Hepatic Dysfunction Quantified by HepQuant DuO Outperforms Child-Pugh Classification in Predicting the Pharmacokinetics of Ampreloxetine

HepQuant tests quantify liver function from clearance of deuterium- and 13C-labeled cholates administered either intravenously and orally (SHUNT) or orally (DuO). Hepatic impairment studies have relied on clinical or laboratory criteria like Child-Pugh classification to categorize the degree of hepatic dysfunction. We compared HepQuant tests with Child-Pugh classification in predicting the pharmacokinetics of ampreloxetine. Twenty-one subjects with hepatic impairment (8 Child-Pugh A, 7 Child-Pugh B, and 6 Child-Pugh C), and 10 age- and sex-matched controls were studied. The pharmacokinetics of ampreloxetine were measured after oral administration of a single dose of 10 mg. Disease severity index (DSI), portal-systemic shunting (SHUNT%), hepatic reserve, and hepatic filtration rates (HFRs) were measured from serum samples obtained after intravenous administration of [24-13C]-cholate and oral administration of [2,2,4,4-2H]cholate. Ampreloxetine plasma exposure (AUC0-inf) was similar to controls in Child-Pugh A, increased 1.7-fold in subjects with Child-Pugh B, and 2.5-fold in subjects with Child-Pugh C and correlated with both Child-Pugh score and HepQuant parameters. The variability observed in ampreloxetine exposure (AUC0-inf) in subjects with moderate (Child-Pugh B) and severe hepatic impairment (Child-Pugh C) was explained by HepQuant parameters. Multivariable regression models demonstrated that DSI, SHUNT%, and Hepatic Reserve from SHUNT and DuO were superior predictors of ampreloxetine exposure (AUC0-inf) compared to Child-Pugh score. HepQuant DSI, SHUNT%, and hepatic reserve were more useful predictors of drug exposure than Child-Pugh class for ampreloxetine and thus may better optimize dose recommendations in patients with liver disease. The simple-to-administer, oral-only DuO version of the HepQuant test could enhance clinical utility.

Efficacy and safety of an inhaled pan-Janus kinase inhibitor, nezulcitinib, in hospitalised patients with COVID-19: results from a phase 2 clinical trial

BACKGROUND

The inhaled lung-selective pan-Janus kinase inhibitor nezulcitinib had favourable safety and potential efficacy signals in part 1 of a phase 2 trial in patients with severe COVID-19, supporting progression to part 2.

METHODS

Part 2 was a randomised, double-blind phase 2 study (NCT04402866). Hospitalised patients aged 18-80 years with confirmed symptomatic COVID-19 requiring supplemental oxygen (excluding baseline invasive mechanical ventilation) were randomised 1:1 to nebulised nezulcitinib 3 mg or placebo for up to 7 days with background standard-of-care therapy (including corticosteroids). Efficacy endpoints included respiratory failure-free (RFF) days through day 28 as the primary endpoint. Secondary endpoints included safety and change from baseline oxygen saturation (SaO2)/fraction of inspired oxygen (FiO2) ratio on day 7, and 28-day mortality rate was a prespecified exploratory endpoint.

RESULTS

Between June 2020 and April 2021, 205 patients were treated (nezulcitinib, 103; placebo, 102). There was no statistically significant difference between nezulcitinib versus placebo in the primary endpoint (RFF days; median, 21.0 vs 21.0; p=0.6137) or secondary efficacy endpoints. Nezulcitinib was generally well tolerated with a favourable safety profile.

CONCLUSIONS

Although the prespecified primary, secondary and exploratory efficacy endpoints, including RFF through day 28, change from baseline SaO2/FiO2 ratio on day 7, and 28-day mortality rate, were not met, nezulcitinib was generally well tolerated and had a favourable safety profile. Further studies are required to determine if treatment with nezulcitinib confers clinical benefit in specific inflammatory biomarker-defined populations of patients with COVID-19.

Phase I study in healthy participants to evaluate safety, tolerability, and pharmacokinetics of inhaled nezulcitinib, a potential treatment for COVID-19

Nezulcitinib (TD‐0903), a lung‐selective pan–Janus‐associated kinase (JAK) inhibitor designed for inhaled delivery, is under development for treatment of acute lung injury associated with coronavirus disease 2019 (COVID‐19). This two‐part, double‐blind, randomized, placebo‐controlled, single ascending dose (part A) and multiple ascending dose (part B) phase I study evaluated the safety, tolerability, and pharmacokinetics (PK) of nezulcitinib in healthy participants. Part A included three cohorts randomized 6:2 to receive a single inhaled dose of nezulcitinib (1, 3, or 10 mg) or matching placebo. Part B included three cohorts randomized 8:2 to receive inhaled nezulcitinib (1, 3, or 10 mg) or matching placebo for 7 days. The primary outcome was nezulcitinib safety and tolerability assessed from treatment‐emergent adverse events (TEAEs). The secondary outcome was nezulcitinib PK. All participants completed the study. All TEAEs were mild or moderate in severity, and none led to treatment discontinuation. Overall (area under the plasma concentration‐time curve) and peak (maximal plasma concentration) plasma exposures of nezulcitinib were low and increased in a dose‐proportional manner from 1 to 10 mg in both parts, with no suggestion of clinically meaningful drug accumulation. Maximal plasma exposures were below levels expected to result in systemic target engagement, consistent with a lung‐selective profile. No reductions in natural killer cell counts were observed, consistent with the lack of a systemic pharmacological effect and the observed PK. In summary, single and multiple doses of inhaled nezulcitinib at 1, 3, and 10 mg were well‐tolerated in healthy participants, with dose‐proportional PK supporting once‐daily administration.

A phase 2 multiple ascending dose study of the inhaled pan-JAK inhibitor nezulcitinib (TD-0903) in severe COVID-19

Severe coronavirus disease 2019 (COVID-19) is characterised by pneumonia with excessive systemic inflammation, referred to as a “cytokine storm” [1–3]. Dexamethasone treatment decreases mortality in patients with COVID-19 receiving respiratory support and is standard of care for severe COVID-19 [4, 5]. However, pulmonary inflammation, which drives COVID-19 morbidity and mortality [3], can persist despite corticosteroid use [6, 7]. Janus kinase (JAK) inhibition blocks signalling by many cytokines in diverse cell types, offering broad immunomodulation [8]. The oral JAK-1/2 inhibitor baricitinib combined with the antiviral remdesivir shows clinical efficacy in patients with severe COVID-19 [9]. Direct delivery of JAK inhibition to the lung via inhalation could overcome corticosteroid-resistant pulmonary inflammation [10], offering the potential for improved responses while minimising risk of excessive systemic immunosuppression. The novel inhaled pan-JAK inhibitor nezulcitinib (TD-0903) was designed to target all JAK isoforms (JAK1, JAK2, JAK3, TYK2; −log inhibition constant ≥9.2) and optimise delivery to the lungs while limiting systemic exposure (R. Sana and co-workers; unpublished results: abstract submitted to ERS International Congress, 2021). We report results from the completed part 1 of a 2-part phase 2 trial (NCT04402866) in hospitalised patients with severe COVID-19.

The inhaled lung-selective pan-JAK inhibitor nezulcitinib appears generally well tolerated in hospitalised patients with severe #COVID-19, with trends for improved oxygenation and clinical status, shortened hospitalisation, and fewer deaths versus placebohttps://bit.ly/35Xs1Rf

Overview of Causality Assessment for Drug-Induced Liver Injury (DILI) in Clinical Trials

Causality assessment for suspected drug-induced liver injury (DILI) during drug development and following approval is challenging. The IQ DILI Causality Working Group (CWG), in collaboration with academic and regulatory subject matter experts (SMEs), developed this manuscript with the following objectives: (1) understand and describe current practices; (2) evaluate the utility of new tools/methods/practice guidelines; (3) propose a minimal data set needed to assess causality; (4) define best practices; and (5) promote a more structured and universal approach to DILI causality assessment for clinical development. To better understand current practices, the CWG performed a literature review, took a survey of member companies, and collaborated with SMEs. Areas of focus included best practices for causality assessment during clinical development, utility of adjudication committees, and proposals for potential new avenues to improve causality assessment. The survey and literature review provided renewed understanding of the complexity and challenges of DILI causality assessment as well as the use of non-standardized approaches. Potential areas identified for consistency and standardization included role and membership of adjudication committees, standardized minimum dataset, updated assessment tools, and best practices for liver biopsy and rechallenge in the setting of DILI. Adjudication committees comprised of SMEs (i.e., utilizing expert opinion) remain the standard for DILI causality assessment. A variety of working groups continue to make progress in pursuing new tools to assist with DILI causality assessment. The minimum dataset deemed adequate for causality assessment provides a path forward for standardization of data collection in the setting of DILI. Continued progress is necessary to optimize and advance innovative tools necessary for the scientific, pharmaceutical, and regulatory community.

Population Pharmacokinetics of Revefenacin in Patients with Chronic Obstructive Pulmonary Disease

BACKGROUND AND OBJECTIVES

Revefenacin is a lung-selective, long-acting muscarinic antagonist indicated for the maintenance treatment of patients with chronic obstructive pulmonary disease. The objectives of this analysis were to evaluate the pharmacokinetics of revefenacin and its major metabolite (THRX-195518) in patients with chronic obstructive pulmonary disease, and identify significant covariates affecting revefenacin disposition using a population pharmacokinetic approach based on plasma concentration-time data obtained after single- and repeated-dose once-daily administration in three phase II and two phase III studies.

METHODS

Plasma concentrations of revefenacin and THRX-195518 following once-daily administration via nebulization at a dose levels ranging from 22-700 μg in 935 patients (488 men, 447 women; age 41-88 years) were analyzed using nonlinear mixed-effects modeling.

RESULTS

Plasma revefenacin pharmacokinetics was best described by a two-compartment model with first-order absorption and elimination. Pharmacokinetic parameters for THRX-195518 were estimated using a sequential approach, where the concentration-time profiles were fit to a combined model. The formation of the metabolite in each subject was estimated to be a fixed fraction of the individually estimated (post-hoc) clearance rate of revefenacin. Four statistically significant covariates were identified: for revefenacin, age on apparent clearance and body weight on apparent intercompartment clearance, for THRX-195518, age on apparent clearance and body weight on the fraction of revefenacin apparent clearance that was metabolized to THRX-195518.

CONCLUSIONS

None of the identified statistically significant covariates were associated with a clinically meaningful effect on revefenacin or THRX-195518 exposure in patients with chronic obstructive pulmonary disease.

REGISTRATION

ClinicalTrials.gov identifier number NCT03064113, NCT01704404, NCT02040792, NCT02459080, and NCT02512510.

Revefenacin Absorption, Metabolism, and Excretion in Healthy Subjects and Pharmacological Activity of Its Major Metabolite

Revefenacin inhalation solution is an anticholinergic indicated for the maintenance treatment of patients with chronic obstructive pulmonary disease. Mass balance, pharmacokinetics, and metabolism of revefenacin were evaluated after intravenous and oral administration of [¹⁴C]-revefenacin in healthy subjects. Pharmacological activity of the major revefenacin metabolite was also assessed. Adult males (n = 9) received 20 μg intravenously of approximately 1 μCi [¹⁴C]-revefenacin and/or a single 200-μg oral solution of approximately 10 μCi [¹⁴C]-revefenacin. Mean recovery of radioactive material was 81.4% after intravenous administration (54.4% in feces; 27.1% in urine) and 92.7% after oral dosing (88.0% in feces, 4.7% in urine). Mean absolute bioavailability of oral revefenacin was low (2.8%). Intact revefenacin accounted for approximately 52.1% and 13.1% of the total radioactivity in plasma after intravenous and oral administration, respectively. Two main circulating metabolites were observed in plasma. After an intravenous dose, a hydrolysis product, THRX-195518 (M2) was observed that circulated in plasma at 14.3% of total radioactivity. After an oral dose, both THRX-195518 and THRX-697795 (M10, N-dealkylation and reduction of the parent compound) were observed at 12.5% of total circulating radioactivity. THRX-195518 was the major metabolite excreted in feces and comprised 18.8% and 9.4% of the administered intravenous and oral dose, respectively. The major metabolic pathway for revefenacin was hydrolysis to THRX-195518. In vitro pharmacological evaluation of THRX-195518 indicated that it had a 10-fold lower binding affinity for the M₃ receptor relative to revefenacin. Receptor occupancy analysis suggested that THRX-195518 has minimal contribution to systemic pharmacology relative to revefenacin after inhaled administration. SIGNIFICANCE STATEMENT: The major metabolic pathway for revefenacin was hydrolysis to the metabolite THRX-195518 (M2), and both revefenacin and THRX-195518 underwent hepatic-biliary and fecal elimination after oral or intravenous administration with negligible renal excretion. Pharmacological evaluation of THRX-195518 indicated that it had a 10-fold lower binding affinity for the M₃ muscarinic receptor relative to revefenacin and that THRX-195518 has minimal contribution to systemic pharmacology after inhaled administration.

Pharmacokinetics and safety of revefenacin in subjects with impaired renal or hepatic function

Purpose

Revefenacin, a long-acting muscarinic antagonist for nebulization, has been shown to improve lung function in patients with chronic obstructive pulmonary disease. Here we report pharmacokinetic (PK) and safety results from two multicenter, open-label, single-dose trials evaluating revefenacin in subjects with severe renal impairment (NCT02578082) and moderate hepatic impairment (NCT02581592).

Subjects and methods

The renal impairment trial enrolled subjects with normal renal function and severe renal impairment (estimated glomerular filtration rate <30 mL/min/1.73 m²). The hepatic impairment trial enrolled subjects with normal hepatic function and moderate hepatic impairment (Child-Pugh class B). Subjects received a single 175-µg dose of revefenacin through nebulization. PK plasma samples and urine collections were obtained at multiple time points for 5 days following treatment; all subjects were monitored for adverse events.

Results

In the renal impairment study, the maximum observed plasma revefenacin concentration (C_max) was up to 2.3-fold higher and area under the concentration–time curve from time 0 to infinity (AUC_inf) was up to 2.4-fold higher in subjects with severe renal impairment compared with those with normal renal function. For THRX-195518, the major metabolite of revefenacin, the corresponding changes in C_max and AUC_inf were 1.8- and 2.7-fold higher, respectively. In the hepatic impairment study, revefenacin C_max and AUC_inf were 1.03- and 1.18-fold higher, respectively, in subjects with moderate hepatic impairment compared with those with normal hepatic function. The corresponding changes in THRX-195518 C_max and AUC_inf were 1.5- and 2.8-fold higher, respectively.

Conclusion

Systemic exposure to revefenacin increased modestly in subjects with severe renal impairment but was similar between subjects with moderate hepatic impairment and normal hepatic function. The increase in plasma exposure to THRX-195518 in subjects with severe renal or moderate hepatic impairment is unlikely to be of clinical consequence given its low antimuscarinic potency, low systemic levels after inhaled revefenacin administration, and favorable safety profile.

Can Bile Salt Export Pump Inhibition Testing in Drug Discovery and Development Reduce Liver Injury Risk? An International Transporter Consortium Perspective

Bile salt export pump (BSEP) inhibition has emerged as an important mechanism that may contribute to the initiation of human drug‐induced liver injury (DILI). Proactive evaluation and understanding of BSEP inhibition is recommended in drug discovery and development to aid internal decision making on DILI risk. BSEP inhibition can be quantified using in vitro assays. When interpreting assay data, it is important to consider in vivo drug exposure. Currently, this can be undertaken most effectively by consideration of total plasma steady state drug concentrations (C_ss,plasma). However, because total drug concentrations are not predictive of pharmacological effect, the relationship between total exposure and BSEP inhibition is not causal. Various follow‐up studies can aid interpretation of in vitro BSEP inhibition data and may be undertaken on a case‐by‐case basis. BSEP inhibition is one of several mechanisms by which drugs may cause DILI, therefore, it should be considered alongside other mechanisms when evaluating possible DILI risk.

Revefenacin, a Long-Acting Muscarinic Antagonist, Does Not Prolong QT Interval in Healthy Subjects: Results of a Placebo- and Positive-Controlled Thorough QT Study

Revefenacin is a novel once‐daily, lung‐selective, long‐acting muscarinic antagonist developed as a nebulized inhalation solution for the maintenance treatment of chronic obstructive pulmonary disease. In a randomized, 4‐way crossover study, healthy subjects received a single inhaled dose of revefenacin 175 µg (therapeutic dose), revefenacin 700 µg (supratherapeutic dose), and placebo via standard jet nebulizer, and a single oral dose of moxifloxacin 400 mg (open‐label) in separate treatment periods. Electrocardiograms were recorded, and pharmacokinetic samples were collected serially after dosing. The primary end point was the placebo‐corrected change from baseline QT interval corrected for heart rate using Fridericia's formula, analyzed at each postdose time. Concentration‐QTc modeling was also performed. Following administration of revefenacin 175  and 700 µg, placebo‐corrected change from baseline QTcF (ΔΔQTcF) values were close to 0 at all times, with the largest mean ΔΔQTcF of 1.0 millisecond (95% confidence interval [CI], −1.2 to 3.1 milliseconds) 8 hours postdose and 1.0 millisecond (95%CI, −1.1 to 3.1 milliseconds) 1 hour postdose after inhalation of revefenacin 175 and 700 µg, respectively. Revefenacin did not have a clinically meaningful effect on heart rate (within ±5 beats per minute of placebo), or PR and QRS intervals (within ±3 and ±1 milliseconds of placebo, respectively). Using concentration‐QTc modeling, an effect of revefenacin > 10 milliseconds can be excluded within the observed plasma concentration range of up to ≈3 ng/mL. Both doses of revefenacin were well tolerated. These results demonstrate that revefenacin does not prolong the QT interval.

Discovery of a novel series of potent non-nucleoside inhibitors of hepatitis C virus NS5B

Hepatitis C virus (HCV) is a major global public health problem. While the current standard of care, a direct-acting antiviral (DAA) protease inhibitor taken in combination with pegylated interferon and ribavirin, represents a major advancement in recent years, an unmet medical need still exists for treatment modalities that improve upon both efficacy and tolerability. Toward those ends, much effort has continued to focus on the discovery of new DAAs, with the ultimate goal to provide interferon-free combinations. The RNA-dependent RNA polymerase enzyme NS5B represents one such DAA therapeutic target for inhibition that has attracted much interest over the past decade. Herein, we report the discovery and optimization of a novel series of inhibitors of HCV NS5B, through the use of structure-based design applied to a fragment-derived starting point. Issues of potency, pharmacokinetics, and early safety were addressed in order to provide a clinical candidate in fluoropyridone 19.

Effect of hepatitis C virus infection on the mRNA expression of drug transporters and cytochrome p450 enzymes in chimeric mice with humanized liver

The expression of drug transporters and metabolizing enzymes is a primary determinant of drug disposition. Chimeric mice with humanized liver, including PXB mice, are an available model that is permissive to the in vivo infection of hepatitis C virus (HCV), thus being a promising tool for investigational studies in development of new antiviral molecules. To investigate the potential of HCV infection to alter the pharmacokinetics of small molecule antiviral therapeutic agents in PXB mice, we have comprehensively determined the mRNA expression profiles of human ATP-binding cassette (ABC) transporters, solute carrier (SLC) transporters, and cytochrome P450 (P450) enzymes in the livers of these mice under noninfected and HCV-infected conditions. Infection of PXB mice with HCV resulted in an increase in the mRNA expression levels of a series of interferon-stimulated genes in the liver. For the majority of genes involved in drug disposition, minor differences in the mRNA expression of ABC and SLC transporters as well as P450s between the noninfected and HCV-infected groups were observed. The exceptions were statistically significantly higher expression of multidrug resistance-associated protein 4 and organic anion-transporting polypeptide 2B1 and lower expression of organic cation transporter 1 and CYP2D6 in HCV-infected mice. Furthermore, the enzymatic activities of the major human P450s were, in general, comparable in the two experimental groups. These data suggest that the pharmacokinetic properties of small molecule antiviral therapies in HCV-infected PXB mice are likely to be similar to those in noninfected PXB mice. However, caution is needed in the translation of this relationship to HCV-infected patients as the PXB mouse model does not accurately reflect the pathology of patients with chronic HCV infection.

Mechanisms underlying saturable intestinal absorption of metformin

The purpose of the study was to elucidate mechanisms of metformin absorptive transport to explain the dose-dependent absorption observed in humans. Apical (AP) and basolateral (BL) uptake and efflux as well as AP to BL (absorptive) transport across Caco-2 cell monolayers were evaluated over a range of concentrations. Transport was concentration-dependent and consisted of saturable and nonsaturable components (K(m) approximately 0.05 mM, J(max) approximately 1.0 pmol min(-1) cm(-2), and K(d, transport) approximately 10 nl min(-1) cm(-2)). AP uptake data also revealed the presence of saturable and nonsaturable components (K(m) approximately 0.9 mM, V(max) approximately 330 pmol min(-1) mg of protein(-1), and K(d, uptake) approximately 0.04 microl min(-1) mg of protein(-1)). BL efflux was rate-limiting to transcellular transport of metformin; AP efflux was 7-fold greater than BL efflux and was not inhibited by N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GW918), a P-glycoprotein inhibitor. AP efflux was trans-stimulated by metformin and prototypical substrates of organic cation transporters, suggesting that a cation-specific bidirectional transport mechanism mediated the AP efflux of metformin. BL efflux of intracellular metformin was much less efficient in comparison with the overall transport, with BL efflux clearance accounting for approximately 7 and approximately 13% of the overall transport clearance at 0.05 and 10 mM metformin concentrations, respectively. Kinetic modeling of cellular accumulation and transport processes supports the finding that transport occurs almost exclusively via the paracellular route (approximately 90%) and that the paracellular transport is saturable. This report provides strong evidence for a saturable mechanism in the paracellular space and provides insight into possible mechanisms for the dose dependence of metformin absorption in vivo.

Saturable Absorptive Transport of the Hydrophilic Organic Cation Ranitidine in Caco-2 Cells: Role of pH-Dependent Organic Cation Uptake System and P-Glycoprotein

PURPOSE

The purpose of this work was to investigate the involvement of carrier-mediated apical (AP) uptake and efflux mechanisms in the absorptive intestinal transport of the hydrophilic cationic drug ranitidine in Caco-2 cells.

METHODS

Absorptive transport and AP uptake of ranitidine were determined in Caco-2 cells as a function of concentration. Permeability of ranitidine in the absorptive and secretory directions was assessed in the absence or presence of the P-glycoprotein (P-gp) inhibitor, GW918. Characterization of the uptake mechanism was performed with respect to inhibitor specificity, pH, energy, membrane potential, and Na+ dependence. Efflux from preloaded monolayers was evaluated over a range of concentrations and in the absence or presence of high extracellular ranitidine concentrations.

RESULTS

Saturable absorptive transport and AP uptake of ranitidine were observed with Km values of 0.27 and 0.45 mM, respectively. The ranitidine absorptive permeability increased and secretory permeability decreased upon inhibition of P-gp. AP ranitidine uptake was inhibited in a concentration-dependent fashion by a diverse set of organic cations including tetraethylammonium, 1-methyl-4-phenylpyridinium, famotidine, and quinidine. AP ranitidine uptake was pH and membrane potential dependent and reduced under conditions that deplete metabolic energy. Efflux of [3H]ranitidine across the basolateral membrane was neither saturable as a function of concentration nor trans stimulated by unlabeled ranitidine.

CONCLUSIONS

Saturable absorptive transport of ranitidine in Caco-2 cells is partially mediated via a pH-dependent uptake transporter for organic cations and is subject to attenuation by P-gp. Inhibition and driving force studies suggest the uptake carrier exhibits similar properties to cloned human organic cation transporters. The results also imply ranitidine transport is not solely restricted to the paracellular space.

Intestinal absorptive transport of the hydrophilic cation ranitidine: a kinetic modeling approach to elucidate the role of uptake and efflux transporters and paracellular vs. transcellular transport in Caco-2 cells

PURPOSE

The mechanism of intestinal drug transport for hydrophilic cations such as ranitidine is complex, and evidence suggests a role for carrier-mediated apical (AP) uptake and saturable paracellular mechanisms in their overall absorptive transport. The purpose of this study was to develop a model capable of describing the kinetics of cellular accumulation and transport of ranitidine in Caco-2 cells, and to assess the relative contribution of the transcellular and paracellular routes toward overall ranitidine transport.

METHODS

Cellular accumulation and absorptive transport of ranitidine were determined in the absence or presence of uptake and efflux inhibitors and as a function of concentration over 60 min in Caco-2 cells. A three-compartment model was developed, and parameter estimates were utilized to assess the expected relative contribution from transcellular and paracellular transport.

RESULTS

Under all conditions, ranitidine absorptive transport consisted of significant transcellular and paracellular components. Inhibition of P-glycoprotein decreased the AP efflux rate constant (k21) and increased the relative contribution of the transcellular transport pathway. In the presence of quinidine, both the AP uptake rate constant (k12) and k21 decreased, resulting in a predominantly paracellular contribution to ranitidine transport. Increasing the ranitidine donor concentration decreased k12 and the paracellular rate constant (k13). No significant changes were observed in the relative contribution of the paracellular and transcellular routes as a function of ranitidine concentration.

CONCLUSIONS

These results suggest the importance of uptake and efflux transporters as determinants of the relative contribution of transcellular and paracellular transport for ranitidine, and provide evidence supporting a concentration-dependent paracellular transport mechanism. The modeling approach developed here may also be useful in estimating the relative contribution of paracellular and transcellular transport for a wide array of drugs expected to utilize both pathways.

Ritonavir, Saquinavir, and Efavirenz, but Not Nevirapine, Inhibit Bile Acid Transport in Human and Rat Hepatocytes

Human immunodeficiency virus-infected patients on antiretroviral drug therapy frequently experience hepatotoxicity, the underlying mechanism of which is poorly understood. Hepatotoxicity from other compounds such as bosentan and troglitazone has been attributed, in part, to inhibition of hepatocyte bile acid excretion. This work tested the hypothesis that antiretroviral drugs modulate hepatic bile acid transport. Ritonavir (28 microM), saquinavir (15 microM), and efavirenz (32 microM) inhibited [(3)H]taurocholate transport in bile salt export pump expressing Sf9-derived membrane vesicles by 90, 71, and 33%, respectively. In sandwich-cultured human hepatocytes, the biliary excretion index (BEI) of [(3)H]taurocholate was maximally decreased 59% by ritonavir, 39% by saquinavir, and 20% by efavirenz. Likewise, in sandwich-cultured rat hepatocytes, the BEI of [(3)H]taurocholate was decreased 100% by ritonavir and 94% by saquinavir. Sodium-dependent and -independent initial uptake rates of [(3)H]taurocholate in suspended rat hepatocytes were significantly decreased by ritonavir, saquinavir, and efavirenz. [(3)H]Taurocholate transport by recombinant NTCP and Ntcp was inhibited by ritonavir (IC(50) = 2.1 and 6.4 microM in human and rat, respectively), saquinavir (IC(50) = 6.7 and 20 microM, respectively), and efavirenz (IC(50) = 43 and 97 microM, respectively). Nevirapine (75 microM) had no effect on bile acid transport in any model system. In conclusion, ritonavir, saquinavir, and efavirenz, but not nevirapine, inhibited both the hepatic uptake and biliary excretion of taurocholate.

Differential substrate and inhibitory activities of ranitidine and famotidine toward human organic cation transporter 1 (hOCT1; SLC22A1), hOCT2 (SLC22A2), and hOCT3 (SLC22A3)

Human organic cation transporters (hOCTs) are expressed in organs of drug absorption and elimination and play an important role in the uptake and elimination of xenobiotics. The purpose of this study was to evaluate the substrate and inhibitory activity of the H2-receptor antagonists ranitidine and famotidine toward hOCTs and to determine the hOCT isoforms involved in the absorption and elimination of these compounds in humans. Inhibition and substrate specificity of hOCT1, hOCT2, and hOCT3 for ranitidine and famotidine were elucidated in cRNA-injected Xenopus laevis oocytes. Ranitidine and famotidine exhibited similarly potent inhibition of [3H]1-methyl-4-phenyl pyridinium uptake into hOCT1-expressing (IC50= 33 and 28 microM, respectively) and hOCT2-expressing oocytes (IC50= 76 and 114 microM, respectively). Famotidine exhibited potent inhibition of hOCT3; in contrast, ranitidine was a moderately weak inhibitor (IC50= 6.7 and 290 microM, respectively). [3H]Ranitidine uptake was stimulated by hOCT1 (Km= 70 +/- 9 microM) and to a much smaller extent by hOCT2. No stimulation of [3H]ranitidine uptake was observed in hOCT3-expressing oocytes. trans-Stimulation and electrophysiology studies suggested that famotidine also is an hOCT1 substrate and exhibits poor or no substrate activity toward hOCT2 and hOCT3. Thus, hOCT1, which is expressed in the intestine and liver, is likely to play a major role in the intestinal absorption and hepatic disposition of ranitidine and famotidine in humans, whereas hOCT2, the major isoform present in the kidney, may play only a minor role in their renal elimination. Famotidine seems to be one of the most potent inhibitors of hOCT3 yet identified.

Photoaffinity Labeling of the Anionic Sites in Caco-2 Cells Mediating Saturable Transport of Hydrophilic Cations Ranitidine and Famotidine

The H(2) antagonists, ranitidine and famotidine, exhibit saturable absorptive transport across Caco-2 cell monolayers and human intestine via a yet unidentified mechanism. A photoreactive derivative of famotidine has been synthesized and evaluated as a photoaffinity probe for the putative transporter protein(s). The probe irreversibly inhibited ranitidine transport across Caco-2 cell monolayers and irreversibly increased the transepithelial electrical resistance (TEER) after UV activation. Photoaffinity labeling was protected by a molar excess of famotidine.