Congener of portimine of the structure: where: R1 is selected from H, acyl, alkyl, alkylene, aryl and aralkyl; R2 is selected from H, acyl, alkyl, alkylene, aryl and aralkyl; M is O, S, N—NH2, N(alkyl)2, N-alkoxy or N(aryl)2; W and X may form a bond, an epoxide, or are independently selected from H, halogen, OH and O-acyl. Y and Z form a bond, an epoxide, or are independently selected from H, halogen (X), OH and O-acyl; and R1 and R2 are not H, M is not O, or W and X or Y and Z do not form a bond.