structure–activity relationships (SARs) of 19 andrographolide analogues which were synthesized by modification at the three hydroxyl groups. A number of the andrographolide analogues showed much higher cytotoxicactivities than that of the parent compound on cancer cells including P-388, KB, COL-2, MCF-7, LU-1 and ASK cells. SAR studies of the synthetic analogues indicated that the introduction of silyl ether or