Title:
Improving oral absorption via drug-loaded nanocarriers: absorption mechanisms, intestinal models and rational fabrication.

Abstract:
Although it is acknowledged that the main impediment of orally administered therapeutic agents is their extensive and changeable pre-systemic metabolism, low absorption and instability in harsh environment of the gastrointestinal (GI) tract are also main influential factors, resulting into inadequate and erratic drug bioavailability. To overcome these shortcomings, nanotechnology has offered new promising strategies to prevent and treat a wide variety of diseases by employing different oral drug-carrier structures capable to enhance therapeutic effects and minimize the toxicity of healthy organs or cells. This review, in general, elucidates some considerable features of in vitro oral drug delivery in three different parts. The first one summarizes the main challenges for oral drug delivery and available absorption mechanisms. The second part embodies an in-depth discussion on the role of the intestinal absorption models used to predict permeability, cellular uptake or even toxicity of nanoparticles, resulting into the design of nanocarriers with optimum efficacy for oral delivery. The third section of the literature is devoted, more particularly, to nanocarriers developed for oral absorption in the past few years, including the behavior of nanovehicles upon oral administration with respect to membrane permeability, retention properties and stability, as well as methods which may lengthen residence time in the GI environment or improve drug absorption.