While LDLs are natural lipoproteins with a proclivity for the tumor cells in which the receptor for the vesicle is overexpressed, liposomes can be viewed as

related synthetic vesicles. Hawthorne and his associates have been involved in a major effort to develop liposomes as boron-delivery agents to tumors. Small unilamellar liposomes have been shown to penetrate the tumor cell membrane and localize intracellularly. Therefore, incorporating boron compounds within such vesicles should provide a method for achieving selectivity between tumor and normal cells. Several of the compounds developed for this purpose are shown in Figure 2.39.19c,26,84

A key question is whether the liposome can pass through the blood/brain barrier (BBB). Unilamellar liposomes with diameters as small as 60 nm are incapable of crossing the BBB. Liposomes have also been used by others to deliver boronated thiouracils to melanoma cells and as a targeting vehicle for BSH through intracerebral administration. An advantage of using liposomes is that the boron species itself does not need to possess tumor-targeting properties. Once the compound is deposited either intracellularly within the tumor cell or interstitially within the tumor by the liposome and has properties that result in cell penetration and binding to subcellular organelles, persistence would be achieved. With respect to brain tumors, the unique restrictive nature of the normal BBB limits penetrability of the CNS, especially for liposomes greater than 60 nm.19c However, in the total scheme of things for all tumors, liposomes may well be an important delivery system for BNCT agents.84

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