In vivo kinetic analysis of covalent binding between N-acetyl-L-cysteine and plasma protein through the formation of mixed disulfide in rats

Purpose. This investigation was undertaken to study the relationship between plasma drug clearance and covalent protein-binding kinetics of N-acetyl-L-cysteine (NAC). Methods. NAC was intravenously administered to rats via a bolus injection or continuous infusion. Plasma concentrations of protein-unbound and total NAC were analyzed using a compartment model, taking into consideration of the protein binding process, and the apparent first-order binding and dissociation rate constants (k(o)n and k(o)ff) were obtained. Results. Plasma total NAC after a bolus injection showed biphasic elimination with an inflection point at 1 hr. After 1 hr, NAC was largely present in the covalent protein-bound form. During the steady state of the infusion, approximately 30%-40% of plasma NAC bound with protein covalently. The k(o)n, k(o)ff, and the elimination rate constant of protein-unbound drug (k(e)) were 0.23, 0.57, and 4.3 hr(-1). The dissociation half-life of NAC from protein estimated from k(o)ff was in agreement with the elimination half-life of plasma total NAC. This suggests that the dissociation of NAC from protein rate-limited the drug elimination in plasma (k(o)ff < k(e)). Conclusion. We demonstrated that plasma total drug clearance is kinetically limited by covalent protein binding. The compartmental model described here is useful for analyzing its kinetics in vivo.