Determination of incorporated amounts of poly(ethylene glycol)-derivatized lipids in liposomes for the physicochemical characterization of stealth liposomes

We describe a method for determining incorporated amounts of poly(ethylene glycol) (PEG)-derivatized lipids in liposomes for the physicochemical characterization of PEG-coated liposomes. This method is based on the spectrophotometric determination of complexes of polyethers with sodium ions after their extraction as picrates into 1,2-dichloroethane, developed by Favretto for measuring levels of polyoxyethylene alkylphenyl-ether non-ionic surfactants in waste water. The same assay was applied to the estimation of PEG-derivatized lipids in liposomes and percent incorporation of PEG-derivatized lipids into liposomes was successfully determined. To prevent the interference from liposomal lipids other than PEG-derivatized lipids in this assay, liposomal samples were diluted at least to a concentration of less than 0.2 mM. The percent incorporation of PEG-lipids varied, depending on the molecular weight of PEG and anchor acyl chain length in PEG-lipids and it was suggested that the percent incorporation of PEG-lipids into liposomes would be a good parameter of quality control of PEG-liposomes in manufacturing facility and the picrate method used in the present study allows for the determination of this parameter without the need for hazardous radioisotopes.

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.

The effects of theanine, as a novel biochemical modulator, on the antitumor activity of adriamycin

We studied the effects of theanine, a component of green tea leaves, on the antitumor activity of adriamycin (ADR) from the biochemical modulation view point. In vitro, theanine inhibited the ADR efflux from Ehrlich ascites carcinoma cells and maintained the ADR concentration in tumor cells. Theanine enhanced the inhibitory effect of ADR on tumor growth by 2.1-fold in vivo, and increased 2.9-fold the ADR concentration in the tumor, compared to the ADR alone group. An increase in ADR concentration was not observed in normal tissues, such as the heart and liver. Theanine did not enhance, rather tended to normalize the increase of lipid peroxide level and reduction of glutathione peroxidase activity as indicators of the ADR-induced side toxicity.

Effects of methylxanthine derivatives on adriamycin concentration and antitumor activity

We studied the mechanism whereby caffeine acts as a biochemical modulator of adriamycin, and examined various methylxanthine derivitives to determine whether they would be of value as biochemical modulators. In an in vitro study of adriamycin efflux in Ehrlich ascites carcinoma cells, theophylline, pentoxifylline, and theobromine inhibited this efflux, while caffeine metabolites did not. The effects of several methylxanthine derivatives on the antitumor activity of adriamycin and on adriamycin concentration in tissue were also examined in CDF1 tumor-bearing mice. Theobromine, which inhibited adriamycin efflux in vitro, increased the antitumor activity of adriamycin and the concentration of adriamycin in tumors. The caffeine metabolites, which had no effect on the adriamycin efflux, did not increase antitumor activity. These results suggest that the metabolism of caffeine may weaken its effect as a biochemical modulator, and that pentoxifylline and theobromine would be of value as biochemical modulators of adriamycin.

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

The distribution of adriamycin (ADR) at a clinically relevant low dose of 2.5 mg/kg was compared to the distribution at a high dose of 7.5 mg/kg (dose often employed in distribution studies). ADR solution (ADRsol), plain liposomal ADR (PLADR) and polyethyleneglycol (PEG)-coated liposomal ADR (PEG-LADR) were injected into the tail vein of Wistar rats. The retention in serum was PEG-LADR > PLADR > ADRsol at both doses. In the high-dose study, ADR concentration in the liver and spleen at 4 h after administrations of PLADR and PEG-LADR were higher than that with ADRsol. On the other hand, in the low-dose study, reticuloendothelial system (RES) uptake of ADR was reduced by liposome encapsulation (liposomalization). Reduced ADR concentrations in the heart due to liposomalization were found only at the low doses. These results indicate that the difference in doses led to different distributions and that there is an optimal dose at which liposomes exhibit their objective distribution. A comparison of the ADR concentration in the spleen to that in the liver indicated RES saturation by increasing doses and the optimal dose being lower than the dose that saturates RES. Thus, distribution of liposome-entrapped ADR must be investigated at doses as close to the clinical dose as possible.

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.