Lipids for therapeutic agent delivery formulations

An ionizable lipid compound of formula I:

• wherein
• n and m are independently 1, 2, 3, or 4;
• R1 and R2 are independently C10-18alkyl or C12-18alkenyl;
• X is —CH2— or absent;
• L is S—C1-4alkylene, —S(O)2—C1-4alkylene, or
• or a pharmaceutically acceptable salt form thereof.