Transporters in Regulatory Science: Notable Contributions from Dr. Giacomini in the Past Two Decades

Utilising Endogenous Biomarkers in Drug Development to Streamline the Assessment of Drug-Drug Interactions Mediated by Renal Transporters: A Pharmaceutical Industry Perspective

The renal secretion of many drugs is facilitated by membrane transporters, including organic cation transporter 2, multidrug and toxin extrusion protein 1/2-K and organic anion transporters 1 and 3. Inhibition of these transporters can reduce renal excretion of drugs and thereby pose a safety risk. Assessing the risk of inhibition of these membrane transporters by investigational drugs remains a key focus in the evaluation of drug-drug interactions (DDIs). Current methods to predict DDI risk are based on generating in vitro data followed by a clinical assessment using a recommended exogenous probe substrate for the individual drug transporter. More recently, monitoring plasma-based and urine-based endogenous biomarkers to predict transporter-mediated DDIs in early phase I studies represents a promising approach to facilitate, improve and potentially avoid conventional clinical DDI studies. This perspective reviews the evidence for use of these endogenous biomarkers in the assessment of renal transporter-mediated DDI, evaluates how endogenous biomarkers may help to expand the DDI assessment toolkit and offers some potential knowledge gaps. A conceptual framework for assessment that may complement the current paradigm of predicting the potential for renal transporter-mediated DDIs is outlined.

Topotecan and Ginkgolic Acid Inhibit the Expression and Transport Activity of Human Organic Anion Transporter 3 by Suppressing SUMOylation of the Transporter

Organic anion transporter 3 (OAT3), expressed at the basolateral membrane of kidney proximal tubule cells, facilitates the elimination of numerous metabolites, environmental toxins, and clinically important drugs. An earlier investigation from our laboratory revealed that OAT3 expression and transport activity can be upregulated by SUMOylation, a post-translational modification that covalently conjugates SUMO molecules to substrate proteins. Topotecan is a semi-synthetic derivative of the herbal extract camptothecin, approved by the FDA to treat several types of cancer. Ginkgolic acid (GA) is one of the major components in the extract of Ginkgo biloba leaves that has long been used in food supplements for preventing dementia, high blood pressure, and supporting stroke recovery. Both topotecan and GA have been shown to affect protein SUMOylation. In the current study, we tested our hypothesis that topotecan and GA may regulate OAT3 SUMOylation, expression, and transport function. Our data show that the treatment of OAT3-expressing cells with topotecan or GA significantly decreases the SUMOylation of OAT3 by 50% and 75%, respectively. The same treatment also led to substantial reductions in OAT3 expression and the OAT3-mediated transport of estrone sulfate, a prototypical substrate. Such reductions in cell surface expression of OAT3 correlated well with an increased rate of OAT3 degradation. Mechanistically, we discovered that topotecan enhanced the association between OAT3 and the SUMO-specific protease SENP2, a deSUMOylation enzyme, which contributed to the significant decrease in OAT3 SUMOylation. In conclusion, this study unveiled a novel role of topotecan and GA in inhibiting OAT3 expression and transport activity and accelerating OAT3 degradation by suppressing OAT3 SUMOylation. During comorbidity therapies, the use of topotecan or Ginkgo biloba extract could potentially decrease the transport activity of OAT3 in the kidneys, which will in turn affect the therapeutic efficacy and toxicity of many other drugs that are substrates for the transporter.

Albumin-bound kynurenic acid is an appropriate endogenous biomarker for assessment of the renal tubular OATs-MRP4 channel

Renal tubular secretion mediated by organic anion transporters (OATs) and the multidrug resistance-associated protein 4 (MRP4) is an important means of drug and toxin excretion. Unfortunately, there are no biomarkers to evaluate their function. The aim of this study was to identify and characterize an endogenous biomarker of the renal tubular OATs-MRP4 channel. Twenty-six uremic toxins were selected as candidate compounds, of which kynurenic acid was identified as a potential biomarker by assessing the protein-binding ratio and the uptake in OAT1-, OAT3-, and MRP4-overexpressing cell lines. OAT1/3 and MRP4 mediated the transcellular vectorial transport of kynurenic acid in vitro. Serum kynurenic acid concentration was dramatically increased in rats treated with a rat OAT1/3 (rOAT1/3) inhibitor and in rOAT1/3 double knockout (rOAT1/3-/-) rats, and the renal concentrations were markedly elevated by the rat MRP4 (rMRP4) inhibitor. Kynurenic acid was not filtered at the glomerulus (99% of albumin binding), and was specifically secreted in renal tubules through the OAT1/3-MRP4 channel with an appropriate affinity (Km) (496.7 μM and 382.2 μM for OAT1 and OAT3, respectively) and renal clearance half-life (t1/2) in vivo (3.7 ± 0.7 h). There is a strong correlation in area under the plasma drug concentration-time curve (AUC0-t) between cefmetazole and kynurenic acid, but not with creatinine, after inhibition of rOATs. In addition, the phase of increased kynurenic acid level is earlier than that of creatinine in acute kidney injury process. These results suggest that albumin-bound kynurenic acid is an appropriate endogenous biomarker for adjusting the dosage of drugs secreted by this channel or predicting kidney injury.

Kidney Drug Transporters in Pharmacotherapy

The kidney functions not only as a metabolite elimination organ but also plays an important role in pharmacotherapy. The kidney tubule epithelia cells express membrane carriers and transporters, which play an important role in drug elimination, and can determine drug nephrotoxicity and drug-drug interactions, as well as constituting direct drug targets. The above aspects of kidney transport proteins are discussed in the review.