Novel lipid

A cationic lipid of formula (Ia):

• or a pharmacologically acceptable salt thereof,
• wherein
• R1 and R2 are each independently a hydrogen atom, a C1-C6 alkyl group optionally substituted with one or more substituents selected from substituent group α, a C2-C6 alkenyl group optionally substituted with one or more substituents selected from substituent group α, a C2-C6 alkynyl group optionally substituted with one or more substituents selected from substituent group α, or a C3-C7 cycloalkyl group optionally substituted with one or more substituents selected from substituent group α, or R1 and R2 form a 3- to 10-membered heterocyclic ring together with the nitrogen atom bonded thereto, wherein the heterocyclic ring is optionally substituted with one or more substituents selected from substituent group α and optionally contains one or more atoms selected from a nitrogen atom, an oxygen atom, and a sulfur atom, in addition to the nitrogen atom bonded to R1 and R2, as atoms constituting the heterocyclic ring;
• R8 is a hydrogen atom or a C1-C6 alkyl group optionally substituted with one or more substituents selected from substituent group α;
• or R1 and R8 together are the group —(CH2)q—;
• substituent group α consists of a halogen atom, an oxo group, a hydroxy group, a sulfanyl group, an amino group, a cyano group, a C1-C6 alkyl group, a C1-C6 halogenated alkyl group, a C1-C6 alkoxy group, a C1-C6 alkylsulfanyl group, a C1-C6 alkylamino group, and a C1-C7 alkanoyl group;
• L1 is a C10-C24 alkyl group optionally substituted with one or more substituents selected from substituent group β1, a C10-C24 alkenyl group optionally substituted with one or more substituents selected from substituent group β1, a C3-C24 alkynyl group optionally substituted with one or more substituents selected from substituent group β1, or a (C1-C10 alkyl)-(Q)k-(C1-C10 alkyl) group optionally substituted with one or more substituents selected from substituent group β1;
• L2 is, independently of L1, a C10-C24 alkyl group optionally substituted with one or more substituents selected from substituent group β1, a C10-C24 alkenyl group optionally substituted with one or more substituents selected from substituent group β1, a C3-C24 alkynyl group optionally substituted with one or more substituents selected from substituent group β1, a (C1-C10 alkyl)-(Q)k-(C1-C10 alkyl) group optionally substituted with having one or more substituents selected from substituent group β1, a (C10-C24 alkoxy)methyl group optionally substituted with one or more substituents selected from substituent group β1, a (C10-C24 alkenyl)oxymethyl group optionally substituted with one or more substituents selected from substituent group β1, a (C3-C24 alkynyl)oxymethyl group optionally substituted with one or more substituents selected from substituent group β1, or a (C1-C10 alkyl)-(Q)k-(C1-C10 alkoxy)methyl group optionally substituted with one or more substituents selected from substituent group β1;
• substituent group β1 consists of a halogen atom, an oxo group, a cyano group, a C1-C6 alkyl group, a C1-C6 halogenated alkyl group, a C1-C6 alkoxy group, a C1-C6 alkylsulfanyl group, a C1-C7 alkanoyl group, a C1-C7 alkanoyloxy group, a C3-C7 alkoxyalkoxy group, a (C1-C6 alkoxy)carbonyl group, a (C1-C6 alkoxy)carboxyl group, a (C1-C6 alkoxy)carbamoyl group, and a (C1-C6 alkylamino)carboxyl group;
• Q is a group of formula (II):
• when L1 and L2 are each substituted with one or more substituents selected from substituent group β1 and substituent group β1 is a C1-C6 alkyl group, a C1-C6 alkoxy group, a C1-C6 alkylsulfanyl group, a C1-C7 alkanoyl group, or a C1-C7 alkanoyloxy group, the substituent or substituents selected from substituent group β1 in L1 and the substituent or substituents selected from substituent group β1 in L2 optionally bind to each other to form a cyclic structure;
• k is 1, 2, 3, 4, 5, 6, or 7;
• m is 0 or 1;
• p is 0, 1, or 2;
• q is 1, 2, 3, or 4;
• and
• r is 0, 1, 2, or 3,
• provided that p+r is 2 or larger, or q+r is 2 or larger.