Cholesteryl oleyl and linoleyl ethers do not trace their ester counterparts in animals with plasma cholesteryl ester transfer activity

Radiolabeled cholesteryl ethers: A need to analyze for biological stability before use

Radiolabeled cholesteryl ethers are widely used as non-metabolizable tracers for lipoproteins and lipid emulsions in a variety of in vitro and in vivo experiments. Since cholesteryl ethers do not leave cells after uptake and are not hydrolyzed by mammalian cellular enzymes, these compounds can act as markers for cumulative cell uptakes of labeled particles. We have employed [³H]cholesteryl oleoyl ether to study the uptake and distribution of triglyceride-rich emulsion particles on animal models. However, questionable unexpected results compelled us to analyze the stability of these ethers. We tested the stability of two commercially available radiolabeled cholesteryl ethers - [³H]cholesteryl oleoyl ether and [³H]cholesteryl hexadecyl ether from different suppliers, employing in vitro, in vivo and chemical model systems. Our results show that, among the two cholesteryl ethers tested, one ether was hydrolyzed to free cholesterol in vitro, in vivo and chemically under alkaline hydrolyzing agent. Free cholesterol, unlike cholesteryl ether, can then re-enter the circulation leading to confounding results. The other ether was not hydrolyzed to free cholesterol and remained as a stable ether. Hence, radiolabeled cholesteryl ethers should be analyzed for biological stability before utilizing them for in vitro or in vivo experiments.

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Tested stability of two commercially available radiolabeled cholesteryl ethers.

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One ether was hydrolyzed to free cholesterol (FC) in vitro and in vivo.

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FC, re-entered circulation giving questionable unexpected results in experiments.

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The other ether was unhydrolyzed in all model systems.

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Radiolabeled cholesteryl ethers should be analyzed for stability before use.

Quantitative In Vitro Assessment of Liposome Stability and Drug Transfer Employing Asymmetrical Flow Field-Flow Fractionation (AF4)

PURPOSE

In the present study we introduce an efficient approach for a size-based separation of liposomes from plasma proteins employing AF4. We investigated vesicle stability and release behavior of the strongly lipophilic drug temoporfin from liposomes in human plasma for various incubation times at 37°C.

METHODS

We used the radioactive tracer cholesteryl oleyl ether (COE) or dipalmitoyl-phosphocholine (DPPC) as lipid markers and (14)C-labeled temoporfin. First, both lipid labels were examined for their suitability as liposome markers. Furthermore, the influence of plasma origin on liposome stability and drug transfer was investigated. The effect of membrane fluidity and PEGylation on vesicle stability and drug release characteristics was also analyzed.

RESULTS

Surprisingly, we observed an enzymatic transfer of (3)H-COE to lipoproteins due to the cholesterol ester transfer protein (CETP) in human plasma in dependence on membrane rigidity and were able to inhibit this transfer by plasma preincubation with the CETP inhibitor torcetrapib. This effect was not seen when liposomes were incubated in rat plasma. DPPC labels suffered from hydrolysis effects during preparation and/or storage. Fluid liposomes were less stable in human plasma than their PEGylated analogues or a rigid formulation. In contrast, the transfer of the incorporated drug to lipoproteins was higher for the rigid formulations.

CONCLUSIONS

The observed effects render COE-labels questionable for in vivo studies using CEPT-rich species. Here, choline labelled (14)C-DPPC was found to be the most promising alternative. Bilayer composition has a high influence on stability and drug release of a liposomal formulation in human plasma.

Enhancement of the in vivo circulation lifetime of L-alpha-distearoylphosphatidylcholine liposomes: importance of liposomal aggregation versus complement opsonization

Incorporation of N-(omega-carboxy)acylamido-phosphatidylethanolamines (-PEs) into large unilamellar vesicles (LUVs) of L-alpha-distearoylphosphatidylcholine (DSPC) was found to dramatically increase the in vivo liposomal circulation lifetime in rats, reaching a maximal effect at 10 mol.% of the total phospholipid. Neither pure DSPC liposomes nor those with the longest circulating derivative, N-glutaryl-dipalmitoylphosphatidylethanolamine (-DPPE), were found to significantly bind complement from serum. Therefore, the relatively short circulation time of pure DSPC liposomes did not appear to be related to greater complement opsonization leading to uptake by the reticuloendothelial system. However, N-(omega-carboxy)acylamido-PEs were particularly efficient inhibitors of a limited aggregation detected for pure DSPC liposomes. The aggregation tendency of DSPC liposomes incorporating various structural analogs of N-glutaryl-DPPE correlated inversely with the circulation lifetimes. Therefore, it is concluded that such PE derivatives enhance the circulation time by preventing liposomal aggregation and avoiding a poorly understood mechanism of clearance that is dependent on size but is independent of complement opsonization. At high concentrations of N-glutaryl-DPPE (above 10 mol.%), the liposomes exhibited strong complement opsonization and were cleared from circulation rapidly, as were other highly negatively charged liposomes. These data demonstrate that both the lack of opsonization and the lack of a tendency to aggregate are required for long circulation. Liposomal disaggregation via N-(omega-carboxy)acylamido-PEs yields a new class of large unilamellar DSPC liposomes with circulation lifetimes that are comparable to those of sterically stabilized liposomes.

Exogenous supply of artificial lipoproteins does not decrease susceptibility to atherosclerosis in cholesterol-fed rabbits

We investigated the effects of reconstituted apo A-I-containing high-density lipoprotein (r-HDL, homologous to small pre-beta-migrating HDL) and reconstituted triglyceride-rich lipoprotein (r-TRL, a mixture of Intralipid and apo E) on atherogenesis in rabbits fed a 0.5% cholesterol diet for 8 weeks. Male Japanese white rabbits (n = 17) were divided into three groups: the control group (n = 7) received a placebo and 2.5 ml of Intralipid 20 h later; the second group (n = 6) received r-HDL containing 18 mg of apo A-I followed by 2.5 ml of Intralipid; and the third group (n = 4) received 18 mg of r-HDL and 2.5 ml of r-TRL containing 4 mg of apo E. Rabbits were injected with the agents weekly and the same interval (20 h) was maintained between the two injections. Three minutes after the injection of r-HDL, a sharp increase in the pre-beta-migrating fraction was observed. The cholesterol-rich diet similarly increased serum lipids in the three groups. No significant changes of the HDL cholesterol and apo A-I concentrations were observed in the three groups. Conversely, there was a 12-fold increase of apo E which correlated positively with the total cholesterol level. Injection of r-HDL and r-TRL caused slight inhibition of fatty streak development and lipid deposition in the aortic wall, but neither change was statistically significant. Lipid accumulation in the liver was similar in all three groups. These results suggest that the physiological properties of artificial and native lipoproteins may differ.

Metabolism of high-density lipoproteins in rainbow trout

Trout high-density lipoproteins have been labelled with residualizing tracers for the lipid and protein moieties ([3H]cholesteryloleyl ether and 125I-tyramine-cellobiose, respectively). Plasma kinetics and tissue site of catabolism were determined for both tracers. The lipid tracer was cleared about twice as fast from the blood as the protein tracer (half lifes were 63.5 and 125.3 h, respectively). This selective removal of lipid from the lipoprotein was mainly accomplished by the higher liver uptake of the cholesteryl ether. The main catabolic site for HDL protein was kidney tissue. This data established the existence of differential HDL catabolism in a lower vertebrate, in which HDL is the dominant plasma lipoprotein. In addition, the findings confirm the importance of fish kidney as a major site of endocytosis of macromolecules, of both exogenous and endogenous origin.

Estimation of HDL cholesteryl ester kinetic parameters in the cebus monkey, an animal species with high plasma cholesteryl ester transfer activity

We studied the kinetic parameters of high density lipoprotein (HDL) cholesteryl esters in the cebus monkey, an animal species with high plasma cholesteryl ester transfer activity. HDL were radiolabeled with cholesteryl [1-14C]oleate and intravenously administered to 4 cebus monkeys. The calculated fractional catabolic rate (FCR) of the HDL cholesteryl esters was 0.081 +/- 0.002 (mean +/- SD) h-1 and the calculated residence time was 12.3 +/- 0.3 h. The production or disposal rate of plasma HDL cholesteryl esters was 34.3 +/- 4.5 mumol/h. The radiolabeled cholesteryl esters were rapidly transferred from the HDL to the very low and low density lipoproteins (VLDL + LDL) and the amount of tracer in the VLDL + LDL had already reached a maximum at 3.5 +/- 0.7 h after tracer administration. The estimated fraction of VLDL + LDL cholesteryl esters derived from the HDL was 0.77 +/- 0.06. We also used radiolabeled [1,2-3H(N)]cholesteryl palmityl ether to trace HDL cholesteryl esters, but the ether tracer was more slowly cleared from the plasma and less readily transferred between plasma lipoproteins than the ester tracer.