In Vitro Plasma Protein Binding of Zileuton and its N-Dehydroxylated Metabolite

Abstract

An ultrafiltration technique or equilibrium dialysis has been used to study the in vitro human plasma protein binding of racemic zileuton, its individual enantiomers, and its pharmacologically inactive metabolite N-dehydroxyzileuton.The plasma protein binding of zileuton and N-dehydroxyzileuton over the concentration range of 0.1 to 100 mg/L averaged 93.1 ± 0.22 and 92.0 ± 0.12%, respectively. However, there appeared to be a stereoselective effect, with the R( +) enantiomer of zileuton demonstrating greater binding to plasma proteins than the S(-) enantiomer (96 vs 88%, respectively) . Zileuton was bound to both human serum albumin (40 gIL) and ell-acid glycoprotein (l g/L), although binding affinity to albumin was approximately 3-fold greater. Displacement interactions of zileuton with warfarin, salicylate, theophylline, naproxen, ibuprofen, prednisone, and terfenadine were minimal. The blood to plasma concentration ratio for zileuton and N-dehydroxyzileuton ranged from 0.65 to 0.68, indicating that these compounds were mainly distributed in the plasma.Thus, zileuton is approximately 93% bound to plasma proteins at expected therapeutic concentrations in vitro, and this figure is largely unaffected by several commonly prescribed agents with which the drug may be coadministered.

Mechanism-Based Inhibition of Human Liver Microsomal Cytochrome P450 1a2 by Zileuton, a 5-Lipoxygenase Inhibitor

“…Use of C max (19 M) and C ave (9.0 M) values for total zileuton in plasma (Awni et al, 1995a) resulted in overestimates of AUC changes (data not shown). After correction for binding of zileuton (f u ϭ 0.069) to human plasma proteins (Machinist et al, 1995a), more reasonable estimates of AUC ratios were obtained (Fig. 4).…”

“…Because dmd.aspetjournals.org the concentrations of both enantiomers in plasma are similar (Wong et al, 1995), the in vitro data suggest that the (S)-enantiomer may play more of a role in vivo. However, f u is higher (0.122 versus 0.037) for the (R)-enantiomer (Machinist et al, 1995a), which could offset the difference in the k inact /K I ratio. Based on the results of this study, it is concluded that zileuton is a dmd.aspetjournals.org weak reversible inhibitor of P450 activities in human liver microsomes.…”

“…The steady-state concentrations of zileuton in plasma (C max and C ave ) were corrected for binding (f u ϭ 0.069) to human plasma proteins in vitro (Awni et al, 1995a;Machinist et al, 1995a). No attempt was made to estimate the concentration of zileuton in the liver or in the portal vein following an oral dose (Ito et al, 1998).…”

Measurement of drug–protein binding by immobilized human serum albumin-HPLC and comparison with ultrafiltration

“…Among those, equilibrium dialysis, gel-filtration, ultra-filtration and ultra-centrifugation [16,[24][25][26][27][28] have been conventionally and most commonly used. These conventional methods suffer from long analysis time.…”

Measurements of drug–protein binding by using immobilized human serum albumin liquid chromatography–mass spectrometry

“…Among those, equilibrium dialysis and ultrafiltration followed by HPLC analysis have been conventionally and most commonly used [7][8][9][10]. Equilibrium dialysis separates molecules across a semipermeable membrane according to molecular size (weight) by utilizing the driving force of concentration differential between solutions on each side of the membrane.…”