Studies on the methodology of the carboxyfluorescein assay and on the mechanism of liposome stabilization by red blood cells in vitro

The stability of small unilamellar liposomes was investigated in human blood, in vitro. Using the carboxyfluorescein technique, interaction between the dye, the detergent Triton X-100, and an as yet unidentified component of human serum grossly interferes with the experiment and necessitates the use of other detergents, preferably sodium deoxycholate. Separation of liposomes and blood cells by centrifugation induces a small leakage from the liposomes and can lead to an underestimation of the real liposome stability. Upon incubation with whole blood, intact liposomes are absorbed nonspecifically to erythrocytes and internalized by leukocytes, the extent and kinetics of the former process being insenstive to the presence of metabolic inhibitors. The stability of liposomes is significantly enhanced in whole blood or in serum containing washed erythrocytes. Similarly, liposome stability in serum could be augmented be presaturating the serum lipoproteins with excess phospholipid. Our work adds support to previous notions that stable liposomes with high affinities for certain blood-cell components might be developed as suitable carrier systems for drug targetting in pathological disorders within the blood stream.