What are described are compounds of formula II. The compound of formula IIconsists of a compound in which R1 and R2 are both a linear alkyl consisting of 1 to 12 carbons, an alkenyl or alkynyl consisting of 2 to 12 carbons； L1 and L2 both consist of a linear alkylene or alkenylene consisting of 5 to 18 carbons； X is S； L3 consists of a bond； R3 consists of a linear or branched alkylene consisting of 1 to 6 carbons； and R4 and R5 are the same or different, each consisting of hydrogen or a linear or branched alkyl consisting of 1 to 6 carbons； or a pharmaceutically acceptable salt thereof.
