Design of Liposomes to Improve Delivery of Macrophage-augmenting Agents to Alveolar Macrophages1

Factors affecting the localization of liposomes injected i.v. in the lung have been studied to identify the optimal type of liposome for delivery of macrophage-activating agents to the lung to augment the tumoricidal activity of alveolar macro phages (AM). Comparison of pulmonary retention of liposomes of differing size, surface charge, and composition following i.v. injection into inbred mice revealed that large multilamellar (MLV) and reversed-phase-evaporation (REV) liposomes are arrested in the lung more efficiently than are small unilamellar liposomes of identical lipid composition. MLV and REV contain ing negatively charged amphiphiles arrest in the lung more efficiently than do neutral MLV's or REV's or MULV's and REV's containing positively charged amphiphiles. Comparison of the ability of liposomes containing a variety of negatively charged amphiphiles to localize in the lung established that optimal localization was achieved using MLV and REV prepared from phosphatidylserine (PS) and phosphatidylcholine (PC) (3: 7 mol ratio) or PS:PC:lysolecithin (4.95:4.95:0.1 mol ratio). The proportion of these liposomes retained in the lung after i.v. injection was constant over a wide dose range (0.02 to 20 ,umol phospholipid per mouse), but hemodilution due to i.v. inocula tion of liposomes in volumes exceeding 0.2 ml reduced reten tion in the lung. Uptake of liposomes by AM was demonstrated by showing that i.v. injection of PS:PC MLV liposomes contain ing fluorescein-labeled bovine serum albumin resulted in local ization of fluorescence within AM recovered by pulmonary lavage. Similarly, AM recovered after i.v. injection of PS:PC MLV liposomes containing lymphokine preparations rich in macrophage-activating factor (MAP) activity exhibited tumori cidal activity. In contrast, macrophages recovered from control animals given injections of unencapsulated MAP or liposomes containing lymphocyte supernatants without MAP activity were devoid of cytotoxic activity. Neutral (PC) MLV liposomes con taining MAP, which show only very limited retention in the lung, were ineffective in activating AM in situ. We conclude that negatively charged MLV liposomes (PS:PC, 3:7 mol ratio) localize efficiently in the lung and that macrophage-activating agents encapsulated within such liposomes can successfully activate lung macrophages in situ.