Sofosbuvir, Velpatasvir and Voxilaprevir

Sofosbuvir is a pan-genotypic inhibitor of the HCV NS5B RNA-dependent RNA polymerase, which is required for viral replication. Sofosbuvir is a nucleotide prodrug that undergoes intracellular metabolism to form the pharmacologically active uridine analogue triphosphate (GS-461203), which can be incorporated into HCV RNA by the NS5B polymerase and acts as a chain terminator. In a biochemical assay, GS-461203 inhibited the polymerase activity of the recombinant NS5B from HCV genotype 1b, 2a, 3a, and 4a. GS-461203 is neither an inhibitor of human DNA and RNA polymerases nor an inhibitor of mitochondrial RNA polymerase.

Velpatasvir is a pan-genotypic HCV inhibitor targeting the HCV NS5A protein, which is required for viral replication.

Voxilaprevir is a pan-genotypic inhibitor of the HCV NS3/4A protease. Voxilaprevir acts as a noncovalent, reversible inhibitor of the NS3/4A protease.

Antiviral activity

The 50% effective concentration (EC~50~) values of sofosbuvir, velpatasvir and voxilaprevir against full-length or chimeric replicons encoding NS5B, NS5A and NS3 protease sequences from the laboratory strains are presented in Table 1. The EC~50~ values of sofosbuvir, velpatasvir and voxilaprevir against clinical isolates are presented in Table 2.

Table 1. Activity of sofosbuvir, velpatasvir and voxilaprevir against full-length or chimeric laboratory replicons:

NA: Not available
^a Mean value from multiple experiments of same laboratory replicon.
^b Stable chimeric 1b replicons carrying NS5B genes from genotype 2b, 5a or 6a were used for testing.
^c Data from various strains of full length NS5A replicons or chimeric NS5A replicons carrying full-length NS5A genes that contain L31 or M31 polymorphisms.
^d Data from a chimeric NS5A replicon carrying NS5A amino acids 9-184.
^e Stable cell lines expressing Renilla luciferase-encoding replicons.
^f Data obtained from transiently transfected replicons.

Table 2. Activity of sofosbuvir, velpatasvir and voxilaprevir against transient replicons containing NS5A, NS5B or NS3 protease from clinical isolates:

NA: Not available

The presence of 40% human serum had no effect on the anti-HCV activity of sofosbuvir but reduced the anti-HCV activity of velpatasvir and voxilaprevir by 13- and 6.8-fold, respectively, against genotype 1a HCV replicons.

Resistance

In cell culture

For sofosbuvir, the NS5B substitution S282T was selected in genotype 1-6 replicons and was associated with 2- to 18-fold reduced susceptibility to sofosbuvir.

For velpatasvir in genotype 1-6 replicons, resistance-associated substitutions selected in 2 or more genotypes were L31I/V and Y93H. Site directed mutagenesis of NS5A resistance associated variants (RAVs) showed that substitutions conferring a >100-fold reduction in velpatasvir susceptibility are M28G, A92K and Y93H/N/R/W in genotype 1a, A92K in genotype 1b, C92T and Y93H/N in genotype 2b, Y93H in genotype 3, and L31V and P32A/L/Q/R in genotype 6. No individual RAV tested in genotypes 2a, 4a or 5a conferred a >100-fold reduction in velpatasvir susceptibility.

For voxilaprevir in genotype 1-6 replicons, resistance-associated substitutions selected in 2 or more genotypes were Q41H, A156V/T/L and D168E/H/Y. Site directed mutagenesis of known NS3 RAVs showed that substitutions conferring a >100-fold reduction in voxilaprevir susceptibility are A156V, A156T or A156L in genotype 1a, 1b, 2a, 3a and 4. No individual RAV tested in genotypes 2b, 5a or 6a conferred a >100-fold reduction in voxilaprevir susceptibility.

For both velpatasvir and voxilaprevir, combinations of RAVs often showed greater reductions in susceptibility than individual RAVs alone.

Cross resistance in cell culture

Voxilaprevir is active in vitro against most of the NS3 RAVs that confer resistance to first generation NS3/4A protease inhibitors. Additionally, velpatasvir is active in vitro against most of the NS5A RAVs that confer resistance to ledipasvir and daclatasvir. Sofosbuvir, velpatasvir, and voxilaprevir were fully active against substitutions associated with resistance to other classes of DAAs with different mechanisms of actions, e.g. voxilaprevir was fully active against NS5A and NS5B NI RAVs.

Absorption

The pharmacokinetic properties of sofosbuvir, GS-331007, velpatasvir and voxilaprevir have been evaluated in healthy adult subjects and in patients with chronic hepatitis C.

Sofosbuvir

Following oral administration of sofosbuvir/velpatasvir/voxilaprevir, sofosbuvir was absorbed quickly and the peak median plasma concentration was observed 2 hours post-dose. Median peak plasma concentration of GS-331007 was observed 4 hours post-dose. Based on the population pharmacokinetic analysis in HCV-infected patients, mean steady-state AUC_0-24 and C_max for sofosbuvir (n=1038) were 1665 ng•hr/mL and 678 ng/mL, respectively; mean steady-state AUC_0-24 and C_max for GS-331007 (n=1593) were 12834 ng•hr/mL and 744 ng/mL, respectively. Sofosbuvir and GS-331007 AUC_0-24 and C_max were similar in healthy adult subjects and patients with HCV infection.

Velpatasvir

Velpatasvir median peak concentrations were observed at 4 hours post-dose. Based on the population pharmacokinetic analysis in HCV-infected patients mean steady-state AUC_0-24 and C_max for velpatasvir (n=1595) were 4041 ng•hr/mL and 311 ng/mL, respectively. Relative to healthy subjects (n=137), velpatasvir AUC_0-24 and C_max were 41% lower and 39% lower, respectively, in HCV-infected patients.

Voxilaprevir

Voxilaprevir median peak concentrations were observed 4 hours post-dose. Based on the population pharmacokinetic analysis in HCV-infected patients mean steady-state AUC_0-24 and C_max for voxilaprevir (n=1591) were 2577 ng•hr/mL and 192 ng/mL, respectively. Relative to healthy subjects (n=63), voxilaprevir AUC_0-24 and C_max were both 260% higher in HCV-infected patients.

Effects of food

When sofosbuvir/velpatasvir/voxilaprevir or its components taken together were administered with food, sofosbuvir AUC_0-inf and C_max were 64% to 144% and 9% to 76% higher, respectively; velpatasvir AUC_0-inf and C_max were 40% to 166% and 37% to 187% higher, respectively; and voxilaprevir AUC_0-inf and C_max were 112% to 435% and 147% to 680% higher, respectively. GS-331007 AUC_0-inf did not change and C_max was 19% to 35% lower when sofosbuvir/velpatasvir/voxilaprevir or its components together were administered with food.

Distribution

Sofosbuvir is approximately 61-65% bound to human plasma proteins and the binding is independent of drug concentration over the range of 1 μg/mL to 20 μg/mL. Protein binding of GS-331007 was minimal in human plasma. After a single 400 mg dose of [¹⁴C]-sofosbuvir in healthy subjects, the blood to plasma ratio of [¹⁴C]-radioactivity was approximately 0.7.

Velpatasvir is >99% bound to human plasma proteins and binding is independent of drug concentration over the range of 0.09 μg/mL to 1.8 μg/mL. After a single 100 mg dose of [¹⁴C]-velpatasvir in healthy subjects, the blood to plasma ratio of [¹⁴C]-radioactivity ranged between 0.5 and 0.7.

Voxilaprevir is approximately >99% bound to human plasma proteins. After a single 100 mg dose of [¹⁴C]-voxilaprevir in healthy subjects, the blood to plasma ratio of [¹⁴C]-radioactivity ranged between 0.5 and 0.8.

Biotransformation

Sofosbuvir is extensively metabolised in the liver to form the pharmacologically active nucleoside analogue triphosphate GS-461203. The metabolic activation pathway involves sequential hydrolysis of the carboxyl ester moiety catalysed by human cathepsin A (CatA) or carboxylesterase 1 (CES1) and phosphoramidate cleavage by histidine triad nucleotide-binding protein 1 (HINT1) followed by phosphorylation by the pyrimidine nucleotide biosynthesis pathway. Dephosphorylation results in the formation of nucleoside metabolite GS-331007 that cannot be efficiently rephosphorylated and lacks anti-HCV activity in vitro. After a single 400 mg oral dose of [¹⁴C]-sofosbuvir, GS-331007 accounted for approximately >90% of total systemic exposure.

Velpatasvir is primarily a substrate of CYP2B6, CYP2C8, and CYP3A4 with slow turnover. Following a single dose of 100 mg [¹⁴C]-velpatasvir, the majority (> 98%) of radioactivity in plasma was parent drug. The monohydroxylated and desmethylated velpatasvir were the metabolites identified in human plasma. Unchanged velpatasvir is the major species present in faeces.

Voxilaprevir is primarily a substrate of CYP3A4 with slow turnover. Following a single dose of 100 mg [¹⁴C]-voxilaprevir, the majority (approximately 91%) of radioactivity in plasma was parent drug. The hydrolysed and dehydrogenated voxilaprevir were the major metabolites identified in human plasma. Unchanged voxilaprevir is the major species present in faeces.

Elimination

Following a single 400 mg oral dose of [¹⁴C]-sofosbuvir, mean total recovery of the [¹⁴C]-radioactivity was greater than 92%, consisting of approximately 80%, 14%, and 2.5% recovered in urine, faeces, and expired air, respectively. The majority of the sofosbuvir dose recovered in urine was GS-331007 (78%) while 3.5% was recovered as sofosbuvir. These data indicate that renal clearance is the major elimination pathway for GS-331007. The median terminal half-lives of sofosbuvir and GS-331007 following administration of sofosbuvir/velpatasvir/voxilaprevir were 0.5 and 29 hours, respectively.

Following a single 100 mg oral dose of [¹⁴C]-velpatasvir, mean total recovery of the [¹⁴C]-radioactivity was 95%, consisting of approximately 94% and 0.4% recovered from the faeces and urine, respectively. Unchanged velpatasvir was the major species in faeces accounting for a mean of 77% of the administered dose, followed by monohydroxylated velpatasvir (5.9%) and desmethylated velpatasvir (3.0%). These data indicate that biliary excretion of parent drug was a major route of elimination for velpatasvir. The median terminal half-life of velpatasvir following administration of sofosbuvir/velpatasvir/voxilaprevir was approximately 17 hours.

Following a single 100 mg oral dose of [¹⁴C]-voxilaprevir, mean total recovery of the [¹⁴C]radioactivity was 94%, with all radioactivity measured in the faeces and none in the urine. Unchanged voxilaprevir was the major species in faeces accounting for a mean of 40% of the administered dose. Voxilaprevir metabolites also identified in faeces included des [methylcyclopropylsulphonamide]voxilaprevir (22.1%), which is formed intestinally, dehydrovoxilaprevir (7.5%), and two des[methylcyclopropylsulphonamide]-oxy-voxilaprevir metabolites (5.4% and 3.9%).Biliary excretion of parent drug was the major route of elimination for voxilaprevir. The median terminal half-life of voxilaprevir following administration of sofosbuvir/velpatasvir/voxilaprevir was approximately 33 hours.

Linearity/non-linearity

Sofosbuvir and GS-331007 AUCs are near dose-proportional over the dose range of 200 mg to 1200 mg. Velpatasvir AUC increases in a greater than proportional manner from 5 to 50 mg and in a less than proportional manner from 50 to 450 mg, indicating velpatasvir absorption is solubility limited. Voxilaprevir (studied under fed conditions) AUC increases in a greater than dose-proportional manner over the dose range of 100 to 900 mg.

In vitro potential for sofosbuvir/velpatasvir/voxilaprevir drug-drug interactions

Sofosbuvir, velpatasvir and voxilaprevir are substrates of drug transporters P-gp and BCRP while GS-331007 is not. Voxilaprevir, and to a lesser extent velpatasvir, are also substrates of OATP1B1 and OATP1B3. In vitro, slow metabolic turnover of velpatasvir primarily by CYP2B6, CYP2C8, and CYP3A4 and of voxilaprevir primarily by CYP3A4 was observed.

Sofosbuvir and GS-331007 are not inhibitors of drug transporters P-gp, BCRP, multidrug resistanceassociated protein 2 (MRP2), bile salt export pump (BSEP), OATP1B1, OATP1B3 and organic cation transporter (OCT) 1 and GS-331007 is not an inhibitor of OAT1, OAT3, OCT2, and multidrug and toxin extrusion protein (MATE) 1. Sofosbuvir and GS-331007 are not inhibitors or inducers of CYP or uridine glucuronosyltransferase (UGT) 1A1 enzymes.

Velpatasvir is an inhibitor of drug transporter P-gp, BCRP, OATP1B1, OATP1B3 and OATP2B1, and its involvement in drug interactions with these transporters is primarily limited to the process of absorption. At clinically relevant concentrations, velpatasvir is not an inhibitor of hepatic transporters BSEP, sodium taurocholate cotransporter protein (NTCP), OATP1A2 or OCT1, renal transporters OCT2, OAT1, OAT3, MRP2 or MATE1, or CYP or UGT1A1 enzymes.

Voxilaprevir is an inhibitor of drug transporters P-gp, BCRP, OATP1B1 and OATP1B3, and its involvement in drug interactions with these transporters is primarily limited to the process of absorption. At clinically relevant concentrations, voxilaprevir is not an inhibitor of hepatic transporters OCT1, renal transporters OCT2, OAT1, OAT3 or MATE1, or CYP or UGT1A1 enzymes.

Pharmacokinetics in special populations

Race and gender

No clinically relevant pharmacokinetic differences due to race or gender have been identified for sofosbuvir, GS-331007, velpatasvir or voxilaprevir.

Elderly

Population pharmacokinetic analysis in HCV-infected patients showed that within the age range (18 to 85 years) analysed, age did not have a clinically relevant effect on the exposure to sofosbuvir, GS-331007, velpatasvir or voxilaprevir. In the 13 patients aged 75 to 84 years with available pharmacokinetic data, mean exposure to voxilaprevir was 93% higher than the mean exposure observed in patients aged 18 to 64 years.

Renal impairment

A summary of the effect of varying degrees of renal impairment (RI) on the exposures of the components of sofosbuvir/velpatasvir/voxilaprevir compared to subjects with normal renal function, as described in the text below, are provided in Table 15.

Table 15. Effect of varying degrees of renal impairment on exposures (AUC) of SOF, GS – 331007, velpatasvir and voxilaprevir compared to subjects with normal renal function:

The pharmacokinetics of sofosbuvir was studied in HCV negative adult patients with mild (eGFR ≥50 and <80 mL/min/1.73 m²), moderate (eGFR ≥30 and <50 mL/min/1.73 m²), severe renal impairment (eGFR <30 mL/min/1.73 m²) and patients with ESRD requiring haemodialysis following a single 400 mg dose of sofosbuvir, relative to patients with normal renal function (eGFR >80 mL/min/1.73 m²). GS-331007 is efficiently removed by haemodialysis with an extraction coefficient of approximately 53%. Following a single 400 mg dose of sofosbuvir, a 4-hour haemodialysis removed 18% of administered dose.

In HCV-infected adult patients with severe renal impairment treated with sofosbuvir 200 mg with ribavirin (n=10) or sofosbuvir 400 mg with ribavirin (n=10) for 24 weeks or ledipasvir/sofosbuvir 90/400 mg (n=18) for 12 weeks, the pharmacokinetics of sofosbuvir and GS-331007 were consistent with that observed in HCV negative adult patients with severe renal impairment.

The pharmacokinetics of velpatasvir were studied with a single dose of 100 mg velpatasvir in HCV negative adult patients with severe renal impairment (eGFR <30 mL/min by Cockcroft-Gault). Voxilaprevir is not renally eliminated.

Additionally, the pharmacokinetics of voxilaprevir were studied with a single dose of 100 mg voxilaprevir in HCV negative adult patients with severe renal impairment (eGFR <30 mL/min by Cockcroft-Gault). The pharmacokinetics of voxilaprevir have not been studied in subjects with ESRD requiring dialysis.

The pharmacokinetics of sofosbuvir, GS-331007, and velpatasvir were studied in HCV-infected patients with ESRD requiring dialysis treated with once daily sofosbuvir/velpatasvir 400/100 mg for 12 weeks, and compared to patients without renal impairment in the sofosbuvir/velpatasvir Phase ⅔ studies.

Although exposures of the fixed-dose combination sofosbuvir, GS-331007, velpatasvir, and voxilaprevir were not directly evaluated in HCV-infected adult patients with ESRD requiring dialysis after administration of sofosbuvir/velpatasvir/voxilaprevir, the exposures of sofosbuvir, GS-331007, and velpatasvir are expected to be similar to those observed after administration of sofosbuvir/velpatasvir 400/100 mg in HCVinfected patients with ESRD requiring dialysis.

Hepatic impairment

The pharmacokinetics of sofosbuvir was studied following 7-day dosing of 400 mg sofosbuvir in HCV-infected adult patients with moderate and severe hepatic impairment (CPT Class B and C). Relative to patients with normal hepatic function, the sofosbuvir AUC_0-24 was 126% and 143% higher in patients with moderate and severe hepatic impairment, while the GS-331007 AUC_0-24 was 18% and 9% higher, respectively. Population pharmacokinetics analysis in HCV-infected adult patients indicated that cirrhosis (CPT Class A) had no clinically relevant effect on the exposure to sofosbuvir and GS-331007.

The pharmacokinetics of velpatasvir were studied with a single dose of 100 mg velpatasvir in HCV negative adult patients with moderate and severe hepatic impairment (CPT Class B and C). Velpatasvir plasma exposure (AUC_inf) was similar in patients with moderate hepatic impairment, severe hepatic impairment, and control subjects with normal hepatic function. Population pharmacokinetic analysis in HCV-infected adult patients indicated that cirrhosis (CPT Class A) had no clinically relevant effect on the exposure of velpatasvir.

The pharmacokinetics of voxilaprevir were studied with a single dose of 100 mg voxilaprevir in HCV negative adult patients with moderate and severe hepatic impairment (CPT Class B and C). Relative to patients with normal hepatic function, the voxilaprevir AUC_inf was 299% and 500% higher in patients with moderate and severe hepatic impairment, respectively. The unbound fraction of voxilaprevir was approximately 2-fold higher in severe hepatic impairment compared with moderate hepatic impairment or normal hepatic function. Population pharmacokinetic analysis in HCV-infected adult patients indicated that patients with cirrhosis (CPT Class A) had 73% higher exposure of voxilaprevir than those without cirrhosis.

Body weight

In adults, body weight did not have a clinically significant effect on sofosbuvir, velpatasvir or voxilaprevir exposure according to a population pharmacokinetic analysis.

Paediatric population

Sofosbuvir, GS-331007, velpatasvir, and voxilaprevir exposures in paediatric patients aged 12 years and older receiving oral once daily doses of sofosbuvir/velpatasvir/voxilaprevir 400 mg/100 mg/100 mg were similar to those in adults.

The pharmacokinetics of sofosbuvir/velpatasvir/voxilaprevir in paediatric patients aged less than 12 years and weighing less than 30 kg have not been established.

Sofosbuvir

Sofosbuvir was not genotoxic in a battery of in vitro or in vivo assays, including bacterial mutagenicity, chromosome aberration using human peripheral blood lymphocytes and in vivo mouse micronucleus assays. No teratogenic effects were observed in the rat and rabbit developmental toxicity studies with sofosbuvir. Sofosbuvir had no adverse effects on behaviour, reproduction, or development of the offspring in the rat pre- and post-natal development study.

Sofosbuvir was not carcinogenic in the 2-year mouse and rat carcinogenicity studies at GS-331007 exposures up to 17 and 10-times higher, respectively than human exposure.

Velpatasvir

Velpatasvir was not genotoxic in a battery of in vitro or in vivo assays, including bacterial mutagenicity, chromosome aberration using human peripheral blood lymphocytes and in vivo rat micronucleus assays.

Velpatasvir was not carcinogenic in the 26-week transgenic mouse and 2-year rat carcinogenicity studies at exposures up to 67- and 5-times higher than human exposure, respectively.

Velpatasvir had no adverse effects on mating and fertility. No teratogenic effects were observed in the mouse and rat developmental toxicity studies with velpatasvir at AUC exposures approximately 23- and 4-fold higher, respectively, than the human exposure at the recommended clinical dose. However, a possible teratogenic effect was indicated in rabbits where an increase in total visceral malformations was seen in exposed animals at AUC exposures up to 0.5 fold the human exposure at recommended clinical dose. The human relevance of this finding is not known. Velpatasvir had no adverse effects on behaviour, reproduction, or development of the offspring in the rat pre- and post-natal development study at AUC exposures approximately 3-fold higher than the human exposure at the recommended clinical dose.

Voxilaprevir

Voxilaprevir was not genotoxic in a battery of in vitro or in vivo assays, including bacterial mutagenicity, chromosome aberration using human peripheral blood lymphocytes and in vivo rat micronucleus assays.

Carcinogenicity studies for voxilaprevir have not been conducted.

Voxilaprevir had no adverse effects on mating and fertility. No teratogenic effects were observed in the rat and rabbit developmental toxicity studies with voxilaprevir at AUC exposures approximately 141- and 4-times higher, respectively, than the human exposure at the recommended clinical dose. Voxilaprevir had no adverse effects on behavior, reproduction, or development of the offspring in the rat pre- and post-natal development study at AUC exposures approximately 238-times higher than the human exposure at the recommended clinical dose.