ABSTRACT
INTRODUCTION The most convenient and used route of administration of drugs for targets within the systemic circulation is oral administration. To obtain a successful oral formulation, the drug has to have properties for entering the circulation in amounts sufficient to produce a therapeutic effect. Low oral availability of a drug is not only a problem for exposure and lack of efficacy but is also often associated with significant variability, both between different treatment occasions and between different patients/individuals. In addition, from a delivery perspective, drugs with problematic absorption characteristics are also difficult to combine with an appropriate formulation, since changes in the formulation behavior might not always result in improved absorbability. Knowledge of the reasons for low and variable absorption/bioavailability is, therefore, of importance for efficacy and to be able to select the candidate with the most appropriate properties for development, ensuring fast and efficient progress in clinical studies.
