Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Determination of intestinal permeability of rigosertib (ON 01910.Na, Estybon): correlation with systemic exposure

Objectives

Rigosertib (ON 01910.Na, Estybon) is a novel, anticancer agent undergoing phase 3 clinical trials for a lead indication against myelodysplastic syndromes (MDS). In this research, the permeability of rigosertib was evaluated using the in-situ perfused rat intestine (IPRI) model to support development of an oral formulation for rigosertib for treating cancer patients.

Methods

Experiments (n = 6 per group) were conducted using male Sprague-Dawley rats. Studies evaluated permeability across various intestinal segments and assessed the dose-linearity of absorption over the entire intestinal length. Drug concentrations in the portal and jugular vein were collected to correlate permeability parameters with presystemic and systemic exposure.

Key findings

Rigosertib permeability was highest in the jejunum, although parameter estimates indicated that rigosertib was a medium permeability compound. The compound displayed nonlinear absorption in the IPRI model, suggesting a saturable transport process. Transport inhibition studies using Caco-2 cells demonstrated that rigosertib was a P-glycoprotein (P-gp) substrate. Absolute bioavailability of rigosertib (10 and 20 mg/kg, 1-h infusion) in rats was estimated to be 10–15%. However, the fraction absorbed in humans predicted from IPRI data (52%) was consistent with published clinical data for rigosertib (35% oral bioavailability).

Conclusions

The results of this research indicated that rigosertib is a promising candidate for oral delivery. Further studies are needed to evaluate the potential impact of P-gp and other intestinal transporters on the oral absorption of this promising anticancer agent.

Renal excretion of apricitabine in rats: ex vivo and in vivo studies

Apricitabine (ATC) is a novel nucleoside reverse transcriptase inhibitor undergoing phase 2/3 clinical development for the treatment of HIV infection. In this investigation, the renal handling of ATC was evaluated in the isolated perfused rat kidney (IPK) model with follow-up in vivo studies. IPK experiments were performed to characterize the renal excretion of ATC, to probe mechanisms of ATC excretion using known inhibitors of organic cation (cimetidine) and organic anion (probenecid) transport systems, and to screen for potential drug-drug interactions between ATC and clinically relevant medications (dapsone, metformin, pentamidine, stavudine, tenofovir and ritonavir). ATC demonstrated net tubular secretion in the IPK with a baseline excretion ratio (XR) of 2.1 ± 0.56. ATC XR decreased 3.6-fold in the presence of cimetidine and 2-fold in the presence of probenecid. Among the clinically relevant medications, metformin produced the greatest inhibitory effect on ATC excretion. In vivo studies were conducted in rats to evaluate ATC disposition upon co-administration with compounds that showed a significant effect on ATC clearance in the IPK model. Co-administration of cimetidine and trimethoprim significantly reduced ATC renal clearance, but resulted in only a moderate increase in plasma exposure. Metformin had no apparent effect on ATC clearance in rats. These findings indicate that the IPK model is more sensitive to secretory inhibition as compared to in vivo. The medications screened showed minimal effects on ATC renal excretion in the IPK, and should thus be excluded as potential in vivo interactants. Overall, this study generated important information on renal handling of ATC to support its development and commercialization.

Evaluation of steady-state pharmacokinetic interactions between ritonavir-boosted BILR 355, a non-nucleoside reverse transcriptase inhibitor, and lamivudine/zidovudine in healthy subjects

WHAT IS KNOWN AND OBJECTIVE

BILR 355 is a second generation non-nucleoside reverse transcriptase inhibitor. It has shown promising in vitro anti-HIV-1 activities and favourable human pharmacokinetic properties after co-administration with ritonavir (RTV). Lamivudine (3TC) is a nucleoside reverse transcriptase inhibitor. It is excreted predominantly in urine by a transporter-mediated pathway. These two drugs are likely to be given together to HIV-infected patients. The objective of this study was to investigate any steady-state pharmacokinetic interactions between RTV-boosted BILR 355 and 3TC/zidovudine (ZDV).

METHODS

This was a randomized, open label, prospective study. In group A, 39 healthy subjects were given 3TC/ZDV (150 mg/300 mg) twice daily (b.i.d.) for 7 days, and then BILR 355 and RTV (BILR 355/r, 150 mg/100 mg) were co-administered with this regimen for an additional 7 days. Intensive blood samples were taken on days 7 and 14 for pharmacokinetic assessments. In group B, 12 healthy subjects were given BILR 355/r (150 mg/100 mg) b.i.d. for 7 days. The pharmacokinetic data from group B were pooled with data from group B subjects in other similar studies performed in parallel (BILR 355 alone group in BILR 355 drug-drug interaction studies with tipranavir, lopinavir/RTV, and emtricitabine/tenofovir DF; BILR 355 regimen was the same).

RESULTS AND DISCUSSION

After co-administration with BILR 355/r, the AUC(12,ss) and C(max,ss) of 3TC increased by 45% and 24%, respectively; the elimination half-life (t(1/2) ,ss) of 3TC was significantly increased. However, the pharmacokinetics of ZDV was unchanged. Co-administration with 3TC/ZDV resulted in a 22% decrease in AUC(12,ss) and a 20% decrease in C(max,ss) for BILR 355. The observed increase in exposure and prolongation of t(1/2,ss) of 3TC is potentially related to inhibition of OCT-mediated urinary excretion of 3TC.

WHAT IS NEW AND CONCLUSION

Concomitant administration of BILR 355 with 3TC/ZDV resulted in a modest decrease in exposure to BILR 355 and a 45% increase in exposure to 3TC.