Compounds of the general formula I or IA or a salt in which X is H, Y is a leaving group, R.sup.1 preferably being an aromatic DNA binding subunit are prodrug analogues of duocarmycin. The compounds are expected to be hydroxylated at the carbon atom to which X is joined, by cytochrome P450, in particular by CYP1B1, expressed at high levels in tumours. The prodrug is expected to be activated preferentially in tumour cells, where it will act as a DNA alkylating agent preventing cell division ##STR00001##