Oral drug delivery biological barriers

Classic in vitro cell culture models were used earlier to test the permeability of drugs themselves (9). Nowadays, these systems display not only an alternative to animal testing in respect to ethical concerns, but are also applied as advanced models to develop new delivery strategies and to study drug delivery properties of nanoparticulate carriers or chemical delivery enhancers. Thereby, interactions with the biological barrier, specifically binding, uptake, and transport can be evaluated in a well-defined and standardized system. While oral delivery of drugs is still the commonly used and most accepted method of choice due to the... 

Figure 9 HPMA copol5nner conjugates designed (i) to deliver drug extracellularly when cleaved by an external trigger (e.g., azo linkers), (ii) to transfer drugs across a biological barrier (e,g., oral peptide vaccine delivery), (iii) for lysosomotropic delivery of anticancer conjugates, or (iv) to deliver macromolecular drugs into the c5d osol via the endosomotropic route.

The skin is biologically intended to be a barrier. Evading this barrier is not easy, because drugs must traverse dead epithelium and live dermis the former is hydrophobic, while lipophilic drugs tends to form a reservoir in the latter. As in oral transmucosal administration, potent drugs, with modest requirements for mass absorbed and reasonable lipophilicity, are the best candidates for transdermal delivery. Fentanyl, nicotine, and scopolamine are good examples. 

---