Targeting approaches to oral drug delivery

Delivery of pharmaceuticals, particularly biotechnology products such as proteins, peptides, genes, oligonucleotides and vaccines, via the oral route remains problematic to this day. Instability in the gastrointestinal environment and poor permeability across the intestinal epithelial cell barrier contribute to poor oral bioavailability for many of these compounds. Current targeting strategies to overcome these issues are focused on three-part systems in which the drug (i) is loaded into a protective particulate carrier (ii) which is coated with target-specific ligands (iii) which mediate site-specific delivery of the drug-carrier complex. Protection from gastrointestinal degradative processes combined with site-specific delivery to absorptive regions of the intestinal tract is purported to yield high local concentrations of the drug of choice in close proximity with the epithelial cell layer and hence, transport across that barrier through a variety of mechanisms. This review examines the impact of cutting-edge technologies such as genomics and combinatorial chemistry on targeted oral drug delivery strategies. The explosion in rate of identification of new targets using genomics, together with high-throughput screening for target-specific ligands using combinatorial chemistry and phage display, has the potential to revolutionise this field. Particular reference is made to advances associated with targeted delivery of vaccines to M-cells or antigen-presenting cells in gut-associated lymphoid tissues.