Pakistan Journal of Pharmaceutical Sciences Vol. 18, No.1, January 2005, pp.43-47 COMPETITIVE BINDING OF IBUPROFEN AND NAPROXEN TO BOVINE SERUM ALBUMIN: MODIFIED FORM OF DRUG–DRUG DISPLACEMENT INTERACTION AT THE BINDING SITE M. M. RAHMAN, M. H. RAHMAN* + AND N.N. RAHMAN* Pharmacy Discipline, Life Science School, Khulna University, Khulna-9208, Bangladesh *Department of Pharmacy, University of Dhaka, Dhaka-1000, Bangladesh The competitive binding characteristics of ibuprofen and naproxen with respect to binding site on bovine serum albumin (BSA) was studied by equilibrium dialysis method at pH 7.4 and 25°C. We studied the effect of one drug on the free concentration of another in vitro during concurrent administration. There was remarkable increase in free concentration of naproxen when ibuprofen (2x10 -5 M) was added to a 1:1 naproxen-BSA mixture (2x10 -5 M: 2x10 -5 M), suggesting that ibuprofen displaced naproxen from its binding sites. However free fraction of naproxen was not increased up to the level expected from direct competitive displacement. Free concentration of ibuprofen was hardly increased by naproxen when naproxen (2x10 -5 M) was added to the ibuprofen-BSA (1:1) mixture. But in both cases, in presence of ranitidine (site I specific probe), the free concentration of the displaced drug increased more prominently compared to that in absence of ranitidine. This result suggests that, ibuprofen displaces naproxen and vice versa from its high affinity binding site (site II) and the displaced drug rebounds to its low affinity binding site (site I) on BSA molecule. This form of modified displacement has been arbitrarily referred to as site-to-site displacement. Keywords: Equilibrium dialysis, bovine serum albumin, ibuprofen, and naproxen. INTRODUCTION The association of drugs with plasma protein and thus formation of drug plasma protein complex is often termed as protein binding. Different investigators have suggested that albumin has limited number of binding sites (Fehske et al., 1979; Hansen, 1981; Naher et al., 1997). On the basis of probe displacement method, it has been detected that there exist at least three relatively high affinity binding sites on BSA. These sites are commonly referred to as the warfarin, the benzodiazepine, and the digoxin site which are also denoted as site I, site II and site III, respectively (Fehske et al., 1981; Sudlow et al., 1975, 1976). Since the number of protein binding sites are limited, competition will exist between two drugs and the drugs with higher affinity will displace the other causing increased free drug concentration (Rahman, 1994). Displacement of drug is defined as reduction in the extent of binding of a drug to protein caused by competition of another drug, the displacer. When two drugs that are capable of binding at the same sites on the protein are administered concurrently this type of competitive displacement is more likely. Pharmacokinetic drug interactions occur when one drug alter the absorption, distribution, metabolism, or excretion of another, thus increasing or reducing the amount of drug available to produce its pharmacological effects. In addition to other mechanisms, displacement of one drug from its binding site on plasma protein by another causes pharmacokinetic drug interaction (Kedderis, 1997; Hooper, 1999; BNF, 2003). Drug-drug interactions, more specifically, displacement interaction will affect the concentrations of free drugs in the plasma. Since the pharmacological activity of a drug is a function of free drug concentration, the displacement of even a small amount of drug bound to plasma protein could produce considerable increase in activity even leading to toxicity. The ability of one drug to inhibit the binding of the other is a function of their relative concentrations, binding affinities, and specificity of binding (Koch-Weser et al., 1976). However, when studying drug-drug displacement interactions, the possibility of displacement of drug from one site to another site should be taken into account. We have demonstrated this effect between dexamethasone phosphate and testosterone phenyl propionate (Rahman et al, 2001). Thus there will exist a difference between free concentration with or without such type of displacement from one binding site to another. Few studies have been carried out on what happens to the displaced fraction of a drug, when a protein bound drug is displaced from its binding site by a second drug. The purpose of the present study is to observe the competitive effect of ibuprofen on binding of naproxen and vice versa when used simultaneously. The study also aims at examining whether these drugs undergo site-to-site displacement when administered concurrently. MATERIALS AND METHODS Ibuprofen and naproxen were obtained from Rhone Poulence Rorer, Bangladesh Limited and Novartis (former + Correspondence