Su1749 Biophysical Evidence That NSAIDs Form Mixed Micelles With Bile Acids as a Possible Mechanism of NSAID-Induced Lower Gut Injury

Abstract

Background: An increasing amount of evidence suggests that consumption of non-aspirin NSAIDs is associated with injury to the lower gut in humans, as has been established in rodent model systems. NSAID-induced lower gut injury may in fact explain the linkage between NSAID consumption and the development of ileitis, diverticulitis and inflammatory bowel disease. Evidence from our and other labs indicates that NSAID-induced injury to the lower gut is dependent on the presence of bile, based upon In Vitro (synthetic membranes and GI cells in culture) and In Vivo (rodent) studies, and we have hypothesized that NSAIDs may form toxic mixed micelles with bile acids (BAs). Aim: To test this hypothesis that NSAIDs and BAs spontaneously form mixed micellar aggregates we used a combination of powerful computational and spectroscopic techniques to gain insights into atomic associations between cholic acid (CA) and ibuprofen (IBU). Methods & Results: Using Molecular Dynamics (MD) simulations in tandem with Nuclear Magnetic Resonance (NMR) spectroscopy, we found that CA and IBU form mixed micelles in a concentration dependent manner, so that multiple CA molecules form ‘pseudo-rings' into which IBUs insert their hydrophobic tails. This ‘β-face-to-tail' packing between the steroid rings of CA and the alkyl carbons of IBU promotes the hydrophobic interaction of these two classes of amphipathic molecules. We also found that the IBU-CA mixed micelles have much larger sizes and higher net surface charges than simple CA micelles, the total charge being the sum of the single negative charge arising from each of the CA and IBU molecules. Furthermore, the IBU-CA mixed micelles adopt amore spherical shape that differs from the oblate shape formed by the pure CA clusters. These results highlight the power of a combined NMR/MD approach for characterizing the molecular interactions in the binarymixtures of NSAIDs and BAs. Conclusions: Our combined NMR/MD analysis yielded compelling structural evidence that NSAIDs and BAs form mixed micelles at concentrations greater than or equal to the CMC of the two species, which likely occur in the small intestinal lumen. The significance of the altered micellar morphology and charge density of these BA/NSAID mixed micelles to the drugs' cytotoxicity to the epithelium of the lower gut is an important area for future investigation. (Supported by NIH grants RC1 DK086304 and P30 DK56338.)