Manipulation of Glutathione-Mediated Degradation of Thiol–Maleimide Conjugates

The retro Michael-type addition and thiol exchange of thioether succinimide click linkages in response to thiol-containing environments offers a novel strategy for the design of glutathione-sensitive degradable hydrogels for controlled drug delivery. Here we characterize the kinetics and extent of the retro Michael-type addition and thiol exchange with changes in both the p K a of the thiols and the identity of N-substituents of maleimides. A series of N-substituted thioether succinimides were prepared through typical Michael-type addition. Model studies ( 1 H NMR, HPLC) of 4-mercaptophenylacetic acid (MPA, p K a 6.6) conjugated to N -ethyl maleimide (NEM), N -phenyl maleimide (NPM), or N -aminoethyl maleimide (NAEM) and then incubated with glutathione showed half-lives of conversion from 3.1 to 18 h, with extents of conversion from approximately 12% to 90%. The variations in the rates of exchange and hydrolytic ring opening appear to be mediated by resonance effects, electron-withdrawing capacity of the N-substituted moiety, as well as the potential for intramolecular catalytic hydrogen bonding of amine substituents with water (particularly in the case of ring opening). Further model studies of 4-mercaptohydrocinnamic acid (MPP, p K a 7.0) and N -acetyl- l -cysteine (NAC, p K a 9.5) conjugated to selected N-substituted maleimides and then incubated with glutathione showed half-lives of conversion from 3.6 to 258 h, with extents of conversion from approximately 1% to 90%. A higher p K a of the thiol decreased the rate of the exchange reaction and limited the impact of other electronic effects of N-substituents on the extents of conversion. Additional factors affecting the conversion kinetics were studied on NEM conjugates. The kinetics of the retro Michael-type addition and exchange reaction were not hindered by thiol traps of lower p K a, but were retarded in conditions of lower pH. These studies shed light into details of thiol and maleimide design that could be used to tune the rates of degradation of drug and polymer conjugates for a variety of applications.