Tumor accumulation of novel RES-avoiding liposomes

Glucuronides: From biological waste to bio-nanomedical applications

Long considered as no more than biological waste meant to be eliminated in urine, glucuronides have recently contributed to tremendous developments in the biomedical field, particularly against cancer. While glucuronide prodrugs monotherapy and antibody-directed enzyme prodrug therapy have been around for some time, new facets have emerged that combine the unique properties of glucuronides notably in the fields of antibody-drug conjugates and nanomedicine. In both cases, glucuronides are utilized as a vector to improve pharmacokinetics and confer localized activation of potent drugs at tumor sites while also decreasing systemic toxicity. Here we will discuss some of the most promising strategies using glucuronides to promote successful anti-tumor therapeutic treatments.

In vitro and in vivo activity of thermosensitive liposomes loaded with doxorubicin and cisplatin

Thermosensitive liposomes loaded with cisplatin and doxorubicin composed of DPPC, DSPC, and DPPE-PEG5000 with different ratios were prepared by thin film hydration method. The Differential Scanning Calorimetry (DSC) curves showed that the liposomes composed of DPPC-DSPC-DPPE-PEG5000 with phospholipid ratio 95:5:0.05 w/w were a suitable formulation as thermosensitive liposomes with a DSC peak at 42.1 °C. The effect of doxorubicin and cisplatin encapsulated non-thermosensitive and thermosensitive liposomes on cellular proliferation and IC50 in SKBR3 & MDA-MB-231 breast cancer and PC-3 & LNcaP prostate cancer cell lines was investigated. The results showed that doxorubicin loaded into thermosensitive liposomes showed 20-fold decrease in the IC50 at 42 °C while comparing it with the same at 37 °C. Also, the results showed a more than 35-fold and 12-fold decrease in the IC50 of cisplatin thermosensitive liposomes at 42 °C, while compared with free cisplatin and cisplatin thermosensitive liposomes at any temperature. The in vivo results showed that the effect of doxorubicin encapsulated thermosensitive liposomes at hyperthermic conditions during the treatment as the tumor growth inhibition was measured 1.5-fold higher than any of the liposomal formulations of doxorubicin. It was also noticed that the tumor volume reduced to 150 mm³ in doxorubicin thermosensitive liposomes (G8) after 3 weeks during the treatment, but increased to 196 mm³ after 4 weeks. The Kaplan-Meir curve showed the 100% survival of the animals from G8 (thermosensitive liposomes containing doxorubicin plus hyperthermia) after 12 weeks. The flow cytometry data revealed more than 25% apoptotic cells and 6.25% necrotic cells in the tumor cells from the tissues of the G8 group of the animals. The results clearly indicate the superior efficacy of doxorubicin and cisplatin containing thermosensitive liposomes treatment during hyperthermia.

Improved Antitumor Activity of Novel Redox-Responsive Paclitaxel-Encapsulated Liposomes Based on Disulfide Phosphatidylcholine

The microtubule inhibitor paclitaxel (PTX) is used to treat a wide range of solid tumors. Due to the poor aqueous solubility of PTX, a continuous demand for safe, efficient PTX formulations with improved antitumor activity exists. Here, we report a novel form of redox-sensitive paclitaxel (PTX)-encapsulated liposomes based on the previously developed disulfide phosphatidylcholine (SS-PC). PTX-loaded stealth liposomes (PTX/SS-LP) composed of SS-PC, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-PEG₂₀₀₀ (DSPE-PEG₂₀₀₀), and cholesterol were prepared using the reverse-phase evaporation method. The characterization of the PTX/SS-LP liposomes using dynamic light scattering and transmission electron microscopy confirmed their uniform particle size and typical unilamellar vesicle structure with an average bilayer thickness of approximately 4 nm. Changes in the size and morphology as well as the rapid release of PTX triggered by the addition of dithiothreitol revealed the redox sensitivity of PTX/SS-LP. Finally, evaluations in MCF-7 and A549 cells in vitro and in BALB/c mice in vivo revealed the improved anticancer efficiency, biodistribution, and safety of PTX/SS-LP compared with those of Taxol and nonredox-sensitive PTX/LP. In conclusion, PTX/SS-LP displays a redox-responsive release of paclitaxel with improved antitumor activity and has great potential as a next-generation stealth liposomal PTX delivery system.

A robust control system for targeting melanoma by a supermolecular DDMC/paclitaxel complex

A DEAE-dextran-MMA copolymer (DDMC)-paclitaxel (PTX) conjugate was prepared using PTX as the guest and DDMC as the host. The resistance of B16F10 melanoma cells to PTX was confirmed, while the DDMC-PTX conjugate showed excellent anticancer activity that followed the Hill equation. The robustness in the tumor microenvironment of the allosteric system was confirmed via BIBO stability. This feedback control system, explained via a transfer function, was very stable and showed the sustainability of the system via a loop, and it showed superior anti-cancer activity without drug resistance from cancer cells. The block diagram of this signal system in the tumor microenvironment used its loop transfer function G(s) and the dN(s) of the external force. This indicial response is an ideal one without a time lag for the outlet response. The cell death rate of DDMC-PTX is more dependent on the Hill coefficient n than on the Michaelis constant Km. This means that this supermolecular reaction with tubulin follows an "induced fit model".

From Conventional to Stealth Liposomes a new Frontier in Cancer Chemotherapy

Many attempts have been made to achieve good selectivity to targeted tumor cells by preparing specialized carrier agents that are therapeutically profitable for anticancer therapy. Among these, liposomes are the most studied colloidal particles thus far applied in medicine and in particular in antitumor therapy. Although they were first described in the 1960s, only at the beginning of 1990s did the first therapeutic liposomes appear on the market. The first-generation liposomes (conventional liposomes) comprised a liposome-containing amphotericin B, Ambisome (Nexstar, Boulder, CO, USA), used as an antifungal drug, and Myocet (Elan Pharma Int, Princeton, NJ, USA), a doxorubicin-containing liposome, used in clinical trials to treat metastatic breast cancer. The second-generation liposomes (“pure lipid approach”) were long-circulating liposomes, such as Daunoxome, a daunorubicin-containing liposome approved in the US and Europe to treat AIDS-related Kaposi's sarcoma. The third-generation liposomes were surface-modified liposomes with gangliosides or sialic acid, which can evade the immune system responsible for removing liposomes from circulation. The fourth-generation liposomes, pegylated liposomal doxorubicin, were called “stealth liposomes” because of their ability to evade interception by the immune system, in the same way as the stealth bomber was able to evade radar. Actually, the only stealth liposome on the market is Caelyx/Doxil (Schering-Plough, Madison NJ, USA), used to cure AIDS-related Kaposi's sarcoma, resistant ovarian cancer and metastatic breast cancer. Pegylated liposomal doxorubicin is characterized by a very long-circulation half-life, favorable pharmacokinetic behavior and specific accumulation in tumor tissues. These features account for the much lower toxicity shown by Caelyx in comparison to free doxorubicin, in terms of cardiotoxicity, vesicant effects, nausea, vomiting and alopecia. Pegylated liposomal doxorubicin also appeared to be less myelotoxic than doxorubicin. Typical forms of toxicity associated to it are acute infusion reaction, mucositis and palmar plantar erythrodysesthesia, which occur especially at high doses or short dosing intervals. Active and cell targeted liposomes can be obtained by attaching some antigen-directed monoclonal antibodies (Moab or Moab fragments) or small proteins and molecules (folate, epidermal growth factor, transferrin) to the distal end of polyethylene glycol in pegylated liposomal doxorubicin. The most promising therapeutic application of liposomes is as non-viral vector agents in gene therapy, characterized by the use of cationic phospholipids complexed with the negatively charged DNA plasmid. The use of liposome formulations in local-regional anticancer therapy is also discussed. Finally, pegylated liposomal doxorubicin containing radionuclides are used in clinical trials as tumor-imaging agents or in positron emission tomography.

Formation and purification of tailored liposomes for drug delivery using a module-based micro continuous-flow system

Liposomes are lipid based bilayer vesicles that can encapsulate, deliver and release low-soluble drugs and small molecules to a specific target site in the body. They are currently exploited in several nanomedicine formulations. However, their development and application is still limited by expensive and time-consuming process development and production methods. Therefore, to exploit these systems more effectively and support the rapid translation of new liposomal nanomedicines from bench to bedside, new cost-effective and scalable production methods are needed. We present a continuous process flow system for the preparation, modification and purification of liposomes which offers lab-on-chip scale production. The system was evaluated for a range of small vesicles (below 300 nm) varying in lipid composition, size and charge; it offers effective and rapid nanomedicine purification with high lipid recovery (> 98%) combined with effective removal of non-entrapped drug (propofol >95% reduction of non-entrapped drug present) or protein (ovalbumin >90% reduction of OVA present) and organic solvent (ethanol >95% reduction) in less than 4 minutes. The key advantages of using this bench-top, rapid, process development tool are the flexible operating conditions, interchangeable membranes and scalable high-throughput yields, thereby offering simultaneous manufacturing and purification of nanoparticles with tailored surface attributes.

Targeting doxorubicin encapsulated in stealth liposomes to solid tumors by non thermal diode laser

BACKGROUND

The use of liposomes as drug delivery systems is the most promising technique for targeting drug especially for anticancer therapy.

METHODS

In this study sterically stabilized liposomes was prepared from DPPC/Cholesterol/PEG-PE encapsulated doxorubicin. The effect of lyophilization on liposomal stability and hence expiration date were studied. Moreover, the effect of diode laser on the drug released from liposomesin vitro and in vivo in mice carrying implanted solid tumor were also studied.

RESULTS

The results indicated that lyophilization of the prepared liposomes encapsulating doxorubicin led to marked stability when stored at 5 °C and it is possible to use the re-hydrated lyophilized liposomes within 12 days post reconstitution. Moreover, the use of low energy diode laser for targeting anticancer drug to the tumor cells is a promising method in cancer therapy.

CONCLUSION

We can conclude that lyophilization of the liposomes encapsulating doxorubicin lead to marked stability for the liposomes when stored at 5 °C. Moreover, the use of low energy diode laser for targeting anticancer drug to the tumor cells through the use of photosensitive sterically stabilized liposomes loaded with doxorubicin is a promising method. It proved to be applicable and successful for treatment of Ehrlich solid tumors implanted in mice and eliminated toxic side effects of doxorubicin.

Medicinal facilities to B16F10 melanoma cells for distant metastasis control with a supramolecular complex by DEAE-dextran-MMA copolymer/paclitaxel

The resistance of cancer cells to chemotherapeutic drugs (MDR) is a major problem to be solved. A supramolecular DEAE-dextran-MMA copolymer (DDMC)/paclitaxel (PTX) complex was obtained by using PTX as the guest and DDMC as the host having 50-300 nm in diameter. The drug resistance of B16F10 melanoma cells to paclitaxel was observed, but there is no drug resistance of melanoma cells to the DDMC/PTX complex in vitro. The cell death rate was determined using Michaelis-Menten kinetics, as the DDMC/PTX complex promoted allosteric supramolecular reaction to tubulin. The DDMC/PTX complex showed a very superior anti-cancer activity to paclitaxel alone in vivo. The median survival time (MST) of the saline, PTX, DDMC/PTX4 (particle size, 50 nm), and DDMC/PTX5 (particle size, 290 nm) groups were 120 h (T/C, 1.0), 176 h (T/C, 1.46), 328 h (T/C, 2.73), and 280 h (T/C, 2.33), respectively. The supramolecular DDMC/PTX complex showed the twofold effectiveness of PTX alone (p < 0.036). Histochemical analysis indicated that the administration of DDMC/PTX complex decreased distant metastasis and increased the survival of mice. A mouse of DDMC/PTX4 group in vivo was almost curing after small dermatorrhagia owing to its anti-angiogenesis, and it will be the hemorrhagic necrotic symptom of tumor by the release of "tumor necrosis factor alpha (TNF-α)" cytokine. As the result, the medicinal action of the DDMC/PTX complex will suppress the tumor-associated action of M2 macrophages and will control the metastasis of cancer cells.

Are PEGylated liposomes better than conventional liposomes? A special case for vincristine

Cancer poses a significant threat to human health worldwide, and many therapies have been used for its palliative and curative treatments. Vincristine has been extensively used in chemotherapy. However, there are two major challenges concerning its applications in various tumors: (1) Vincristine's antitumor mechanism is cell-cycle-specific, and the duration of its exposure to tumor cells can significantly affect its antitumor activity and (2) Vincristine is widely bio-distributed and can be rapidly eliminated. One solution to these challenges is the encapsulation of vincristine into liposomes. Vincristine can be loaded into conventional liposomes, but it quickly leak out owing to its high membrane permeability. Numerous approaches have been attempted to overcome this problem. Vincristine has been loaded into PEGylated liposomes to prolong circulation time and improve tumor accumulation. These liposomes indeed prolong circulation time, but the payout characteristic of vincristine is severer, resulting in a compromised outcome rather than a better efficacy compared to conventional sphingomyelin (SM)/cholesterol (Chol) liposomes. In 2012, the USA Food and Drug Administration (FDA) approved SM/Chol liposomal vincristine (Marqibo®) for commercial use. In this review, we mainly focus on the drug's rapid leakage problem and the potentially relevant solutions that can be applied during the development of liposomal vincristine and the reason for conventional liposomal vincristine rather than PEGylated liposomes has access to the market.

Anticancer efficacy of a supramolecular complex of a 2-diethylaminoethyl-dextran-MMA graft copolymer and paclitaxel used as an artificial enzyme

The anticancer efficacy of a supramolecular complex that was used as an artificial enzyme against multi-drug-resistant cancer cells was confirmed. A complex of diethylaminoethyl-dextran-methacrylic acid methylester copolymer (DDMC)/paclitaxel (PTX), obtained with PTX as the guest and DDMC as the host, formed a nanoparticle 50-300 nm in size. This complex is considered to be useful as a drug delivery system (DDS) for anticancer compounds since it formed a stable polymeric micelle in water. The resistance of B16F10 melanoma cells to PTX was shown clearly through a maximum survival curve. Conversely, the DDMC/PTX complex showed a superior anticancer efficacy and cell killing rate, as determined through a Michaelis-Menten-type equation, which may promote an allosteric supramolecular reaction to tubulin, in the same manner as an enzymatic reaction. The DDMC/PTX complex showed significantly higher anticancer activity compared to PTX alone in mouse skin in vivo. The median survival times of the saline, PTX, DDMC/PTX4 (particle size 50 nm), and DDMC/PTX5 (particle size 290 nm) groups were 120 h (treatment (T)/control (C), 1.0), 176 h (T/C, 1.46), 328 h (T/C, 2.73), and 280 h (T/C, 2.33), respectively. The supramolecular DDMC/PTX complex showed twice the effectiveness of PTX alone (p < 0.036). Above all, the DDMC/PTX complex is not degraded in cells and acts as an intact supramolecular assembly, which adds a new species to the range of DDS.

NANOMATERIALS FOR PROTEIN MEDIATED THERAPY AND DELIVERY

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

In-vitro cytotoxicity, in-vivo biodistribution and anti-tumour effect of PEGylated liposomal topotecan

In attempt to increase the accumulation of topotecan in tumours and improve its anti-cancer activity, PEGylated liposome (H-PEG) containing topotecan was prepared. The in-vitro cytotoxicity, in-vivo biodistribution pattern and anti-tumour effect of H-PEG were studied systemically. Compared with free topotecan or conventional liposome (H-Lip), H-PEG improved the cytotoxic effect of topotecan against human ovarian carcinoma A2780 and human colon carcinoma HCT-8 cells. The IC50 value (concentration leading to 50% cell-killing) of H-PEG decreased 5 fold (P&lt;0.01) and 9 fold (P&lt;0.01) against A2780 and HCT-8 cells compared with H-Lip, respectively. The results of biodistribution studies in sarcoma S180 tumour-bearing mice showed that liposomal encapsulation increased the concentration of total topotecan and the ratio of lactone form in plasma. H-PEG resulted in a 70-fold and 3.7-fold increase in AUC0→24h compared with free topotecan and H-Lip, respectively. Moreover, H-PEG increased the accumulation of topotecan in tumours and the relative tumour uptake ratio compared with free topotecan was 5.2, and higher than that of H-Lip. The anti-cancer effect studies in murine heptocarcinoma H22 tumour-bearing mice showed that H-PEG improved the therapeutic efficiency of topotecan and decreased the toxicity of topotecan to a certain extent compared with H-Lip. These results indicated that PEG-modified liposome might be an efficient carrier of topotecan.

Polymer migration among phospholipid liposomes

Complexation of phospholipid lipsomes with a cationic polymer, poly(N-ethyl-4-vinylpyridinium bromide) (PEVP), and subsequent interliposomal migration of the adsorbed macromolecules, have been investigated. Liposomes of two different charge types were examined: (a) a liposomal system, with an overall charge near zero, consisting of zwitterionic phosphatidylcholine (egg lecithin, EL) with added doubly anionic phospholipid, cardiolipin (CL(2-)), and cationic dihexadecyldimethylammonium bromide (HMAB(+)), in a CL(2-)/HMAB(+) charge-to-charge ratio of 1:1; (b) an anionic liposomal system composed of an EL/CL(2-) mixture plus polyoxyethylene monocetyl ether (Brij 58). Both three-component systems were designed specifically to preclude liposomal aggregation upon electrostatic association with the PEVP, a phenomenon that had complicated analysis of data from several two-component liposomes. PEVP macromolecules were found from fluorescence experiments to migrate among the charge-neutral EL/CL(2-)/HMAB(+) liposomes. In the case of anionic EL/CL(2-)/Brij liposomes, a combination of fluorescence and laser microelectrophoresis methods showed that PEVP macromolecules travel from liposome to liposome while being electrostatically associated with anionic lipids.

Improved biochemical strategies for targeted delivery of taxoids

Paclitaxel (Taxol) and docetaxel (Taxotere) are very important anti-tumor drugs in clinical use for cancer. However, their clinical utility is limited due to systemic toxicity, low solubility and inactivity against drug resistant tumors. To improve chemotherapeutic levels of these drugs, it would be highly desirable to design strategies which bypass the above limitations. In this respect various prodrug and drug targeting strategies have been envisioned either to improve oral bioavailability or tumor specific delivery of taxoids. Abnormal properties of cancer cells with respect to normal cells have guided in designing of these protocols. This review article records the designed biochemical strategies and their biological efficacies as potential taxoid chemotherapeutics.

Polymeric micelles with a pH-responsive structure as intracellular drug carriers

Polymeric micelles based on poly(L-lactide)-b-poly(2-ethyl-2-oxazoline)-b-poly(L-lactide) (PLLA-PEOz-PLLA) ABA triblock copolymers were designed as intracellular drug carriers. The PLLA-PEOz-PLLA micelles adopt a "flower-like" arrangement with A-blocks at the core and a B-block on the shell under neutral condition. The deformation of the core-shell structure is then promoted by the aggregation of PEOzs due to the formation of inter- and intra-hydrogen bonding between protonated nitrogen and carbonyl groups. The experiments on in vitro release have confirmed that the release of doxorubicin (DOX) from micelles was successfully inhibited at pH 7.4. In contrast, an accelerated release of DOX from micelles was observed at acidic conditions. The results of growth inhibition assay indicated that the cell-killing rate of DOX-loaded micelles gradually approached that of free DOX as increasing the concentration and the incubation time. The overlay of fluorescent images on CLSM observation clearly demonstrated the colocalization of DOX with acidic compartments, suggesting that the drug release was successfully triggered in the acidic organelles by means of micelle deformation.

Doxorubicin loaded pH-sensitive polymeric micelles for reversal of resistant MCF-7 tumor

In order to overcome multidrug resistance in solid tumors, doxorubicin (DOX) loaded pH-sensitive micelles of which surface was decorated with folate (PHSM/f) were evaluated both in vitro and in vivo experiments. PHSM/f were fabricated from a mixture of two block copolymers of poly(L-histidine) (M(n): 5K)-b-PEG (M(n): 2K)-folate (polyHis/PEG-folate) (75 wt.%) and poly(L-lactic acid) (M(n): 3K)-b-PEG (M(n): 2K)-folate (PLLA/PEG-folate) (25 wt.%). The PHSM/f showed more than 90% cytotoxicity of DOX resistant MCF-7 (MCF-7/DOX(R)) when cultured with PHSM/f at a concentration of 10 microg/ml DOX. The result was interpreted by a sequential event of active internalization of PHSM/f via folate-receptor mediated endocytosis and ionization of His residues which result in micelle destabilization and probably disturbance of endosomal membranes. This potential mechanism may endow the drug carriers to bypass Pgp efflux pump and sequestration of DOX in acidic intracellular compartments, yielding high cytotyoxicity. Experimental evaluation of tumor regression was carried out in a small animal model bearing s.c. MCF-7 or MCF-7/DOX(R) xenografts. The tumor (MCF-7/DOX) volumes of mice treated with PHSM/f were significantly less than control groups treated with free DOX or similar micelles but without folate (PHSM). In the MCF-7/DOX(R) xenograft model, the accumulated DOX level of PHSM/f in solid tumors was 20 times higher than free DOX group, and 3 times higher than PHSM group. The results demonstrate that PHSM/f is a viable means for treating drug resistant tumors.

Liposomalized Oligopeptides in Cancer Therapy

Organ-specific delivery of biofunctional agents is thought to enhance their activity and to reduce their side effects. Liposomes have been used as drug carriers in cancer chemotherapy, since they accumulate passively in tumor tissues due to an enhanced permeability and retention (EPR) effect. In addition, modification of liposomes with specific ligands enables active targeting. A small peptide having a high affinity for a certain antigen is suitable for modification of liposomes, since it is biocompatible, biodegradable, and less antigenic compared with antibody and other modifiers. Oligopeptide-modified liposomes are prepared by using lipophilic derivatives of the peptide, which are synthesized easily and incorporated readily into the liposomal bilayer. We describe two examples of the use of liposomal oligopeptides: one for antimetastatic therapy and the other for antineovascular therapy. Arg-Gly-Asp (RGD)-related peptides are known to contribute various cellular functions such as adhesion and invasion and to inhibit tumor metastasis. However, peptide drugs are generally rapidly hydrolyzed and eliminated from the bloodstream. Liposomal RGD enables the half-lives and affinity to be improved, resulting in enhancement of antimetastatic activity. We then describe the usefulness of liposomal Ala-Pro-Arg-Pro-Gly (APRPG) for tumor treatment, which is specific for tumor angiogenic vessels. APRPG is originally isolated by use of a phage-displayed peptide library. Adriamycin encapsulated in APRPG-modified liposomes accumulated specifically in and damage tumor neovessels, resulting in notable antitumor efficacy.

Glucuronate-Modified, Long-Circulating Liposomes for the Delivery of Anticancer Agents

Liposomes are useful as drug carriers in drug delivery systems, especially for drugs with severe side effects such as antitumor agents. The conventional formulations of liposomes are opsonized by plasma proteins in the bloodstream and trapped in the reticuloendothelial system (RES). Therefore, liposomes with reduced opsonization are expected to have prolonged circulation and to accumulate in tumor tissue due to the leaky endothelium of the tissue. To avoid RES trapping of liposomes, two approaches have been considered. Liposomes may mimic cells circulating in the blood to escape host recognition as foreign substances, or liposomes may be covered with a hydrophilic barrier to escape recognition. For the latter purpose, poly(ethylene glycol) is widely used. For the former purpose, here we focus on the characteristics, in vivo trafficking, and usage in cancer therapy of glucuronate-modified liposomes. Glucuronate-modified liposomes bind to a lower extent to macrophage-like cells in vitro and passively accumulate in tumor tissue evaluated by a technique using positron emission tomography. Glucuronate-modified liposomes with extended circulation are useful for delivering anticancer agents to tumors and reducing the toxic side effects of the agents.

Incorporation of hydrophobic porphyrins into liposomes: characterization and structural requirements

The ability of photosensitisers to give reactive oxygenated products is considered decisive for photodynamic applications, but the hydrophobic nature of many porphyrins makes necessary to obtain suitable pharmaceutical formulations. This paper reports the structural photosensitiser features that allow the preparation of stable liposomal formulations. Metallated and non-metallated TPPs and TPyPs and different lipid/porphyrin ratios were considered in order to procure liposomal preparations containing porphyrin concentrations adequate to necessary doses. The results show that the incorporation of porphyrins into liposomes can be related with their ability to form aggregates in a watery media. Thus, ZnTPP, which structural properties avoid the formation of aggregates, was efficiently incorporated into stable liposomes. Moreover, the efficient generation of singlet oxygen by ZnTPP liposomal suspensions has been shown. Because of this, the synthesis of hydrophobic porphyrin derived structures or other sensitisers, which do not aggregate in a watery media and with Q-bands shifted to higher lambda values than ZnTPP, will be efficiently incorporated into liposomes and useful for clinical applications.

Anti-neovascular therapy using novel peptides homing to angiogenic vessels

Cancer chemotherapy targeted to angiogenic vessels is expected to cause indirect tumor regression through the damage of the neovasculature without the induction of drug resistance. To develop a tool for neovasculature-specific drug delivery, we isolated novel peptides homing to angiogenic vessels formed by a dorsal air sac method from a phage-displayed peptide library. Three distinct phage clones that markedly accumulated in murine tumor xenografts presented PRPGAPLAGSWPGTS-, DRWRPALPVVLFPLH- or ASSSYPLIHWRPWAR-peptide respectively. After the determination of the epitope sequences of these peptides, we modified liposomes with epitope penta-peptides. Liposome modified with APRPG-peptide showed high accumulation in murine tumor xenografts, and APRPG-modified liposome encapsulating adriamycin effectively suppressed experimental tumor growth. Finally, specific binding of APRPG-modified liposome to human umbilical endothelial cells, and that of PRP-containing peptide to angiogenic vessels in human tumors, i.e., islet cell tumor and glioblastoma, were demonstrated. The present study indicates the usefulness of APRPG-peptide as a tool for anti-neovascular therapy, a novel modality of cancer treatment.

Intestinal mucosal immune response in chickens following intraocular immunization with liposome-associated Salmonella enterica serovar enteritidis antigen

Induction of intestinal mucosal immune responses against Salmonella enterica serovar enteritidis was studied by immunizing chickens with liposome-associated antigen. An ultrasonicated whole cell extract of the bacteria was used for immunizing antigen. Intraocular immunization induced serum IgA, IgG and IgM responses. Also, significant IgA and IgG antibodies were detected in the intestinal tract. Immunization with antigen alone induced only IgG response in the intestine. Salmonella enteritidis-specific antibody-secreting lymphocytes were detected in the spleen and lamina propria of the intestinal tract of immunized chickens. Immunoglobulin (Ig) fractions extracted from intestines of immunized chickens inhibited the adherence of S. enteritidis to cultured HeLa cells. These results indicate that intraocular immunization with liposome-associated S. enteritidis elicits specific antibody-producing lymphocytes in the intestinal tract, and that Ig secreted in the intestine inhibits adherence of the bacteria to intestinal epithelial cells, suppressing the spread of bacterial infection in the host.

Uptake of long-circulating immunoliposomes, directed against colon adenocarcinoma cells, by liver metastases of colon cancer

Radiolabeled ([3H]cholesteryloleyl ether) immunoliposomes directed against rat colon adenocarcinoma CC531 cells were prepared by random coupling of a tumor cell-specific antibody, CC52, via a thio ether bond. In vitro binding experiments demonstrated a saturable and specific interaction of CC52-immunoliposomes, which could be inhibited by free non-coupled CC52 but not by irrelevant antibodies. The in vivo targeting potential of CC52-immunoliposomes, which were pegylated to achieve prolonged circulation times, was tested in an established rat liver CC531 metastasis model. Twenty-four hours after injection of the liposomes, 25% of the CC52-immunoliposomes were still present in the blood, which was comparable with the control liposomes (either with or without antibody). Liposomes were mainly taken up from the blood by the liver and the spleen, although hepatic uptake of the immunoliposomes was higher and splenic uptake was lower as compared to liposomes without antibody. Within the metastatic tumor nodules in the liver, uptake of both the CC52-immunoliposomes and non-specific immunoliposomes was significantly higher than that of control liposomes without antibody. Visualization of fluorescently or gold labeled CC52-immunoliposomes revealed that, although targeting to liver metastases was achieved, the immunoliposomes were mostly not associated with tumor cells but rather localized in tumor associated cells, probably macrophages.

Potential usage of thermosensitive liposomes for site-specific delivery of cytokines

Long-circulating liposomes reside long in the bloodstream, and transient swelling during phase transition of liposomes with hyper-osmotic internal aqueous phase causes release of macromolecules. Here we examined the applicability of long-circulating thermosensitive liposomes for delivery of tumor necrosis factor (TNF) by the heating of a local tumor-growing site after injection of TNF-loaded liposomes into tumor-bearing mice. Glucuronate modified thermosensitive liposomes with internal solution of two-fold higher osmotic pressure and sized through 200 nm-pore, released encapsulated [(131)I] human serum albumin at 42 degrees C in vitro and showed long-circulating character in vivo. Cytotoxic action of TNF encapsulated in long-circulating thermosensitive-liposomes (LCTS-liposomes) against L929 fibrosarcoma cells was enhanced at 42 degrees C in vitro. Furthermore, the tumor growth tended to be inhibited more by hyperthermia of mice bearing Meth A sarcoma than without heating after injection of TNF encapsulated in LCTS-liposomes. These results suggest that the cytokine can be released at the tumor site from the circulating CLTS-liposomes.

Liposphere based lipoprotein-mimetic delivery system for 6-mercaptopurine

Long-circulating lipospheres containing 6-mercaptopurine (6-MP) were prepared by solidification of warm microemulsion at low temperature. Palmitoyl PEG was incorporated in the system to confer stealth-type nature. The size of lipospheres was in the range of 60-70 nm and was inversely proportional to sonication time. The size range was attained after 8 h. of sonication. The entrapped 6-MP contained 0.12 mmol/mole of lipid. The coating efficiency of 63-71% was attained. The zeta potential substantially decreased after PEG coating, however, the lipospheres were stable due to steric repulsion and exhibited no aggregation. The release of 6-MP was found to be 18-25% of administered dose in 24 h. and followed a mixed profile for stealth lipospheres. The percent dose remaining in plasma was found to be high even after 24 h as compared to control, indicating an increase in circulation time of lipospheres. Tissue accumulation of drug correlated with the pharmacokinetic behavior of lipospheres. The system seems to be an ideal carrier for anticancer drug delivery.

Anticancer therapy using glucuronate modified long-circulating liposomes

Since conventional liposomes tend to be trapped by the reticuroendothelial systems (RES), their use as drug carriers is limited when the targets are not RES cells. Therefore, many attempts have been made to avoid the RES-trapping of liposomes. Favorable results were obtained by a modification of liposomes with a glucuronic acid derivative, PGlcUA, and polyethyleneglycol. These liposomes have a long-circulating character, and showed the further advantage for passive targeting to tumor tissues, since the vasculature in tumor tissues is leaky enough for small-sized liposomes to extravasate. Thus long-circulating liposomes are useful for tumor imaging and treatment. PGlcUA-modified liposomes were actually found to accumulate effectively in tumor tissue, and showed enhanced efficacy of antitumor agents, such as adriamycin and vincristine when they were encapsulated into the liposomes. Usefulness of PGlcUA liposomes as drug carriers was also observed in photodynamic therapy and in treatment of cancer by amphiphilic novel antitumor agents.

Pharmacokinetics of anticancer drugs, plasmid DNA, and their delivery systems in tissue-isolated perfused tumors

To achieve an optimal chemotherapy or gene therapy against tumors or to realize rational design of delivery systems for cancer therapy, pharmacokinetic information in tumor should be obtained. A tissue-isolated tumor preparation is a useful experimental system to investigate the intratumoral disposition of drugs, carriers, and their complexes. The disposition of drugs in the solid tumor was analyzed in this system after intraarterial infusion (systemic route) or by intratumoral injection (topical route). Here the results of low-molecular weight drugs, their macromolecular prodrugs, lipid carriers like fat emulsions and liposomes, and plasmid DNA and its complexes, are addressed. Pharmacokinetic analyses in the tumor clearly indicate that the intratumoral fate of drugs and delivery systems are determined by (i) the anatomical and physiological properties of the tissue and (ii) the physicochemical characteristics of drugs and delivery systems such as molecular weight, size, lipophilicity, and electrical charge. These approaches are useful for designing and developing optimized drug delivery systems.

The size of liposomes: a factor which affects their targeting efficiency to tumors and therapeutic activity of liposomal antitumor drugs

The size of liposomes has been shown to be an important factor in the efficient delivery of an antitumor agent to a tumor. In this paper, the effects of the size of liposomes on the pharmacokinetics of liposomes and liposome-encapsulated drugs are discussed with reference to: (1) the circulation amount and residence time of liposomes in the blood, (2) the accumulation of liposomes in the tumor, and (3) in vivo drug release from liposomes. In addition, the effect of size on therapeutic activity (antitumor efficacy and toxicity) of a liposomal anticancer preparation is discussed. Finally we discuss the importance of liposome size in the design of a more effective liposomal antitumor preparation.

Detection and determination of surface levels of poloxamer and PVA surfactant on biodegradable nanospheres using SSIMS and XPS

The surface chemical characterisation of sub-200 nm poly(DL-lactide co-glycolide) nanospheres has been carried out using the complementary analytical techniques of static secondary ion mass spectrometry (SSIMS) and X-ray photoelectron spectroscopy (XPS). The nanospheres, which are of interest for site-specific drug delivery, were prepared using an emulsification-solvent evaporation technique with poly(vinyl alcohol), Poloxamer 407 and Poloxamine 908 respectively as stabilisers. The presence of surfactant molecules on the surface of cleaned biodegradable colloids was confirmed and identified on a qualitative molecular level (SSIMS) and from a quantitative elemental and functional group analysis (XPS) perspective. SSIMS and XPS data were also used in combination with electron microscopy to monitor the effectiveness of cleaning procedures in removing poorly bound surfactant molecules from the surface of nanospheres. The findings are discussed with respect to the development of nanoparticle delivery systems, particularly the composition of the surface for extending blood circulation times and achieving site-specific deposition.

Delivery of contrast agents for positron emission tomography imaging by liposomes

Liposomes encapuslating positron emitters are applicable for diagnostic imaging and are useful to investigate the real-time liposomal trafficking in vivo. Long-circulating liposomes encapsulaing [2-(18)F]-2-fluoro-2-deoxyglucose were administrated to tumor-bearing mice, and a PET scan was performed. Small-sized long-circulating liposomes (100 nm) tended to accumulate in tumor tissues of tumor-bearing mice as compared with conventional liposomes. Then the size effect on trafficking of long-circulating liposomes was investigated. Large-sized liposomes (>300 nm) accumulated in liver and spleen in a time dependent manner. On the contrary, small-sized ones (<200 nm) were transiently accumulated in the liver right after injection, but the accumulation decreased time dependently, suggesting that, although the majority of small long-circulating liposomes remain in bloodstream, some extravasate once into interstitial spaces in liver which re-enter into bloodstream again. Next the trafficking of so-called long-circulating liposomes, i.e., liposomes modified with ganglioside GM1, palmityl glucuronide (PGlcUA), and polyethylene glycol (PEG), in tumor-bearing mice was examined. The accumulation of all three kinds of long-circulating liposomes in liver decreased time-dependently, and PGlcUA-liposomes could avoid liver-trapping the most efficiently. Tumor accumulation of liposomes was obvious for PGlcUA-liposomes and PEG-liposomes from immediately after injection, but not for GM1-liposomes. Finally, the trafficking of differently charged liposomes was investigated in normal mice. The accumulation of positively charged liposomes containing 1,2-dimyristyloxypropyl-3-dimethyl-hydroxyethyl bromide was different from that of neutral and negatively charged DCP-liposomes. The agglutinability of and serum protein ginding to positively charged liposomes were marked, suggesting that these factors affect the high accumulation of DMRIE-liposomes in liver. Non-invasive PET analysis of liposomal trafficking is beneficial for obtaining information about liposomal drug delivery, and long-circulating liposomes might be useful for diagnostic tumor imaging by PET.

Effect of particle size and charge on the disposition of lipid carriers after intratumoral injection into tissue-isolated tumors

PURPOSE

Pharmacokinetic properties of various lipid carriers (liposome and emulsions) after intratumoral injection were studied in perfusion experiments using tissue-isolated tumor preparations of Walker 256 carcinosarcoma.

METHODS

Four types of lipid carriers, large emulsion (254 nm), small emulsion (85 nm), neutral liposomes (120 nm) and cationic liposomes (125 nm) were prepared. We quantified their recovery from the tumor, leakage from the tumor surface and venous outflow after intratumoral injection into perfused tissue-isolated tumors, and analyzed venous appearance curves based on a pharmacokinetic model.

RESULTS

In contrast to the small emulsion and neutral liposomes, which immediately appeared in the venous outflow perfusate following intratumoral injection, the appearance of the cationic liposomes and the large emulsion was highly restricted, clearly demonstrating that intratumoral clearance of these formulations can be greatly retarded by the cationic charge and large particle size, respectively. The venous appearance rate-time profiles were fitted to equations derived from a two-compartment model by nonlinear regression analysis. When the calculated parameters were compared among these four formulations, the venous appearance rate did not exhibit such a large difference; however, the rate of transfer from the injected site to the compartment which involves clearance by venous outflow was all very different.

CONCLUSIONS

The results of this study indicate that the determining factor which alters the pharmacokinetic properties of these lipid carriers after intratumoral injection is not the rate of transfer from the interstitial space to the vascular side but the rate of intratumoral transfer from the injection site to the well-vascularized region.

Improvement of pharmacokinetics and antitumor activity against human hepatoma cell line by using adriamycin-entrapped stealth liposomes

Preferential accumulation in the reticuloendothelial system is one of the major obstacles to the use of liposomes as a drug carrier for targeting therapy. To reduce their uptake, ganglioside GM1 was introduced into the components of conventional liposomes that had been used in our targeting experiments. Two types of such liposomes were prepared. Tissue distribution studies on Adriamycin entrapped in both types of liposomes clearly indicated that the uptake of Adriamycin by liver and spleen decreased to the level comparable to that of free Adriamycin administration. By contrast, the level of Adriamycin in the serum remains high, and some increase was observed in the accumulation to the tumor. Furthermore, Adriamycin in these liposomes, which were conjugated with anti-alpha-fetoprotein (AFP) antibody, inhibited the growth of AFP-positive human hepatoma Li-7 more efficiently than free Adriamycin or Adriamycin in antibody-conjugated conventional liposomes.

Liposomal Arg-Gly-Asp analogs effectively inhibit metastatic B16 melanoma colonization in murine lungs

Analogs of a synthetic peptide having the L-arginine-L-glycine-L-aspartic acid (RGD) sequence have been found to decrease metastatic colonization. To enhance the metastasis-suppressing efficacy of these analogs, we sought to stabilize these analogs and to prolong their circulation time by incorporating them into a liposomal formulation. Various structures of RGD analogs grafted to hydrophobic groups were synthesized and then incorporated into liposomes. Liposomes composed of distearoylphosphatidylcholine, cholesterol, dipalmitoylphosphatidylglycerol and appropriate RGD analogs were injected intravenously along with B16BL6 murine melanoma cells into mice. Liposomal RGD (0.6 mumol of the analog equivalent to ca. 200 micrograms RGD peptides) inhibited lung colonization up to 76%. This dose is an order of magnitude lower than that for comparable inhibition reported for free RGD. Multi-dose administration of liposomal RGD (0.15 mumol of the analog) also inhibited the spontaneous lung metastasis of cells from a primary tumor site of B16BL6 cells subcutaneously implanted into the footpad of mice. Taken together, our data indicate that liposomal RGD may serve as a useful anti-metastatic agent.

Antitumor activity of vincristine encapsulated in glucuronide-modified long-circulating liposomes in mice bearing Meth A sarcoma

Liposomes modified with the uronic acid derivative palmityl-D-glucuronide (PGlcUA) have a long circulation time and tend to accumulate in the tumors of tumor-bearing mice. Taking advantage of this character, we investigated the therapeutic effect of vincristine (VCR) encapsulated in liposomes containing PGlcUA (dipalmitoylphosphatidylcholine/cholesterol/PGlcUA = 4:4:1 as a molar ratio) on tumor-bearing mice. VCR was loaded into liposomes by a remote loading method, and then free or liposomal VCR was injected intravenously into BALB/c mice bearing Meth A sarcoma implanted subcutaneously 5 days before hand. Single-dose administration of VCR (3.0 mg/kg) in PGlcUA-liposomes significantly suppressed tumor growth, and prolonged the survival time (T/C = 1.37). Furthermore, two-dose administration of the liposomes cured one third of the animals. The therapeutic effect of PGlcUA-liposomes was greater than that of control liposomes containing dipalmitoylphosphatidylglycerol instead of PGlcUA. PGlcUA-liposomes might thus be a useful tool for delivering antitumor agents to tumor tissues.

Real-time analysis of liposomal trafficking in tumor-bearing mice by use of positron emission tomography

Long-circulating liposomes are known to accumulate passively in tumor tissues of tumor-bearing animals. To evaluate the in vivo behavior of such liposomes, we investigated the real-time liposomal trafficking by a non-invasive method using position emission tomography (PET). Liposomes composed of dipalmitoylphosphatidylcholine, cholesterol, and palmityl-D-glucuronide (PGlcUA) in a molar ratio of 4:4:1 were prepared in the presence of 2-[18F]fluoro-2-deoxyglucose ([2-18F]FDG). [2-18F]FDG-labeled liposomes sized by extrusion through a filter with various-sized pores were administered to mice bearing Meth A sarcoma, and a PET scan was performed for 120 min. Small-sized, long-circulating liposomes (100 nm in diameter) constructed with PGlcUA tended to accumulate in the tumor tissues. On the contrary, control liposomes (100 nm in diameter) containing dipalmitoylphosphatidylglycerol instead of PGlcUA accumulated in the liver. Large-sized PGlcUA-containing liposomes (> 300 nm) also accumulated in the liver, as well as in the spleen. Time-activity curves indicated that the small long-circulating liposomes (< 200 nm) transiently accumulated in the liver right after the injection but that the accumulation there decreased time-dependently. These data suggest that, although the majority of small long-circulating liposomes remain in the bloodstream, some extravasate once into the interstitial spaces in the liver re-enter the bloodstream again, and finally accumulate in the tumor tissues. This PET technique might be useful for studying real-time liposomal trafficking and for tumor imaging.

The importance of liposomes as models and tools in the understanding of photosensitization mechanisms

The various applications of liposomes in understanding photosensitization are described in this paper, with particular emphasis on the various kinds of information that these models allow to obtain in phototherapy. Liposomes are simple vesicles in which an aqueous phase is enclosed by a phospholipidic membrane. They are suitable models mimicking specific situations occurring in vivo and they allow study of the influence of physicochemical, photobiological and biochemical factors on the uptake of photosensitizers by tissues, their mechanisms of action and the subsequent photoinduced tumor necrosis. Moreover, solubilization of the sensitizer into the bilayer seems to improve its tumoral selectivity and its photodynamic efficiency.