[Show abstract][Hide abstract] ABSTRACT: Raman and NMR studies are performed to characterize the solution structures of complexes between heparin and a group of amidated acids, which act as delivery agents that facilitate the gastrointestinal absorption of orally administered heparin. At concentrations typically employed for the oral drug delivery of heparin, the contact points between heparin complexed with the delivery agents include points near the OH groups of heparin. The results suggest that heparin interacts rather nonspecifically with the amidated acids as monomers and with self-associated complexes of the delivery agents. It is also found that the carboxyl groups of at least one of the bioactive delivery agents easily protonates when it forms complexes with itself or heparin. This attribute may be one reason why this class of compounds is effective in the oral delivery of heparin.
[Show abstract][Hide abstract] ABSTRACT: The spectroscopic and solution properties of a series of amidated acids (delivery agents), which promote the gastrointestinal absorption of USP heparin and other drugs that show poor oral bioavailability, are investigated using Raman and NMR spectroscopy. The results show evidence for self-association at low concentrations of delivery agents that increases as the concentration of the delivery agent is increased. The self-associate is characterized by ring-ring stacking interactions, and the best geometrical arrangement for the stacking is the parallel-shifted arrangement of the rings. In addition, the amide group participates in the formation of intermolecular hydrogen bonds in the self-associate. Unlike the rigid ring, the tails of these delivery agents remain relatively flexible in the self-associate. It is suggested that the limited solubility of the delivery agents at physiological pH arises from a percentage of protonated carboxyls. Their presence promotes the formation of intermolecular hydrophobic and ring stacking interactions, which are otherwise weakened by an ionized carboxyl group.