Labelled (Z)-2-Ethyl-3-methylmaleic Acid (E923510). A maleic acid/anhydride derivative potentially useful as building blocks for the synthesis of a diverse range of structurally interesting, complex and bioactive molecules/polymers with tailored material characteristics. A maleic acid derivative that has an inhibitory activity against metallo-β-lactamase. The use of the compound in combination with a β-lactam antibiotic enables to recover the anti-bacterial activity of the β-lactam antibiotic against a bacterium capable of producing metallo-β-lactamase.