Cathepsin A is the major hydrolase catalyzing the intracellular hydrolysis of the antiretroviral nucleotide phosphonoamidate prodrugs GS-7340 and GS-9131

GS-7340 and GS-9131 {9-[(R)-2-[[(S)-[[(S)-1-(isopropoxycarbonyl)ethyl]amino]phenoxyphosphinyl]methoxy]-propyl]adenine and 9-(R)-4'-(R)-[[[(S)-1-[(ethoxycarbonyl)ethyl]amino]phenoxyphosphinyl]methoxy]-2'-fluoro-1'-furanyladenine, respectively} are novel alkylalaninyl phenyl ester prodrugs of tenofovir {9-R-[(2-phosphonomethoxy)propyl]adenine} (TFV) and a cyclic nucleotide analog, GS-9148 (phosphonomethoxy-2'-fluoro-2', 3'-dideoxydidehydroadenosine), respectively. Both prodrugs exhibit potent antiretroviral activity against both wild-type and drug-resistant human immunodeficiency virus type 1 strains and excellent in vivo pharmacokinetic properties. In this study, the main enzymatic activity responsible for the initial step in the intracellular activation of GS-7340 and GS-9131 was isolated from human peripheral blood mononuclear cells and identified as lysosomal carboxypeptidase A (cathepsin A [CatA]; EC 3.4.16.5). Biochemical properties of the purified hydrolase (native complex and catalytic subunit molecular masses of 100 and 29 kDa, respectively; isoelectric point [pI] of 5.5) matched those of CatA. Recombinant CatA and the isolated prodrug hydrolase displayed identical susceptibilities to inhibitors and identical substrate preferences towards a panel of tenofovir phosphonoamidate prodrugs. Incubation of both enzymes with 14C-labeled GS-7340 or [3H]difluorophosphonate resulted in the covalent labeling of identical 29-kDa catalytic subunits. Finally, following a 4-h incubation with GS-7340 and GS-9131, the intracellular concentrations of prodrug metabolites detected in CatA-negative fibroblasts were approximately 7.5- and 3-fold lower, respectively, than those detected in normal control fibroblasts. Collectively, these data demonstrate the key role of CatA in the intracellular activation of nucleotide phosphonoamidate prodrugs and open new possibilities for further improvement of this important class of antiviral prodrugs.     

Hepatitis C Virus Genotype 1 to 6 Protease Inhibitor Escape Variants: In Vitro Selection,Fitness, and Resistance Patterns in the Context of the Infectious Viral Life Cycle

Hepatitis C virus (HCV) NS3 protease inhibitors (PIs) are important components of novel HCV therapy regimens. Studies of PI resistance initially focused on genotype 1. Therefore, knowledge about the determinants of PI resistance for the highly prevalent genotypes 2 to 6 remains limited. Using Huh7.5 cell culture-infectious HCV recombinants with genotype 1 to 6 NS3 protease, we identified protease positions 54, 155, and 156 as hot spots for the selection of resistance substitutions under treatment with the first licensed PIs, telaprevir and boceprevir. Treatment of a genotype 2 isolate with the newer PIs vaniprevir, faldaprevir, simeprevir, grazoprevir, paritaprevir, and deldeprevir identified positions 156 and 168 as hot spots for resistance; the Y56H substitution emerged for three newer PIs. Substitution selection also depended on the specific recombinant. The substitutions identified conferred cross-resistance to several PIs; however, most substitutions selected under telaprevir or boceprevir treatment conferred less resistance to certain newer PIs. In a single-cycle production assay, across genotypes, PI treatment primarily decreased viral replication, which was rescued by PI resistance substitutions. The substitutions identified resulted in differential effects on viral fitness, depending on the original recombinant and the substitution. Across genotypes, fitness impairment induced by resistance substitutions was due primarily to decreased replication. Most combinations of substitutions that were identified increased resistance or fitness. Combinations of resistance substitutions with fitness-compensating substitutions either rescued replication or compensated for decreased replication by increasing assembly. This comprehensive study provides insight into the selection patterns and effects of PI resistance substitutions for HCV genotypes 1 to 6 in the context of the infectious viral life cycle, which is of interest for clinical and virological HCV research.     

Substitutions at NS3 Residue 155, 156, or 168 of Hepatitis C Virus Genotypes 2 to 6 Induce Complex Patterns of Protease Inhibitor Resistance

Various protease inhibitors (PIs) currently are becoming available for treatment of hepatitis C virus (HCV). For genotype 1, substitutions at NS3 protease positions 155, 156, and 168 are the main determinants of PI resistance. For other genotypes, similar substitutions were selected during PI treatment but were not characterized systematically. To elucidate the impact of key PI resistance substitutions on genotypes 2 to 6, we engineered the substitutions R155A/E/G/H/K/Q/T, A156G/S/T/V, and D/Q168A/E/G/H/N/V into HCV recombinants expressing genotype 2 to 6 proteases. We evaluated viral fitness and sensitivity to nine PIs (telaprevir, boceprevir, simeprevir, asunaprevir, vaniprevir, faldaprevir, paritaprevir, deldeprevir, and grazoprevir) in Huh7.5 cells. We found that most variants showed decreased fitness compared to that of the original viruses. Overall, R155K, A156G/S, and D/Q168A/E/H/N/V variants showed the highest fitness; however, genotype 4 position 168 variants showed strong fitness impairment. Most variants tested were resistant to several PIs. Resistance levels varied significantly depending on the specific substitution, genotype, and PI. For telaprevir and boceprevir, specific 155 and 156, but not 168, variants proved resistant. For the remaining PIs, most genotype 2, 4, 5, and 6, but not genotype 3, variants showed various resistance levels. Overall, grazoprevir (MK-5172) had the highest efficacy against original viruses and variants. This is the first comprehensive study revealing the impact of described key PI resistance substitutions on fitness and PI resistance of HCV genotypes 2 to 6. In conclusion, the studied substitutions induced resistance to a panel of clinically relevant PIs, including the newer PIs paritaprevir, deldeprevir, and grazoprevir. We discovered complex patterns of resistance, with the impact of substitutions varying from increased sensitivity to high resistance.     

Using Pharmacokinetic and Viral Kinetic Modeling To Estimate the Antiviral Effectiveness of Telaprevir,Boceprevir, and Pegylated Interferon during Triple Therapy in Treatment-Experienced Hepatitis C Virus-Infected Cirrhotic Patients

Triple therapy combining a protease inhibitor (PI) (telaprevir or boceprevir), pegylated interferon (PEG-IFN), and ribavirin (RBV) has dramatically increased the chance of eradicating hepatitis C virus (HCV). However, the efficacy of this treatment remains suboptimal in cirrhotic treatment-experienced patients. Here, we aimed to better understand the origin of this impaired response by estimating the antiviral effectiveness of each drug. Fifteen HCV genotype 1-infected patients with compensated cirrhosis, who were nonresponders to prior PEG-IFN/RBV therapy, were enrolled in a nonrandomized study. HCV RNA and concentrations of PIs, PEG-IFN, and RBV were frequently assessed in the first 12 weeks of treatment and were analyzed using a pharmacokinetic/viral kinetic model. The two PIs achieved similar levels of molar concentrations (P = 0.5), but there was a significant difference in the 50% effective concentrations (EC₅₀) (P = 0.008), leading to greater effectiveness for telaprevir than for boceprevir in blocking viral production (99.8% versus 99.0%, respectively, P = 0.002). In all patients, the antiviral effectiveness of PEG-IFN was modest (43.4%), and there was no significant contribution of RBV exposure to the total antiviral effectiveness. The second phase of viral decline, which is attributed to the loss rate of infected cells, was slow (0.19 day⁻¹) and was higher in patients who subsequently eradicated HCV (P = 0.03). The two PIs achieved high levels of antiviral effectiveness. However, the suboptimal antiviral effectiveness of PEG-IFN/RBV and the low loss of infected cells suggest that a longer treatment duration might be needed in cirrhotic treatment-experienced patients and that a future IFN-free regimen may be particularly beneficial in these patients.     

Phenotypic characterization of resistant Val36 variants of hepatitis C virus NS3-4A serine protease

In patients chronically infected with hepatitis C virus (HCV) strains of genotype 1, rapid and dramatic antiviral activity has been observed with telaprevir (VX-950), a highly selective and potent inhibitor of the HCV NS3-4A serine protease. HCV variants with substitutions in the NS3 protease domain were observed in some patients during telaprevir dosing. In this study, purified protease domain proteins and reconstituted HCV subgenomic replicons were used for phenotypic characterization of many of these substitutions. V36A/M or T54A substitutions conferred less than eightfold resistance to telaprevir. Variants with double substitutions at Val36 plus Thr54 had approximately 20-fold resistance to telaprevir, and variants with double substitutions at Val36 plus Arg155 or Ala156 had >40-fold resistance to telaprevir. An X-ray structure of the HCV strain H protease domain containing the V36M substitution in a cocomplex with an NS4A cofactor peptide was solved at a 2.4-A resolution. Except for the side chain of Met36, the V36M variant structure is identical to that of the wild-type apoenzyme. The in vitro replication capacity of most variants was significantly lower than that of the wild-type replicon in cells, which is consistent with the impaired in vivo fitness estimated from telaprevir-dosed patients. Finally, the sensitivity of these replicon variants to alpha interferon or ribavirin remained unchanged compared to that of the wild-type.     

Telaprevir for the treatment of chronic hepatitis C infection

Telaprevir is an NS3/4A protease inhibitor that has recently received US FDA approval for the treatment of chronic HCV infection. Telaprevir is given in combination with peg-IFN-α and ribavirin and is indicated for both treatment-naive and treatment-experienced patients with genotype 1 infection. Along with the other first generation NS3/4A protease inhibitor boceprevir, these combination regimens have immediately become the standard of care for genotype 1 patients. The adverse event profile for the combination regimen remains dominated by peg-IFN-α and ribavirin, but there is additional anemia and rash with telaprevir. Owing to telaprevir’s metabolism by the CYP3/4A pathway, drug–drug interactions could lead to toxicity from other medications or decreased efficacy of telaprevir. Viral resistance can develop during treatment with telaprevir, and patients will need to be educated on their role in adherence to minimize the risk of resistance and improve their chances of cure of HCV infection.     

Lack of Clinically Significant Pharmacokinetic Interaction between the Thrombopoietin Receptor Agonist Eltrombopag and Hepatitis C Virus Protease Inhibitors Boceprevir and Telaprevir

Eltrombopag is an orally bioavailable thrombopoietin receptor agonist approved for the treatment of thrombocytopenia associated with chronic immune (idiopathic) thrombocytopenic purpura and chronic hepatitis C virus (HCV) infection. This study evaluated the potential drug-drug interactions between eltrombopag and the HCV protease inhibitors boceprevir and telaprevir. In this open-label, 3-period, single-sequence, and crossover study, 56 healthy adult subjects were randomized 1:1 to cohort 1 (boceprevir) or 2 (telaprevir). The dosing was as follows: period 1, single 200-mg dose of eltrombopag; period 2, 800 mg boceprevir or 750 mg telaprevir every 8 hours (q8h) for 10 days; and period 3, single 200-mg dose of eltrombopag with either 800 mg boceprevir or 750 mg telaprevir q8h (3 doses). All doses were administered with food, and eltrombopag was administered specifically with low-calcium food. There was a 3-day washout between periods 1 and 2 and no washout between periods 2 and 3. Serial pharmacokinetic samples were collected for 72 h in periods 1 and 3 and for 8 h in period 2. The coadministration of eltrombopag increased the rate of boceprevir absorption, resulting in a 20% increase in the maximum concentration in plasma (C_max), a 1-h-earlier time to C_max (T_max) for boceprevir, a 32% decrease in the concentration at the end of the dosing interval (C_τ), and no change in the area under the concentration-time curve over the dosing interval (AUC_0-τ). The coadministration of eltrombopag did not alter telaprevir pharmacokinetics, and the coadministration of boceprevir or telaprevir did not alter eltrombopag pharmacokinetics. Dysgeusia, headache, and somnolence occurred in ≥2 subjects. One subject withdrew because of nausea, headache, dizziness, sinus pressure, and vomiting. There were no severe or serious adverse events. Dose adjustment is not required when eltrombopag is coadministered with boceprevir or telaprevir given the lack of clinically significant pharmacokinetic interaction.     

Telaprevir for the treatment of chronic hepatitis C infection

Telaprevir is an NS3/4A protease inhibitor that has recently received US FDA approval for the treatment of chronic HCV infection. Telaprevir is given in combination with peg-IFN-α and ribavirin and is indicated for both treatment-naive and treatment-experienced patients with genotype 1 infection. Along with the other first generation NS3/4A protease inhibitor boceprevir, these combination regimens have immediately become the standard of care for genotype 1 patients. The adverse event profile for the combination regimen remains dominated by peg-IFN-α and ribavirin, but there is additional anemia and rash with telaprevir. Owing to telaprevir's metabolism by the CYP3/4A pathway, drug-drug interactions could lead to toxicity from other medications or decreased efficacy of telaprevir. Viral resistance can develop during treatment with telaprevir, and patients will need to be educated on their role in adherence to minimize the risk of resistance and improve their chances of cure of HCV infection.     

New antiviral therapies in the management of HCV infection

Improved knowledge of the HCV life cycle and of structural features of HCV proteins have led to the discovery of numerous potential targets for antiviral therapy. Viral replication and polyprotein processing have been tagged as promising viral targets. Clathrin-mediated endocytosis, fusion of HCV with cellular membranes, translation of viral RNA, virus production and release as well as several host cell factors may provide alternative targets for future anti-HCV therapies. Several compounds are currently under investigation in clinical trials and showed high antiviral activity in patients with chronic hepatitis C. Recently, Phase III studies for two protease inhibitors, telaprevir and boceprevir, each given in combination with pegylated interferon (standard of care [SOC]), were completed. In HCV-genotype-1-infected patients, the addition of telaprevir or boceprevir to SOC increased sustained virological response rates from <50% to >70%. Nucleoside/nucleotide inhibitors of the HCV NS5B polymerase have shown antiviral activity against different HCV genotypes, and have a higher barrier to resistance than protease inhibitors. In addition, several allosteric binding sites have been identified for non-nucleoside inhibitors of the NS5B polymerase. Inhibitors of NS5A are potentially active against all HCV genotypes. Among the different host cell-targeting compounds, cyclophilin inhibitors have shown promising results. Future hope lies in the combination of direct-acting antiviral agents with the possibility of interferon-free treatment regimens.     

Protease inhibitors: silver bullets for chronic hepatitis C infection?

Recent trials evaluated the safety and efficacy of two protease inhibitors, boceprevir (Victrelis) and telaprevir (Incivek), added to standard care with pegylated interferon and ribavirin, in patients with chronic hepatitis C virus (HCV) infection. These drugs open the door for triple therapy and other new therapies involving combinations of other direct-acting antiviral agents to become the new standard of care for this population.     

Restoration of the Activated Rig-I Pathway in Hepatitis C Virus (HCV) Replicon Cells by HCV Protease,Polymerase, and NS5A Inhibitors In Vitro at Clinically Relevant Concentrations

Development of persistent hepatitis C virus (HCV) infection may be mediated by HCV NS3 · 4A protease-dependent inhibition of host innate immunity. When double-stranded RNA (dsRNA) is detected in virus-infected cells, host innate immunity mounts an antiviral response by upregulating production of type I interferons (α/β interferon [IFN-α/β]); HCV counters by cleaving the IFN-β stimulator 1 (IPS-1) adaptor protein, decreasing synthesis of IFN-α/β. We evaluated HCV protease (telaprevir, boceprevir, and TMC435350), polymerase (HCV-796 and VX-222), and NS5A (BMS-790052) inhibitors for the ability to restore IPS-1-mediated Rig-I signaling by measuring Sendai virus-induced IFN-β promoter activation in HCV replicon cells after various exposure durations. All direct-acting HCV antivirals tested restored mitochondrial localization of IPS-1 and rescued Sendai virus-induced IRF3 signaling after 7 days by inhibiting HCV replication, thereby reducing the abundance of HCV NS3 · 4A protease. With 4-day treatment, HCV protease inhibitors, but not polymerase inhibitors, restored mitochondrial localization of IPS-1 and rescued IFN-β promoter activation in the presence of equivalent levels of NS3 protein in protease or polymerase inhibitor-treated cells. The concentrations of HCV protease and polymerase inhibitors needed to rescue IRF3-mediated signaling in vitro were in the range of those observed in vivo in the plasma of treated HCV patients. These findings suggest that (i) HCV protease, polymerase, and NS5A inhibitors can restore virus-induced IRF3 signaling by inhibiting viral replication, thereby reducing NS3 protease levels, and (ii) HCV protease inhibitors can restore innate immunity by directly inhibiting NS3 protease-mediated cleavage of IPS-1 at clinically achievable concentrations.     

Selection and characterization of hepatitis C virus replicons dually resistant to the polymerase and protease inhibitors HCV-796 and boceprevir (SCH 503034)

HCV-796 is a nonnucleoside inhibitor of the hepatitis C virus (HCV) nonstructural protein 5B (NS5B) polymerase, and boceprevir is an inhibitor of the NS3 serine protease. The emergence of replicon variants resistant to the combination of HCV-796 and boceprevir was evaluated. Combining the inhibitors greatly reduced the frequency with which resistant colonies arose; however, some resistant replicon cells could be isolated by the use of low inhibitor concentrations. These replicons were approximately 1,000-fold less susceptible to HCV-796 and 9-fold less susceptible to boceprevir. They also exhibited resistance to anthranilate nonnucleoside inhibitors of NS5B but were fully sensitive to inhibitors of different mechanisms: a pyranoindole, Hsp90 inhibitors, an NS5B nucleoside inhibitor, and pegylated interferon (Peg-IFN). The replicon was cleared from the combination-resistant cells by extended treatment with Peg-IFN. Mutations known to confer resistance to HCV-796 (NS5B C316Y) and boceprevir (NS3 V170A) were present in the combination-resistant replicons. These changes could be selected together and coexist in the same genome. The replicon bearing both changes exhibited reduced sensitivity to inhibition by HCV-796 and boceprevir but had a reduced replicative capacity.     

Molecular assessment of the potential for renal drug interactions between tenofovir and HIV protease inhibitors

BACKGROUND: Active renal secretion of tenofovir (TFV) across proximal tubules occurs via uptake by human organic anion transporters 1 and 3 (hOAT1 and hOAT3) coupled with efflux by multidrug resistance protein 4 (MRP4). Co-administration of some HIV protease inhibitors (PIs) with tenofovir disoproxil fumarate (TDF), an oral prodrug of TFV, has been shown to increase systemic levels of TFV, leading to a hypothesis that PIs may affect tubular secretion of TFV and potentially alter the renal safety of TDF. METHODS: The effect of PIs on the transport of TFV by hOAT1, hOAT3 and MRP4 was assessed using in vitro cell-based transport models. RESULTS: At concentrations equal to their therapeutic peak plasma levels (Cmax) all PIs showed <20% inhibition of TFV transport by hOAT1. hOAT3 was more sensitive to Pls with ritonavir (RTV) and lopinavir being the most potent inhibitors of TFV transport (62% and 37% inhibition, respectively, at their Cmax). In the absence of human serum, RTV at concentrations exceeding its therapeutic Cmax also exhibited a minor effect on the cellular efflux of TFV by MRP4 (<30% inhibition at 20 microM). However, no effects of PIs on hOAT1, hOAT3 or MRP4 were detected in the presence of human serum with the exception of RTV that inhibited hOAT3 by approximately 35% at its Cmax. In addition, PIs did not affect the cytotoxicity of TFV or TDF in MRP4- or MRP2-overexpressing cells. CONCLUSION: These data indicate a low potential of PIs to interfere with the active tubular secretion of TFV and to alter the clinical renal safety profile of TDF.     

New pharmacotherapy for hepatitis C

Chronic hepatitis C infection remains a major global public health burden associated with substantial morbidity and mortality. Recent advances in antiviral therapy with the US Food and Drug Administration (FDA) approval of the oral protease inhibitors boceprevir and telaprevir introduce a new era of treatment for hepatitis C based on directly acting antiviral agents, which are associated with significant improvements in viral eradication rates in combination with pegylated interferon plus ribavirin. Newer classes targeting the hepatitis C virus (HCV) protease, polymerase, NS5A, and other components of the viral genome demonstrate great promise to further enhance viral eradication with superior efficacy, improved tolerability, shorter duration of therapy, and diminished requirement for interferon. Current and future strategies for HCV pharmacotherapy are reviewed.     

Differential Mechanisms of Tenofovir and Tenofovir Disoproxil Fumarate Cellular Transport and Implications for Topical Preexposure Prophylaxis

Intravaginal rings releasing tenofovir (TFV) or its prodrug, tenofovir disoproxil fumarate (TDF), are being evaluated for HIV and herpes simplex virus (HSV) prevention. The current studies were designed to determine the mechanisms of drug accumulation in human vaginal and immune cells. The exposure of vaginal epithelial or T cells to equimolar concentrations of radiolabeled TDF resulted in over 10-fold higher intracellular drug levels than exposure to TFV. Permeability studies demonstrated that TDF, but not TFV, entered cells by passive diffusion. TDF uptake was energy independent but its accumulation followed nonlinear kinetics, and excess unlabeled TDF inhibited radiolabeled TDF uptake in competition studies. The carboxylesterase inhibitor bis-nitrophenyl phosphate reduced TDF uptake, suggesting saturability of intracellular carboxylesterases. In contrast, although TFV uptake was energy dependent, no competition between unlabeled and radiolabeled TFV was observed, and the previously identified transporters, organic anion transporters (OATs) 1 and 3, were not expressed in human vaginal or T cells. The intracellular accumulation of TFV was reduced by the addition of endocytosis inhibitors, and this resulted in the loss of TFV antiviral activity. Kinetics of drug transport and metabolism were monitored by quantifying the parent drugs and their metabolites by high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS). Results were consistent with the identified mechanisms of transport, and the exposure of vaginal epithelial cells to equimolar concentrations of TDF compared to TFV resulted in ∼40-fold higher levels of the active metabolite, tenofovir diphosphate. Together, these findings indicate that substantially lower concentrations of TDF than TFV are needed to protect cells from HIV and HSV-2.     

Effects of human immunodeficiency virus protease inhibitors on the intestinal absorption of tenofovir disoproxil fumarate in vitro

Human immunodeficiency virus protease inhibitors (PIs) modestly affect the plasma pharmacokinetics of tenofovir (TFV; -15% to +37% change in exposure) following coadministration with the oral prodrug TFV disoproxil fumarate (TDF) by a previously undefined mechanism. TDF permeation was found to be reduced by the combined action of ester cleavage and efflux transport in vitro. Saturable TDF efflux observed in Caco-2 cells suggests that at pharmacologically relevant intestinal concentrations, transport has only a limited effect on TDF absorption, thus minimizing the magnitude of potential intestinal drug interactions. Most tested PIs increased apical-to-basolateral TDF permeation and decreased secretory transport in MDCKII cells overexpressing P-glycoprotein (Pgp; MDCKII-MDR1 cells) and Caco-2 cells. PIs were found to cause a multifactorial effect on the barriers to TDF absorption. All PIs showed similar levels of inhibition of esterase-dependent degradation of TDF in an intestinal subcellular fraction, except for amprenavir, which was found to be a weaker inhibitor. All PIs caused a dose-dependent increase in the accumulation of a model Pgp substrate in MDCKII-MDR1 cells. Pgp inhibition constants ranged from 10.3 microM (lopinavir) to >100 microM (amprenavir, indinavir, and darunavir). Analogous to hepatic cytochrome P450-mediated drug interactions, we propose that the relative differences in perturbations in TFV plasma levels when TDF is coadministered with PIs are based in part on the net effect of inhibition and induction of intestinal Pgp by PIs. Combined with prior studies, these findings indicate that intestinal absorption is the mechanism for changes in TFV plasma levels when TDF is coadministered with PIs.