ASSESSMENT OF LIPOSOME DELIVERY USING SCINTIGRAPHIC IMAGING

ABSTRACT

Scintigraphic imaging is a valuable tool for the development of liposome-based therapeutic agents. It provides the ability to non-invasively track and quantitate the distribution of liposomes in the body. Liposomes labeled with technetium-99 m (^99mTc) are particularly advantageous for imaging studies because of their favorable physical characteristics. Examples of how scintigraphic imaging studies have contributed to the evaluation and development of a variety of liposome formulations will be presented. These include liposomes for targeting processes with inflammation associated increased vascular permeability such as healing bone fractures and viral infections; liposomes for intraarticular delivery; and liposomes for delivery of agents to lymph nodes located in the extremities, the mediastinum and the peritoneum. Scintigraphic studies of liposome distribution are very informational and often suggest new drug delivery applications for liposomes.     

The use of scintigraphic imaging as a tool in the development of liposome formulations

Scintigraphic imaging is a valuable tool that can be used during the development of liposome-based therapeutic agents. It provides the ability to non-invasively track and quantitate the distribution of liposomes in the body. This review article provides a general overview of the methods involved in producing scintigraphic images as well as methods of radiolabeling liposomes. Liposomes labeled with technetium-99m ((99m)Tc) are particularly useful for scintigraphic imaging due to the physical characteristics of (99m)Tc, which provides a high quality image. Examples of how scintigraphic imaging studies have contributed to the development of a variety of liposome-based formulations are covered in this article. These liposome formulations include long-circulating liposome-based oxygen carriers, liposome-based anti-cancer drugs, liposomes encapsulating antibiotics and anti-fungals, and liposomes targeted to lymph nodes. Studies using scintigraphic imaging for the investigation of immune responses to liposomes are also discussed. These examples demonstrate the usefulness of scintigraphic imaging for the development of novel liposome formulations.     

In vivo visualizing of organs and tissues with liposomes

Abstract

The application of liposomes as carriers for imaging agents is considered. Liposomes loaded with the appropriate contrast agents have been shown to be suitable for gamma-, magnetic resonance (MR), computed tomography (CT) and ultrasound imaging. The methods are briefly described to prepare liposomes loaded with different contrast agents, as well as some data on their biodistribution. The application of contrast-loaded liposomes for liver/spleen, tumor, lymph nodes, infection and inflammation sites, myocardial infarction, and blood pool imaging is briefly reviewed together with some data available on the use of liposome for the ophtalmological imaging. New trends in the use of contrast-loaded liposomes are also considered, such as the application of long-circulating polymer-modified liposomes for imaging purposes and development of new lipid-coated liposome-like contrast agents.     

A simple method for producing a technetium-99m-labeled liposome which is stable In Vivo

A new method for labeling preformed liposomes with technetium-99m (^99mTc) has been developed which is simple to perform and stable in vivo. Previous ^99mTc-liposome labels have had variable labeling efficiencies and stability. This method consistently achieves high labeling efficiencies (> 90%) with excellent stability. A commercially available radiopharmaceutical kit—hexamethylpropyleneamine oxime (HM-PAO)—is reconstituted with ^99mTcO^−₄ and then incubated with preformed liposomes that encapsulate glutathione. The incubation takes only 30 min at room temperature. Liposomes that co-encapsulate other proteins such as hemoglobin or albumin, in addition to glutathione, also label with high efficiency. Both in vitro and in vivo studies indicate good stability of this label. Rabbit images show significant spleen and liver uptake at 2 and 20 h after liposome infusion without visualization of thyroid, stomach or bladder activity.This labeling method can be used to study the biodistribution of a wide variety of liposome preparations that are being tested as novel drug delivery systems. This method of labeling liposomes with ^99mTc may also have applications in diagnostic imaging.     

A tumor vasculature targeted liposome delivery system for combretastatin A4: Design, characterization, and in vitro evaluation

The objective of this study was to develop an efficient tumor vasculature targeted liposome delivery system for combretastatin A4, a novel antivascular agent. Liposomes composed of hydrogenated soybean phosphatidylcholine (HSPC), cholesterol, distearoyl phosphoethanolamine-polyethylene-glycol-2000 conjugate (DSPE-PEG), and DSPE-PEG-maleimide were prepared by the lipid film hydration and extrusion process. Cyclic RGD (Arg-Gly-Asp) peptides with affinity for αvβ3-integrins expressed on tumor vascular endothelial cells were coupled to the distal end of PEG on the liposomes sterically stabilized with PEG (long circulating liposomes, LCL). The liposome delivery system was characterized in terms of size, lamellarity, ligand density, drug loading, and leakage properties. Targeting nature of the delivery system was evaluated in vitro using cultured human umbilical vein endothelial cells (HUVEC). Electron microscopic observations of the formulations revealed presence of small unilamellar liposomes of ∼120 nm in diameter. High performance liquid chromatography determination of ligand coupling to the liposome surface indicated that more than 99% of the RGD peptides were reacted with maleimide groups on the liposome surface. Up to 3 mg/mL of stable liposomal combretastatin A4 loading was achieved with ∼80% of this being entrapped within the liposomes. In the in vitro cell culture studies, targeted liposomes showed significantly higher binding to their target cells than non-targeted liposomes, presumably through specific interaction of the RGD with its receptors on the cell surface. It was concluded that the targeting properties of the prepared delivery system would potentially improve the therapeutic benefits of combretastatin A4 compared with nontargeted liposomes or solution dosage forms.     

pH-Sensitive Liposomes

Abstract

pH sensitive liposomes are lipid compositions that can be destabilized when the external pH is changed; usually from a neutral or slightly alkaline pH to an acidic pH. They are designed to circumvent delivery of liposome contents to the lysosomes of cells following internalization of the vesicle via the endocytic pathway. In the majority of compositions, a lipid containing a pH titratable group is mixed with phosphatidylethanolamine containing unsaturated acyl chains in a molar ratio (pH sensitive component/PE) of 1/4 or greater. There are five major groups of phosphatidylethanolamine containing pH-senstive lipid compositions. These can be classified by their acid-titratable component: phospholipids, acylated amino acids, fatty acids, cholesterol derivatives and miscellaneous double chain amphiphiles. The biophysical mechanism of action involves a transition of the lipids from the lamellar phase to the hexagonal phase. In cell culture, pH sensitive vesicles can increase the delivery of fluorescent markers, proteins, cytotoxic compounds, RNA and DNA into the cytoplasm. The mechanism of delivery is suggested to involve the destabilization of the liposome in the endosome as the pH is reduced from 7.4 to 5.0 and subsequent destabilization of, or fusion with, the endosomal membrane; some of the liposome contents are introduced into the cytoplasm. In most cases, the extent of liposome contents delivery into the cytoplasm is less than 1% of the amount that becomes cell associated. However further studies, with more reliable assays to differentiate cytoplasmic from lysosomal delivery, are required to place an exact value on this efficiency. The efficiency of pH sensitive liposomes in vivo is limited by stability of certain of the liposome compositions in serum and targeting to the appropriate cell. Cholesterol hemisuccinate is a particularly attractive component for in vivo use since it stabilizes the liposome when in serum at pH 7.4. The use of pH sensitive liposomes in drug delivery should continue to expand due to the increasing number of macromolecular therapeutic agents with intracellular targets.     

Immunoliposomes and PEGylated Immunoliposomes: Possible Use for Targeted Delivery of Imaging Agents

Liposomes can be made target-specific by immobilizing antibodies on their surface against characteristic components of target organ or tissue. Possible schemes of antibody immobilization on liposomes are briefly considered. The use of immunoliposomes for the targeted delivery of diagnostic and therapeutic agents within the cardiovascular system is discussed. Immunoliposomes are shown to be suitable carriers for targeting blood vessel injuries, lung endothelium, and myocardial infarction. The role of polyethylene glycol in the preparation of long-circulation liposomes is investigated, and a hypothesis explaining the mechanism of polymer protective action in terms of physicochemical properties of diluted polymeric solutions is suggested. Polyethylene glycol-coated liposomes are investigated as possible carriers of imaging agents for gamma and MR visualization of different areas of interest in the body, including lymph nodes. The possibility of simultaneous immobilization of protective polymer and antibody on the liposome surface is proved, and the long-circulating targeted immunoliposomes are used for the targeted delivery of radiolabel to necrotic areas in rabbits with experimental myocardial infarction.     

Engineering Lipid Vesicles of Enhanced Intratumoral Transport Capabilities: Correlating Liposome Characteristics with Penetration into Human Prostate Tumor Spheroids

Liposomes have been widely used delivery systems, particularly relevant to the development of cancer therapeutics. Numerous liposome-based drugs are in the clinic or in clinical trials today against multiple tumor types; however, systematic studies of liposome interactions with solid or metastatic tumor nodules are scarce. This study is describing the in vitro interaction between liposomes and avascular human prostate (LNCaP-LN3) tumor spheroids. The ability of fluorescently labelled liposomal delivery systems of varying physicochemical characteristics to penetrate within multicellular tumor spheroids has been investigated by confocal laser scanning microscopy. A variety of liposome characteristics and experimental parameters were investigated, including lipid bilayer composition, duration of liposome-spheroid interaction, mean liposome size, steric stabilization of liposomes. Electrostatic binding between cationic liposomes and spheroids was very efficient; however, it impeded any significant penetration of the vesicles within deeper layers of the tumor spheroid. Small unilamellar liposomes of neutral surface character did not bind as efficiently but exhibited enhanced penetrative transport capabilities closer to the tumor core. Polymer-coated (sterically stabilised) liposomes exhibited almost no interaction with the spheroid, indicating that their limited diffusion within avascular tissues may be a limiting step for their use against micrometastases. Multicellular tumor spheroids were used as models of solid tumor interstitium relevant to delivery systems able to extravasate from the microcapillaries or as models of prevascularized micrometastases. This study illustrates that interactions between liposomes and other drug delivery systems with multicellular tumor spheroids can offer critically important information with respect to optimizing solid or micrometastatic tumor delivery and targeting strategies.     

Engineering lipid vesicles of enhanced intratumoral transport capabilities: correlating liposome characteristics with penetration into human prostate tumor spheroids

Liposomes have been widely used delivery systems, particularly relevant to the development of cancer therapeutics. Numerous liposome-based drugs are in the clinic or in clinical trials today against multiple tumor types; however, systematic studies of liposome interactions with solid or metastatic tumor nodules are scarce. This study is describing the in vitro interaction between liposomes and avascular human prostate (LNCaP-LN3) tumor spheroids. The ability of fluorescently labelled liposomal delivery systems of varying physicochemical characteristics to penetrate within multicellular tumor spheroids has been investigated by confocal laser scanning microscopy. A variety of liposome characteristics and experimental parameters were investigated, including lipid bilayer composition, duration of liposome-spheroid interaction, mean liposome size, steric stabilization of liposomes. Electrostatic binding between cationic liposomes and spheroids was very efficient; however, it impeded any significant penetration of the vesicles within deeper layers of the tumor spheroid. Small unilamellar liposomes of neutral surface character did not bind as efficiently but exhibited enhanced penetrative transport capabilities closer to the tumor core. Polymer-coated (sterically stabilised) liposomes exhibited almost no interaction with the spheroid, indicating that their limited diffusion within avascular tissues may be a limiting step for their use against micrometastases. Multicellular tumor spheroids were used as models of solid tumor interstitium relevant to delivery systems able to extravasate from the microcapillaries or as models of prevascularized micrometastases. This study illustrates that interactions between liposomes and other drug delivery systems with multicellular tumor spheroids can offer critically important information with respect to optimizing solid or micrometastatic tumor delivery and targeting strategies.     

Effect of dehydration and rehydration of the pH-sensitive liposomes containing chimeric gag-V3 virus like particle on their long-term stability

One of the practical limitations with the use of liposomes for delivery of the pharmaceutical substances such as antigens is that liposomes are relatively unstable in storage. In order to extend the stability of liposome in storage without affecting their functional activity, solution-type liposomes were dehydrated to form a structurally intact dry liposomes. Comparative immunological evaluation was carried out for both dry and solution-type liposomes containing gag-V3 chimera, consequently it was found that dry liposomes elicited both humoral and cellular response as efficiently as solution-type liposomes did against the same gag-V3 antigen. Especially, long-term stability of the liposomes was remarkably enhanced by the dehydration made to liposomes without a significant change in its ability to elicit immune responsein vivo. These results indicate that dry pH-sensitive liposome may become an effective delivery and adjuvant system for general vaccine development.     

Liposomes in the Treatment of Parasitic,Viral, Fungal and Bacterial Infections

Abstract

The applicability of liposomes as carriers of immunomodulatory agents or antibiotics for improvement of treatment of severe infections is under investigation. The use of “classical'’ liposomes for targeting of macrophage modulators to enhance non-specific host resistance to infections caused by a variety of micro-organisms shows good results. The therapeutic prospects of “classical'’ liposomes as carriers of antibiotics are good, however are limited to the treatment of intracellular infections in mononuclear phagocyte system (MPS) tissues. The recent development of liposome formulations with reduced affinity to the MPS and long circulation half-lives creates new possibilities for obtaining improved delivery of antibiotics to infected tissues in general including infections in non-MPS tissues.     

Targeting of Liposomes Within Cardiovascular System

Abstract

Our studies on the targeting of liposomes and liposome-associated pharmaceuticals within the cardiovascular system are reviewed. The delivery of diagnostic and therapeutic agents in plain liposomes, immunoliposomes, long-circulating liposomes and long-circulating immunoliposomes into the sites of vascular injuries and myocardial infarction is discussed. In vitro, ex vivo, and in vivo experiments present a general view on the advantages and limitations of using liposome-mediated targeting. Liposomes capable of targeting pathological areas of the blood vessel wall both, in vitro and ex vivo are described, as well as liposome able to be internalized by normal endothelial cells. Liposome-mediated drug targeting to compromised myocardium is reviewed with a primary impact on liposomes with anti-cardiac myosin antibodies. Targeted visualization of myocardial infarction with diagnostic liposomes is discussed. Efficient accumulation of long-circulating immunoliposomes in the infarct zone is demonstrated, and a relative importance of different variables, such as liposome size, targetability, and prolonged circulation time, for target accumulation is analyzed. The use of immunoliposomes for targeted sealing of hypoxia-caused damages in plasmic membranes of cardiocytes is considered as a new approach in the therapeutic use of liposomes.     

Dual-targeting for brain-specific liposomes drug delivery system: Synthesis and preliminary evaluation

The treatment of glioma has become a great challenge because of the existence of brain barrier (BB). In order to develop an efficient brain targeting drug delivery system to greatly improve the brain permeability of anti-cancer drugs, a novel brain-targeted glucose-vitamin C (Glu-Vc) derivative was designed and synthesized as liposome ligand for preparing liposome to effectively deliver paclitaxel (PTX). The liposome was prepared and its particle size, zeta potential, encapsulation efficiency, release profile, stability, hemolysis and cytotoxicity were also characterized. What’s more, the cellular uptake of CFPE-labeled Glu-Vc-Lip on GLUT₁- and SVCT₂-overexpressed C6 cells was 4.79-, 1.95-, 4.00- and 1.53-fold higher than that of Lip, Glu-Lip, Vc-Lip and Glu?+?Vc-Lip. Also, the Glu-Vc modified liposomes showed superior targeting ability in vivo evaluation compared with naked paclitaxel, non-coated, singly-modified and co-modified by physical blending liposomes. The relative uptake efficiency was enhanced by 7.53 fold to that of naked paclitaxel, while the concentration efficiency was up to 7.89 times. What’s more, the Glu-Vc modified liposomes also displayed the maximum accumulation of DiD-loaded liposomes at tumor sites with the strongest fluorescence in the brain in vivo imaging. Our results suggest that chemical modification of liposomes with warheads of glucose and vitamin C represents a promising and efficient strategy for the development of brain-specific liposomes drug delivery system by utilizing the endogenous transportation mechanism of the warheads.     

Novel liposome-based formulations for prolonged delivery of proteins and vaccines

Abstract

Two strategies for increasing liposome stability in vivo are described in this review. The first strategy involves the encapsulation of liposomes within polymeric microcapsules of alginate-poly(L-lysine) that retained the liposomes inside but allowed the outward diffusion of proteins of 100 kDa or less, once they were released from the encapsulated liposomes. In vivo studies revealed that the microencapsulated liposome systems (MELs) extended the delivery of a model antigen, bovine serum albumin (BSA), for more that 80 days, resulting in the prolonged production of high levels of antigen-specific antibodies. The antibody levels were higher that those obtained with rats injected with BSA in complete Freund's adjuvant, or in liposomes. The unique construction of MELs enabled also the enzymatically-triggered pulsatile delivery of proteins from encapsulated liposomes, which was not possible before with liposomes.     

Pros and Cons of the Liposome Platform in Cancer Drug Targeting

Coating of liposomes with polyethylene-glycol (PEG) by incorporation in the liposome bilayer of PEG-derivatized lipids results in inhibition of liposome uptake by the reticulo-endothelial system and significant prolongation of liposome residence time in the blood stream. Parallel developments in drug loading technology have improved the efficiency and stability of drug entrapment in liposomes, particularly with regard to cationic amphiphiles such as anthracyclines. An example of this new generation of liposomes is a formulation of pegylated liposomal doxorubicin known as Doxil® or Caelyx®, whose clinical pharmacokinetic profile is characterized by slow plasma clearance and small volume of distribution. A hallmark of these long-circulating liposomal drug carriers is their enhanced accumulation in tumors. The mechanism underlying this passive targeting effect is the phenomenon known as enhanced permeability and retention (EPR) which has been described in a broad variety of experimental tumor types. Further to the passive targeting effect, the liposome drug delivery platform offers the possibility of grafting tumor-specific ligands on the liposome membrane for active targeting to tumor cells, and potentially intracellular drug delivery. The pros and cons of the liposome platform in cancer targeting are discussed vis-à-vis nontargeted drugs, using as an example a liposome drug delivery system targeted to the folate receptor.     

Synthesis and characterization of an ELP-conjugated liposome with thermo-sensitivity for controlled release of a drug

Liposome, one of various drug carriers, has been extensively studied as an inert carrier for the delivery of protein, DNA, and biologically active agents into cells. Recently, much effort has been directed to the development of stimuli-sensitive liposomes that are able to respond to certain internal or external stimuli, such as, pH, electricity, temperature, magnet, or light. Among them, to obtain liposomes which release the contents in response to ambient temperature, liposomes have been modified with chemically synthetic polymers having various lower critical solution temperatures (LCST). In this study, instead of chemically synthetic polymers, a biologically produced elastin-like polypeptide (ELP), which was composed of oligomeric repeats of the pentapeptide sequence (Val-Pro-Gly-Val- Gly), was used for endowing the liposome with thermosensitivity. A model drug was encapsulated in the ELPconjugated liposomes and the release behavior of the drug caused by the liposome disruption due to the aggregation of ELPs was investigated. In addition, conjugation of ELP to liposome was identified with Fourier Transformed Infrared (FT-IR) and Scanning Electron Microscope (SEM) analyses.     

Lyophilization of TC-99m-Hynic Labeled Peg-Liposomes

Abstract

The development of long circulating liposomes represented a major step forward towards the use of radiolabeled liposomes in nuclear medicine. The long circulation property markedly improves their uptake and consequently visualization of sites of infection and inflammation. Previously, we have developed a rapid and convenient method to label polyethylene glycol (PEG)-lipo-somes with technetium-99m (Tc-99m). PEG-liposomes containing the technetium-chelator hydrazino nicotinamide (HYNIC) could be labeled with Tc-99m with high efficiency. We showed that these Tc-99m-HYNIC labeled PEG-liposomes have excellent in vivo imaging characteristics in several pre-clinical and clinical studies. However, an important limitation associated with the use of HYNIC-PEG-liposome formulation as radiopharmaceutical is that their labeling efficiency decreases markedly within 3 months. In this paper we present a lyophilization method for HYNIC-PEG-liposomes using sucrose as a lyopro-tectant. The long-term stability of these liposomes in terms of the particle size and labeling efficiency upon reconstitution were determined. Additionally, the in vivo behavior of reconstituted radiolabeled liposomes in a rat model of focal infection was studied at two time-points after preparation.

Increasing the duration of the dehydration step significantly reduced the mean particle size upon reconstitution. Increasing the storage temperature from -20^°C to +4^°C also improved the particle size distribution upon reconstitution. The labeling efficiency for both freeze-dried preparations remained high during the 1 year-storage period and was always higher than 86%, but decreased for the control liposomes. Eight months after preparation, these liposomes had a labeling efficiency as low as 6%, whereas both freeze-dried preparations could still be labeled with an efficiency of 90%. The in vivo studies showed that there was no major difference in the biodistribution of the radiolabeled liposomes between 3 and 30 weeks post-preparation in rats with an Staphylococcus aureus abscess, indicating an acceptable long-term shelf-life of both freeze-dried liposome preparations. Abscesses were visualized from 2 hours post injection onwards.

In conclusion, a freeze-drying method which improved the long term shelf-life of HYNIC-PEG-liposomes is presented. The in vivo behavior of Tc-99m-PEG-liposomes, reconstituted 30 weeks after preparation, was similar to the biodistribution obtained with the non-freeze-dried preparation. The splenic uptake of these liposomes was slightly increased.     

Carboxylated phytosterol derivative-introduced liposomes for skin environment-responsive transdermal drug delivery system

Abstract

Transdermal drug delivery systems are a key technology for skin-related diseases and for cosmetics development. The delivery of active ingredients to an appropriate site or target cells can greatly improve the efficacy of medical and cosmetic agents. For this study, liposome-based transdermal delivery systems were developed using pH-responsive phytosterol derivatives as liposome components. Succinylated phytosterol (Suc-PS) and 2-carboxy-cyclohexane-1-carboxylated phytosterol (CHex-PS) were synthesized by esterification of hydroxy groups of phytosterol. Modification of phytosterol derivatives on 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes was confirmed by negatively zeta potentials at alkaline pH and the change of zeta potentials with decreasing pH. In response to acidic pH and temperatures higher than body temperature, Suc-PS-containing and CHex-PS-containing liposomes exhibited content release at intracellular acidic compartments of the melanocytes at the basement membrane of the skin. Phytosterol-derivative-containing liposomes were taken up by murine melanoma-derived B16-F10 cells. These liposomes delivered their contents into endosomes and cytosol of B16-F10 cells. Furthermore, phytosterol-derivative-containing liposomes penetrated the 3?D skin models and reached the basement membrane. Results show that pH-responsive phytosterol-derivative-containing DMPC liposomes are promising for use in transdermal medical or cosmetic agent delivery to melanocytes.     

Protein-liposome conjugates using cysteine-lipids and native chemical ligation

Liposomes have become popular drug delivery vehicles and have more recently also been applied as contrast agents for molecular imaging. Most current methods for functionalization of liposomes with targeting proteins rely on reactions of amine or thiol groups at the protein exterior, which generally result in nonspecific conjugation at multiple sites on the protein. In this study, we present native chemical ligation (NCL) as a general method to covalently couple recombinant proteins in a highly specific and chemoselective way to liposomes containing cysteine-functionalized phospholipids. A cysteine-functionalized phospholipid (Cys-PEG-DSPE) was prepared and shown to readily react with the MESNA thioester of EYFP, which was used as a model protein. Characterization of the EYFP-liposomes using fluorescence spectroscopy showed full retention of the fluorescent properties of conjugated EYFP and provides a lower limit of 120 proteins per liposome. The general applicability of NCL was further tested using CNA35, a collagen-binding protein recently applied in fluorescent imaging of collagen. NCL of CNA35 thioester yielded liposomes containing approximately 100 copies of CNA35 per liposome. The CNA35-liposomes were shown to be fully functional and bind collagen with a 150-fold higher affinity compared to CNA35. Our results show that NCL is an attractive addition to existing conjugation methods that allows direct, covalent, and highly specific coupling of recombinant proteins to liposomes and other lipid-based assemblies.     

Pros and cons of the liposome platform in cancer drug targeting

Coating of liposomes with polyethylene-glycol (PEG) by incorporation in the liposome bilayer of PEG-derivatized lipids results in inhibition of liposome uptake by the reticulo-endothelial system and significant prolongation of liposome residence time in the blood stream. Parallel developments in drug loading technology have improved the efficiency and stability of drug entrapment in liposomes, particularly with regard to cationic amphiphiles such as anthracyclines. An example of this new generation of liposomes is a formulation of pegylated liposomal doxorubicin known as Doxil or Caelyx, whose clinical pharmacokinetic profile is characterized by slow plasma clearance and small volume of distribution. A hallmark of these long-circulating liposomal drug carriers is their enhanced accumulation in tumors. The mechanism underlying this passive targeting effect is the phenomenon known as enhanced permeability and retention (EPR) which has been described in a broad variety of experimental tumor types. Further to the passive targeting effect, the liposome drug delivery platform offers the possibility of grafting tumor-specific ligands on the liposome membrane for active targeting to tumor cells, and potentially intracellular drug delivery. The pros and cons of the liposome platform in cancer targeting are discussed vis-à-vis nontargeted drugs, using as an example a liposome drug delivery system targeted to the folate receptor.