Stability and clearance of small unilamellar liposomes. Studies with normal and lipoprotein-deficient mice.

The effect of high density lipoproteins (HDL) on the stability and clearance of injected liposomes was investigated under conditions of abnormal lipoprotein metabolism in vivo. Small unilamellar liposomes composed of phosphatidylcholine and containing quenched carboxyfluorescein were injected intravenously or intraperitoneally into normal mice or mice previously made lipoprotein deficient with 4-aminopyrazolo[3,4d]pyrimidine (4-APP). As evidenced from quenched carboxyfluorescein values in the blood, levels of stable liposomes in the circulation were increased and clearance rates reduced considerably in lipoprotein-deficient animals indicating increased bilayer integrity. This was confirmed by the demonstration that transfer of liposomal phosphatidylcholine to HDL, occurring in the presence of normal mouse plasma, was virtually abolished in the presence of plasma from lipoprotein deficient mice. The role of other lipoprotein species in destabilizing liposomes was also investigated. Plasma from lipoprotein-deficient mice was supplemented with increasing amounts of HDL, LDL + IDL or VLDL (to cover the physiological range of lipoprotein concentrations in mouse blood) prior to the addition of phosphatidylcholine liposomes, and incubated at 37 degrees C. It was shown that among the lipoprotein species studied only HDL was detrimental to liposomal stability under the conditions employed. Our results indicate that use of liposomal drugs in the treatment of patients must take into account HDL fluctuations in their blood as these could after liposomal membrane permeability to the drugs and thus upset therapeutic efficiency.