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 C1-4 alkyl groups and C2-4 hydroxyalkyl and C2-4 dihydroxyalkyl groups, or R′ and R″ together are a C2-6 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 pKa in the range of −3.0 (minus 3.0) to 9.0.