We have developed the first total syntheses of marine natural products ma'edamines A (18) and B (20). Structurally, they contain a pyrazine-2-(1H)-one core and were screened for antiproliferative activity on several cancer cell lines. Out of the six cell lines tested, ma'edamines A and B showed significant cytotoxicity against human colon cancer line COLO 205 (IC(50) 7.9 and 10.3 μM, respectively), breast cancer cell line MCF-7 (IC(50): 6.9 and 10.5 μM, respectively) and human lung adenocarcinoma cell line A549 (IC(50): 12.2 and 15.4 μM, respectively). The apoptotic effect of ma'edamines was confirmed by comet assay. Hence ma'edamines are likely to be useful as leads for development of a new class of anti-cancer agents.