A compound of the formula ##STR1## wherein: R.sub.1 and R.sub.2 are defined as follows: R.sub.1 is hydrogen, an alkyl group, or an alkyl group having one or more hydroxy groups; R.sub.2 is an alkyl group having one or more hydroxy groups, or an alkyl group containing an amino group in which the amino function is substituted with hydrogen or one or two individual alkyl groups same or different, or two alkyl groups which together form a cyclic amine, e.g. piperidine; R.sub.3 is hydrogen or an alkyl group; and R.sub.4 is hydrogen or an alkyl group. The compounds have high radiosensitizing activity and selectivity for hypoxic cells.