The effect of dose on the distribution of adriamycin encapsulated in polyethyleneglycol-coated liposomes

The effect of injected dose on localized tumor accumulation and cardiac uptake of doxorubicin in a Vx2 rabbit tumor model using MR-HIFU mild hyperthermia and thermosensitive liposomes

PURPOSE

When doxorubicin (DOX) is administered via lyso-thermosensitive liposomes (LTLD), mild hyperthermia enhances localized delivery to heated vs. unheated tumors. The optimal LTLD dose and the impact of different doses on systemic drug distribution are unknown.

Materials and methods: In this study, we evaluated local and systemic DOX delivery with three LTLD doses (0.1, 0.5, and 2.5 mg/kg) in a Vx2 rabbit tumor model. Temporally and spatially accurate controlled hyperthermia was achieved using a clinical MR-HIFU system for the intended heating duration (40 min).

Results: DOX concentration in tissues delivered from LTLD combined with MR-HIFU mild hyperthermia are dose-dependent, including heated/unheated tumor, heart, and other healthy organs. Higher DOX accumulation and tumor-to-heart drug concentration ratio, defined as the ratio of DOX delivered into the tumor vs the heart, were observed in heated tumors compared to unheated tumors in all three tested doses. The DOX uptake efficiency for each mg/kg of LTLD injected IV of heated tumor was significantly higher than that of unheated tumor and heart within the tested dose range (0.1-2.5 mg/kg). The DOX uptake for the heart linearly scaled up as a function of dose while that for the heated tumor showed some evidence of saturation at the high dose of 2.5 mg/kg.

Conclusions: These results provide guidance on clinical protocol design of hyperthermia-triggered drug delivery.

Core/Shell Gene Carriers with Different Lengths of PLGA Chains to Transfect Endothelial Cells

In order to improve the transfection efficiency and reduce the cytotoxicity of gene carriers, many strategies have been used to develop novel gene carriers. In this study, five complex micelles (MSP(2 k), MSP(4 k), MSP(6 k), MSP(8 k), and MSP(10 k)) were prepared from methoxy-poly(ethylene glycol)-b-poly(d,l-lactide-co-glycolide) (mPEG-b-PLGA) and sorbitol-poly(d,l-lactide-co-glycolide)-graft-PEI (sorbitol-PLGA-g-PEI, where the designed molecular weights of PLGA chains were 2 kDa, 4 kDa, 6 kDa, 8 kDa, and 10 kDa, respectively) copolymers by a self-assembly method, and the mass ratio of mPEG-b-PLGA to sorbitol-PLGA-g-PEI was 1/3. These complex micelles and their gene complexes had appropriate sizes and zeta potentials, and pEGFP-ZNF580 (pDNA) could be efficiently internalized into EA.hy926 cells by their gene complexes (MSP(2 k)/pDNA, MSP(4 k)/pDNA, MSP(6 k)/pDNA, MSP(8 k)/pDNA, and MSP(10 k)/pDNA). The MTT assay results demonstrated that the gene complexes had low cytotoxicity in vitro. When the hydrophobic PLGA chain increased above 6 kDa, the gene complexes showed higher performance than that prepared from short hydrophobic chains. Moreover, the relative ZNF580 protein expression levels in MSP(6 k)/pDNA, MSP(8 k)/pDNA, and MSP(10 k)/pDNA) groups were 79.6%, 71.2%, and 73%, respectively. These gene complexes could promote the transfection of endothelial cells, while providing important information and insight for the design of new and effective gene carriers to promote the proliferation and migration of endothelial cells.

Characterization of Novel Mixed Polyethyleneglycol Modified Liposomes

In our previous paper, mixed polyetheleneglycol (PEG) modification of liposomes by a mixture of 1-monomethoxypolyethyleneglycol-2,3-distearoylglycerol (PEG-DSG) with short polyoxyethylene chain and PEG-DSG with long one was shown to increase fixed aqueous layer thickness (FALT) around the liposomal membrane, and this was useful in vivo. In this study, we investigated the characterization of mixed PEG modification of liposomes with different anchors (PEG2000-DSG and PEG2000-cholesterol (CHO)). When the liposomes was modified by a mixture of PEG2000-DSG and PEG2000-CHO, FALT was increased compared to that of each single PEG-lipids modification and the most suitable mix modification (PEG2000-DSG:PEG2000-CHO = 3:1) showed a maximum FALT. This phenomenon was speculated to be based on the difference in the insertion state of the PEG anchor unit in the liposomal membrane. PEG-CHO-modified liposomes (single or mixed PEG-modified liposomes) were easily incorporated into the liposomal membranes compared with that of single PEG-DSG-modified liposomes. Namely, it was considered that the cholesterol anchor as a single chain was able to be easily introduced, compared with the DSG anchor as two chains, and induced some interaction with both PEG modification. In conclusion, it is expected that novel PEG-modified liposomes with PEG2000-DSG and PEG2000-CHO (3:1) had superior physicochemical properties.

Characterization and cytotoxicity of mixed polyethyleneglycol modified liposomes containing doxorubicin

Liposomes are recognized as one of the useful drug carriers, but have many problems to overcome before their clinical application. Liposomes, bonding peculiarly with serum protein (opsonization), are taken up by reticuloendothelial system (RES) cells in the liver and spleen. It is known that polyethyleneglycol (PEG) modification of the liposome surface induces the formation of a fixed aqueous layer around the liposomes due to the interaction between the PEG-polymer and water molecule, and thus prevents the attraction of opsonins. Namely, PEG-modified liposomes are able to escape trapping by the RES cells, and have a prolonged circulation time. In this study, the effects of different anchors with the same PEG molecular weight on the cell uptake and cytotoxicity of mixed PEG-modified liposomal doxorubicin (DOX) were examined. The fixed aqueous layer thickness (FALT) of liposomes covered with mixtures of PEG-molecules which differ in their chain length were increased, compared to that of the single PEG2000-modified liposome. Mixed PEG-modification of liposomes with different anchors (PEG2000-(1-monomethoxypolyethyleneglycol-2,3-distearoylglycerol (DSG): cholesterol (CHO)=1:1)-modified liposome) led to an increase in the FALT, compared to that of each single PEG-modification. The uptake of DOX into Ehrlich ascites carcinoma cells by the liposomes covered with PEG-CHO was higher than the other liposomes. Thus, liposomes covered with PEG-DSG and PEG-CHO have an enhanced cytotoxicity. In conclusion, it was confirmed that mix-modified liposomes using PEG-lipid with different anchors were superior.

Enhanced efficacy of anti-tumor liposomal doxorubicin by hyperthermia

The efficiency of novel tumor chemotherapeutics could be increased using targeted drug delivery by hyperthermia. In this paper, the 3D liposomal doxorubicin distribution in the tumor tissue enhanced by local hyperthermia was quantitatively studied in real time using laser confocal microscopy. Results showed that the thermally induced liposomal doxorubicin extravasation was non-uniform and more excessive in the peripheral region than that in the tumor center. The effect of the thermally targeted drug delivery was also investigated. On the 1st, 3rd day after the thermally targeted drug treatment, histological examination showed that many nucleolus were condensed and collapsed in the peripheral region. But, in the tumor center, there were no such changes found until the 3rd day. While on the 6th day, tumor cells in both the peripheral and center region were found necrotic. The enhancement of the nanoparticle anti-tumor drug effect was significant. A theoretical analysis of liposomal doxorubicin diffusion to the tumor cells in vivo was performed. Results showed that it took more than 40 hrs for the doxorubicin to get into the tumor cells in the center region from the periphery region. The theoretical results well explained the experimental observations.

The phototoxicity of photofrin was enhanced by PEGylated liposome in vitro

In recent years, photodynamic therapy (PDT) with a photosensitizer and laser has been given attention, especially for the treatment of superficial cancers, such as lung, gastric, bladder and cervical cancer. In this study, in order to enhance the efficacy of PDT, photofrin liposome (PF-Lip) was prepared with dimyristoylphosphatidylcholine, dimyristoylphosphatidylglycerol and cholesterol. Polyethyleneglycol modified photofrin liposome (PF-PEG-Lip) was prepared by modification of PF-Lip with monomethoxypolyethyleneglycol-2.3-dimyristoylglycerol. PF-Lip and PF-PEG-Lip entrapped with photofrin with 81.0+/-5.9 and 81.2+/-9.2%, respectively. The particle size of each liposome was 114.3+/-5.7nm (PF-Lip) and 118+/-3.5nm (PF-PEG-Lip), respectively. It was suggested that PEGylated liposomes has no effect on the trapping ratio of PF and particle size. Phototoxicity was enhanced by liposomalization, especially PEG-modification. However, PF-PEG-Lip inhibited the uptake of photofrin into tumor cells. The amount of singlet oxygen from photofrin solution (PF-sol) and each liposome was PF-PEG-Lip=PF-Lip>PF-sol. The photofrin release revel of PF-PEG-Lip was lower than that of PF-Lip. In conclusion, the phototoxicity of PF-PEG-Lip was significantly higher than that of PF-sol or PF-Lip. It is expected that formation of a fixed aqueous layer on the liposome membrane by PEGylation physically changed it into the stable state of PF-PEG-Lip.

[Characterization and cytotoxicity of mixed PEG-DSG modified liposomes]

It is known that polyethyleneglycol (PEG) modification of the liposome surface leads to the formation of a fixed aqueous layer around the liposomes due to interaction between the PEG polymer and water molecules, which prevents the attraction of opsonins. When a combination of PEG-distearolyglycerol (PEG-DSG) whose characteristics are remarkably different is used, interaction between molecules occurs, leading to increased fixed aqueous layer thickness (FALT). From this speculation, we studied the effect of both modification of PEG900-DSG and PEG2000-DSG modified liposome on FALT, cell uptake and biodistribution. The FALT of mixed PEG modified liposome increased, compared to that of each single PEG modified liposome. In this mixed modification, maximum FALT was shown at liposome modified by added PEG-2000:PEG-900=2:1. This most suitable additional ratio was equal to actual incorporated ratio. On the other hand, cell uptake of mixed modified liposome containing doxorubicin (DOX) was similar with that of PEG2000 modified liposome. Furthermore, mixed PEG modification of liposome was tendency to increase cytotoxicity, compared to that of other modifications. After DOX contained liposome treatment, DOX distribution in the tumor and antitumor activity of DOX increase by mixed PEG modification. In conclusion, it was suggested that mixed PEG liposome (PEG-2000:PEG-900=2:1) was useful for cancer chemotherapy.

Liposomal photofrin enhances therapeutic efficacy of photodynamic therapy against the human gastric cancer

Photodynamic therapy (PDT) has been established as a potent and less invasive treatment for gastrointestinal tumors. The aim of the present study was to investigate whether or not liposomalization of the photosensitizer enhanced the therapeutic efficacy of PDT. Photofrin (PF) was entrapped in multilammelar liposomes. Mice implanted with a human gastric cancer xenograft, were divided into a PF group and a liposomal photofrin (LPF) group and intravenously administered 10 mg/kg of PF or LPF (as a dose of PF), respectively. At 8 h after injection PF level in tumor tissue in the LPF group was significantly higher level by 2.4-fold of that in the PF group, whereas the PF levels in the skin were almost equal. Irradiation was performed with the excimer dye laser at 150 mW/cm(2), total dose 40 J, at 8 h after PF or LPF administration. The results revealed that the volume of necrotic tumor tissue was significantly higher in the LPF group than in the PF group. The apoptotic index of the tumor was also significantly higher in the LPF group. In conclusion, the liposomalization of the photosensitizer increased its tumor accumulation, with a resulting enhancement of the therapeutic effect of PDT.

Study on the characterization of mixed polyethyleneglycol modified liposomes containing doxorubicin

Previously, we suggested that mixed polyethyleneglycol (PEG)-modification of liposomes with mixture of 1-monomethoxypolyethyleneglycol-2,3-distearoylglycerol (PEG-DSG) with short and long polyoxyethylene chains led to increasing fixed aqueous layer thickness (FALT) and was useful in vivo. FALT is expected to be one of the important factors that influence the pharmacokinetics of liposomes. In this study, we investigated the connection between FALT and some parameters in vitro. In both mixed and single PEG-modified liposomes, the incorporation ratios of PEG-DSGs were the same, the residual amount of PEG-DSGs with serum proteins was not affected by molecular weight. Consequently, the effective properties in vivo of mixed PEG-modification may be the increase in the absolute amount of PEG-DSGs on the liposome membranes, decrease of slipping out action taken PEG-DSGs with long polyoxyethylene chains by serum protein, and thus maintenance of FALT. While, it has been suggested that mixed PEG-modified liposomes are effective in vitro, too, because of the lower leakage and same level of doxorubicin (DOX) uptake as plain liposomal DOX.

Effect of polyethyleneglycol (PEG) chain on cell uptake of PEG-modified liposomes

The liposomalization and polyethyleneglycol (PEG) modification of antitumor agents prolongs their circulation in the blood and increases their accumulation in the tumor. It is expected that modification of the liposome surface with PEG-Lipid will prevent connection of liposome and tumor cell, so we examined the effect of PEG chain length and anchor length on liposome uptake into the tumor cell. It was obvious that modification of the liposome surface with PEG-Lipid did not prevent liposome uptake into tumor cells, but rather, promoted it. It was suggested that the increase in liposome uptake into the tumor cell was induced by modification of PEG-lipids with apparent stability. In other words, PEG 2,000-DPG, which had a high rate of residual PEG-Lipid on liposomal membrane depending on the re-uptake to liposomal membrane, met to this requirement.

Effect of dihydrokainate on the antitumor activity of doxorubicin

For biochemical modulation, components of green tea have been shown to be useful modulators in combination with doxorubicin (DOX). We have confirmed that theanine enhances the antitumor activity of DOX due to inhibition of DOX efflux from tumor cells. Because theanine is a glutamate analogue, we found that it is associated with a change in the drug transport system on the tumor cell membrane, in particular glutamate transporters. We examined the effect of dihydrokainate (DHK), one of the useful glutamate transporter inhibitors. DHK also inhibits DOX efflux significantly and reduces the glutamate uptake by Ehrlich ascites carcinoma cells. The potential contribution of glutamate transporters not only to glutamate uptake but also to cell membrane export of DOX has been shown. In addition, the combination of DHK with DOX significantly enhances the antitumor activity of DOX, by 1.8-fold (P<0.001). The DOX concentration in tumors significantly increases on combination with DHK and is correlated with the reduced tumor weight. On the other hand, DHK tends to reduce the DOX concentration in normal tissues. We expect that DHK has different actions in tumor and normal tissues because different isoforms of glutamate transporters are expressed in the two tissues. Thus, the results suggest that DHK is a novel and useful modulator for inducing enhancement of antitumor activity.

Effects of mixed polyethyleneglycol modification on fixed aqueous layer thickness and antitumor activity of doxorubicin containing liposome

Polyethyleneglycol (PEG) has often been used for the modification of liposomes, but it is difficult to insert PEG on the surface of liposomes, and the effects of modification are not marked enough. In this study, we examined the fixed aqueous layer thickness (FALT) of single or mixed PEG (molecular weight, 340, 500, 900, 2000)-modified doxorubicin (DOX) liposomes, and physical character and biological properties of these liposomes. On single PEG-modification, as the PEG-molecular weight increased, FALT also increased, but the ratio of the increase was reduced. While on modification by a mixture of PEG2000 and PEG with a short polyoxyethylene chain (PEG340 or PEG500), FALT increased compared with the single PEG2000-modified value. Moreover, when liposomes were modified by mixture of PEG2000 and PEG500, we recognized the most suitable mixed rate (PEG2000, 500=2:1), showed the maximum FALT. On the other hand, in vivo, as increase of FALT, DOX concentrations increased in the plasma and in the tumor, decreased in the liver. Furthermore, liposomes with remarkable increase of FALT showed enhancement of antitumor activity. As a result, the connection among increase of FALT and improvement of circulation in blood, the involvement of antitumor activity of DOX of these liposomes was suggested.

Stealth monensin immunoliposomes as potentiator of immunotoxins in vitro

Stealth monensin liposomes (SML) were prepared using dipalmitoyl phosphatidylcholine, cholesterol, distearoyl glycerophosphoethanolamine coupled to polyethylene glycol, stearylamine, and N-succinimidyl pyridodithiopropionate linked to stearyl amine, in the molar ratio of 10:5:1.4:1.4:1.5. SML was conjugated to the anti-MY9 antibody by a disulfide linkage to form stealth monensin immunoliposomes (SMIL) by an already established procedure. The encapsulation concentrations of monensin in SML and SMIL were 10(-7) and 4.9x10(-8) M, respectively. More than 20% of monensin remained in circulation after 24 h in BALB/c mice. The ability of SML and SMIL to potentiate the effect of anti-MY9 immunotoxin (anti-MY9-IT) was tested against human leukemia HL-60 sensitive and resistant tumor cells in vitro. SML and SMIL potentiated the activity of anti-MY9-IT by 10-20 times against HL-60 sensitive tumor cell lines. However, greater potentiation of anti-MY9-IT was observed in combination with SML and SMIL against HL-60 resistant tumor cells, found to be 200 and 500 times, respectively. The potentiation of anti-MY9-IT by SMIL was more than two-fold compared with SML against both HL-60 sensitive and resistant tumor cells. Transmission electron microscopy studies conducted with HL-60 resistant cells incubated with anti-MY9-IT and monensin liposomes showed significant dilation of the golgi, which was reversible after re-incubation in fresh medium. Our studies show that SML and SMIL can be successfully used to potentiate the activity of ricin based anti-MY9-IT in vitro, and further in vivo studies will demonstrate the usefulness of this approach.

Intraperitoneal administration of doxorubicin encapsulating liposomes against peritoneal dissemination

To improve therapy for peritoneal dissemination, and the distributions of doxorubicin (DOX) in the abdominal cavity, solid tumor and normal tissues after intraperitoneal administration of DOX-encapsulating liposomes was examined. In small negatively charged liposomes, lipid composition did not affect the clearance or stability of liposomes in the abdominal cavity. Whereas, for the treatment of solid tumor and the reduction of side effects, L-alpha-distearoylphosphatidylcholine-containing liposomes were most effective. On the other hand, large liposomes (DS(L)-Lip) were most abundant in the abdominal cavity. As the DOX levels in the heart, liver and solid tumor after DS(L)-Lip injection were lower than the corresponding values for the small liposome group, we considered that DS(L)-Lip were disrupted in the abdominal cavity and DOX was released from the liposomes. DS(L)-Lip remain in the abdominal cavity for a long time inducing cytotoxicity. The survival of Ehrlich ascites carcinoma-bearing mice was considered to be prolonged by DS(L)-Lip. Liposomes, both small and large in size appear to be effective against solid tumors except in the abdominal cavity, and against peritoneal dissemination in the abdominal cavity, respectively.

Membrane transport and antitumor activity of pirarubicin, and comparison with those of doxorubicin

We have compared the membrane transport and antitumor activity of pirarubicin with those of doxorubicin in M5076 ovarian sarcoma, which exhibits low sensitivity to doxorubicin. Pirarubicin was rapidly taken up by M5076 cells and the intracellular concentration of pirarubicin reached more than 2.5-fold that of doxorubicin. In terms of the 50% cell growth-inhibitory concentration in vitro, pirarubicin was more effective than doxorubicin. Thus, the intracellular concentration influenced the cytotoxicity of these anthracycline agents. On comparison of the nuclear uptake of pirarubicin and doxorubicin, the nucleus/cell ratio of pirarubicin was found to be about 40%, whereas that of doxorubicin reached more than 80%. As the intranuclear concentration of pirarubicin is dependent on nuclear transport, the increases in not only cell membrane transport, but also nuclear membrane transport contributed to the enhancement of the efficacy of pirarubicin. In M5076 solid tumor-bearing mice, pirarubicin reduced the tumor weight to 60% of the control level, although doxorubicin had no effect. These results were supported by the intracellular uptake of pirarubicin. Moreover, theanine, which inhibited the pirarubicin efflux from M5076 cells, increased by 1.3-fold the pirarubicin concentration in the tumor and enhanced the therapeutic efficacy of pirarubicin 1.7-fold. In conclusion, our results suggest that an increase in the concentration of an anthracycline derivative in tumor cells due to alteration of cell membrane transport results in enhancement of the antitumor activity.

Does the amount of an antitumor agent entrapped in liposomes influence its tissue distribution and cell uptake?

The effects of the amount of a drug entrapped in liposomes and polyethyleneglycol (PEG) modification on the tissue distribution in vivo and cell uptake in vitro have been examined. An increase in the amount of doxorubicin (DOX) entrapped in liposomes induced an increase in the DOX level in the plasma and tumor and a decrease in this level in the liver. The high amount of DOX entrapped demonstrated the usefulness of DOX liposomes for tumor cell uptake in vitro. The cell uptake of the liposomes depended on additional amounts of DOX and liposomal lipid. Furthermore, PEG modification of the surface of the liposomes facilitated the initial rate of liposome uptake into the tumor cells. This facilitation was attributed to the lipohydrophilic property of PEG and the fixed aqueous layer around the liposomes. Therefore, a higher amount of DOX entrapped in liposomes and PEG modification have been confirmed to be beneficial.

Effect of liposomalization on the antitumor activity, side-effects and tissue distribution of CPT-11

We have examined the efficacy of liposomalization and polyethyleneglycol (PEG) modification of liposomes on the antitumor activity, side-effects and tissue distribution of irinotecan hydrochloride (CPT-11). PEG-liposome was confirmed to elevate the plasma circulation of CPT-11 and SN-38 (active metabolite) concentrations. The tumor accumulation of CPT-11 and SN-38 was increased by the PEG-modified liposomes. The antitumor activity of CPT-11 increased due to the elevated tumor distribution of CPT-11 and SN-38 levels by the PEG-modified liposomes. In the tumor, CPT-11 was converted to SN-38. Thus, it is considered that passive targeting to the tumor by liposomalization elevated the SN-38 level in the tumor especially and increased the antitumor activity of CPT-11. Furthermore, intestinal disorder, a side toxicity of CPT-11, decreased dependent on the CPT-11 and SN-38 concentrations in the bile by liposomalization. Although the liposomes induce improved tissue distribution of the prodrug, the tissue distribution of active metabolites does not always improve. However, CPT-11-entrapped liposome was useful, as CPT-11 is converted to SN-38 in the tumor. These results suggested that the usefulness of CPT-11 could be extended.

Pharmacokinetic and pharmacological profiles of free and liposomal recombinant human erythropoietin after intravenous and subcutaneous administrations in rats

PURPOSE

Recombinant human erythropoietin (Epo) is used frequently through intravenous (i.v.) and subcutaneous (s.c.) administration for the clinical treatment of the last stage of renal anemia. We encapsulated Epo in liposomes to develop an alternative administration route. The purpose of our study was to evaluate the pharmacokinetics and the pharmacological effects of liposomal Epo in comparison with the Epo after i.v. and s.c. administration to rats.

METHODS

Epo was encapsulated in liposomes composed of dipalmitoylphosphatidylcholine (DPPC) and soybean-derived sterol mixture (SS) prepared by the reversed-phase evaporation vesicle method. After filtration through a 0.1 micron polycarbonate membrane, liposomes were gel filtered (Epo/liposomes).

RESULTS

Epo/liposomes showed higher pharmacological activity than Epo/liposomes before gel filtration after i.v. administration to rats. Non-encapsulated Epo lost its activity, whereas encapsulated Epo in liposomes retained it. The pharmacological effects of Epo/liposomes were greater than those of Epo after i.v. administration. Epo/liposomes afforded 3-9 times higher AUC, lower clearance and lower steady-state volume of distribution than Epo after both i.v. and s.c. administrations. Epo/liposomes had an improved pharmacokinetics profile compared with Epo. S.c. administration of Epo/liposomes at 7 h may penetrate primarily (40% of dose) through the blood as a liposome and partly (7% of dose) in lymph. CONCLUSIONS. Epo/liposomes may reduce the frequency of injections required for a certain reticulocyte effect in comparison to Epo. The lower clearance of Epo/liposomes may increase the plasma concentrations of Epo, which increases the efficacy.

Effects of administered route on tissue distribution and antitumor activity of polyethyleneglycol-coated liposomes containing adriamycin

Tissue distribution, antitumor activity and side effects after intraperitoneal administration of polyethyleneglycol-coated liposomes containing adriamycin (PEG-LADR) were examined and compared to that after intravenous treatment. Plain liposomes (PLADR) and PEG-LADR appeared to maintain blood circulation by intraperitoneal injection and suggested usefulness in passive targeting. Because of the accumulation of ADR in the pancreas found after intraperitoneal treatment, this administered route of PLADR and PEG-LADR was expected to be useful as a method of targeting the pancreas. The side effects of ADR in the heart and liver were suppressed by the liposomalization and PEG-modification. The antitumor effect of ADR was increased by the liposomalization, and PEG-modification after intraperitoneal administration was superior to that after intravenous administration. The slowly disappearing pattern of PLADR and PEG-LADR from the abdominal cavity was similar. It is suggested that PLADR and PEG-LADR were absorbed intact from the abdominal cavity and transferred into the blood circulation.

Determination of the thickness of the fixed aqueous layer around polyethyleneglycol-coated liposomes

The zeta potentials of adriamycin-encapsulating liposomes containing 1-(monomethoxy polyethyleneglycol)-2,3-dimyristoylglycerol (PEG-DMG) were measured in an isotonic solution of 10mM lactate buffer (pH 4.0) with sodium chloride and sucrose. The negativity of the zeta potentials of adriamycin-encapsulating liposomes containing PEG-DMG decreased with increases in NaCl concentration more steeply than that of adriamycin-encapsulating liposomes without PEG coating. From this observation, the electrical potential distributions near the membrane surfaces were shown to be different between adriamycin-encapsulating liposomes with and without PEG coating. Based on these zeta potential data, the thickness of the fixed aqueous layer around PEG-DMG-containing adriamycin liposomes was determined from the slope of 1n zeta potential versus Debye-Hückel parameter plot. As a result, a correlation was indicated to exist between the circulation time of liposomes and the thickness of the aqueous layer around the liposomes.