This invention relates to antitumoral ecteinascidin derivatives that contain a fused ecteinascidin five ring system with a 1,4-bridge having the structure of formula (VIa) or (VIb) as described herein and compounds in which the —NH2 or —OH of the 1,4-bridge is derivatized, and related pharmaceutical compositions and methods. Such ecteinascidin derivatives include, but are not limited to, those compounds having formula (XVIIb): in which R1 and R4 together form a group of formula (VIa) or (VIb) as described herein, and R5, R7, R8, R14a, R14b, R15, and R21 are as defined herein.