adical or a C1-C4 alkyl radical substituted with a nitrogenous group; R1 and R2 may also form, with the nitrogen atom that bears them, a 5- or 6-membered nitrogenous heterocycle optionally substituted with one or more alkyl, hydroxyl or ureido groups; R3 represents a hydrogen atom, a halogen atom such as a chlorine atom, a C1-C4 alkyl radical, a sulfo radical, a carboxy radical, a C1-C4 monohydrox