Maleimide

Maleimide is a chemical compound with the formula H₂C₂(CO)₂NH (see diagram). This unsaturated imide is an important building block in organic synthesis. The name is a contraction of maleic acid and imide, the -C(O)NHC(O)- functional group. Maleimides are also a class of derivatives of the parent maleimide where the NH group is replaced with alkyl or aryl groups such as a methyl or phenyl, respectively. The substituent can also be a small molecule (such as biotin, a fluorescent dye, an oligosaccharide, or a nucleic acid), a reactive group, or a synthetic polymer such as polyethylene glycol. Human hemoglobin chemically modified with maleimide-polyethylene glycol is a blood substitute called MP4.

Many analogues of maleimide are prepared by treating maleic anhydride with amines followed by dehydration.

A defining feature of the reactivity of maleimides is their susceptibility to additions across the double bond either by Michael additions or via Diels-Alder reactions. In this context, bismaleimides, compounds with two maleimide groups connected by the nitrogen atoms via a linker, are used as crosslinking reagents in thermoset polymer chemistry. Compounds containing a maleimide group linked with another reactive group, such as an activated N-hydroxysuccinimide ester, are called maleimide heterobifunctional reagents .

Maleimide is weakly acidic, with a pKa estimated at 10.

One natural maleimide is the cytotoxic showdomycin from Streptomyces showdoensis, and pencolide from Pe. multicolor – have been reported. Farinomalein was first isolated in 2009 from the entomopathogenic fungus Isaria farinosa (Paecilomyces farinosus) – source H599 (Japan).

Maleimide-mediated methodologies are among the most used in bioconjugation. Due to fast reactions and high selectivity towards cysteine residues in proteins, a large variety of maleimide heterobifunctional reagents are used for the preparation of targeted therapeutics, assemblies for studying proteins in their biological context, protein-based microarrays, or proteins immobilisation. For instance, antibody-drug conjugates, are constituted of three main components: a monoclonal antibody, a cytotoxic drug, and a linker molecule often containing a maleimide group, which conjugates the drug through thiols or dienes to the antibody.

Maleimides linked to polyethylene glycol chains are often used as flexible linking molecules to attach proteins to surfaces. The double bond readily undergoes a retro-Michael reaction with the thiol group found on cysteine to form a stable carbon-sulfur bond. Cysteines are often used for site-selective modifications for therapeutic purposes because of the rapid rate of complete bioconjugation with sulfhydryl groups, allowing for higher levels of cytotoxic drug incorporations. Attaching the other end of the polyethylene chain to a bead or solid support allows for easy separation of protein from other molecules in solution, provided these molecules do not also possess thiol groups.

Maleimide-functionalised polymers and liposomes exhibit enhanced ability to adhere to mucosal surfaces (mucoadhesion) due to the reactions with thiol-containing mucins. This could be applicable in the design of dosage forms for transmucosal drug delivery.