The typical PEG-maleimide-thiolreaction results in thiosuccinimide formation which is reversible, with PEG-maleimide elimination occurring slowly under biologically relevant conditions. The reversed reaction leads to the breakage of the maleimide-thiol linkage and the loss of PEG from the thiol/cysteine-containing molecules. If the maleimide-thiol chemistry does not work for your application, you may consider alternatives with much more stable bioconjugation.
Here are three examples:
- Carbonylacrylic PEG reagents as an improved alternative to PEG-maleimide. mPEG-Carbonylacrylic (mPEG-CA): Carbonylacrylic (CA) PEG reagents are for chemoselective cysteine bioconjugation and undergo rapid thiol Michael-addition under biocompatible conditions in stoichiometric amounts. Importantly, the conjugates formed between Carbonylacrylic PEG and thiol/cysteine-containing molecules are resistant to degradation in physiological conditions.
- PEG-Vinyl Pyridinium (PEG-VIP)reagents are a new class of thiol-selective PEGylation tools, and it offers an efficient and irreversible cysteine-selective bioconjugation method. The thiol selectivity and reactivity of the vinyl group is enabled by the quaternization of the nitrogen of the pyridine group, making the vinyl pyridinium an extremely reactive electrophile towards thiols. Importantly, the conjugates formed are resistant towards thiol exchange reactions or degradation.
- Self-hydrolyzing maleimide: Many antibody-drug conjugates (ADCs) are unstable
in vivo because they are formed from maleimide-containing components conjugated to reactive thiols. These thiosuccinimide linkages undergo two competing reactions in plasma: elimination of the maleimide through a retro-Michael reaction, which results in loss of drug-linker from the ADC, and hydrolysis of the thiosuccinimide ring, which results in a derivative that is resistant to the elimination reaction. Drug-linker incorporating a basic amino group adjacent to the maleimide, positioned to provide intramolecular catalysis of thiosuccinimide ring hydrolysis. This basic group induces the thiosuccinimide to undergo rapid hydrolysis at neutral pH and room temperature. Once hydrolyzed, the drug-linker is no longer subject to maleimide elimination reactions, preventing nonspecific deconjugation.