Antibody-directed liposomes: the development of a cell-specific cytotoxic agent.

that (2 h) of similar cholesterol-rich liposomes (Senior & Gregoriadis, 19826). When liposomes were injected intraperitoneally into normal animals, no latent carboxyfluorescein could be recovered in the circulation ; however, when injected similarly into lipoprotein-deficient animals, latent dye was detected in the blood, with a peak of 10% of dose detected after 2 h. Maintenance of liposomal stability in lipoprotein-deficient animals is also supported by studies in oitro, which indicate that [3H]phosphatidylcholinelabelled liposomes incubated with lipoprotein-deficient plasma at 37°C retain both entrapped carboxyfluorescein and [3H]phosphatidylcholine to a greater extent than do those incubated in normal plasma. This was shown by gel chromatography on Ultrogel AcA 34, which separates liposomally bound and HD-lipoprotein-associated [3H]phosphatidylcholine marker, and also entrapped and free carboxyfluorescein (Senior et al., 1983). To determine whether other lipoproteins in addition to HD lipoprotein are active in destabilizing liposomes, lipoprotein-deficient plasma (0.5 ml) from 4-aminopyrazolopyrimidine-treated mice was supplemented with mouse HD (188-750pg), lowand intermediate-density (1434pg) and very-low-density (18-5Opg of protein) lipoproteins in concentrations covering the physiological range and incubated at 37°C with 50p1 of liposomes containing quenched carboxyfluorescein. Duplicate lop1 samples from the incubation mixtures were diluted phosphate-buffered saline at time intervals, and latent carboxyfluorescein was measured. Only HD lipoproteins were found to destabilize liposomes, as other lipoproteins did not augment loss of latency above that seen by the action of lipoproteindeficient plasma alone. Furthermore, when lipoproteindeficient plasma was heated at 56°C for 30min, and then, before to incubation with liposomes, supplemented with HD lipoproteins, its liposome-destabilizing activity was not restored to its initial value. This confirmed that there is a heat-labile factor in the plasma acting with HD lipoprotein to bring about maximum vesicle destabilization (Senior et al., 1983; Damen et al., 1981). An example of plasma lipoprotein deficiency occurring in man is phosphatidylcholine :cholesterol acyltransferase deficiency characterized by low concentrations of plasma HD lipoproteins. A sample of plasma from a patient with phosphatidylcholine :cholesterol acyltransferase deficiency on incubation with phosphatidylcholine liposomes was less active in promoting solute release than was plasma from a normal control. Our results suggest that clinical application of drug-containing liposomes must take into account fluctuations in plasma HD lipoproteins, and perhaps other factors which could alter liposomal membrane permeability in vivo and thus influence drug fate and effect.