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.

Gut-Selective Design of Orally Administered Izencitinib (TD-1473) Limits Systemic Exposure and Effects of Janus Kinase Inhibition in Nonclinical Species.

Izencitinib (TD-1473), an oral, gut-selective pan-Janus kinase (JAK) inhibitor under investigation for treatment of inflammatory bowel diseases, was designed for optimal efficacy in the gastrointestinal tract while minimizing systemic exposures and JAK-related safety findings. The nonclinical safety of izencitinib was evaluated in rat and dog repeat-dose and rat and rabbit reproductive and developmental toxicity studies. Systemic exposures were compared with JAK inhibitory potency to determine effects at or above pharmacologic plasma concentrations (≥1× plasma average plasma concentration [Cave]:JAK 50% inhibitory concentration [IC50] ratio). In rats and dogs, 1000 and 30 mg/kg/day izencitinib, respectively, produced minimal systemic findings (ie, red/white cell changes) and low systemic concentrations (approximately 1× plasma Cave:JAK IC50 ratio) with an 8× nonclinical:clinical systemic area under the curve (AUC) margin compared with exposures at the highest clinically tested dose (300 mg, quaque die, once daily, phase 1 study in healthy volunteers). In dogs, it was possible to attain sufficient systemic exposures to result in immunosuppression characteristic of systemic JAK inhibition, but at high AUC margins (43×) compared with systemic exposures observed at the highest tested dose in humans. No adverse findings were observed in the gastrointestinal tract or systemic tissues. Izencitinib did not affect male or female fertility. Izencitinib did not affect embryonic development in rats and rabbits as commonly reported with systemic JAK inhibition, consistent with low maternal systemic concentrations (2-6× plasma Cave:JAK IC50 ratio, 10-33× nonclinical:clinical AUC margin) and negligible fetal exposures. In conclusion, the izencitinib gut-selective approach resulted in minimal systemic findings in nonclinical species at pharmacologic, clinically relevant systemic exposures, highlighting the impact of organ-selectivity in reducing systemic safety findings.

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.

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.

Pharmacokinetics of Ampreloxetine, a Norepinephrine Reuptake Inhibitor, in Healthy Subjects and Adults with Attention-Deficit/Hyperactive Disorder or Fibromyalgia Pain.

BACKGROUND AND OBJECTIVE: Ampreloxetine is a novel norepinephrine reuptake inhibitor in development for the treatment of symptomatic neurogenic orthostatic hypotension. The objectives of this analysis were to define the pharmacokinetics of once-daily oral ampreloxetine and provide dose recommendations for clinical development. METHODS: We fitted a population pharmacokinetic model to ampreloxetine plasma concentrations from single- and multiple-ascending dose trials in healthy subjects and two phase II studies in adult subjects with attention-deficit/hyperactive disorder or fibromyalgia at doses of 2-50 mg. RESULTS: Ampreloxetine pharmacokinetics was best described by a two-compartment model with first-order absorption and elimination. The terminal half-life was 30-40 h, resulting in sustained drug concentrations for the entire 24-h dosing interval at steady state. Covariates of age, weight, or renal impairment did not impact ampreloxetine exposure. Cytochrome P450 2D6 phenotype had no influence on ampreloxetine exposure. Sex and smoking status were identified as statistically significant covariates, suggesting a role for cytochrome P450 1A2 in the elimination of ampreloxetine. Despite statistical significance, differences in ampreloxetine exposure in male vs female subjects and smokers vs non-smokers were not clinically meaningful at the recommended dose. At the 10-mg dose, > 75% norepinephrine transporter inhibition and < 50% serotonin transporter inhibition are anticipated for adult subjects. CONCLUSIONS: The population pharmacokinetic model effectively described the plasma concentration-time profile of ampreloxetine after single and multiple doses. Population pharmacokinetic/pharmacodynamic analysis justified using a fixed dosing regimen with no dose adjustments across a broad population and can be used to inform dosing strategies in future clinical studies. CLINICAL TRIAL REGISTRATION: ClinicalTrials.gov identifier numbers NCT01693692 (fibromyalgia); NCT01458340 (attention-deficit/hyperactive disorder).

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.

Safety, Pharmacokinetics, and Pharmacodynamics of TD-0714, a Novel Potent Neprilysin Inhibitor in Healthy Adult and Elderly Subjects.

TD-0714 is an orally active, potent, and selective inhibitor of human neprilysin (NEP) in development for the treatment of chronic heart failure. Oral administration of TD-0714 in rats resulted in dose-dependent and sustained increases in plasma cyclic guanosine monophosphate (cGMP) over 24 hours consistent with NEP target engagement. Randomized, double-blind, placebo controlled, single ascending dose (50-600 mg TD-0714) and multiple ascending dose (10-200 mg TD-0714 q.d. for 14 days) studies were conducted in healthy volunteers. TD-0714 was generally well-tolerated and no serious adverse events or clinically significant effects on vital signs or electrocardiogram parameters were observed. TD-0714 exhibited dose-proportional pharmacokinetics (PKs) with high oral bioavailability, minimal accumulation after once daily dosing, and negligible renal elimination. Pharmacodynamic (PD) responses were observed at all dose levels studied, as reflected by statistically significant increases in plasma cGMP concentrations. The increases in cGMP were significantly above the baseline (~ 50-100%) on day 14 for the entire 24-hour interval indicating that sustained cGMP elevations are achieved at steady-state. Maximal steady-state cGMP response was observed in plasma and urine at doses ≥ 50 mg. The TD-0714 PK-PD relationship and safety profile were similar in elderly vs. younger adult subjects. The TD-0714 PK and PD profiles support further clinical development of TD-0714 and suggest the potential for once-daily administration and predictable exposure in patients with cardiorenal diseases regardless of their renal function.

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.

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 (Cmax) was up to 2.3-fold higher and area under the concentration-time curve from time 0 to infinity (AUCinf) 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 Cmax and AUCinf were 1.8- and 2.7-fold higher, respectively. In the hepatic impairment study, revefenacin Cmax and AUCinf 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 Cmax and AUCinf 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 (Css,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.

Pharmacodynamics, pharmacokinetics and safety of revefenacin (TD-4208), a long-acting muscarinic antagonist, in patients with chronic obstructive pulmonary disease (COPD): Results of two randomized, double-blind, phase 2 studies.

BACKGROUND: Revefenacin (TD-4208) is a potent, lung-selective, long-acting muscarinic antagonist currently in development as a once-daily nebulized therapy for chronic obstructive pulmonary disease (COPD). We evaluated the pharmacodynamics (bronchodilator activity), pharmacokinetics (PK) and safety of single- and multiple-dose administrations of revefenacin in two clinical trials (Study 0059 and Study 0091) in patients with moderate to severe COPD. METHODS: In Study 0059, 32 patients were randomized to receive a single dose of revefenacin (350 or 700 µg), active control ipratropium (500 µg) or placebo inhalation solution administered via standard jet nebulizer in a double-blind, crossover fashion. In Study 0091, 59 patients were randomized to receive once-daily inhalations of revefenacin (22, 44, 88, 175, 350 or 700 µg) or placebo for 7 days in a double-blind, incomplete block, five-way crossover design. The primary efficacy endpoint was change from baseline in peak (0-6 h) forced expiratory volume in 1 s (FEV1) in Study 0059, and trough FEV1 after the final dose (Day 7) in Study 0091. In both studies, secondary endpoints included area under the FEV1-time curve (FEV1 AUC) values from time 12-24 h post dose and FEV1 AUC values from time zero to 24 h post dose. RESULTS: Revefenacin demonstrated a rapid onset and sustained duration of bronchodilator action in both studies. In Study 0059, mean peak FEV1 was significantly higher (p < 0.001) for revefenacin and ipratropium compared to placebo, with differences of 176.8 mL for 350 µg revefenacin, 162.2 mL for 700 µg revefenacin and 190.6 mL for ipratropium. In Study 0091, mean trough FEV1 on Day 7 was significantly higher (p < 0.006) for all revefenacin doses compared to placebo, with differences ranging from 53.5 mL (22 µg dose) to 114.2 mL (175 µg dose). The results for the other spirometry endpoints were consistent with the primary endpoint for each study, demonstrating that the bronchodilator effect of revefenacin lasted more than 24 h following nebulized administration. Revefenacin was rapidly absorbed and extensively metabolized, followed by a slow apparent terminal elimination and minimal accumulation with repeated dosing. In both studies, adverse events were generally mild and occurred with similar frequencies in all groups, with no indication of significant systemic anti-muscarinic activity at any dose. CONCLUSIONS: Following single or multiple nebulized-dose administration in patients with COPD, revefenacin demonstrates a rapid onset and sustained duration of bronchodilator effect over 24 h following once-daily administration, with a PK profile that is commensurate with low systemic exposure.

Biotransformation and bioactivation reactions of alicyclic amines in drug molecules.

Aliphatic nitrogen heterocycles such as piperazine, piperidine, pyrrolidine, morpholine, aziridine, azetidine, and azepane are well known building blocks in drug design and important core structures in approved drug therapies. These core units have been targets for metabolic attack by P450s and other drug metabolizing enzymes such as aldehyde oxidase and monoamine oxidase (MAOs). The electron rich nitrogen and/or α-carbons are often major sites of metabolism of alicyclic amines. The most common biotransformations include N-oxidation, N-conjugation, oxidative N-dealkylation, ring oxidation, and ring opening. In some instances, the metabolic pathways generate electrophilic reactive intermediates and cause bioactivation. However, potential bioactivation related adverse events can be attenuated by structural modifications. Hence it is important to understand the biotransformation pathways to design stable drug candidates that are devoid of metabolic liabilities early in the discovery stage. The current review provides a comprehensive summary of biotransformation and bioactivation pathways of aliphatic nitrogen containing heterocycles and strategies to mitigate metabolic liabilities.

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.

Prediction of human serotonin and norepinephrine transporter occupancy of duloxetine by pharmacokinetic/pharmacodynamic modeling in the rat.

Translation of central nervous system occupancy and clinical effect from animal models to humans has remained elusive for many pharmacological targets. The current studies evaluate the ability of a rodent pharmacokinetic/pharmacodynamic (PK/PD) modeling approach to translate ex vivo occupancy determined in rats to that observed after positron emission tomography (PET) imaging in humans for the dual serotonin transporter (SERT) and norepinephrine transporter (NET) inhibitor duloxetine. Ex vivo transporter occupancy in rat spinal cord was evaluated after single oral doses of 0.3 to 60 mg/kg. A novel methodology, based on the initial rates of association of transporter selective radioligands to tissue homogenates, was developed and validated for the assessment of ex vivo transporter occupancy. Duloxetine exhibited selectivity for occupancy of SERT over NET in rat spinal cord with ED(50) values of 1 and 9 mg/kg, respectively. Corresponding EC(50) values for the inhibition of SERT and NET based on unbound duloxetine spinal cord concentrations were 0.5 and 8 nM. An effect compartment PK/PD modeling approach was used to analyze the relationship between the time course of duloxetine plasma concentration and SERT and NET occupancy. Duloxetine inhibited SERT and NET in rat spinal cord with a plasma EC(50) of 2.95 and 59.0 ng/ml, respectively. Similar plasma EC(50) values for the inhibition of SERT (2.29-3.7 ng/ml) have been reported from human PET studies. This study illustrates the value of translational PK/PD modeling approaches and suggests that the preclinical modeling approach used in the current study is capable of predicting plasma concentrations associated with 50% occupancy of SERT in the human central nervous system.

5-HT(4) receptor agonist mediated enhancement of cognitive function in vivo and amyloid precursor protein processing in vitro: A pharmacodynamic and pharmacokinetic assessment.

There remains an urgent need for therapeutic agents that provide improved symptomatic treatment and attenuate disease progression in patients with Alzheimer's disease (AD). 5-HT(4) receptors are widely expressed in those CNS areas which receive substantial cholinergic input and are involved in cognition. The ability of 5-HT(4) receptor agonists to increase acetylcholine (ACh) release and reduce cognitive impairment in both animals and humans has been demonstrated. In addition, 5-HT(4) receptor agonist modulation of levels of the amyloid precursor protein (APP) derived peptides, soluble amyloid precursor protein (sAPPα) and amyloid beta protein (Aß) in the CNS has been reported. In this study, the preclinical properties of three structurally-distinct 5-HT(4) receptor selective agonists, PRX-03140, velusetrag and TD-8954, were studied to assess their potential for symptomatic and disease-modifying benefit in the treatment of AD. All three compounds exhibited high affinity for the rat 5-HT(4) receptor but could be discriminated on the basis of their agonist activity. In cAMP accumulation and sAPPα secretion assays using recombinant HEK293f-5-HT(4(d))-APP(695) cells, velusetrag and TD-8954 were potent, full agonists, relative to 5-HT, whereas PRX-03140 was a partial agonist (intrinsic activity 18%, relative to 5-HT). In a guinea pig colon isolated tissue preparation, TD-8954 exhibited lower intrinsic activity than velusetrag, and PRX-03140 had negligible agonist activity. In the rat Morris water maze (MWM) cognition test, velusetrag and TD-8954 (0.1 mg/kg), but not PRX-03140 (0.03-1 mg/kg), significantly reversed the scopolamine-induced spatial learning deficit via activation of 5-HT(4) receptors. Coadministration of subefficacious doses of the acetylcholinesterase inhibitor (AChEi), donepezil (0.1 mg/kg, i.p.), and either velusetrag or TD-8954 (0.01 mg/kg i.p.) resulted in reversal of the scopolamine-induced cognitive deficit. Pharmacokinetic data indicated that the CNS penetration for all three 5-HT(4) receptor agonists was relatively low. However, the pharmacodynamic-pharmacokinetic relationships in the MWM model for velusetrag and TD-8954 were consistent with their respective receptor pharmacology (binding affinity and intrinsic efficacy) and CNS penetration properties. Collectively, these findings support a potential role for potent and efficacious 5-HT(4) receptor agonists in the treatment of AD.

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.