What is described is a compound of formula (I), 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, in which the compound facilitates the intracellular delivery of biologically active and therapeutic molecules.
