As for the US protocol, the COM of the drug or prodrug molecule of interest was pulled away from the COM of the PLGA matrix towards the aqueous solution and starting structures for the umbrella simulations were extracted at 0.2 nm intervals for a total of 60 adjacent windows. Before the production run, each starting configuration was equilibrated for 100 ns with constraints applied to the compound embedded within a PLGA matrix. Each window was next sampled for 1 µs, with the distance between the COM of the compound and that of the PLGA matrix restrained by using a biasing potential with a spring constant of 1,000 kJ mol -1 nm -2 . Calculations were performed using the stochastic dynamics (SD) integrator and a timestep of 2 fs. The weighted histogram analysis method (WHAM) was employed to combine the probability densities associated with sampled windows spanning the reaction coordinate and derive the free energy profile.