Effect of the cholesterol content of small unilamellar liposomes on their stability in vivo and in vitro

Abstract

Small unilamellar neutral, negatively and positively charged liposomes composed of egg phosphatidylcholine, various amounts of cholesterol and, when appropriate, phosphatidic acid or stearylamine and containing 6-carboxyfluorescein were injected into mice, incubated with mouse whole blood, plasma or serum or stored at 4 degrees C. Liposomal stability, i.e. the extent to which 6-carboxyfluorescein is retained by liposomes, was dependent on their cholesterol content. (1) Cholesterol-rich (egg phosphatidylcholine/cholesterol, 7:7 molar ratio) liposomes, regardless of surface charge, remained stable in the blood of intravenously injected animals for up to at least 400min. In addition, stability of cholesterol-rich liposomes was largely maintained in vitro in the presence of whole blood, plasma or serum for at least 90min. (2) Cholesterol-poor (egg phosphatidylcholine/cholesterol, 7:2 molar ratio) or cholesterol-free (egg phosphatidylcholine) liposomes lost very rapidly (at most within 2min) much of their stability after intravenous injection or upon contact with whole blood, plasma or serum. Whole blood and to some extent plasma were less detrimental to stability than was serum. (3) After intraperitoneal injection, neutral cholesterol-rich liposomes survived in the peritoneal cavity to enter the blood circulation in their intact form. Liposomes injected intramuscularly also entered the circulation, although with somewhat diminished stability. (4) Stability of neutral and negatively charged cholesterol-rich liposomes stored at 4 degrees C was maintained for several days, and by 53 days it had declined only moderately. Stored liposomes retained their unilamellar structure and their ability to remain stable in the blood after intravenous injection. (5) Control of liposomal stability by adjusting their cholesterol content may help in the design of liposomes for effective use in biological systems in vivo and in vitro.

Tissue specific opsonins for phagocytic cells and their different affinity for cholesterol‐rich liposomes

“…lb). This may be attributed to the fact that cholesterol-free liposomes are degraded in serum [2][3][4][5][6], as a consequence a smaller number of intact liposomes may be available to cells for phagocytosis which may apparently result in the poor uptake [4]. Among cholesterolcontaining liposomes, cholesterol-poor are taken up much more by hepatic and splenic cells in the absence of serum than cholesterol-rich liposomes.…”

“…Inclusion of cholesterol in liposomes is known to increase their half-life in the bloodstream [2,3]. This is achieved by cholesterol playing a dual role [4], firstly, it stabilizes the liposomal membrane and makes it less susceptible to destruction by various serum components [5,6]. Secondly it makes liposomes unfavourable for phagocytosis by hepatic Kupffer cells [4,7,8].…”

Noninvasive and persistent transfollicular drug delivery system using a combination of liposomes and iontophoresis

“…However, it was obvious that cationic-liposomes containing DOPE were not only transfer into the hair follicles but penetrate through the skin surface. It has been reported that Chol stabilizes liposomal membranes (Damen et al, 1981;Kirby et al, 1980), whereas DOPE, a fusogenic lipid, destabilizes the lipid bilayers by promoting transition from a lamellar to a hexagonal II phase (Guo and Szoka, 2001). As shown in Table I, although the physical characteristics (size, ζ-potential and polydispersity) of cationic liposomes consisted of DOTAP/EPC = 1:1 and DOTAP/EPC/DOPE = 2:1:1 were very similar, the distribution of lipid marker in the skin after iontophoresis was entirely different (Fig.…”

Stability of liposomes in circulation is markedly enhanced by structural modification of their phospholipid component

“…lA, solid circles), which was quite unhke that observed in egg PCKZH hposomes (fig, 1A, sotid triangles). The faster clearance of 6-CF from the egg PC/CH iiposomes may result from the plasma-induced leakage of the entrapped dye [7]. This is evident from the differences between the rates of clearance of the liposomal iJC and the latent 6CF (fig.…”

“…Disruption of liposomes in the circulation is mainly caused by the transfer of their lipid constituents to the high density fraction of plasma lipoproteins [4,5]. This effect of the lipoprotein is reduced by increasing the CH content of the liposomes [6,7] or by an appropriate choice of the phospholipid component [g,lll. in an attempt to design liposomes that would be stable in blood, we have recently modified phosphatidylcholines by introducing one NH residue adjacent to the carbon atom of their C-2 ester group [ 121. This change rendered the resulting phospho~ipids selectively resistant to phospholipase A2 [12], without adversely affecting the physico-chemical properties of the liposomes Abbreviations: PC, phosphatidylchoiine; MPC, l-pal mitoyt-2-heptadec-lO-cis-enylcarbamyloxy-sn-glycero-3-phosphorylcholine; CH, cholesterol; 6-CF, Gcarboxyfluorescein; HDL, high density fraction of plasma lipoproteins * To whom correspondence should be addressed…”