Kinetic analysis of saturable hepatic uptake of digoxin and its inhibition by rifampicin

Pharmacokinetics of Tenofovir Alafenamide When Coadministered With Other HIV Antiretrovirals

BACKGROUND

Tenofovir alafenamide (TAF), a prodrug of the nucleotide analogue tenofovir (TFV), is an antiretroviral (ARV) agent approved either as a complete regimen [elvitegravir/cobicistat/emtricitabine (F)/TAF, rilpivirine/F/TAF, bictegravir/F/TAF], or for use with other ARVs (F/TAF), for treatment of HIV. TAF is a substrate of P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) transporters. Disposition of TAF may be altered by comedications that can inhibit or induce P-gp or BCRP transporters. The effects of ARVs on the pharmacokinetics of TAF were evaluated in 3 studies.

METHODS

Healthy participants received TAF administered alone or with rilpivirine in study 1, with dolutegravir, ritonavir-boosted atazanavir (ATV + RTV), lopinavir (LPV/RTV), or darunavir (DRV + RTV) in study 2, and with the pharmacokinetic enhancer cobicistat or efavirenz in study 3.

RESULTS

Across the 3 studies, 98 participants received treatment with TAF and a coadministered agent (n = 10-34/cohort). All study treatments were well tolerated. TAF and TFV exposures were unaffected after co-administration with rilpivirine and dolutegravir. Coadministration with P-gp/BCRP inhibitors such as cobicistat or PI-based regimens (ATV + RTV, LPV/r, or DRV + RTV) resulted in a range of 6%-183% increases in TAF and 105%-316% increases in TFV exposure, whereas coadministration with a P-gp inducer, efavirenz, resulted in a 15%-24% decrease in TAF and TFV exposure.

CONCLUSIONS

Evaluation of the drug interaction between TAF and other commonly prescribed boosted and unboosted ARVs provides characterization of the susceptibility of TAF and/or TFV pharmacokinetics to inhibitors or inducers of P-gp/BCRP transporters.

Functional characterization of hepatic transporters using intravital microscopy

A better understanding of the role of hepatic transporters in drug elimination is of crucial importance for drug development and therapy. This study examined the usefulness of intravital microscopy to quantitatively evaluate the function of hepatic transporters in the exposed liver of anesthetized rats. In one experiment the function of the organic anion transporting polypeptide (Oatp) in sinusoidal uptake was investigated by administering an Oatp inhibitor, rifampicin, prior to the probe substrate Na-fluorescein. In another experiment, rhodamine 123 was used to quantify the biliary canalicular transporter P-glycoprotein (P-gp, Abcb1a/b) with cyclosporin A as an inhibitor of P-gp activity. Calibrated fluorescence intensity time curves measured in sinusoids and hepatocytes together with cumulative biliary excretion data from control and inhibitor treated animals were analyzed with a three-compartment model. A robust parameter estimation was achieved using nonlinear mixed effects modeling. Rifampicin reduced the hepatic uptake clearance of Na-fluorescein to 25% of the control (p<0.05) without affecting other parameters. In the presence of cyclosporin A, biliary excretion of rhodamine 123 decreased to 7% of the control (p<0.01). The novelty of this approach is that it allows a quantitative evaluation of transporter function in the in vivo rat liver.

Understanding the interplay of drug transporters involved in the disposition of rosuvastatin in the isolated perfused rat liver using a physiologically-based pharmacokinetic model

The role of hepatic uptake (Oatp1a1 and Oatp1b4) and efflux (Bcrp and Mrp2) transporters in the disposition of rosuvastatin were investigated using the isolated perfused rat liver (IPRL). A simple physiologically-based pharmacokinetic model was developed to quantitatively determine the interplay between the individual transporters. Uptake and elimination of rosuvastatin in the IPRL was rapid and extensive. In the presence of rifamycin (an equipotent inhibitor of both Oatp1a1 and Oatp1a4) the perfusate clearance of rosuvastatin was reduced, but rifampicin (a potent inhibitor of Oatp1a4) had no effect upon the perfusate clearance. This might indicate a limited role for Oatp1a4, but it is possible that Oatp1a1 (or other uptake transporters) may have redundancy in their affinity for rosuvastatin. In the presence of GF120918 (a potent inhibitor of Bcrp) and in the Wistar TR- rat (a naturally occurring mutant not expressing Mrp2) the biliary clearance was reduced and virtually abolished in the TR- pre-incubated GF120918. Bcrp and Mrp2 appear to represent the primary efflux mechanisms for rosuvastatin in the rat. Rosuvastatin disposition in the IPRL is mediated in part by Oatp1a1 and efflux is almost entirely by Mrp2 and Bcrp. Other uptake processes may be involved.

In vivo probes of drug transport: commonly used probe drugs to assess function of intestinal P-glycoprotein (ABCB1) in humans

Intestinal P-glycoprotein (P-gp, ABCB1) may significantly influence drug absorption and elimination. Its expression and function is highly variable, regio-selective and influenced by genetic polymorphisms, drug interactions and intestinal diseases. An in vivo probe drug for intestinal P-gp should a registered, safe and well tolerated nonmetabolized selective substrate with low protein binding for which P-gp is rate-limiting during absorption. Other P-gp dependent processes should be of minor influence. The mechanism(s) and kinetics of intestinal uptake must be identified and quantified. Moreover, the release properties of the dosage form should be known. So far, the cardiac glycoside digoxin and the ß₁-selective blocker talinolol have been used in mechanistic clinical studies, because they meet most of these criteria. Digoxin and talinolol are suitable in vivo probe drugs for intestinal P-gp under the precondition, that they are used as tools in carefully designed pharmacokinetic studies with adequate biometrically planning of the sample size and that several limitations are considered in interpreting and discussion of the study results.

Drug structure-transport relationships

Malcolm Rowland has greatly facilitated an understanding of drug structure–pharmacokinetic relationships using a physiological perspective. His view points, covering a wide range of activities, have impacted on my own work and on my appreciation and understanding of our science. This overview summarises some of our parallel activities, beginning with Malcolm’s work on the pH control of amphetamine excretion, his work on the disposition of aspirin and on the application of clearance concepts in describing the disposition of lidocaine. Malcolm also spent a considerable amount of time developing principles that define solute structure and transport/pharmacokinetic relationships using in situ organ studies, which he then extended to involve the whole body. Together, we developed a physiological approach to studying hepatic clearance, introducing the convection–dispersion model in which there was a spread in blood transit times through the liver accompanied by permeation into hepatocytes and removal by metabolism or excretion into the bile. With a range of colleagues, we then further developed the model and applied it to various organs in the body. One of Malcolm’s special interests was in being able to apply this knowledge, together with an understanding of physiological differences in scaling up pharmacokinetics from animals to man. The description of his many other activities, such as the development of clearance concepts, application of pharmacokinetics to the clinical situation and using pharmacokinetics to develop new compounds and delivery systems, has been left to others.

An improved nonlinear model describing the hepatic pharmacokinetics of digoxin: evidence for two functionally different uptake systems and saturable binding

PURPOSE

To develop a semi-distributed liver model for the evaluation of saturable sinusoidal uptake and binding kinetics of the Oatp1a4 substrate digoxin.

METHODS

In the perfused rat liver, two successive digoxin doses of 42 and 125 microg were administered, and the outflow concentration was determined by LC/MS/MS. [14C]-sucrose was used as vascular reference. The data were analyzed simultaneously by a population approach using sucrose to determine the sinusoidal mixing of digoxin.

RESULTS

The results suggest the existence of a high-affinity, low-capacity system, and a low-affinity, high-capacity system for sinusoidal uptake with apparent Michaelis constants (K(M)) of 0.24 and 332 microg/ml, respectively. Incorporation of saturable sinusoidal binding of digoxin considerably improved the fit, and the parameter estimates were consistent with those of binding to hepatic Na,K-ATPase. Simpler models that neglect the concentration gradient in flow direction failed to describe the outflow data in the high dose range.

CONCLUSION

The semi-distributed liver model with saturable uptake should be useful for a functional characterization of transporters in the in situ rat liver.

Digoxin – a therapeutic agent and mechanistic probe: review of liquid chromatographic mass spectrometric methods and recent nuances in the clinical pharmacology attributes of digoxin

Digoxin is an important therapeutic agent for the treatment of congestive cardiac failure. In spite of its narrow therapeutic index, digoxin has been used extensively by the medical community and, lately, the use of digoxin as a mechanistic probe for p-glycoprotein transporter activity has increased. This review describes recent trends in the bioanalysis of digoxin, where scores of liquid chromatographic–mass spectrometric assays have been successfully employed to measure digoxin in preclinical, clinical and mechanistic studies. It provides various considerations such as internal standard selection, extraction schemes, matrix effect, selectivity evaluation and optimization of mass spectral conditions, for example, to enable the development of sound bioanalytical methods for digoxin. Some recent updates with regard to clinical pharmacology, absorption and disposition aspects of digoxin have been included. Overall, liquid chromatographic–mass spectrometric assays represent an important tool for many future preclinical, clinical and mechanistic probe studies that would probe digoxin with or without other coadministered substrates.