Interactions of immunoliposomes with target cells

A convenient method for the relative and absolute quantification of lipid components in liposomes by 1H- and 31P NMR-spectroscopy

OBJECTIVE

Liposomes are promising delivery systems for pharmaceutical applications and have been used in medicine in the recent past. Preparation of liposomes requires reliable characterization and quantification of the phospholipid components for which the traditional cumbersome molybdate method is used frequently. The objective was to improve relative and absolute quantification of lipid components from liposomes.

METHODS

A reliable method for quantification of lipid composition in liposome formulations in the 1-10 μmol range with 1H- and 31P NMR spectroscopy at 600 MHz has been developed. The method is based on three crystalline small-molecule standards (Ph3PO4, (Tol)3PO4, and Ph3PO) in CDCl3.

RESULTS

Excellent calibration linearity and chemical stability of the standards was observed. The method was tested in blind fashion on liposomes containing POPC, PEG-ceramide and a pH-sensitive trans-aminocyclohexanol-based amphiphile (TACH).1 Relative quantification (percentage of components) as well as determination of absolute lipid amount was possible with excellent reproducibility with an average error of 5%. Quantification (triplicate) was accomplished in 15 min based on 1H NMR and in 1 h based on 31P NMR. Very little change in mixture composition was observed over multiple preparative steps.

CONCLUSION

Liposome preparations containing POPC, POPE, DOPC, DPPC, TACH, and PEG-ceramide can be reliably characterized and quantified by 1H NMR and 31P NMR spectroscopy at 600 MHz in the μmol range.

Combination of Physicochemical Tropism and Affinity Moiety Targeting of Lipid Nanoparticles Enhances Organ Targeting

Two camps have emerged for targeting nanoparticles to specific organs and cell types: affinity moiety targeting and physicochemical tropism. Here we directly compare and combine both using intravenous (IV) lipid nanoparticles (LNPs) designed to target the lungs. We utilized PECAM antibodies as affinity moieties and cationic lipids for physicochemical tropism. These methods yield nearly identical lung uptake, but aPECAM LNPs show higher endothelial specificity. LNPs combining these targeting methods had >2-fold higher lung uptake than either method alone and markedly enhanced epithelial uptake. To determine if lung uptake is because the lungs are the first organ downstream of IV injection, we compared IV vs intra-arterial (IA) injection into the carotid artery, finding that IA combined-targeting LNPs achieve 35% of the injected dose per gram (%ID/g) in the first-pass organ, the brain, among the highest reported. Thus, combining the affinity moiety and physicochemical strategies provides benefits that neither targeting method achieves alone.

Dectin-2-Targeted Antifungal Liposomes Exhibit Enhanced Efficacy

Invasive fungal diseases caused by Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus have mortality rates ranging from 10 to 95%. Individual patient costs may exceed $100,000 in the United States. All antifungals in current use have serious limitations due to host toxicity and/or insufficient fungal cell killing that results in recurrent infections. Few new antifungal drugs have been introduced in the last 2 decades. Hence, there is a critical need for improved antifungal therapeutics. By targeting antifungal-loaded liposomes to α-mannans in the extracellular matrices secreted by these fungi, we dramatically reduced the effective dose of drug. Dectin-2-coated liposomes loaded with amphotericin B bound 50- to 150-fold more strongly to C. albicans, C. neoformans, and A. fumigatus than untargeted liposomes and killed these fungi more than an order of magnitude more efficiently. Targeting drug-loaded liposomes specifically to fungal cells has the potential to greatly enhance the efficacy of most antifungal drugs.

Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus cause life-threatening candidiasis, cryptococcosis, and aspergillosis, resulting in several hundred thousand deaths annually. The patients at the greatest risk of developing these life-threatening invasive fungal infections have weakened immune systems. The vulnerable population is increasing due to rising numbers of immunocompromised individuals as a result of HIV infection or immunosuppressed individuals receiving anticancer therapies and/or stem cell or organ transplants. While patients are treated with antifungals such as amphotericin B, all antifungals have serious limitations due to lack of sufficient fungicidal effect and/or host toxicity. Even with treatment, 1-year survival rates are low. We explored methods of increasing drug effectiveness by designing fungicide-loaded liposomes specifically targeted to fungal cells. Most pathogenic fungi are encased in cell walls and exopolysaccharide matrices rich in mannans. Dectin-2 is a mammalian innate immune membrane receptor that binds as a dimer to mannans and signals fungal infection. We coated amphotericin-loaded liposomes with monomers of Dectin-2’s mannan-binding domain, sDectin-2. sDectin monomers were free to float in the lipid membrane and form dimers that bind mannan substrates. sDectin-2-coated liposomes bound orders of magnitude more efficiently to the extracellular matrices of several developmental stages of C. albicans, C. neoformans, and A. fumigatus than untargeted control liposomes. Dectin-2-coated amphotericin B-loaded liposomes reduced the growth and viability of all three species more than an order of magnitude more efficiently than untargeted control liposomes and dramatically decreased the effective dose. Future efforts focus on examining pan-antifungal targeted liposomal drugs in animal models of fungal diseases.

IMPORTANCE Invasive fungal diseases caused by Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus have mortality rates ranging from 10 to 95%. Individual patient costs may exceed $100,000 in the United States. All antifungals in current use have serious limitations due to host toxicity and/or insufficient fungal cell killing that results in recurrent infections. Few new antifungal drugs have been introduced in the last 2 decades. Hence, there is a critical need for improved antifungal therapeutics. By targeting antifungal-loaded liposomes to α-mannans in the extracellular matrices secreted by these fungi, we dramatically reduced the effective dose of drug. Dectin-2-coated liposomes loaded with amphotericin B bound 50- to 150-fold more strongly to C. albicans, C. neoformans, and A. fumigatus than untargeted liposomes and killed these fungi more than an order of magnitude more efficiently. Targeting drug-loaded liposomes specifically to fungal cells has the potential to greatly enhance the efficacy of most antifungal drugs.

Surface-Functionalized Nanoparticles as Efficient Tools in Targeted Therapy of Pregnancy Complications

Minimizing exposure of the fetus to medication and reducing adverse off-target effects in the mother are the primary challenges in developing novel drugs to treat pregnancy complications. Nanomedicine has introduced opportunities for the development of novel platforms enabling targeted delivery of drugs in pregnancy. This review sets out to discuss the advances and potential of surface-functionalized nanoparticles in the targeted therapy of pregnancy complications. We first describe the human placental anatomy, which is fundamental for developing placenta-targeted therapy, and then we review current knowledge of nanoparticle transplacental transport mechanisms. Meanwhile, recent surface-functionalized nanoparticles for targeting the uterus and placenta are examined. Indeed, surface-functionalized nanoparticles could help prevent transplacental passage and promote placental-specific drug delivery, thereby enhancing efficacy and improving safety. We have achieved promising results in targeting the placenta via placental chondroitin sulfate A (plCSA), which is exclusively expressed in the placenta, using plCSA binding peptide (plCSA-BP)-decorated nanoparticles. Others have also focused on using placenta- and uterus-enriched molecules as targets to deliver therapeutics via surface-functionalized nanoparticles. Additionally, we propose that placenta-specific exosomes and surface-modified exosomes might be potential tools in the targeted therapy of pregnancy complications. Altogether, surface-functionalized nanoparticles have great potential value as clinical tools in the targeted therapy of pregnancy complications.

Long-Circulating, pH-Sensitive Liposomes

A major limiting factor for the wide application of pH-sensitive liposomes is their recognition and sequestration by the phagocytes of the reticuloendothelial system, which conditions a very short circulation half-life. Typically prolonged circulation of liposomes is achieved by grafting their membranes with pegylated phospholipids (PEG-lipids), which have been shown, however, to deteriorate membrane integrity on one hand and to hamper the pH-responsiveness on the other. Hence, the need for novel alternative surface modifying agents to ensure effective half-life prolongation of pH-sensitive liposomes is a subject of intensive research. A series of copolymers having short blocks of lipid-mimetic units has been shown to sterically stabilize conventional liposomes based on different phospholipids. This has prompted us to broaden their utilization to pH-sensitive liposomes, too. The present contribution gives a thorough account on the chemical synthesis of these copolymers their incorporation in DOPE:CHEMs pH-sensitive liposomes and detailed explanation on the battery of techniques for the biopharmaceutical characterization of the prepared formulations in terms of pH-responsiveness, cellular internalization, in vivo pharmacokinetics and biodistribution.

Targeted small interfering RNA-immunoliposomes as a promising therapeutic agent against highly pathogenic Avian Influenza A (H5N1) virus infection

This study describes a proof-of-concept study on the use of small interfering RNA (siRNA)-immunoliposomes as a therapeutic agent against H5N1 influenza virus infection. siRNA specific for influenza virus nucleoprotein (NP) mRNA was employed as the key antiviral agent to inhibit viral replication in this study. A humanized single-chain Fv antibody (huscFv) against the hemagglutinin (HA) of H5N1 highly pathogenic avian influenza virus (HPAI) was used as the targeting molecule to HA of H5N1 virus, which is abundantly expressed on the surface of infected cells (the HA target cells). The huscFv was applied to cationic polyethylene glycol-conjugated 3β-[N-(N',N'-dimethylaminoethane) carbamoyl] cholesterol-dioleoylphosphatidyl ethanolamine (PEGylated DC-Chol-DOPE) liposomes to generate immunoliposomes for siRNA delivery. The immunoliposomes were shown to specifically bind HA-expressing Sf9 cells and demonstrated enhanced siRNA transfection efficiency. The siRNA transfection efficiency was significantly reduced after preincubation of the HA target cells with an excess amount of free huscFv. These results therefore demonstrated that the enhanced siRNA delivery by use of immunoliposomes was mediated via targeting by huscFv. Furthermore, the siRNA silencing effect was more pronounced when the immunoliposomes were administered 6 to 12 h post-H5N1 infection in MDCK cells compared with the nontargeted liposomes. This proof-of-concept study may contribute to the future design and development of an siRNA delivery system for combating viral infectious diseases in humans.

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.

Preclinical photodynamic therapy in Spain 2: Liposome vectorization of photosensitizers; Different strategies, different outcomes

Photodynamic therapy is an emerging modality of cancer treatment based on the use of photosensitizing drugs, which accumulate selectively in tumor cells. Exposure to visible light induces local cytotoxic effects that lead selectively to tumor cell death in the irradiated region, thereby minimizing the risk and extension of unwanted secondary effects. One of the goals sought in the development of photodynamic therapy drugs is the selective targeting of tumor cells. As a general trend, the indiscriminate delivery of drugs is being increasingly substituted by the selective delivery to pathological tissues which can be achieved by embedding them into transporters that actively recognize differential factors of tumor cells and tissues as compared to healthy ones. Likewise, the chemical modification of the photosensitizers is a valid strategy to change the subcellular localization of the drug. The use of liposomes as transporters for targeted delivery of drugs has attracted particular attention during the last two decades. After a period characterized by the skepticism expressed by certain scientists in the field of drug delivery, interest in liposomes was rejuvenated by the introduction of fresh ideas from membrane biophysics.

Nanostructure-based drug delivery systems for brain targeting

CONTEXT

It is well-known fact that blood brain barrier (BBB) hinders the penetrance and access of many pharmacotherapeutic agents to central nervous system (CNS). Many diseases of the CNS remain undertreated and the inability to treat most CNS disorders is not due to the lack of effective CNS drug discovery, rather, it is due to the ineffective CNS delivery. Therefore, a number of nanostructured drug delivery carriers have been developed and explored over the past couple of years to transport the drugs to brain.

OBJECTIVE

The present review will give comprehensive details of extensive research being done in field of nanostructured carriers to transport the drugs through the BBB in a safe and effective manner.

METHODS

The method includes both the polymeric- and lipid-based nanocarriers with emphasis on their utility, methodology, advantages, and the drugs which have been worked on using a particular approach to provide a noninvasive method to improve the drug transport through BBB.

RESULTS

Polymeric- and lipid-based nanocarriers enter brain capillaries before reaching the surface of the brain microvascular endothelial cells without the disruption of BBB. These systems are further modified with specific ligands vectors and pegylation aiming to target and enhance their binding with surface receptors of the specific tissues inside brain and increase long circulatory time which favors interaction and penetration into brain endothelial cells.

CONCLUSION

This review would give an insight to the researchers working on neurodegenerative and non-neurodegenerative diseases of the CNS including brain tumor.

Liposome technology for industrial purposes

Liposomes, spherical vesicles consisting of one or more phospholipid bilayers, were first described in the mid 60s by Bangham and coworkers. Since then, liposomes have made their way to the market. Today, numerous lab scale but only a few large-scale techniques are available. However, a lot of these methods have serious limitations in terms of entrapment of sensitive molecules due to their exposure to mechanical and/or chemical stress. This paper summarizes exclusively scalable techniques and focuses on strengths, respectively, limitations in respect to industrial applicability. An additional point of view was taken to regulatory requirements concerning liposomal drug formulations based on FDA and EMEA documents.

In vivo evaluation of pH-sensitive polymer-based immunoliposomes targeting the CD33 antigen

The purpose of this study was to evaluate in vivo a targeted pH-sensitive liposomal formulation tailored to promote the efficient intracellular delivery of 1-beta-d-arabinofuranosylcytosine (ara-C) to human myeloid leukemia cells. Specifically, pH-sensitive immunoliposomes were obtained by anchoring a copolymer of dioctadecyl, N-isopropylacrylamide and methacrylic acid in bilayers of PEGylated liposomes (LP) and by coupling the whole anti-CD33 monoclonal antibody (mAb) or its Fab' fragments. Their pharmacokinetic and biodistribution profiles were assessed in Balb/c and leukemic HL60-bearing immunodepressed (SCID) mice. In naive mice, nontargeted and pH-sensitive Fab'-LP had longer circulation times than LP with whole mAb. In SCID/HL60 (CD33(+)) mice, the pharmacokinetic and biodistribution profiles of LP and encapsulated ara-C were comparable between nontargeted and pH-sensitive Fab'-LP. In leukemic mice, only pH-insensitive, ara-C-loaded Fab' induced prolonged survival times. The apparent absence of pH-sensitive Fab'-LP effect could be related to lower exposure to ara-C in SCID mice.

Long-circulating, pH-sensitive liposomes

A major limiting factor for the wide application of pH-sensitive liposomes is their recognition and sequestration by the phagocytes of the reticulo-endothelial system, which conditions a very short circulation half-life. Typically prolonged circulation of liposomes is achieved by grafting their membranes with pegylated phospholipids (PEG-lipids), which have been shown, however, to deteriorate membrane integrity on one hand and to hamper the pH-responsiveness on the other. Hence, the need for novel alternative surface modifying agents to ensure effective half-life prolongation of pH-sensitive liposomes is a subject of intensive research. A series of copolymers having short blocks of lipid-mimetic units has been shown to sterically stabilize conventional liposomes based on different phospholipids. This has prompted us to broaden their utilization to pH-sensitive liposomes, too. The present contribution gives thorough account on the chemical synthesis of these copolymers their incorporation in DOPE:CHEMs pH-sensitive liposomes and detailed explanation on the battery of techniques for the biopharmaceutical characterization of the prepared formulations in terms of pH-responsiveness, cellular internalization, in vivo pharmacokinetics and biodistribution.

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

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

Long-circulating, pH-sensitive liposomes sterically stabilized by copolymers bearing short blocks of lipid-mimetic units

A major hurdle towards in vivo utilization of pH-sensitive liposomes is their prompt sequestration by reticuloendothelial system and hence short circulation time. Prolonged circulation of liposomes is usually achieved by incorporation of pegylated lipids, which have been frequently reported to deteriorate the acid-triggered release. In this study we evaluate the ability of four novel nonionic copolymers, bearing short blocks of lipid-mimetic units to provide steric stabilization of DOPE:CHEMs liposomes. The vesicles were prepared using the lipid film hydration method and extrusion, yielding liposomes of 120-160 nm in size. Their pH-sensitivity was monitored via the release of encapsulated calcein. The incorporation of the block copolymers at concentration up to 10 mol% did not deteriorate the pH-sensitivity of the liposomes. A selected formulation was tested for stability in presence of 25% human plasma and proved to significantly outclass the plain DOPE:CHEMs liposomes. The ability of calcein-loaded liposomes to deliver their cargo inside EJ cells was investigated using fluorescent microscopy and the results show that the surface-modified vesicles are as effective to ensure intracellular delivery as plain liposomes. The pharmacokinetics and organ distribution of a selected formulation, containing a copolymer bearing four lipid anchors was investigated in comparison to plain liposomes and PEG (2000)-DSPE stabilized liposomes. The juxtaposition of the blood clearance curves and the calculated pharmacokinetic parameters show that the block copolymer confers superior longevity in vivo. The block copolymers utilized in this study can be consider as promising sterically stabilizing agents for pH-sensitive liposomes.

Haloperidol-associated Stealth Liposomes

Sigma receptors are membrane-bound proteins that are overexpressed in certain human malignancies including breast cancer. These receptors show very high affinity for various sigma ligands including neuroleptics like haloperidol. We hypothesized that in associating haloperidol-linked lipid into the cationic lipid-DNA complex, we can specifically target and deliver genes to breast cancer cells that overexpress sigma receptors. In the present study, haloperidol was chemically modified to conjugate at the distal end of the polyethylene glycollinked phospholipid, which was then incorporated into the cationic liposome known to condense and deliver genes inside cells. The resulting haloperidol-conjugated targeted lipoplex showed at least 10-fold higher (p < 0.001) reporter gene expression in MCF-7 cells than control lipoplex. The reporter gene expression of the targeted lipoplex was significantly blocked by haloperidol (p < 0.001) and by another sigma ligand, 1,3-ditolylguanidine (p < 0.001) in the majority of cationic lipid to DNA charge ratios (+/-). Spironolactone-mediated sigma receptor down-regulation enabled MCF-7 to show 10-fold lower transgene expression with targeted lipoplex compared with that obtained in spironolactone-untreated cells. The targeted lipoplex generated nonspecific gene expression in sigma receptor-nonexpressing human cancer cells such as Hela, KB, HepG2, and Chinese hamster ovary cells. Moreover, the transgene expression remained unabated in physiologically relevant serum concentrations. This is the first study to demonstrate that haloperidol-targeted gene delivery systems can mediate efficient targeting of genes to sigma receptor-overexpressing breast cancer cells, thereby becoming a novel class of therapeutics for the treatment of human cancers.

Targeting of immunoliposomes to endothelial cells using a single-chain Fv fragment directed against human endoglin (CD105)

We generated immunoliposomes targeting proliferating endothelial cells by chemically coupling a single-chain Fv fragment (scFv A5) directed against human endoglin to the liposomal surface. For this purpose, we introduced an additional cysteine residue at the C-terminus of the scFv fragment. This scFv' fragment was expressed in soluble form in bacteria and allowed for a site-directed coupling to sulfhydryl-reactive lipids incorporated into the lipid bilayer. The immunoliposomes (ILA5) showed rapid and strong binding to human endoglin-expressing endothelial cells (HUVEC, HDMEC), while no binding was observed with various endoglin-negative cell lines and blood lymphocytes. In vitro, ILA5 were stable for several hours in serum- or plasma-containing medium. Incubation of endothelial cells with ILA5 at 37 degrees C led to increased binding and internalisation of the liposomes as evidenced by a perinuclear accumulation. In vitro, doxorubicin-loaded ILA5 showed an increased cytotoxicity towards endothelial cells compared to untargeted liposomes and free doxorubicin. Since the vasculature of tumours is easily accessible to drug carrier systems, the described endothelial cell-specific immunoliposomes may be useful for the development of efficacious and safe vascular targeting agents in cancer therapy.

Ligand-targeted liposomal anticancer drugs

Antibody or ligand-mediated targeting of liposomal anticancer drugs to antigens expressed selectively or over-expressed on tumor cells is increasingly being recognized as an effective strategy for increasing the therapeutic indices of anticancer drugs. This review summarizes some recent advances in the field of ligand-targeted liposomes (LTLs) for the delivery of anticancer drugs. New approaches used in the design and optimization of LTLs is discussed and the advantages and potential problems associated with their therapeutic applications are described. New technologies are widening the spectrum of ligands available for targeting and are allowing choices to be made regarding affinity, internalization and size. The time is rapidly approaching where we will see translation of anticancer drugs entrapped in LTLs to the clinic.

Translocation of liposomes into cancer cells by cell-penetrating peptides penetratin and tat: a kinetic and efficacy study

Unlike conventional liposomes, sterically stabilized liposomes, with their smaller volume of distribution and reduced clearance, preferentially convey encapsulated drugs into tumor sites. Despite these improvements, intracellular delivery is hampered by the stable drug retention of the liposomes, which diminishes the efficacy of the liposomal drug. To facilitate uptake of liposomal drugs into cells, two cell-penetrating peptides, penetratin (PEN) and TAT, derived from the HIV-1 TAT protein, were studied. In contrast to control peptides, both TAT and PEN enhanced the translocation efficiency of liposomes in proportion to the number of peptides attached to the liposomal surface. A peptide number of as few as five could enhance the intracellular delivery of liposomes. The kinetics of uptake was peptide- and cell-type dependent. Intracellular accumulation of TAT-liposomes increased with incubation time, but PEN-liposomes peaked at 1 h and then declined gradually. After treatment with 1 microg/ml doxorubicin equivalents of liposome for 2 h, TAT increased the doxorubicin uptake of A431 cells by 12-fold. However, the improvement of uptake of liposomal doxorubicin was not reflected by cytotoxicity in vitro or tumor control in vivo. Our results demonstrated that merely adding CPP to a liposome encapsulating anticancer drug was inadequate in improving its antitumor activity. An additional approach to enhance the intracellular release of the encapsulated drug is obviously necessary.

New systems for delivery of drugs to the brain in neurological disease

The restricted or regulated entry of most blood-borne substances into the brain has been recognised for more than a century. The blood-brain barrier (BBB)-shielding function provided by endothelial cells is important in the treatment of neurological diseases because this exclusion of foreign substances also restricts entry of many potentially therapeutic agents into the brain. The recent identification of several neuroactive proteins of potential therapeutic value has highlighted the crucial need for effective and safe transcapillary delivery methods to the brain. One promising method is delivery through brain capillaries by augmentation of pinocytotic vesicles delivery systems that use this cellular mechanism are in development. Recent investigations in animal models show that large molecules of neurotherapeutic potential can be conjugated to peptidomimetic ligands, which bind to selected peptide receptors, and are then internalised and transported in small vesicles across the cytoplasmic brain capillary barrier. These conjugates have been shown to remain functionally active and effective in animal models of neurological disease.

Polymer based pH-sensitive carriers as a means to improve the cytoplasmic delivery of drugs

pH-sensitive niosomal and liposomal formulations bearing alkylated N-isopropylacrylamide (NIPAM) copolymers were characterized with regard to vesicle-polymer interaction, pH-responsiveness and stability in human serum. The interactions between the pH-sensitive NIPAM copolymer and the vesicles were studied by spectrofluorimetry, using covalently-attached pyrene as a probe. In contrast to liposomes, where complexation of copolymer to the lipid bilayer is essentially mediated by hydrophobic interactions, the binding between niosomes and PNIPAM was mainly driven by hydrogen bonding. Both formulations were found to rapidly release their contents under mildly acidic conditions. However, the niosomes lost their pH-sensitivity after incubation in serum, whereas liposomes maintained their ability to respond to pH only when complexed with a copolymer containing a high proportion of hydrophobic anchor. The ability of pH-sensitive liposome/polymer complexes to enhance the cytotoxicity of cytosine arabinofuranoside (ara-C) was evaluated in vitro using macrophage-like J774 cells. Ara-C encapsulated in pH-sensitive liposomes exhibited a higher cytotoxicity than the control formulation. This study showed that both niosomes and liposomes can be rendered pH-sensitive by anchoring a randomly-alkylated NIPAM copolymer to their surface. The interactions that take place between the polymer and the vesicles strongly depend on the vesicle nature. pH-sensitive PNIPAM-based liposomes can improve the in vitro efficiency of ara-C.

Trends and developments in liposome drug delivery systems

Since the discovery of liposomes or lipid vesicles derived from self-forming enclosed lipid bilayers upon hydration, liposome drug delivery systems have played a significant role in formulation of potent drugs to improve therapeutics. Currently, most of these liposome formulations are designed to reduce toxicity and to some extent increase accumulation at the target site(s) in a number of clinical applications. The current pharmaceutical preparations of liposome-based therapeutics stem from our understanding of lipid-drug interactions and liposome disposition mechanisms including the inhibition of rapid clearance of liposomes by controlling size, charge, and surface hydration. The insight gained from clinical use of liposome drug delivery systems can now be integrated to design liposomes targeted to tissues and cells with or without expression of target recognition molecules on liposome membranes. Enhanced safety and heightened efficacy have been achieved for a wide range of drug classes, including antitumor agents, antivirals, antifungals, antimicrobials, vaccines, and gene therapeutics. Additional refinements of biomembrane sensors and liposome delivery systems that are effective in the presence of other membrane-bound proteins in vivo may permit selective delivery of therapeutic compounds to selected intracellular target areas.

N-isopropylacrylamide copolymers for the preparation of pH-sensitive liposomes and polymeric micelles

Hydrophobically-modified copolymers of N-isopropylacrylamide bearing a pH-sensitive moiety were investigated for the preparation of pH-responsive liposomes and polymeric micelles. The copolymers having the hydrophobic anchor randomly distributed within the polymeric chain were found to more efficiently destabilize egg phosphatidylcholine (EPC)/cholesterol liposomes than the alkyl terminated polymers. Release of both a highly-water soluble fluorescent contents marker, pyranine, and an amphipathic cytotoxic anti-cancer drug, doxorubicin, from copolymer-modified liposomes was shown to be dependent on pH, the concentration of copolymer, the presence of other polymers such as polyethylene glycol, and the method of preparation. Both polymers were able to partially stabilize EPC liposomes in human serum. These polymers were found to self-assemble to form micelles. The critical association concentration was low (9--34 mg/l) and influenced by the position of the alkyl chains. In phosphate buffered saline, the micelles had a bimodal size distribution with the predominant population having a mean diameter of 35 nm. The polymeric micelles were studied as a delivery system for the photosensitizer aluminum chloride phthalocyanine, (AlClPc), currently evaluated in photodynamic therapy. pH-Responsive polymeric micelles loaded with AlClPc were found to exhibit increased cytotoxicity against EMT-6 mouse mammary cells in vitro than the control Cremophor EL formulation.

Liposome targeting to tumors using vitamin and growth factor receptors

Liposome-encapsulated anticancer drugs reveal their potential for increased therapeutic efficacy and decreased nonspecific toxicities due to their ability to enhance the delivery of chemotherapeutic agents to solid tumors. Advances in liposome technology have resulted in the development of ligand-targeted liposomes capable of selectively increasing the efficacy of carried agents against receptor-bearing tumor cells. Receptors for vitamins and growth factors have become attractive targets for ligand-directed liposomal therapies due to their high expression levels on various forms of cancer and their ability to internalize after binding to the liposomes conjugated to receptors' natural ligands (vitamins) or synthetic agonists (receptor-specific antibodies and synthetic peptides). This chapter summarizes various strategies and advances in targeting liposomes to vitamin and growth factor receptors in vitro and in vivo with special emphasis on two extensively studied liposome-targeting systems utilizing folate receptor and HER2/neu growth factor receptor.

Site-directed lipid modification of IgG-binding protein by intracellular bacterial lipoprotein process

IgG-binding protein was genetically expressed and lipid-modified in a site-directed manner in Escherichia coli. The DNA sequence encoding the signal peptide and the nine N-terminal amino acid residues of the major lipoprotein of E. coli (lpp) was fused to the sequence of B-domain which was one of the IgG binding domains of Staphylococcal Protein A (SpA). The N-terminal cysteine residue of the resulting protein was enzymatically linked with lipids in the bacterial membrane. The lipid-modified protein was translocated at the bacterial membrane in a manner similar to native bacterial lipoprotein, and it was purified with IgG-Sepharose by affinity chromatography. The lipid modified proteins (lppB1 and lppB5) showed a similar IgG binding activity to unmodified proteins, which was estimated by competitive ELISA. Proteoliposomes of lipid modified proteins were prepared in an elegant fashion so that the IgG binding site should be properly oriented on the surface of an individual liposome by anchoring the lipid-tail into the hydrophobic layer of the liposome membrane. As compared with the unmodified one, the lipid modified protein incorporated into the proteoliposome exhibited higher IgG binding activity.

Reduction of FK-506 requirements by combination with polyethylene glycol superoxide dismutase in orthotopic rat liver transplantation

Reperfusion after ischemia results in endothelial cell injury and Kupffer cell activation. Inflammatory cytokines thus released can induce major histocompatibility complex antigens and increase the immunogenecity of the graft. An orthotopic rat liver allotransplant model was used to test the hypothesis that prevention of reperfusion injury by infusion of polyethylene glycol superoxide dismutase (PEG-SOD) would result in long-term allograft survival in the presence of subthreshold immunosuppressive dosages. ACI rats were used as donors, and Lewis strain rats as recipients. Orthotopic liver transplantation was initially performed to identify a subthreshold dose of the immunosuppressant FK-506, which would be unable to extend survival longer than control untreated rats with this strain combination. After testing three intramuscular FK-506 doses of 0.04, 0.08, and 0.16 mg/kg, it was observed that an FK-506 dose of 0.04 mg/kg/day for 14 days was unable to extend survival longer than in untreated recipients. This dose of FK-506 was used in combination with PEG-SOD at doses of 1000, 3000, 10,000, or 30,000 units. Recipient animals were treated intravenously with PEG-SOD as a loading dose to facilitate tissue penetration on day 1, and beginning on the day of transplantation, every 2 days for the duration of the study. Results of histologic studies and mean survival time were compared in untreated recipients and in rats treated with PEG-SOD plus 0.04 mg/kg/day FK-506.(ABSTRACT TRUNCATED AT 250 WORDS)

Endocytosis does not necessarily augment the cytotoxicity of adriamycin encapsulated in immunoliposomes

We studied the relationship between endocytosis and cytoxicity of adriamycin (ADM) encapsulated in antibody-coated liposomes (immunoliposomes, IL) which are called chemoimmunoliposomes (CIL), by using several human cancer cell lines. IL coated with a monoclonal antibody, HBJ127 (IgG), which recognizes human gp125 antigen, specifically bound to gp125-positive target cancer cell lines, KU-1, T24, MKN-7, SKBr-3 and LS174T. Flow cytometric analysis using IL encapsulating carboxyfluorescein (CF) revealed that efficiencies of endocytosis varied among different cancer cells. The rate of IL internalization was in the order KU-1 > T24 > MKN-7 > SKBr-3 > LS174T. In 1 h incubation at 37 degrees C, all the four cell lines other than LS174T internalized about 60% of IL which were bound on their cell surfaces. KU-1, T24 and MKN-7, but not SKBr-3, significantly processed IL in endosome or lysosome. On the contrary, 80% of CIL bound to LS174T remained on the cell surface even after 2 h incubation. Furthermore, we evaluated the cytotoxic activities of CIL against the same panels of cancer cells. CIL inhibited the growth of all cancer cells tested in antibody-dependent manner, but, contrary to our expectation, KU-1 and T24 cells, which showed significant endocytosis activity, required a 7-14-fold higher amount of ADM binding than LS174T cells with low endocytosis activity for 50% cell growth inhibition. The difference of sensitivity to free ADM was only within 2.3-fold among those cancer cells. These results showing that liposomal ADM endocytosed is less effective than that remaining on the cell surface suggest that endocytosis is not necessarily required for cytotoxicity of CIL.

Interactions of liposome bilayers composed of 1,2-diacyl-3-succinylglycerol with protons and divalent cations

Bilayer liposomes were prepared by using pure DOSG (1,2-dioleoyl-3-succinylglycerol) or DPSG (1,2-dipalmitoyl-3-succinylglycerol) at pH 7.4 or above. These liposomes undergo destabilization upon incubation with acid. When calcein was used as an entrapped aqueous marker, half maximal content leakage was observed between pH 5.8-6.3. Differential scanning calorimetry showed that at pH 7.4, the chain-melting temperature (Tm) of DPSG was 60.4 degrees C, and increased with decreasing pH (Tm = 57.0 degrees C and 62.7 degrees C at pH 8.9 and 6.7, respectively). Below pH 6.7, extensive phase separation occurred as the major chain melting peak split into three peaks. These three peaks coalesced into one peak below pH 5. Freeze fracture electron micrographs of DOSG liposomes at pH 4 showed the formation of non-bilayer as well as hexagonal phase structures. The effects of divalent cations, such as Ca2+ and Mg2+, on the destabilization of DASG bilayers have also been studied. Differential scanning calorimetry studies of bilayers composed of DPSG showed that both Ca2+ and Mg2+ could increase the Tm of DPSG with increasing concentrations. However, under identical conditions Mg2+ was more effective than Ca2+ in increasing the Tm of DPSG. X-ray diffraction indicated that both Ca2+ and Mg2+ could induce DPSG bilayers to undergo a complete lamellar to hexagonal phase transition. There was a size-dependency on the plasma stability of DOSG liposomes. DOSG liposomes that were smaller in size were more stable in plasma than the larger ones. After incubation with plasma, DOSG liposomes became less acid-sensitive. DOSG immunoliposomes entrapping diphtheria toxin A chain were used as a model for cytoplasmic delivery of the novel pH-sensitive liposomes. The delivery activity was comparable to that of the conventional pH-sensitive liposomes containing unsaturated phosphatidylethanolamine. Our data indicate that the mechanism of liposome destabilization involves extensive bilayer phase separation as well as the formation of non-bilayer structures.