Piperidine compound or salt thereof

A method for potentiating an anti-tumor effect of a microtubule-targeting agent, comprising:

• administering an effective dose of a piperidine compound of formula (I) or a salt thereof:
• wherein R1 is a carboxyl group, —C(═O)NR5R6, or an oxadiazolyl group optionally having a C1-C6 alkyl group or a trifluoromethyl group as a substituent;
• R2 is a halogen atom or a C1-C6 alkoxy group;
• R3 is a phenyl group optionally having 1 to 3 same or different group(s) selected from the group consisting of a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, and a trifluoromethyl group as a substituent;
• R4 is a hydrogen atom or a C1-C6 alkyl group;
• R5 and R6 are the same or different and are each independently a hydrogen atom, a C1-C6 alkyl group, or a C3-C6 cycloalkyl group, or R5 and R6 optionally form a 3 to 6-membered nitrogen-containing saturated heterocyclic group together with a nitrogen atom to which R5 and R6 are bound.

A method for treating cancer, comprising:

• administering an effective dose of a piperidine compound of formula (I) or a salt thereof and an effective dose of a microtubule-targeting agent:
• wherein R1 is a carboxyl group, —C(═O)NR5R6, or an oxadiazolyl group optionally having a C1-C6 alkyl group or a trifluoromethyl group as a substituent;
• R2 is a halogen atom or a C1-C6 alkoxy group;
• R3 is a phenyl group optionally having 1 to 3 same or different group(s) selected from the group consisting of a halogen atom, a C1-C6 alkyl group, a C1-C6 alkoxy group, and a trifluoromethyl group as a substituent;
• R4 is a hydrogen atom or a C1-C6 alkyl group;
• R5 and R6 are the same or different and are each independently a hydrogen atom, a C1-C6 alkyl group, or a C3-C6 cycloalkyl group, or R5 and R6 optionally form a 3 to 6-membered nitrogen-containing saturated heterocyclic group together with a nitrogen atom to which R5 and R6 are bound.