The present invention provides a VDAC regulator comprising a compound represented by the formula (I): wherein R1 is a halogen atom, an optionally substituted heterocyclic group, an optionally substituted hydroxy group, an optionally substituted thiol group or an optionally substituted amino group; A is an optionally substituted acyl group, an optionally substituted heterocyclic group, an optionally substituted hydroxy group or an optionally esterified or amidated carboxyl group; B is an optionally substituted aromatic group; X is an oxygen atom, a sulfur atom or an optionally substituted nitrogen atom; and Y is a divalent hydrocarbon group or heterocyclic group, a salt thereof or a prodrug thereof, which is useful as an agent for the prophylaxis or treatment of Down's syndrome and the like, and the like.