Disclosed is a drug delivery system for administration of at least one pharmaceutically active substance that is a cationic amphiphile by itself, which pharmaceutically active substance is present in the drug delivery system in particles of a complex between said pharmaceutically active substance and a sodium salt of the methyl ester of N-all-trans-retinoyl cysteic acid, a sodium salt of methyl ester of N-13-cis-retinoyl cysteic acid, or a combination thereof, said particles of said complex having an effective average particle size of less than about 100 nm, wherein - the particles of said complex are essentially amorphous - the particles of said complex are entrapped in nanoparticles formed of a sodium salt of the methyl ester of N-all-trans-retinoyl cysteic acid, sodium salt of methyl ester of N-13-cis-retinoyl cysteic acid, or a combination thereof and - the weight-to-weight ratio of said sodium salt of methyl ester of N-all-transretinoyl cysteic acid, sodium salt of methyl ester of N-13-cis-retinoyl cysteic acid, or combination thereof, to the weight of said complex is in the range from about 0.5:1 to about 20:1. Also disclosed is a pharmaceutical composition comprising the drug delivery system and the use of a sodium salt of the methyl ester of N-all-trans-retinoyl cysteic acid, a sodium salt of the methyl ester of N-13-cis-retinoyl cysteic acid, or a combination thereof, in the preparation of the drug delivery system and a method for preparing the drug delivery system and the use of the drug delivery system in the manufacture of a medicament for the treatment of cancer.
