Exploring Integrin α5ß1 as a Potential Therapeutic Target for Pulmonary Arterial Hypertension: Insights from Comprehensive Multicenter Preclinical Studies.

Pulmonary arterial hypertension (PAH) is characterized by obliterative vascular remodeling of the small pulmonary arteries (PA) and progressive increase in pulmonary vascular resistance (PVR) leading to right ventricular (RV) failure. Although several drugs are approved for the treatment of PAH, mortality remains high. Accumulating evidence supports a pathological function of integrins in vessel remodeling, which are gaining renewed interest as drug targets. However, their role in PAH remains largely unexplored. We found that the arginine-glycine-aspartate (RGD)-binding integrin α5ß1 is upregulated in PA endothelial cells (PAEC) and PA smooth muscle cells (PASMC) from PAH patients and remodeled PAs from animal models. Blockade of the integrin α5ß1 or depletion of the α5 subunit resulted in mitotic defects and inhibition of the pro-proliferative and apoptosis-resistant phenotype of PAH cells. Using a novel small molecule integrin inhibitor and neutralizing antibodies, we demonstrated that α5ß1 integrin blockade attenuates pulmonary vascular remodeling and improves hemodynamics and RV function in multiple preclinical models. Our results provide converging evidence to consider α5ß1 integrin inhibition as a promising therapy for pulmonary hypertension. One sentence summary: The α5ß1 integrin plays a crucial role in pulmonary vascular remodeling.

Selective inhibition of integrin αvß6 leads to rapid induction of urinary bladder tumors in cynomolgus macaques.

Administration of a novel and selective small molecule integrin αvß6 inhibitor, MORF-627, to young cynomolgus monkeys for 28 days resulted in the rapid induction of epithelial proliferative changes in the urinary bladder of 2 animals, in the absence of test agent genotoxicity. Microscopic findings included suburothelial infiltration by irregular nests and/or trabeculae of epithelial cells, variable cytologic atypia, and high mitotic rate, without invasion into the tunica muscularis. Morphologic features and patterns of tumor growth were consistent with a diagnosis of early-stage invasive urothelial carcinoma. Ki67 immunohistochemistry demonstrated diffusely increased epithelial proliferation in the urinary bladder of several monkeys, including those with tumors, and αvß6 was expressed in some epithelial tissues, including urinary bladder, in monkeys and humans. Spontaneous urothelial carcinomas are extremely unusual in young healthy monkeys, suggesting a direct link of the finding to the test agent. Inhibition of integrin αvß6 is intended to locally and selectively block transforming growth factor beta (TGF-ß) signaling, which is implicated in epithelial proliferative disorders. Subsequent in vitro studies using a panel of integrin αvß6 inhibitors in human bladder epithelial cells replicated the increased urothelial proliferation observed in monkeys and was reversed through exogenous application of TGF-ß. Moreover, analysis of in vivo models of liver and lung fibrosis revealed evidence of epithelial hyperplasia and cell cycle dysregulation in mice treated with integrin αvß6 or TGF-ß receptor I inhibitors. The cumulative evidence suggests a direct link between integrin αvß6 inhibition and decreased TGF-ß signaling in the local bladder environment, with implications for epithelial proliferation and carcinogenesis.

HCV RdRp, sofosbuvir and beyond.

The therapeutic targeting of the nonstructural protein 5B (NS5B) RNA-dependent RNA polymerase (RdRp) of the Hepatitis C Virus (HCV) with nucleotide analogs led to a deep understanding of this enzymes structure, function and substrate specificity. Unlike previously studied DNA polymerases including the reverse transcriptase of Human Immunodeficiency Virus, development of biochemical assays for HCV RdRp proved challenging due to low solubility of the full-length protein and inefficient acceptance of exogenous primer/templates. Despite the poor apparent specific activity, HCV RdRp was found to support rapid and processive transcription once elongation is initiated in vitro consistent with its high level of viral replication in the livers of patients. Understanding of the substrate specificity of HCV RdRp led to the discovery of the active triphosphate of sofosbuvir as a nonobligate chain-terminator of viral RNA transcripts. The ternary crystal structure of HCV RdRp, primer/template, and incoming nucleotide showed the interaction between the nucleotide analog and the 2'-hydroxyl binding pocket and how an unfit mutation of serine 282 to threonine results in resistance by interacting with the uracil base and modified 2'-position of the analog. Host polymerases mediate off-target toxicity of nucleotide analogs and the active metabolite of sofosbuvir was found to not be efficiently incorporated by host polymerases including the mitochondrial RNA polymerase (POLRMT). Knowledge from studying inhibitors of HCV RdRp serves to advance antiviral drug discovery for other emerging RNA viruses including the discovery of remdesivir as an inhibitor of severe acute respiratory syndrome coronavirus 2 (SARS-CoV2), the virus that causes COVID-19.

Species differences in liver accumulation and metabolism of nucleotide prodrug sofosbuvir.

Sofosbuvir (SOF) is a nucleotide prodrug which has been used as a backbone for the clinical treatment of hepatitis C viral infection. Because sofosbuvir undergoes complex first pass metabolism, including metabolic activation to form its pharmacologically active triphosphate (GS-331007-TP) to inhibit the viral RNA polymerase in the liver, it is difficult to project the human dose for clinical evaluation based on preclinical data. Selecting an appropriate animal model for drug exposure in the target tissue is challenging due to differences in absorption, stability, hepatic uptake, and intracellular activation across species. Efficient liver delivery has been established in human liver following administration in a clinical trial of patients receiving sofosbuvir prior to liver transplantation. Using the clinical liver exposure as a benchmark, we assessed and compared the pharmacokinetic profile in mouse, rat, hamster, dog and monkey. Liver accumulation was also assessed in the PXB mouse model in which the liver is mostly populated with human hepatocytes. At human equivalent dose, the hepatic concentrations of GS-331007-TP in dog and PXB mouse were comparable to those observed in the human livers. In these species, high and sustained levels of GS-331007-TP were observed in both primary hepatocytes in vitro and the liver in vivo.

Biochemical characterization of tirabrutinib and other irreversible inhibitors of Bruton's tyrosine kinase reveals differences in on - and off - target inhibition.

BACKGROUND: Bruton's tyrosine kinase (BTK) is a key component of the B-cell receptor (BCR) pathway and a clinically validated target for small molecule inhibitors such as ibrutinib in the treatment of B-cell malignancies. Tirabrutinib (GS-4059/ONO-4059) is a selective, once daily, oral BTK inhibitor with clinical activity against many relapsed/refractory B-cell malignancies. METHODS: Covalent binding of tirabrutinib to BTK Cys-481 was assessed by LC-MSMS analysis of BTK using compound as a variable modification search parameter. Inhibition potency of tirabrutinib, ibrutinib, acalabrutinib, and spebrutinib against BTK and related kinases was studied in a dose-dependent manner either after a fixed incubation time (as used in conventional IC50 studies) or following a time course where inactivation kinetics were measured. RESULTS: Tirabrutinib irreversibly and covalently binds to BTK Cys-481. The inactivation efficiency kinact/Ki was measured and used to calculate selectivity among different kinases for each of the four inhibitors studied. Tirabrutinib showed a kinact/Ki value of 2.4 ± 0.6 × 104 M-1 s-1 for BTK with selectivity against important off-targets. CONCLUSIONS: For the BTK inhibitors tested in this study, analysis of the inactivation kinetics yielded a more accurate measurement of potency and selectivity than conventional single-time point inhibition measurements. Subtle but clear differences were identified between clinically tested BTK inhibitors which may translate into differentiated clinical efficacy and safety. GENERAL SIGNIFICANCE: This is the first study that offers a detailed side-by-side comparison of four clinically-relevant BTK inhibitors with respect to their inactivation of BTK and related kinases.

Inhibition of Acetyl-CoA Carboxylase by Phosphorylation or the Inhibitor ND-654 Suppresses Lipogenesis and Hepatocellular Carcinoma.

The incidence of hepatocellular carcinoma (HCC) is rapidly increasing due to the prevalence of obesity and non-alcoholic fatty liver disease, but the molecular triggers that initiate disease development are not fully understood. We demonstrate that mice with targeted loss-of-function point mutations within the AMP-activated protein kinase (AMPK) phosphorylation sites on acetyl-CoA carboxylase 1 (ACC1 Ser79Ala) and ACC2 (ACC2 Ser212Ala) have increased liver de novo lipogenesis (DNL) and liver lesions. The same mutation in ACC1 also increases DNL and proliferation in human liver cancer cells. Consistent with these findings, a novel, liver-specific ACC inhibitor (ND-654) that mimics the effects of ACC phosphorylation inhibits hepatic DNL and the development of HCC, improving survival of tumor-bearing rats when used alone and in combination with the multi-kinase inhibitor sorafenib. These studies highlight the importance of DNL and dysregulation of AMPK-mediated ACC phosphorylation in accelerating HCC and the potential of ACC inhibitors for treatment.

Transporters in Drug Development: 2018 ITC Recommendations for Transporters of Emerging Clinical Importance.

This white paper provides updated International Transporter Consortium (ITC) recommendations on transporters that are important in drug development following the 3rd ITC workshop. New additions include prospective evaluation of organic cation transporter 1 (OCT1) and retrospective evaluation of organic anion transporting polypeptide (OATP)2B1 because of their important roles in drug absorption, disposition, and effects. For the first time, the ITC underscores the importance of transporters involved in drug-induced vitamin deficiency (THTR2) and those involved in the disposition of biomarkers of organ function (OAT2 and bile acid transporters).

GS-0976 Reduces Hepatic Steatosis and Fibrosis Markers in Patients With Nonalcoholic Fatty Liver Disease.

BACKGROUND & AIMS: De novo lipogenesis is increased in livers of patients with nonalcoholic steatohepatitis (NASH). Acetyl-coenzyme carboxylase catalyzes the rate-limiting step in this process. We evaluated the safety and efficacy of GS-0976, an inhibitor of acetyl-coenzyme A carboxylase in liver, in a phase 2 randomized placebo-controlled trial of patients with NASH. METHODS: We analyzed data from 126 patients with hepatic steatosis of at least 8%, based on the magnetic resonance imaging-estimated proton density fat fraction (MRI-PDFF), and liver stiffness of at least 2.5 kPa, based on magnetic resonance elastography measurement or historical biopsy result consistent with NASH and F1-F3 fibrosis. Patients were randomly assigned (2:2:1) to groups given GS-0976 20 mg, GS-0976 5 mg, or placebo daily for 12 weeks, from August 8, 2016 through July 18, 2017. Measures of hepatic steatosis, stiffness, serum markers of fibrosis, and plasma metabolomics were evaluated. The primary aims were to confirm previous findings and evaluate the relation between dose and efficacy. RESULTS: A relative decrease of at least 30% from baseline in MRI-PDFF (PDFF response) occurred in 48% of patients given GS-0976 20 mg (P = .004 vs placebo), 23% given GS-0976 5 mg (P = .43 vs placebo), and 15% given placebo. Median relative decreases in MRI-PDFF were greater in patients given GS-0976 20 mg (decrease of 29%) than those given placebo (decrease of 8%; P = .002). Changes in magnetic resonance elastography-measured stiffness did not differ among groups, but a dose-dependent decrease in the fibrosis marker tissue inhibitor of metalloproteinase 1 was observed in patients given GS-0976 20 mg. Plasma levels of acylcarnitine species also decreased in patients with a PDFF response given GS-0976 20 mg. GS-0976 was safe, but median relative increases of 11% and 13% in serum levels of triglycerides were observed in patients given GS-0976. CONCLUSIONS: In a randomized placebo-controlled trial of patients with NASH, we found 12-week administration of GS-0976 20 mg decreased hepatic steatosis, selected markers of fibrosis, and liver biochemistry. ClinicalTrials.gov ID NCT02856555.

Nucleotide Prodrug Containing a Nonproteinogenic Amino Acid To Improve Oral Delivery of a Hepatitis C Virus Treatment.

Delivery of pharmacologically active nucleoside triphosphate analogs to sites of viral infection is challenging. In prior work we identified a 2'-C-methyl-1'-cyano-7-deaza-adenosine C-nucleotide analog with desirable selectivity and potency for the treatment of hepatitis C virus (HCV) infection. However, the prodrug selected for clinical development, GS-6620, required a high dose for meaningful efficacy and had unacceptable variability due to poor oral absorption as a result of suboptimal solubility, intestinal metabolism, and efflux transport. While obtaining clinical proof of concept for the nucleotide analog, a more effective prodrug strategy would be necessary for clinical utility. Here, we report an alternative prodrug of the same nucleoside analog identified to address liabilities of GS-6620. A phosphoramidate prodrug containing the nonproteinogenic amino acid methylalanine, an isopropyl ester and phenol in the (S) conformation at phosphorous, GS2, was found to have improved solubility, intestinal stability, and hepatic activation. GS2 is a more selective substrate for hepatically expressed carboxyl esterase 1 (CES1) and is resistant to hydrolysis by more widely expressed hydrolases, including cathepsin A (CatA) and CES2. Unlike GS-6620, GS2 was not cleaved by intestinally expressed CES2 and, as a result, was stable in intestinal extracts. Levels of liver triphosphate following oral administration of GS2 in animals were higher than those of GS-6620, even when administered under optimal conditions for GS-6620 absorption. Combined, these properties suggest that GS2 will have better oral absorption in the clinic when administered in a solid dosage form and the potential to extend the clinical proof of concept obtained with GS-6620.

Acetyl-CoA Carboxylase Inhibition Reverses NAFLD and Hepatic Insulin Resistance but Promotes Hypertriglyceridemia in Rodents.

Pharmacologic inhibition of acetyl-CoA carboxylase (ACC) enzymes, ACC1 and ACC2, offers an attractive therapeutic strategy for nonalcoholic fatty liver disease (NAFLD) through simultaneous inhibition of fatty acid synthesis and stimulation of fatty acid oxidation. However, the effects of ACC inhibition on hepatic mitochondrial oxidation, anaplerosis, and ketogenesis in vivo are unknown. Here, we evaluated the effect of a liver-directed allosteric inhibitor of ACC1 and ACC2 (Compound 1) on these parameters, as well as glucose and lipid metabolism, in control and diet-induced rodent models of NAFLD. Oral administration of Compound 1 preferentially inhibited ACC enzymatic activity in the liver, reduced hepatic malonyl-CoA levels, and enhanced hepatic ketogenesis by 50%. Furthermore, administration for 6 days to high-fructose-fed rats resulted in a 20% reduction in hepatic de novo lipogenesis. Importantly, long-term treatment (21 days) significantly reduced high-fat sucrose diet-induced hepatic steatosis, protein kinase C epsilon activation, and hepatic insulin resistance. ACCi treatment was associated with a significant increase in plasma triglycerides (approximately 30% to 130%, depending on the length of fasting). ACCi-mediated hypertriglyceridemia could be attributed to approximately a 15% increase in hepatic very low-density lipoprotein production and approximately a 20% reduction in triglyceride clearance by lipoprotein lipase (P ≤ 0.05). At the molecular level, these changes were associated with increases in liver X receptor/sterol response element-binding protein-1 and decreases in peroxisome proliferator-activated receptor-α target activation and could be reversed with fenofibrate co-treatment in a high-fat diet mouse model. Conclusion: Collectively, these studies warrant further investigation into the therapeutic utility of liver-directed ACC inhibition for the treatment of NAFLD and hepatic insulin resistance.

ITC Commentary on Metformin Clinical Drug-Drug Interaction Study Design That Enables an Efficacy- and Safety-Based Dose Adjustment Decision.

Metformin drug-drug interaction (DDI) studies are conducted during development of drugs that inhibit organic cation transporters and/or multidrug and toxin extrusion proteins (OCTs/MATEs). Monitoring solely changes in systemic exposure, the typical DDI study endpoint appears inadequate for metformin, which is metabolically stable, has poor passive membrane permeability, and undergoes transporter-mediated tissue distribution and clearance. Evaluation of renal clearance, antihyperglycemic effects, and potentially lactate as an exploratory safety marker, can support rational metformin dose adjustment. The proposed DDI study design aims to adequately inform metformin dosing during comedication.

Acetyl-CoA Carboxylase Inhibitor GS-0976 for 12 Weeks Reduces Hepatic De Novo Lipogenesis and Steatosis in Patients With Nonalcoholic Steatohepatitis.

BACKGROUND & AIMS: Increased de novo lipogenesis (DNL) contributes to the pathogenesis of nonalcoholic steatohepatitis (NASH). Acetyl-CoA carboxylase catalyzes the rate-limiting step in DNL. We evaluated the safety and efficacy of GS-0976, a small molecule inhibitor of acetyl-CoA carboxylase, in patients with NASH. METHODS: In an open-label prospective study, patients with NASH (n = 10) received GS-0976 20 mg orally once daily for 12 weeks. NASH was diagnosed based on a proton density fat fraction estimated by magnetic resonance imaging (MRI-PDFF) ≥10% and liver stiffness by magnetic resonance elastography (MRE) ≥2.88 kPa. The contribution from hepatic DNL to plasma palmitate was measured by 14 days of heavy water labeling before and at the end of treatment. We performed the same labelling protocol in an analysis of healthy volunteers who were not given DNL (controls, n = 10). MRI-PDFF and MRE at baseline, and at weeks 4 and 12 of GS-0976 administration, were measured. We analyzed markers of liver injury and serum markers of fibrosis. RESULTS: The contribution of hepatic DNL to plasma palmitate was significantly greater in patients with NASH compared with controls (43% vs 18%) (P = .003). After 12 weeks administration of GS-0976, the median hepatic DNL was reduced 22% from baseline in patients with NASH (P = .004). Compared with baseline, reductions in MRI-PDFF at week 12 (15.7% vs 9.1% at baseline; P = .006), liver stiffness by MRE (3.4 kPa vs 3.1 kPa at baseline; P = .049), TIMP metallopeptidase inhibitor 1 (275 ng/mL vs 244 ng/mL at baseline; P = .049), and serum level of alanine aminotransferase (101 U/L vs 57 U/L at baseline; P = .23) were consistent with decreased hepatic lipid content and liver injury. At week 12, 7 patients (70%) had a ≥30% decrease in MRI-PDFF. CONCLUSION: In an open-label study, patients with NASH given GS-0976 for 12 weeks had reduced hepatic DNL, steatosis, and markers of liver injury. ClinicalTrials.gov no: NCT02856555.

Coronavirus Susceptibility to the Antiviral Remdesivir (GS-5734) Is Mediated by the Viral Polymerase and the Proofreading Exoribonuclease.

Emerging coronaviruses (CoVs) cause severe disease in humans, but no approved therapeutics are available. The CoV nsp14 exoribonuclease (ExoN) has complicated development of antiviral nucleosides due to its proofreading activity. We recently reported that the nucleoside analogue GS-5734 (remdesivir) potently inhibits human and zoonotic CoVs in vitro and in a severe acute respiratory syndrome coronavirus (SARS-CoV) mouse model. However, studies with GS-5734 have not reported resistance associated with GS-5734, nor do we understand the action of GS-5734 in wild-type (WT) proofreading CoVs. Here, we show that GS-5734 inhibits murine hepatitis virus (MHV) with similar 50% effective concentration values (EC50) as SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV). Passage of WT MHV in the presence of the GS-5734 parent nucleoside selected two mutations in the nsp12 polymerase at residues conserved across all CoVs that conferred up to 5.6-fold resistance to GS-5734, as determined by EC50 The resistant viruses were unable to compete with WT in direct coinfection passage in the absence of GS-5734. Introduction of the MHV resistance mutations into SARS-CoV resulted in the same in vitro resistance phenotype and attenuated SARS-CoV pathogenesis in a mouse model. Finally, we demonstrate that an MHV mutant lacking ExoN proofreading was significantly more sensitive to GS-5734. Combined, the results indicate that GS-5734 interferes with the nsp12 polymerase even in the setting of intact ExoN proofreading activity and that resistance can be overcome with increased, nontoxic concentrations of GS-5734, further supporting the development of GS-5734 as a broad-spectrum therapeutic to protect against contemporary and emerging CoVs.IMPORTANCE Coronaviruses (CoVs) cause severe human infections, but there are no approved antivirals to treat these infections. Development of nucleoside-based therapeutics for CoV infections has been hampered by the presence of a proofreading exoribonuclease. Here, we expand the known efficacy of the nucleotide prodrug remdesivir (GS-5734) to include a group ß-2a CoV. Further, GS-5734 potently inhibits CoVs with intact proofreading. Following selection with the GS-5734 parent nucleoside, 2 amino acid substitutions in the nsp12 polymerase at residues that are identical across CoVs provide low-level resistance to GS-5734. The resistance mutations decrease viral fitness of MHV in vitro and attenuate pathogenesis in a SARS-CoV animal model of infection. Together, these studies define the target of GS-5734 activity and demonstrate that resistance is difficult to select, only partial, and impairs fitness and virulence of MHV and SARS-CoV, supporting further development of GS-5734 as a potential effective pan-CoV antiviral.

Sofosbuvir and Ribavirin Liver Pharmacokinetics in Patients Infected with Hepatitis C Virus.

Sofosbuvir and ribavirin exert their anti-hepatitis C virus (anti-HCV) activity following metabolic activation in the liver. However, intrahepatic concentrations of the pharmacologically active nucleotide metabolites in humans are poorly characterized due to the inaccessibility of tissue and technical challenges with measuring nucleotide levels. A clinical study assessing the efficacy of sofosbuvir and ribavirin administered prior to liver transplantation to prevent HCV recurrence provided a unique opportunity to quantify nucleotide concentrations in human liver. We analyzed nucleotides using high-performance liquid chromatography coupled to tandem mass spectrometry in liver tissue from 30 HCV-infected patients with hepatocellular carcinoma who were administered sofosbuvir (400 mg/day) and ribavirin (1,000 to 1,200 mg/day) for 3 to 52 weeks prior to liver transplantation. Median total hepatic metabolite concentrations (the sum of nucleoside and mono-, di-, and triphosphates) were 77.1 µM for sofosbuvir and 361 µM for ribavirin in patients on therapy at the time of transplantation. Ribavirin and sofosbuvir efficiently loaded the liver, with total hepatic metabolite concentrations exceeding maximal levels in plasma by approximately 30-fold. Ribavirin metabolite levels suggest that its monophosphate is in great excess of its inhibition constant for IMP dehydrogenase and that its triphosphate is approaching the binding constant for incorporation by the HCV NS5B RNA-dependent RNA polymerase. In accordance with the potent antiviral activity of sofosbuvir, these results demonstrate that the liver triphosphate levels achieved following sofosbuvir administration greatly exceed the inhibition constant for HCV NS5B. In conclusion, this study expands the quantitative understanding of the pharmacology of sofosbuvir and ribavirin by establishing efficient hepatic delivery in the clinic. (This study has been registered at ClinicalTrials.gov under identifier NCT01559844.).

Transporter Expression in Noncancerous and Cancerous Liver Tissue from Donors with Hepatocellular Carcinoma and Chronic Hepatitis C Infection Quantified by LC-MS/MS Proteomics.

Protein expression of major hepatobiliary drug transporters (NTCP, OATPs, OCT1, BSEP, BCRP, MATE1, MRPs, and P-gp) in cancerous (C, n = 8) and adjacent noncancerous (NC, n = 33) liver tissues obtained from patients with chronic hepatitis C with hepatocellular carcinoma (HCV-HCC) were quantified by LC-MS/MS proteomics. Herein, we compare our results with our previous data from noninfected, noncirrhotic (control, n = 36) and HCV-cirrhotic (n = 30) livers. The amount of membrane protein yielded from NC and C HCV-HCC tissues decreased (31%, 67%) relative to control livers. In comparison with control livers, with the exception of NTCP, MRP2, and MATE1, transporter expression decreased in NC (38%-76%) and C (56%-96%) HCV-HCC tissues. In NC HCV-HCC tissues, NTCP expression increased (113%), MATE1 expression decreased (58%), and MRP2 expression was unchanged relative to control livers. In C HCV-HCC tissues, NTCP and MRP2 expression decreased (63%, 56%) and MATE1 expression was unchanged relative to control livers. Compared with HCV-cirrhotic livers, aside from NTCP, OCT1, BSEP, and MRP2, transporter expression decreased in NC (41%-71%) and C (54%-89%) HCV-HCC tissues. In NC HCV-HCC tissues, NTCP and MRP2 expression increased (362%, 142%), whereas OCT1 and BSEP expression was unchanged. In C HCV-HCC tissues, OCT1 and BSEP expression decreased (90%, 80%) relative to HCV-cirrhotic livers, whereas NTCP and MRP2 expression was unchanged. Expression of OATP2B1, BSEP, MRP2, and MRP3 decreased (56%-72%) in C HCV-HCC tissues in comparison with matched NC tissues (n = 8), but the expression of other transporters was unchanged. These data will be helpful in the future to predict transporter-mediated hepatocellular drug concentrations in patients with HCV-HCC.

Beyond drug-drug interactions: effects of transporter inhibition on endobiotics, nutrients and toxins.

INTRODUCTION: Membrane transport proteins play a central role in regulating the disposition of endobiotics, dietary nutrients and environmental toxins. The inhibition of transporters by drugs has potential physiologic consequences. The full extent of the effect of drugs on the function of transporters is poorly understood because only a small subset of the hundreds of transporters expressed in humans - primarily those mediating the rate-determining step in the elimination of specific drugs - are assessed during clinical development. Areas covered: We provide a comprehensive overview of literature reports implicating the inhibition of transporters as the mechanism for off-target effects of drugs. Expert opinion: Transporter inhibition, the mechanism of action of many marketed drugs, appears to play an underappreciated role in a number of side effects including vitamin deficiency, edema, dyslipidemia, cholestasis and gout. Cell systems more broadly expressing transporter networks and methods like unbiased metabolomics should be incorporated into the screening paradigm to expand our understanding of the impact of drugs on the physiologic function of transporters and to allow for these effects to be taken into account in drug discovery and clinical practice.

Broad-spectrum antiviral GS-5734 inhibits both epidemic and zoonotic coronaviruses.

Emerging viral infections are difficult to control because heterogeneous members periodically cycle in and out of humans and zoonotic hosts, complicating the development of specific antiviral therapies and vaccines. Coronaviruses (CoVs) have a proclivity to spread rapidly into new host species causing severe disease. Severe acute respiratory syndrome CoV (SARS-CoV) and Middle East respiratory syndrome CoV (MERS-CoV) successively emerged, causing severe epidemic respiratory disease in immunologically naïve human populations throughout the globe. Broad-spectrum therapies capable of inhibiting CoV infections would address an immediate unmet medical need and could be invaluable in the treatment of emerging and endemic CoV infections. We show that a nucleotide prodrug, GS-5734, currently in clinical development for treatment of Ebola virus disease, can inhibit SARS-CoV and MERS-CoV replication in multiple in vitro systems, including primary human airway epithelial cell cultures with submicromolar IC50 values. GS-5734 was also effective against bat CoVs, prepandemic bat CoVs, and circulating contemporary human CoV in primary human lung cells, thus demonstrating broad-spectrum anti-CoV activity. In a mouse model of SARS-CoV pathogenesis, prophylactic and early therapeutic administration of GS-5734 significantly reduced lung viral load and improved clinical signs of disease as well as respiratory function. These data provide substantive evidence that GS-5734 may prove effective against endemic MERS-CoV in the Middle East, circulating human CoV, and, possibly most importantly, emerging CoV of the future.

Discovery of a 2'-fluoro-2'-C-methyl C-nucleotide HCV polymerase inhibitor and a phosphoramidate prodrug with favorable properties.

A series of 2'-fluorinated C-nucleosides were prepared and tested for anti-HCV activity. Among them, the triphosphate of 2'-fluoro-2'-C-methyl adenosine C-nucleoside (15) was a potent and selective inhibitor of the NS5B polymerase and maintained activity against the S282T resistance mutant. A number of phosphoramidate prodrugs were then prepared and evaluated leading to the identification of the 1-aminocyclobutane-1-carboxylic acid isopropyl ester variant (53) with favorable pharmacokinetic properties including efficient liver delivery in animals.

GS-5734 and its parent nucleoside analog inhibit Filo-, Pneumo-, and Paramyxoviruses.

GS-5734 is a monophosphate prodrug of an adenosine nucleoside analog that showed therapeutic efficacy in a non-human primate model of Ebola virus infection. It has been administered under compassionate use to two Ebola patients, both of whom survived, and is currently in Phase 2 clinical development for treatment of Ebola virus disease. Here we report the antiviral activities of GS-5734 and the parent nucleoside analog across multiple virus families, providing evidence to support new indications for this compound against human viruses of significant public health concern.

Discovery and Synthesis of a Phosphoramidate Prodrug of a Pyrrolo[2,1-f][triazin-4-amino] Adenine C-Nucleoside (GS-5734) for the Treatment of Ebola and Emerging Viruses.

The recent Ebola virus (EBOV) outbreak in West Africa was the largest recorded in history with over 28,000 cases, resulting in >11,000 deaths including >500 healthcare workers. A focused screening and lead optimization effort identified 4b (GS-5734) with anti-EBOV EC50 = 86 nM in macrophages as the clinical candidate. Structure activity relationships established that the 1'-CN group and C-linked nucleobase were critical for optimal anti-EBOV potency and selectivity against host polymerases. A robust diastereoselective synthesis provided sufficient quantities of 4b to enable preclinical efficacy in a non-human-primate EBOV challenge model. Once-daily 10 mg/kg iv treatment on days 3-14 postinfection had a significant effect on viremia and mortality, resulting in 100% survival of infected treated animals [ Nature 2016 , 531 , 381 - 385 ]. A phase 2 study (PREVAIL IV) is currently enrolling and will evaluate the effect of 4b on viral shedding from sanctuary sites in EBOV survivors.