Contrast agents for magnetic resonance imaging

Contrast agent for imaging by magnetic resonance imaging comprising a chelating ligand and a metal ion transition, carrying said ligand a substituent which can react by chemical or biochemical pathway with a target substance causing a change of spin state, characterized it comprises: a) or formula (I) ** (See formula) ** in which: - R4 represents a hydrogen atom or an alkyl group of 1 to 4 carbon atoms, linear or branched, or a radical aryl; - W represents a group - (CH2) m-, m being equal to 0 or 1; - R5 represents a group capable of reacting with the target substance to cause a break between the N atom of the macrocycle and the carbon carrying said group R 5, or represents a hydrogen atom, provided that: i) when R5 represents a group can react with the target substance to cause a break between the N atom of the macrocycle and the carbon carrying said group R 5 (complexes Ia), substituents R1 to R3, designated by Ri, represent, independently of each other, an atom hydrogen or a group selected to adjust the solubility properties of dispersibility in biological media and of the magnetic moment of the complex, and can one of the Ri also represent a group can experience an electronic modification of the neighboring pyridyl group by reaction with the target substance ; ii) when R5 represents a hydrogen atom (Ib complex), one of the groups Ri represents a group can experience an electronic modification pyridyl group neighbor by reaction with the target substance, can represent the Ri groups remaining, independently of each other , a hydrogen atom or chosen to adjust the properties of solubility, dispersibility in biological media and of the magnetic moment of the complex; b) or formula (II) ** (See formula) ** in which: - R1 and R2, designated by Ri, represent, independently of one another, a hydrogen atom or a group chosen to adjust the properties of solubility, dispersibility in biological media and of magnetic moment of the complex; - R4 and W are as defined above; - R5 represents a (-C6X4-Y-R6), in which X is an electron withdrawing group, Y is a heteroatom selected from an oxygen atom, a nitrogen atom or a sulfur atom, and R6 represents a group capable of undergoing cleavage by a glycosidase, or an aminoacyl group capable of undergoing cleavage by an aminopeptidase, or susceptible acyl group undergoing cleavage by an esterase or a lipase, or a group capable of undergoing a retroaldol transformation by a natural aldolase , or a group capable of undergoing transformation by a phosphatase; - Z is a divalent group which forms with the two nitrogen atoms carrying as an aromatic cycle benzotriazole, triazole, tetrazole, pyrazole, aromatic ring may bear one substituent NO2.