Cytotoxic, antiviral indolocarbazoles from a blue-green alga belonging to the Nostocaceae.

6-Cyano-5-methoxy-12-methylindolo[2,3-alpha]carbazole and 6-cyano-5-methoxyindolo[2,3-alpha]carbazole are responsible for most of the cytotoxicity and antiviral activity associated with the blue-green alga Nostoc sphaericum EX-5-1. The compounds are active against HSV II and show weak cytotoxicity against KB and LoVo human carcinoma cell lines.