The metabolic distinctiveness of emulsified lipid particles in the bloodstream and its clinical implications

Population Semiphysiologic Kinetic Modeling and Simulation of Plasma Triglyceride Levels After Soybean Oil-Based Intravenous Lipid Emulsion Administration in Rats

BACKGROUND

Soybean oil-based intravenous lipid emulsion (SO-ILE) has clinical utility as an energy source and in lipid rescue therapy. However, an excessive infusion rate of SO-ILE in routine use and in lipid rescue therapy may cause serious side effects. There is little information about plasma triglyceride (TG) kinetics following SO-ILE administration. The present study aimed to develop a population semiphysiologic kinetic model of TG and to predict the TG kinetics even at extremely high concentrations in rats.

MATERIALS AND METHODS

TG concentration profiles after intravenous bolus (0.1, 0.25, 0.5, 1.0, 1.5, and 2.0 g/kg) or infusion (3.0 g/kg/h for 1 hour) of SO-ILE to rats were analyzed by a kinetic model constructed with 4 pathways: apolipoprotein acquisitions, zero-order catabolism, first-order uptake to storage sites, and zero-order secretion from storage sites. The developed model was subjected to internal and external validation.

RESULTS

Plasma TG concentrations appeared to decline in a biphasic manner with nonlinear TG kinetics. The developed kinetic model was well validated and found to accurately predict the external validation data.

CONCLUSIONS

The proposed kinetic model accurately described TG concentrations after SO-ILE administration at various infusion rates, including a lipid rescue regimen. The maximum acceptable infusion rate of SO-ILE in routine use should correspond to the maximum velocity of the apolipoprotein acquisition: 0.619 g/kg/h in rats. The prediction of TG kinetics at extremely high concentrations will provide useful information for lipid rescue therapy.

Lipid transfer protein transports compounds from lipid nanoparticles to plasma lipoproteins

Nanometer-sized lipid emulsion particles with a diameter of 25-50 nm, called Lipid Nano-Sphere (LNS), are expected as a promising drug carrier to show prolonged plasma half-life of an incorporating drug. In terms of successful drug delivery using LNS, a drug should be incorporated into the lipid particles and remain within the particle, not only in the formulation in vitro but also after administration into the systemic blood circulation. In this study, we showed that phospholipids and some water-insoluble molecules also moved from lipid particles to plasma lipoproteins or albumin in serum and plasma half-lives of these compounds did not reflect that of the drug carriers. It was suggested that phospholipid or its derivative were transferred from LNS particles to plasma lipoproteins by lipid transfer proteins (LTP) in the circulation. These phenomena leaded to unsuccessful delivery of the drug with lipid-particulate drug carriers. On the other hand, lipophilic derivatives with cholesterol pro-moiety tested in this study were not released from LNS particles and showed prolonged plasma half-lives. Lipophilicity is known to be an important parameter for incorporating drugs into lipid particles but substrate specificity for LTP seems to be another key to success promising drug design using lipid emulsion particulate delivery system.

Effect of injection volume on the pharmacokinetics of oil particles and incorporated menatetrenone after intravenous injection as O/W lipid emulsions in rats

Oil-in-water lipid emulsions are promising drug carriers for lipophilic drugs, however, the pharmacokinetics after entering the circulation should be clarified at clinical injection volume in order to utilize them in a clinical situation. In the present study, the standard lipid emulsions, consisting of soybean oil, egg yolk phosphatides and menatetrenone with diameters of about 150 nm, were prepared using a microfluidizer system. The pharmacokinetics of menatetrenone and the oil particles after intravenous injection as standard lipid emulsions at various injection volumes, from the clinical injection volume (0.1 ml/kg) to the experimental injection volume (3.0 ml/kg), were examined in rats. The plasma concentrations of menatetrenone and the oil particles were similar after administration, showing that menatetrenone was not released even after entering the circulation. Menatetrenone was delivered to the liver and spleen at the clinical injection volume, and more menatetrenone was delivered to the liver at clinical injection volume compared with the experimental volume. Moreover, additional information on injection volume-dependency was also obtained from these findings. These results at various injection volumes suggested that the standard lipid emulsions can be utilized as a useful drug delivery system at the clinical injection volume, especially for liver and spleen targeting.

Lipid microspheres incorporated by U937 cells via their specific receptors

Lipid microspheres (LM), currently in clinical use as drug carriers, mainly consist of soybean oil as a core and lecithin as a surfactant. The purpose of our study wass to determine whether or not LM incorporation is receptor-mediated. U937 cells resuspended in a serum-free medium abundantly took up unmodulated LM. A binding study showed that U937 cells had a single binding site for LM (410 sites/cell at 24°C; 100 sites/cell at 4°C). Inhibition assays revealed that lecithin liposome, lysophosphatidylcholines, activated α2-macroglobulin, and HDL did not affect the binding of LM to U937 cells. VLDL strongly, and LDL and AcLDL moderately, inhibited the binding of LM to U937 cells. Ligand blotting analysis revealed that unmodulated LM in an apoprotein-free buffer directly bound to a 40 kDa protein in the cell membrane fraction. These results suggest that LM that is not modulated by any protein is incorporated by specific cells via receptor-mediated processes.Key words: lipid emulsion, drug delivery system, monocyte, free fatty acids.

Preparation of fine emulsified fat particles without glycerol for intravenous nutrition

A method of preparing fine emulsified fat particles without glycerol for intravenous nutrition was investigated. The factors assessed were the oil phase ratio, the glucose level of the aqueous phase and the temperature of high-pressure homogenization. The particle size decreased with an increase in the oil phase ratio and it went below 250 nm only in the emulsion with a 50% oil phase ratio. The weight-weighted particle size (dw)/number-weighted particle size (dn) value reflected the particle size distribution. The emulsion with a 50% oil phase ratio had a very narrow distribution of particle sizes and the dw/dn value was below 1.1. With the use of glucose solutions for the aqueous phase, smaller particle sizes and narrower distributions were obtained with increasing glucose concentrations. The controlled temperature of 50 degrees C was appropriate for high-pressure homogenization, producing particles below 160 nm. The rate of the layer separation was a function of particle size. The particle sizes below 180 nm can be expected to suppress the separation of the formulation which consisted of 10.0% soybean oil, 1.2% phospholipids and 5.0% glucose. The stability studies were conducted at 40 degrees C for 3 months and the fat emulsion was stable during storage. These investigations contribute to the preparation of a new caloric source for peripheral parenteral nutrition.

Effect of composition on biological fate of oil particles after intravenous injection of O/W lipid emulsions

Plasma concentrations of oil particles after intravenous injection of oil-in-water (O/W) lipid emulsions were monitored based on the plasma concentration of phospholipids (PL) and triglycerides (TG), and the light scattering intensity (LSI) of plasma. Previously, we found that their time profiles after injection of the standard O/W lipid emulsion composed of soybean oil (SO) and egg yolk phosphatides (EYP) were similar and suggested that the oil particles with diameter of about 200 nm were entrapped by reticuloendothelial system (RES). Herein, in order to develop a delivery system to avoid the RES uptake by using the lipid emulsions, biological fate of lipid emulsions with oil particles of various sizes or those emulsified by surfactants with polyoxyethylene segments were subjected to the investigations. Lipid emulsions with oil particles of various sizes (about 150-550 nm) were prepared by altering EYP content. The oil particles were stable in plasma in vitro, but oil particle size decreased time-dependently after intravenous injection. Plasma clearance of oil particles depended on their initial size and was decreased by pretreatment with dextran sulfate 500 (DS500), a known RES suppressor. These results suggested that oil particles are still entrapped by RES, even for small-sized oil particles (about 150 nm). Lipid emulsion with small-sized oil particles was also prepared using medium chain triglycerides. The oil particles were stable in vitro, but the time profiles of plasma concentrations of PL and TG, and LSI of plasma were different, and oil particle size decreased time-dependently after intravenous injection. Plasma clearance of the oil particles also depended on their initial size and was decreased by DS500, suggesting that in vivo instability could be due to RES-mediated processes. Artificial surfactants with polyoxyethylene segments, HCO-60 (HCO60) and polysorbate 80 (PS80), were used for RES avoidance. HCO60 resulted in drastic reduction of the plasma clearance of the oil particles for both lipid emulsions composed of soybean oil and medium chain triglycerides. The time-dependent decrease of oil particle size after intravenous injection was marginal. In contrast, PS80 could not prolong the circulation time of the oil particles, and their size decreased time-dependently after intravenous injection.

O/W lipid emulsions for parenteral drug delivery. III. Lipophilicity necessary for incorporation in oil particles even after intravenous injection

The potential usefulness of oil-in-water (O/W) lipid emulsions as injectable drug delivery systems was examined. Plasma concentrations of oil particles after intravenous injection of a standard lipid emulsion composed of soybean oil and egg yolk phosphatides were monitored based on the plasma concentrations of phospholipids and triglycerides, and the light scattering intensity of the plasma. Their time profiles were similar to each other, and the oil particle size decreased time-dependently. Pretreatment with dextran sulfate, a known reticuloendothelial system (RES) suppressor, resulted in marked reduction of the plasma clearance of the oil particles and of the time-dependent alteration of oil particle size, suggesting that oil particles were trapped by RES. The lipophilicity of the drug needed for its incorporation in the oil particles even after intravenous injection was found to be clog P > 8, where clog P is the calculated logarithm of the partition coefficient between n-octanol and water. In the case of sudan II (clog P = 5.4), the release from the oil particles was very quick after intravenous injection, resulting in slight alteration in biodistribution when compared with its micellar solution. In contrast, menatetrenone (clog P = 9.5) was selectively delivered to the liver, lungs and spleen, being consistent with the oil particles taken up by RES.