should also address some of the conceptual drawbacks of drug targeting to tumors, and that strategies should be developed to overcome these shortcomings.

Anticancer nanomedicine and tumor vascular permeability; Where is the missing link?

Anticancer nanomedicine was coined to describe anticancer delivery systems such as polymer conjugates, liposomes, micelles, and metal nanoparticles. These anticancer delivery platforms have been developed with the enhanced permeability and retention (EPR) effect as a central mechanism for tumor targeting. EPR based nanomedicine has demonstrated, beyond doubt, to selectively target tumor tissues in animal models. However, over the last two decades, only nine anticancer agents utilizing this targeting strategy have been approved for clinical use. In this review, we systematically analyze various aspects that explain the limited clinical progress yet achieved. The influence of nanomedicine physicochemical characteristics, animal tumor models, and variations in tumor biology, on EPR based tumor targeting is closely examined. Furthermore, we reviewed results from over one hundred publications to construct patterns of factors that can influence the transition of EPR based anticancer nanomedicine to the clinic.

Analysis on the current status of targeted drug delivery to tumors

Targeted drug delivery to tumor sites is one of the ultimate goals in drug delivery. Recent progress in nanoparticle engineering has certainly improved drug targeting, but the results are not as good as expected. This is largely due to the fact that nanoparticles, regardless of how advanced they are, find the target as a result of blood circulation, like the conventional drug delivery systems do. Currently, the nanoparticle-based drug delivery to the target tumor tissues is based on wrong assumptions that most of the nanoparticles, either PEGylated or not, reach the target by the enhanced permeation and retention (EPR) effect. Studies have shown that so-called targeting moieties, i.e., antibodies or ligands, on the nanoparticle surface do not really improve delivery to target tumors. Targeted drug delivery to tumor sites is associated with highly complex biological, mechanical, chemical and transport phenomena, of which characteristics vary spatiotemporally. Yet, most of the efforts have been focused on design and surface manipulation of the drug carrying nanoparticles with relatively little attention to other aspects. This article examines the current misunderstandings and the main difficulties in targeted drug delivery.

Polymeric nucleic acid carriers: current issues and novel design approaches

To deliver nucleic acids including plasmid DNA (pDNA) and short interfering RNA (siRNA), polymeric gene carriers equipped with various functionalities have been extensively investigated. The functionalities of these polymeric vectors have been designed to overcome various extracellular and intracellular hurdles that nucleic acids and their carriers encounter during their journey from injection site to intracellular target site. This review briefly introduces known extracellular and intracellular issues of nucleic acid delivery and their solution strategies. We examine significant yet overlooked factors affecting nucleic acid delivery (e.g., microenvironmental pH, polymer/siRNA complexation, and pharmaceutical formulation) and highlight our reported approaches to solve these problems.

Nanoparticle delivery systems for cancer therapy: advances in clinical and preclinical research

Conventional anticancer drugs display significant shortcomings which limit their use in cancer therapy. For this reason, important progress has been achieved in the field of nanotechnology to solve these problems and offer a promising and effective alternative for cancer treatment. Nanoparticle drug delivery systems exploit the abnormal characteristics of tumour tissues to selectively target their payloads to cancer cells, either by passive, active or triggered targeting. Additionally, nanoparticles can be easily tuned to improve their properties, thereby increasing the therapeutic index of the drug. Liposomes, polymeric nanoparticles, polymeric micelles and polymer- or lipid-drug conjugate nanoparticles incorporating cytotoxic therapeutics have been developed; some of them are already on the market and others are under clinical and preclinical research. However, there is still much research to be done to be able to defeat the limitations of traditional anticancer therapy. This review focuses on the potential of nanoparticle delivery systems in cancer treatment and the current advances achieved.

Biodistribution and Tumor Localization of PEG-Modified Dendritic Poly(L-Lysine) Oligonucleotide Complexes

A poly(ethylene glycol) (PEG)-modified dendritic poly(L-lysine) (PEG-WeKG6) containing tryptophan residues in its core was synthesized as an oligonucleotide carrier to tumors after systemic injection. PEG- WeKG6 formed a stable complex with double-stranded deoxyoligonucleotide (ODN). The size and the zeta-potential of the complex were smaller than those of a dendritic poly(L-lysine) without PEG (WeKG6). To study the biodistribution of the complexes in tumor-bearing mice after intravenous injection, the den- drimers and the oligonucleotide were labeled with gadolinium and Cy5, respectively. Our results show that PEG modification of the dendrimer improved the stability of ODN in blood circulation. Effective accumulation of the PEG-WeKG6/ODN complex in the tumor tissue was found 24 h after the injection. These results indicate that PEG-WeKG6 is suitable for forming a complex with any genetic or therapeutic material for efficient delivery to tumors.

Cancer stem cells and drug resistance: the potential of nanomedicine

Properties of the small group of cancer cells called tumor-initiating or cancer stem cells (CSCs) involved in drug resistance, metastasis and relapse of cancers can significantly affect tumor therapy. Importantly, tumor drug resistance seems to be closely related to many intrinsic or acquired properties of CSCs, such as quiescence, specific morphology, DNA repair ability and overexpression of antiapoptotic proteins, drug efflux transporters and detoxifying enzymes. The specific microenvironment (niche) and hypoxic stability provide additional protection against anticancer therapy for CSCs. Thus, CSC-focused therapy is destined to form the core of any effective anticancer strategy. Nanomedicine has great potential in the development of CSC-targeting drugs, controlled drug delivery and release, and the design of novel gene-specific drugs and diagnostic modalities. This review is focused on tumor drug resistance-related properties of CSCs and describes current nanomedicine approaches, which could form the basis of novel combination therapies for eliminating metastatic and CSCs.

"Targeting" nanoparticles: the constraints of physical laws and physical barriers

In comparison to the complexities of the body, its organs, its normal and aberrant cells, many nanoparticles will appear to be relatively simple objects. This view is deceptive because the physicochemical properties of nanosystems, although quite well understood on the basis of material science, surface science and colloid theory, are far from simple in practice. While their properties are largely controllable in vitro, often purportedly "designed", their administration by any route changing environments conspires to produce additional layers of complexity. Some of the key physical laws and physicochemical parameters governing the fate of nanoparticles on their journey from point of intravenous administration to desired destinations such as tumors are discussed. Much of the science relevant to nanocarrier based targeting has been elaborated in studying purely physical phenomena, but there can be found therein many analogies with biological systems. These include factors that impede quantitative targeting: diffusion in complex media, aggregation and flocculation, hindered behavior of particles in confined spaces, jamming and dispersion in flow. All of these have the ability to influence fate and destination. Most of the critical processes are particle size dependent but not always linearly so. Virtually all processes in vivo involve an element of probability. Particle size and properties can be controlled to a large extent, but stochastic processes cannot by definition. Progress has been made, but the quantitative delivery of a nanocarrier to defined sites in tumors is neither inevitable nor yet predictable.

Image-guided drug delivery with magnetic resonance guided high intensity focused ultrasound and temperature sensitive liposomes in a rabbit Vx2 tumor model

Clinical-grade doxorubicin encapsulated low temperature sensitive liposomes (LTSLs) were combined with a clinical magnetic resonance-guided high intensity focused ultrasound (MR-HIFU) platform to investigate in vivo image-guided drug delivery. Plasma pharmacokinetics were determined in 3 rabbits. Fifteen rabbits with Vx2 tumors within superficial thigh muscle were randomly assigned into three treatment groups: 1) free doxorubicin, 2) LTSL and 3) LTSL + MR-HIFU. For the LTSL + MR-HIFU group, mild hyperthermia (40-41 °C) was applied to the tumors using an MR-HIFU system. Image-guided non-invasive hyperthermia was applied for a total of 30 min, completed within 1h after LTSL infusion. High-pressure liquid chromatography (HPLC) analysis of the harvested tumor and organ/tissue homogenates was performed to determine doxorubicin concentration. Fluorescence microscopy was performed to determine doxorubicin spatial distribution in the tumors. Sonication of Vx2 tumors resulted in accurate (mean = 40.5 ± 0.1 °C) and spatially homogenous (SD = 1.0 °C) temperature control in the target region. LTSL + MR-HIFU resulted in significantly higher tumor doxorubicin concentrations (7.6- and 3.4-fold greater compared to free doxorubicin and LTSL respectively, p<0.05, Newman-Keuls). This improved tumor concentration was achieved despite heating <25% of the tumor volume. Free doxorubicin and LTSL treatments appeared to deliver more drug in the tumor periphery as compared to the tumor core. In contrast, LTSL + MR-HIFU treatment suggested an improved distribution with doxorubicin found in both the tumor periphery and core. Doxorubicin bio-distribution in non-tumor organs/tissues was fairly similar between treatment groups. This technique has potential for clinical translation as an image-guided method to deliver drug to a solid tumor.

Nanoscale drug delivery systems for enhanced drug penetration into solid tumors: current progress and opportunities

Poor penetration of anticancer drags into solid tumors significantly limits their efficacy. This phenomenon has long been observed for small-molecule chemotherapeutics, and it can be even more pronounced for nanoscale therapies. Nanoparticles have enormous potential for the treatment of cancer due to their wide applicability as drug delivery and imaging vehicles and their size-dependent accumulation into solid tumors by the enhanced permeability and retention (EPR) effect. Further, synthetic nanoparticles can be engineered to overcome barriers to drag delivery. Despite their promise for the treatment of cancer, relatively little work has been done to study and improve their ability to diffuse into solid tumors following passive accumulation in the tumor vasculature. In this review, we present the complex issues governing efficient penetration of nanoscale therapies into solid tumors. The current methods available to researchers to study nanoparticle penetration into malignant tumors are described, and the most recent works studying the penetration of nanoscale materials into solid tumors are summarized. We conclude with an overview of the important nanoparticle design parameters governing their tumor penetration, as well as by highlighting critical directions in this field.

Polymeric conjugates for drug delivery

The field of polymer therapeutics has evolved over the past decade and has resulted in the development of polymer-drug conjugates with a wide variety of architectures and chemical properties. Whereas traditional non-degradable polymeric carriers such as poly(ethylene glycol) (PEG) and N-(2-hydroxypropyl methacrylamide) (HPMA) copolymers have been translated to use in the clinic, functionalized polymer-drug conjugates are increasingly being utilized to obtain biodegradable, stimuli-sensitive, and targeted systems in an attempt to further enhance localized drug delivery and ease of elimination. In addition, the study of conjugates bearing both therapeutic and diagnostic agents has resulted in multifunctional carriers with the potential to both "see and treat" patients. In this paper, the rational design of polymer-drug conjugates will be discussed followed by a review of different classes of conjugates currently under investigation. The design and chemistry used for the synthesis of various conjugates will be presented with additional comments on their potential applications and current developmental status.

Hybrid nanocrystals: University of Kentucky US20060280680A1

This patent application claims an interesting and novel combination of passive accumulation of drug nanocrystals within diseased tissue, in combination with active uptake of the nanocrystals by diseased cells. The patent application further claims the hybrid nanocrystals combining imaging or stabilizing molecules as inclusions in the crystal matrix. There is a focus on cancer chemotherapy and imaging, but the initial claims are not disease specific. In this patent evaluation, the novelty and utility of this application is examined, while the state of the art in nanocrystal formulations and formulation is discussed.

Association of chemotherapeutic drugs with dendrimer nanocarriers: an assessment of the merits of covalent conjugation compared to noncovalent encapsulation

Cancer is a leading cause of death within developed nations, and part of this morbidity is due to difficulties associated with its treatment. Currently, anticancer therapy relies heavily upon the administration of small molecule cytotoxic drugs that attack both cancerous and noncancerous cells due to limited selectivity of the drugs and widespread distribution of the cytotoxic molecules throughout the body. The antitumor efficacy and systemic toxicity of existing chemotherapeutic drugs can, however, be improved by employing formulation and particle engineering approaches. Thus, drug delivery systems can be developed that more specifically target tumor tissue using both passive (such as the enhanced permeation and retention effect) and active (through the use of cancer targeting ligands) modalities. Dendrimers are one such system that can be developed with high structural monodispersity, long plasma circulation times and precise control over surface structure and biodistribution properties. Chemotherapeutic drugs can be associated with dendrimers via covalent conjugation to the surface, or via encapsulation of drugs within the structure. Each of these approaches has demonstrated therapeutic benefit relative to the administration of free drug. Thus far, however, there has not been a systematic review toward which drug association approach will provide the best outcomes in terms of antitumor efficacy and systemic toxicity. Hence, the current literature is reviewed here and recommendations are proposed as to the suggested approach to develop dendrimers as tumor targeted drug-delivery vectors.

Drug targeting systems for inflammatory disease: one for all, all for one

In various systemic disorders, structural changes in the microenvironment of diseased tissues enable both passive and active targeting of therapeutic agents to these tissues. This has led to a number of targeting approaches that enhance the accumulation of drugs in the target tissues, making drug targeting an attractive strategy for the treatment of various diseases. Remarkably, the strategic principles that form the basis of drug targeting are often employed for tumor targeting, while chronic inflammatory diseases appear to draw much less attention. To provide the reader with a general overview of the current status of drug targeting to inflammatory diseases, the passive and active targeting strategies that have been used for the treatment of rheumatoid arthritis (RA) and multiple sclerosis (MS) are discussed. The last part of this review addresses the dualism of platform technology-oriented ("one for all") and disease-oriented drug targeting research ("all for one"), both of which are key elements of effective drug targeting research.

Reductionism and complexity in nanoparticle-vectored drug targeting

This paper briefly discusses reductionism as a process for dissecting the complexities of drug targeting mediated by nanoparticulate carriers. While reductionism has been said to have been a drawback to enhanced appreciation and understanding of complex biological systems, it is concluded here that the dissection of the individual stages of the procession from injection to final destination in specific targets in a living complex organism is essential. It should allow a decrease in the empiricism from laudable and inventive efforts to achieve high levels of drug delivery to specific diseased targets such as tumours. At the stage of development of the field there have perhaps been fewer than desirable detailed experimental or theoretical investigations of these individual stages. However, there are frequently analogies in the literature from which to draw at least tentative conclusions about the physics, physical chemistry and biology which underpin the processes involved.

The journey of a drug-carrier in the body: an anatomo-physiological perspective

Recent advances in chemistry and material sciences have witnessed the emergence of an increasing number of novel and complex nanosized carriers for the delivery of drugs and imaging agents. Nevertheless, this raise in complexity does not necessarily offer more efficient systems. The lack of performance experienced by several colloidal drug carriers during the preclinical and clinical development processes can be explained by inadequate pharmacokinetic/biodistribution profiles and/or unacceptable toxicities. A comprehensive understanding of the body characteristics is necessary to predict and prevent these problems from the early stages of nanomaterial conception. In this manuscript, we review and discuss the anatomical and physiological elements which must be taken into account when designing new carriers for delivery or imaging purposes. This article gives a general overview of the main organs involved in the elimination of nanosized materials and briefly summarizes the knowledge acquired over more than 30 years of research and development in the field of drug targeting.

In vivo antitumor effect of cromolyn in PEGylated liposomes for pancreatic cancer

A PEGylated liposomal formulation of cromolyn, composed of dipalmitoylphosphatidylcholine (DPPC), dimyristoylphosphatidylcholine (DMPC), distearoylphosphatidylcholine (DSPC) and 1,2-distearoyl-sn-glycero-3-phospho-ethanolamine-N-[methoxy(polyethyleneglycol)-2000] (DSPE-mPEG2000), has been developed with the purpose of improving the antitumor activity of cromolyn for human pancreatic adenocarcinoma. In stability study, the amount of proteins adsorbed onto the PEGylated liposomes encapsulating cromolyn was 4.5-fold lower than the non-PEGylated liposome. In vitro study showed that the cromolyn in PEGylated liposome exhibited better anti-proliferative effect in BxPC-3 cells than in Panc-1 cells, which indicates higher level of endogenous S100P protein in BxPC-3 cells than in Panc-1 cells as a target protein for this drug. Moreover, the combination of cromolyn with gemcitabine in PEGylated liposomes demonstrated the strongest cytotoxicity to BxPC-3 pancreatic cancer cells in vitro and the highest anti-tumor activity against the BxPC-3 tumor bearing nude mice in vivo. Thus, this PEGylated liposomal formulation of cromolyn is expected to provide a novel approach to the treatment of pancreatic cancer in the future.