The present invention relates to indole derivatives represented by formula (I):
wherein R1 and R2 independently represent hydrogen, lower alkyl, hydroxy, lower alkoxy, halogen, -O-(CH2)n-OR5, or
R3 represents hydrogen, lower alkyl, or
R4 represents hydroxy, lower alkoxy, substituted or unsubstituted aryloxy, or -NR10R11, or pharmaceutically acceptable salts thereof.