What is described is a compound of formula (1) 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 carbon atoms; L1 and L2 are the same or different, each a linear alkyl having 5 to 18 carbon atoms, or form a heterocycle with N; X1 is a bond, or is —CO—O— whereby L2-CO—O—R2 is formed; X2 is S or O; L3 is a bond or a lower alkyl, or form a heterocycle with N; R3 is a lower alkyl; and R4 and R5 are the same or different, each a lower alkyl; or a pharmaceutically acceptable salt thereof.