Modulation of reticuloendothelial activity with liposomes.

The effect of multilamellar vesicles (MLV) on reticuloendothelial (RE) activity, as measured by the carbon clearance method, was investigated in mice. MLV were prepared with a mixture of dipalmitoyl phosphatidylcholine, cholesterol and a charged agent (7:2:1 molar ratio). RE activity was moderately depressed shortly (1-4 hr) after i.v. injection of all MLV tested; negatively charged MLV with dicetyl phosphate, positively charged MLV with stearylamine and neutral MLV without a charged agent. In the later time, either MLV with dicetyl phosphate or MLV with stearylamine caused a marked stimulation of reticulo-endothelial system (RES), but neutral MLV did not. A peak of RE activity resulted from the treatment of MLV with dicetyl phosphate appeared 48 hr after injection and then gradually returned to the normal level within 7 days. The depression and the enhancement of RE activity induced by the treatment of MLV with dicetyl phosphate were observed to be dose-related and significant at a dose of 1 mg/mouse and 0.3 mg/mouse, respectively. These results should be taken into consideration upon the use of liposomes as a drug carrier, or for blocking RES.