Circular dichroism simulation shows a site-II-to-site-I displacement of human serum albumin-bound diclofenac by ibuprofen.
OBJECTIVE
The purpose of this study was to confirm the hypothesis that a site-II-to-site-I displacement takes place when some nonsteroidal anti-inflammatory drugs are displaced by another drug from their high-affinity binding site to a site of lower affinity on human serum albumin (HSA).


METHODS
Diclofenac, sodium salt, was used as a representative example because of its prominent reversal of the Cotton effect. Effects of site-specific drugs on the free fraction of diclofenac were determined by equilibrium dialysis, and effects on induced circular dichroism (CD) of diclofenac bound to HSA were studied by CD and CD simulation techniques.


RESULTS
Ibuprofen, a site-II-specific drug, altered the CD spectrum of the diclofenac-HSA complex at a molar ratio of 0.5 : 1 to that obtained at a higher ratio (5:1) without ibuprofen. The induced CD spectrum obtained in the presence of ibuprofen was very similar to one that assumed that all diclofenac displaced from its high-affinity binding site (site II) became rebound to a lower-affinity site (site I). The rebinding could be influenced by a free energy linkage between the two sites which would make site I (or parts thereof) more suitable for diclofenac binding.


CONCLUSIONS
We have confirmed the existence of a site II-to-site I displacement, which is very striking and pharmacologically important, because the concentration of unbound drug being displaced is much lower than expected for a competitive mechanism.
Yamasaki
K
K
Department of Pharmaceutics, Faculty of Pharmaceutical Sciences, Kumamoto University, Japan.
Rahman
M H
MH
Tsutsumi
Y
Y
Maruyama
T
T
Ahmed
S
S
Kragh-Hansen
U
U
Otagiri
M
M
D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000737:chemistry / Q000378:metabolism; D001665:Binding Sites / Q000187:drug effects; D001667:Binding, Competitive / Q000187:drug effects; D002942:Circular Dichroism / Q000379:methods*; D003198:Computer Simulation; D004008:Diclofenac / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D007052:Ibuprofen / Q000737:chemistry / Q000378:metabolism*; D008956:Models, Chemical; D011485:Protein Binding / Q000187:drug effects; D012709:Serum Albumin / Q000737:chemistry / Q000378:metabolism*
D000894:Anti-Inflammatory Agents, Non-Steroidal; D012709:Serum Albumin; D004008:Diclofenac; D007052:Ibuprofen