A solution for a model of mass diffusion from a drug-eluting stent to the arterial wall is addressed. The coating layer is described as a porous reservoir where the drug is initially loaded in polymer-encapsulated solid-phase, and then released both to the coating and to the arterial tissue in a liquid-phase. The endothelium, intima, internal elastic lamina and media are all treated as homogeneous porous media and the drug transfer through them is modelled by a non-homogeneous set of coupled partial differential equations that describe a local mass non-equilibrium diffusion problem. Drug concentration levels and mass profiles in each layer at various times are computed as a spectral decomposition: numerical results show a delayed release depending on the physico-chemical drug properties combined with the microstructure of the polymeric-coated stents.