Retro-inverted forms of GIT targeting agents that target specific receptor sites in vivo and/or promote uptake of active agents and/or enhance active agent delivery across the GIT into the systemic circulation are provided. These retro-inverted peptides and compositions containing these retro-inverted peptides can be used to deliver an active agent, such as a drug or a drug-containing nano- or microparticle for treatment of a condition in a subject in need of the drug, in vivo. Additionally, the invention provides antibodies which are capable of immunospecifically binding the retro-inverted peptides.