title: Thermodynamics of lipophilic drug binding to intestinal fatty acid binding protein and permeation across membranes.
abstract: Intestinal fatty acid binding protein (I-FABP) is present at high levels in the absorptive cells of the intestine (enterocytes), where it plays a role in the intracellular solubilization of fatty acids (FA). However, I-FABP has also been shown to bind to a range of non-FA ligands, including some lipophilic drug molecules. Thus, in addition to its central role in FA trafficking, I-FABP potentially serves as an important intracellular carrier of lipophilic drugs. In this study we provide a detailed thermodynamic analysis of the binding and stability properties of I-FABP in complex with a series of fibrate and fenamate drugs to provide an insight into the forces driving drug binding to I-FABP. Drug binding and selectivity for I-FABP are driven by the interplay of protein-ligand interactions and solvent processes. The Gibbs free energies (deltaGo) determined from dissociation constants at 25 degrees C ranged from -6.2 to -10 kcal/mol. The reaction energetics indicate that drug binding to I-FABP is an enthalpy-entropy driven process. The relationship between I-FABP stability and drug binding affinity was examined by pulse proteolysis. There is a strong coupling between drug binding and I-FABP stability. The effect of an I-FABP protein sink on the kinetics and thermodynamics of tolfenamic acid permeation across an artificial phospholipid membrane were investigated. I-FABP significantly decreased the energy barrier for desorption of tolfenamic acid from the membrane into the acceptor compartment. Taken together, these data suggest that the formation of stable drug-I-FABP complexes is thermodynamically viable under conditions simulating the reactant concentrations likely observed in vivo and maybe a significant biochemical process that serves as a driving force for passive intestinal absorption of lipophilic drugs.
authors.affiliation: Medicinal Chemistry and Drug Action, Monash Institute of Pharmaceutical Sciences, Monash University (Parkville Campus), 381 Royal Parade, Parkville, Victoria 3052, Australia. Tony.Velkov@pharm.monash.edu.au
authors.forename: Tony
authors.initials: T
authors.lastname: Velkov
mesh_terms: D000818:Animals; D001665:Binding Sites; D002995:Clofibric Acid / Q000737:chemistry / Q000494:pharmacology*; D020895:Enterocytes / Q000378:metabolism; D050556:Fatty Acid-Binding Proteins / Q000737:chemistry* / Q000378:metabolism*; D005227:Fatty Acids / Q000378:metabolism; D054361:Fenamates / Q000737:chemistry / Q000494:pharmacology*; D000960:Hypolipidemic Agents / Q000737:chemistry / Q000494:pharmacology*; D007408:Intestinal Absorption; D007413:Intestinal Mucosa / Q000378:metabolism; D008567:Membranes, Artificial; D011487:Protein Conformation; D051381:Rats; D013816:Thermodynamics
chemical_list: D050556:Fatty Acid-Binding Proteins; D005227:Fatty Acids; D054361:Fenamates; D000960:Hypolipidemic Agents; D008567:Membranes, Artificial; D002995:Clofibric Acid