A drug delivery system (DDS) for administration of a water soluble, cationic, and amphiphilic pharmaceutically active substance (API) which DDS comprises poorly water soluble nanoparticles formed by the API together with a Na-salt of N-all- trans-retinoyl cysteic acid methyl ester and/or a Na-salt of N-13-cis-retinoyl cys- teic acid methyl ester. A pharmaceutical composition comprising such a DDS. Methods for preparation of such a DDS and such a pharmaceutical composition. Use of such a DDS and pharmaceutical composition for treatment of cancer.
