Pyridine derivatives

A compound of formula (I) or a salt thereof:

• wherein
• R1 is C1-10 alkyl optionally having 1 to 5 substituents selected from group G1;
• C2-10 alkenyl optionally having 1 to 5 substituents selected from group G1;
• C2-10 alkynyl optionally having 1 to 5 substituents selected from group G1;
• -lower alkylene-X-(lower alkyl optionally having 1 to 5 substituents selected from group G1);
• R4;
• 5- or 6-membered heterocyclic group optionally having 1 to 5 substituents selected from group G3;
• -lower alkylene-R4;
• -lower alkenylene-R4;
• -lower alkylene-X—R4;
• or -lower alkylene-X-lower alkylene-R4,
• or R1 forms, together with R21 or R22 on the pyridine ring bonded thereto, a hydrocarbon ring fused with the pyridine ring, represented by any of the following formulas (i) to (iv):
• wherein the hydrocarbon ring optionally has 1 to 4 substituents selected from the group consisting of lower alkyl optionally having 1 to 5 substituents selected from group G1, cycloalkyl optionally having 1 to 5 substituents selected from group G2, -lower alkylene-R4, and —O-lower alkylene-R4, or the hydrocarbon ring optionally forms a spiro ring with C3-8 cycloalkane,
• R21 and R22 are the same or different and each are H;
• lower alkyl optionally having 1 to 5 substituents selected from group G1;
• —X-lower alkyl;
• —X-halogeno lower alkyl;
• cycloalkyl optionally having 1 to 5 substituents selected from group G2;
• or -lower alkylene-(cycloalkyl optionally having 1 to 5 substituents selected from group G2), R3 is lower alkyl optionally having 1 to 5 substituents selected from group G1;
• -lower alkylene-X-(lower alkyl optionally having 1 to 5 substituents selected from group G1);
• -lower alkylene-X-(lower alkenyl optionally having 1 to 5 substituents selected from group G1);
• -lower alkylene-(cycloalkyl optionally having 1 to 5 substituents selected from group G2);
• -lower alkylene-X-(cycloalkyl optionally having 1 to 5 substituents selected from group G2);
• or -lower alkylene-X-lower alkylene-(cycloalkyl optionally having 1 to 5 substituents selected from group G2),
• each X is independently O or S,
• each R4 is independently cycloalkyl optionally having 1 to 5 substituents selected from group G2;
• cycloalkenyl optionally having 1 to 5 substituents selected from group G2;
• or aryl optionally having 1 to 5 substituents selected from group G3, group G1 consists of halogen, OH, —O-lower alkyl, —S-lower alkyl, —O-halogeno lower alkyl, and CN,
• group G2 consists of the groups of group G1, lower alkyl optionally having 1 to 5 substituents selected from group G1, and -lower alkylene-(cycloalkyl optionally substituted by 1 to 4 lower alkyl groups), and
• group G3 consists of the groups of group G1, lower alkyl optionally having 1 to 5 substituents selected from group G1, and benzyloxycarbonyl.