The invention relates to pellets having a mean particle size of between 300 and 1100 µm, containing a pharmaceutically active substance integrated into a polymer matrix consisting of at least one water-insoluble polymer. The invention is characterised in that the polymer matrix also contains between 10 and 90 wt. % of an anionic polymer, in the release test according to USP, provided that the pellets release no more than 10 % of the active substance in the artificial gastric juice at a pH of 1.2 after 120 min, and release at least 50 % of the active substance after a total of another 300 min at a pH of 6.8 and/or 7.5.
