A compound of formula (I): in which A is a C alkylene group with a chain length between NH and N(O)R'R'' of at least 2 carbon atoms and R' and R'' are each separately selected from C.sub.1-4 alkyl groups and C.sub.2-4 hydroxyalkyl and C.sub.2-4 dihydroxyalkyl groups, or R' and R'' together are a C.sub.2-6 alkylene group, is formulated so that upon dissolution in aqueous solution the pH of the solution is in the range of 5 to 9. The compound may be in the form of salt with a physiologically acceptable acid having a pK.sub.a in the range of -3.0 (minus 3.0) to 9.0 ##STR00001##