What is described is a compound wherein R1 and R2 are the same or different, each a linear or branched alkyl consisting of 1 to 20 carbons, or a linear or branched alkenyl or alkynyl consisting of 2 to 20 carbons; L1 and L2 are the same or different, each a bond, a linear alkylene of 1-18 carbons, or a linear alkenylene consisting of 2 to 18 carbons; L3 is a bond or a linear or branched alkylene consisting of 1 to 6 carbons; L4 is a bond or a methylene; X is S or O, R3 is a linear or branched alkylene consisting of 1 to 6 carbons, and R4 and R5 are the same or different, each a hydrogen or a linear or branched alkyl consisting of 1 to 6 carbons; or a pharmaceutically acceptable salt thereof.