Liposome composition for delivery of nucleic acid

A composition for administration of a nucleic acid, comprising:

• (a) liposomes comprised of
• (i) a lipid having the formula
• where each of R1 and R2 is an alkyl or alkenyl chain having between 8-24 carbon atoms, and each of R1 or R2 are independently selected;
• n=0-20;
• L is selected from the group consisting of (1) —X—(C═O)—Y—[[CH2—]], (2) —X—(C═O)—, and (3) —X—[[CH2—]], where X and Y are independently selected from oxygen, NH and a direct bond;
• Z is a weakly basic moiety that has a pk of less than 7.4 and greater than about 4.0;
• and
• (ii) a compound having the general structure:
• wherein R3 is a hydrophilic polymer comprising a linkage for attachment to the dithiobenzyl moiety;
• R4 is selected from the group consisting of H, alkyl and aryl;
• R5 is selected from the group consisting of O(C═O)R7, S(C═O)R7, and O(C═S)R7;
• R7 comprises an amine-containing lipid;
• and R6 is selected from the group consisting of H, alkyl and aryl;
• and where orientation of CH2—R5 is selected from the ortho position and the para position;
• and
• (b) a nucleic acid associated with said liposomes.