A drug delivery system (DDS) for administration of a water soluble, cationic, and amphiphilic pharmaceutically active substance (API) which DDS comprises amorphous particles of <；100 nm of a poorly water soluble complex of the API with a Na-salt of N-all-trans-retinoyl cysteic acid methyl ester and/or a Na-salt of N-13-cis-retinoyl cysteic acid methyl ester, which particles are entrapped in nanoparticles formed a Na-salt of N-all-trans-retinoyl cysteic acid methyl ester and/or a Na-salt of N-13-cis-retinoyl cysteic acid methyl ester, the w/w-ratio of Na-salt of N-alltrans-retinoyl cysteic acid methyl ester and/or a Na-salt of N-13-cis-retinoyl cysteic acid methyl ester to the complex is about 0.5:1 to about 20:1. 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.
