Tuning H₂S Release by Controlling Mobility in a Micelle Core

Drug delivery from polymer micelles has been widely studied, but methods to precisely tune rates of drug release from micelles are limited. Here, the mobility of hydrophobic micelle cores was varied to tune the rate at which a covalently bound drug was released. This concept was applied to cysteine-triggered release of hydrogen sulfide (H₂S), a signaling gas with therapeutic potential. In this system, thiol-triggered H₂S donor molecules were covalently linked to the hydrophobic blocks of self-assembled polymer amphiphiles. Because release of H₂S is triggered by cysteine, diffusion of cysteine into the hydrophobic micelle core was hypothesized to control the rate of release. We confirmed this hypothesis by carrying out release experiments from H₂S-releasing micelles in varying compositions of EtOH/H₂O. Higher EtOH concentrations caused the micelles to swell, facilitating diffusion in and out of their hydrophobic cores and leading to faster H₂S release from the micelles. To achieve a similar effect without addition of organic solvent, we prepared micelles with varying core mobility via incorporation of a plasticizing co-monomer in the core-forming block. The glass transition temperature (T_g) of the core block could therefore be precisely varied by changing the amount of the plasticizing co-monomer in the polymer. In aqueous solution under identical conditions, the release rate of H₂S varied over 20-fold (t_½ = 0.18 - 4.2 h), with the lowest T_g hydrophobic block resulting in the fastest H₂S release. This method of modulating release kinetics from polymer micelles by tuning core mobility may be applicable to many types of physically encapsulated and covalently linked small molecules in a variety of drug delivery systems.