Compounds of the formula (I): ##STR1## in which the nitrogen-containing hetero ring portion is a 1,4-dihydropyridine ring or a pyridine ring, R.sup.1 is only present when the nitrogen-containing hetero ring is a 1,4-dihydropyridine ring, and Z is either a CO.sub.2 R.sup.8 group or a group of the formula (II): ##STR2## in which R.sup.4 and R.sup.5 may be the same or different and together may form one divalent group, are effective agents for enhancing the antitumor effect or antitumor drugs.