Etherlipid-containing multiple lipid liposomes

A liposome having a lipid bilayer which comprises:

• (a) a phosphatidylcholine;
• (b) a sterol;
• (c) a headgroup-derivatized lipid comprising a phosphatidylethanolamine linked at the ethanolamine group to a dicarboxylic acid;
• and, (d) an etherlipid having the formula:
• ##STR11## wherein R.sub.1 is Y.sub.1 Y.sub.2, Y.sub.2 is CH.sub.3 or CO.sub.2 H, Y.sub.1 is (CH.sub.2).sub.n1 (CH.dbd.CH).sub.n2 (CH.sub.2).sub.n3 (CH.dbd.CH).sub.n4 (CH.sub.2).sub.n5 (CH.dbd.CH).sub.n6 (CH.sub.2).sub.n7 (CH.dbd.CH).sub.n8 (CH.sub.2).sub.n9, the sum of n1+2n2+n3+2n4+n5+2n6+n7+2n8+n9 is an integer of from 3 to 23, n1 is zero or an integer of from 1 to 23, n3 is zero or an integer of from 1 to 20, n5 is zero or an integer of from 1 to 17, n7 is zero or an integer of from 1 to 14, n9 is zero or an integer of from 1 to 11, and each of n2, n4, n6 and 8 is independently zero or 1;
• wherein Z is oxygen or sulfur and R.sub.2 is CH.sub.3 ;
• wherein R.sub.3 is --O--P(O).sub.2 --O--CH.sub.2 CH.sub.2 N(CH.sub.3).sub.3 ;
• and wherein the phosphatidylethanolamine-dicarboxylic acid comprises from about 5 mole percent to about 20 mole percent of the lipid bilayer and the etherlipid comprises from greater than about 10 mole percent to less than about 30 mole percent of the lipid bilayer.