Derivatives of a novel cyclopeptolide. 2. Synthesis, activity against multidrug resistance in CHO and KB cells in vitro, and structure-activity relationships.

A series of derivatives of the novel cyclopeptolide 1 was prepared, and their ability to chemosensitize multi drug resistant CHO and KB cells in vitro was evaluated. In contrast to the parent compound, several of the derivatives were found to be highly active. In particular, conversion of the R-lactic acid residue of 1 into its S-isomer via lactone ring cleavage and recyclization with inversion resulted in a marked enhancement of activity. Some of these derivatives (e.g., 15a, SDZ 280.446) belong to the most potent resistance modulating compounds known so far.