Sink conditions do not guarantee the absence of saturation effects

Limited drug solubility effects can play a major role for the control of drug release from a variety of drug delivery systems, e.g. tablets, pellets, implants and microparticles. Importantly, such saturation effects can occur inside and/or outside the dosage form. This is true for drug release occurring in vitro and in vivo. In vivo, released drug might be rapidly transported away from the site of administration, e.g. due to absorption into the blood stream. In vitro, many frequently used experimental set-ups are "closed systems" and eventually drug saturation effects in the surrounding release medium might artificially occur, "falsifying" the resulting release kinetics. To avoid such errors, often "sink conditions" are provided: Selecting appropriate release medium volumes, renewal rates and/or "open systems", it is assured that the maximum concentration in the release medium does not exceed about 20% of the drug solubility. However, this does not mean that drug saturation effects within the dosage form are also avoided. It should clearly be distinguished between potential limited drug solubility effects inside versus outside the drug delivery system. This articles aims at: (i) giving a brief overview on the underlying physico-chemical phenomena involved in drug dissolution and drug release, (ii) clarifying some key terms, and (iii) presenting several examples of dosage forms in which drug saturation effects within the system are of importance, even when providing sink conditions in the surrounding bulk fluid. Interestingly, this can also include highly hydrated delivery systems containing freely water-soluble drugs.