HUMAN RECOMBINANT APOLIPOPROTEIN E-ENRICHED LIPOSOMES CAN MIMIC LOW-DENSITY LIPOPROTEINS AS CARRIERS FOR THE SITE-SPECIFIC DELIVERY OF ANTITUMOR AGENTS

Progressive hypocholesterolemia is a feature associated with a number of cancers of different origin, and it is caused by the high expressio n of low-density lipoprotein (LDL) receptors (LDLrs) on many tumor cel l types. Selective delivery of chemotherapeutics using LDL as a carrie r has therefore been proposed, but the endogenous nature of LDL hamper s its pharmaceutical application. In the current study, we explored th e possibility of synthesizing liposomes that mimic LDL from commercial ly available lipids and proteins. Small unilamellar liposomes were cre ated (28.9 +/- 0.9 nm) and complexed with 5.8 +/- 0.7 molecules of hum an recombinant apolipoprotein E (apoE). On intravenous injection into rats, the liposomes retained their aqueous core, structural integrity, and the majority of the preassociated apoE. [H-3]Cholesteryl oleate-l abeled apoE-enriched liposomes showed a relatively long serum half-lif e (>5 hr), and a low uptake by cells of the reticuloendothelial system was observed (<0.8% of the injected dose at 30 min after injection). Pretreatment of rats with 17 alpha-ethinyl estradiol, which induces th e expression of the LDLr on the liver and adrenals, led to a 2.5-fold accelerated serum clearance (t 1/2 = 123 +/- 10 min) and a selectively increased uptake of liposomes by the liver (2.0-fold) and adrenals (3 .8-fold). The liver association of the liposomes was coupled to the ly sosomal uptake route, similarly as for LDL. In vitro studies using B16 melanoma cells showed that the liposomes bound exclusively to the LDL r via their apoE moiety (90,000 liposomes/cell), with a 14-fold higher affinity (K-d = 0.77 +/- 0.09 nM) than LDL itself. Because of their f avorable properties, we anticipate that these apoE-enriched liposomes are advantageous compared with native LDL in the development of a sele ctive LDLr-targeted antitumor therapy.