What is described is a compound wherein R1 and R2 are the same or different, each a linear or branched alkyl with 1-9 carbons, or an alkenyl or alkynyl with 2 to 11 carbons; L1 and L2 are the same or different, each a linear alkyl having 3 to 18 carbons; X1 and X3 are —CO—O—; X3 is S or O; L3 is a lower alkyl; R3 is a lower alkyl; and R4 and R5 are the same or different, each a lower alkyl; or a pharmaceutically acceptable salt thereof.