A 1,4-dihydropyridine derivative represented by the formula ##STR1## wherein R.sup.1 is a hydrogen atom, an alkoxy group having 1 to 4 carbon atoms, a mercapto group, an alkylcarbonylthio group in which the alkyl group has 1 to 4 carbon atoms or a benzoylthio group, R.sup.2 is a mercapto group, an alkylcarbonylthio group in which the alkyl group has 1 to 4 carbon atoms, a benzoylthio group or a 2-cyanoethylthio group, A and B are the same or different and are each an alkylene group having 1 to 4 carbon atoms, and X is a hydrogen atom, a halogen atom, a nitro group or a trifluoromethyl group is disclosed. These compounds increase the therapeutic effect of drug-resistant cancers.