The release and absorption profile of an oral medication is influenced by the physicochemical properties of the drug and its formulation, as well as by the anatomy and physiology of the gastrointestinal (GI) tract. During drug development the bioavailability of a new drug is typically assessed in early clinical studies in a healthy adult population. However, many disease conditions are associated with an alteration of the anatomy and/or physiology of the GI tract. The same holds true for some subpopulations, such as paediatric or elderly patients, or populations with different ethnicity. The variation in GI tract conditions compared to healthy adults can directly affect the kinetics of drug absorption, and thus, safety and efficacy of an oral medication. This review provides an overview of GI tract properties in special populations compared to healthy adults and discusses how drug absorption is affected by these conditions. Particular focus is directed towards non-disease dependent conditions (age, sex, ethnicity, genetic factors, obesity, pregnancy), GI diseases (ulcerative colitis and Crohn's disease, celiac disease, cancer in the GI tract, Roux-en-Y gastric bypass, lactose intolerance, Helicobacter pylori infection, and infectious diseases of the GI tract), as well as systemic diseases that change the GI tract conditions (cystic fibrosis, diabetes, Parkinson's disease, HIV enteropathy, and critical illness). The current knowledge about GI conditions in special populations and their impact on drug absorption is still limited. Further research is required to improve confidence in pharmacokinetic predictions and dosing recommendations in the targeted patient population, and thus to ensure safe and effective drug therapies.

Patient acceptability of a medicinal product is a key aspect in the development and prescribing of medicines. Children and older adults differ in many aspects from the other age subsets of population and require particular considerations in medication acceptability. This review highlights the similarities and differences in these two age groups in relation to factors affecting acceptability of medicines. New and conventional formulations of medicines are considered regarding their appropriateness for use in children and older people. Aspects of a formulation that impact acceptability in these patient groups are discussed, including, for example, taste/smell/viscosity of a liquid and size/shape of a tablet. A better understanding of the acceptability of existing formulations highlights opportunities for the development of new and more acceptable medicines and facilitates safe and effective prescribing for the young and older populations.

Objectives: The aim of this review was to map the currently available evidence on acceptability of oral paediatric medicines to aid in the selection of suitable platform formulations for the development of new acceptable paediatric products. Methods: This process used a defined search strategy of indexed publications and included methods to assess the quality of the evidence retrieved. Key findings: Taste/palatability was the most extensively studied area of paediatric medicine acceptability yet standard methods or criteria that define what is classed as acceptable to children is still to be defined. There have been many reports on the acceptability of medicines to paediatric populations yet major gaps in the acceptability knowledge base exist including the shape and dimensions of tablets, minitablets and capsules swallowed whole in infants and children; size and overall volume of multiparticulates; volume of liquids completely swallowed in infants and children; duration of retention within the oral cavity, size and taste of orodispersible tablets, lozenges and chewable tablets and the number of solid units dosed at each time point. Conclusions: The review highlights where further information is required to support knowledge around acceptability of age-appropriate medicines. An algorithm to aid in selection of a formulation that is likely to be acceptable based on the age range to be treated by the medicine is presented as a result of this review.