What is described is a compound of formula III or IVwherein R1 is a branched alkyl with 12 to 20 carbons；R2 is a linear alkyl with 5 to 10 carbons or a branched alkyl with 12 to 20 carbons；L1 and L2 are each a bond or a linear alkylene having 1 to 3 carbon atoms, when L3 is a bond；L1 and L2 are each a linear alkylene or alkenylene 5 to 18 carbon atoms, when L3 is an alkylene of 5 to 18 carbons；X is S or O；R3 is of a lower alkyl； andR4 and R5 are the same or different, each a lower alkyl；or a pharmaceutically acceptable salt thereof.
