The Impact of Plasma Protein Binding on the Renal Transport of Organic Anions

Abstract

Drugs and xenobiotics bind to plasma proteins with varying degrees of affinity, and the amount of binding has a direct effect on free drug concentration and subsequent pharmacokinetics. Multiple active and facilitative transport systems regulate the excretion of anionic compounds from the blood in excretory and barrier tissues. Assumptions are made about in vivo substrate affinity and route of elimination based on data from plasma protein-free in vitro assays, particularly following expression of cloned transporters. Ochratoxin A (OTA), a fungal mycotoxin, is a high-affinity substrate for several renal secretory organic anion transporters (OATs), and literature suggests that this elimination pathway is the route of entry leading to proximal tubule-targeted toxicity. However, OTA is known to bind to several plasma proteins with a high affinity, particularly serum albumin, which may impact elimination. In this study, we have systematically examined the handling of OTA and other organic anions, estrone sulfate (ES) and methotrexate (MTX), by OATs in the presence of serum albumin. Increasing concentrations of albumin markedly reduced uptake of OTA by both Xenopus laevis oocytes expressing OATs 1, 3, and 4 and organic anion-transporting polypeptide 1. For all transporters tested, virtually all mediated OTA uptake was eliminated by an albumin concentration equivalent to 10% of that present in the blood plasma. Thus, OTA uptake is dependent on the free substrate concentration and severely limited by binding to human serum albumin. MTX and ES uptake were likewise dependent on free concentration.The proximal tubule is the primary site for renal secretory elimination of drugs, toxins, and their metabolites from the body. In addition, the proximal tubule exhibits a very high metabolic demand since it mediates not only secretion of drugs and xenobiotics but also the reabsorption of 60 to 80% of the solute and water filtered by the glomerulus (Schnellmann, 2001). Thus, the proximal tubule is particularly susceptible to injury by drugs and toxins.Elimination of drugs, toxins, and their metabolites is handled by several facilitative and active epithelial transport proteins expressed in the proximal tubule. Since the advent of cloning and the extensive study of these transporters in vitro, the impact of plasma protein has generally been overlooked when predicting physiological roles of transporters in the renal elimination of drugs. Nevertheless, plasma proteins are abundant, and the potential for altered transporter efficacy is substantial. For example, serum albumin, a major transport/carrier plasma protein [4% (w/v)], makes up approximately 60% of total plasma protein (Peters, 1996;Gekle, 2005). Indeed, it has long been known that to predict the efficacy of a drug in vivo, it is critical to account for plasma protein binding because it will determine the availability of free drug, its half-life, and its subsequent renal elimination (Weiner et al., 1964).Certainly, the impact of binding will vary depending upon the af...

Organic Anion Transporter 1 Is Inhibited by Multiple Mechanisms and Shows a Transport Mode Independent of Exchange

“…Given telmisartan's high degree of plasma protein binding (;99.5%; Stangier et al, 2000), the plasma protein likely acted as a sink to more quickly remove telmisartan from the cells. These data are consistent with those of Bow et al (2006), who showed that plasma protein can profoundly influence the interaction of ligands with renal organic anion transporters, including OAT1. Together, these data indicate that telmisartan binding to OAT1 and its inhibitory effect are reversible with time and that the relatively long-lasting inhibitory effect is due to the slow rate at which telmisartan leaves the cells.…”

Prediction of Human Renal Clearance from Preclinical Species for a Diverse Set of Drugs That Exhibit Both Active Secretion and Net Reabsorption

“…Bow et al (2006) recently demonstrated that the in vitro uptake of the acid molecule Ochratoxin A by organic anion transporters (OATs) 1, 3, and 4 is virtually eliminated by an albumin concentration equivalent to 10% of that present in plasma. These recent findings suggest that the extent of plasma protein binding can affect the active secretion of drugs.…”

Physiology, structure, and regulation of the cloned organic anion transporters

“…For this reason, Bow et al (2006) assessed the impact of protein binding on several OAT transporters expressed in vitro. These experiments demonstrated that uptake of two highly albumin bound anions (OTA and ES, K b >10 5 ) is very much less than that seen in the absence of albumin, i.e., that uptake reflects the limited free fraction of substrate in the presence of albumin.…”