A pellet or pellets having an average particle size in the range from 300 to 1,100 μm, comprising a pharmaceutically active substance embedded in a polymer matrix of one or more water-insoluble polymers, wherein the polymer matrix additionally contains 10 to 90% by weight of an anionic polymer and with the proviso that the pellets release no more than 10% of the active compound contained in the release test according to USP in artificial gastric juice at pH 1.2 after 120 min and release at least 50% of the active compound contained after altogether a further 300 min at pH 6.8 and/or pH 7.5.