Structural Insights into the Competitive Binding of Diclofenac and Naproxen by Equine Serum Albumin

The binding modes to equine serum
albumin (ESA) of two nonsteroidal
anti-inflammatory drugs (NSAIDs), diclofenac (Dic) and naproxen (Nps),
were studied by X-ray crystallography and isothermal titration calorimetry.
On the basis of the crystal structure of ESA/Dic determined to a resolution
of 1.92 Å and the structure of the previously described ESA/Nps
complex (2.42 Å), it was found that both NSAIDs bind within drug
site 2 (DS2) of ESA and both occupy secondary binding sites in separate
cavities of domain II (Nps) and domain III (Dic). The two structures
of the ternary complex ESA/Dic/Nps, obtained by competitive cocrystallization
(2.19 Å) and through a displacement experiment (2.35 Å),
were determined to investigate possible competition of these widely
used pharmaceutical drugs in binding to ESA. In these complexes Nps
occupies the DS2 pocket common for both drugs, whereas the other distinct
binding sites of Dic and Nps remain unaffected. These results suggest
that combined application of both drugs may result in increased concentration
of free diclofenac in plasma.