Mechanism of Action
Glecaprevir
Glecaprevir is an inhibitor of the HCV NS3/4A protease, which is necessary for the proteolytic cleavage of the HCV encoded polyprotein (into mature forms of the NS3, NS4A, NS4B, NS5A, and NS5B proteins) and is essential for viral replication. In a biochemical assay, glecaprevir inhibited the proteolytic activity of recombinant NS3/4A enzymes from clinical isolates of HCV genotypes 1a, 1b, 2a, 2b, 3a, 4a, 5a, and 6a with IC50 values ranging from 3.5 to 11.3 nM.
Pibrentasvir
Pibrentasvir is an inhibitor of HCV NS5A, which is essential for viral RNA replication and virion assembly. The mechanism of action of pibrentasvir has been characterized based on cell culture antiviral activity and drug resistance mapping studies.
Antiviral Activity
In HCV replicon assays, glecaprevir had median EC50 values of 0.08-4.6 nM against laboratory and clinical isolates from subtypes 1a, 1b, 2a, 2b, 3a, 3b, 4a, 4d, 5a, and 6a. Pibrentasvir had median EC50 values of 0.5-15.6 pM against laboratory and clinical isolates from subtypes 1a, 1b, 2a, 2b, 3a, 3b, 4a, 4b, 4d, 5a, 6a, 6e and 6p.
Combination Antiviral Activity
Evaluation of combination of glecaprevir and pibrentasvir showed no antagonism in antiviral activity in HCV genotype 1 replicon cell culture assays.
Resistance
In Cell Culture
Selection of HCV genotype 1a, 1b, 2a, 3a, 4a or 6a replicons for reduced susceptibility to glecaprevir resulted in the emergence of amino acid substitutions most commonly at NS3 positions A156 or D/Q168. Individual substitutions at NS3 amino acid position A156 introduced into HCV replicons by site-directed mutagenesis generally caused the greatest reductions (>100-fold) in susceptibility to glecaprevir. Individual substitutions at NS3 position D/Q168 had varying effects on glecaprevir susceptibility depending on HCV genotype/subtype and specific amino acid change, with the greatest reductions (>30-fold) observed in genotypes 1a (D168F/Y), 3a (Q168R) and 6a (D168A/G/H/V/Y). Combinations of NS3 Y56H plus D/Q168 substitutions resulted in greater reductions in glecaprevir susceptibility. An NS3 Q80R substitution in genotype 3a caused a 21-fold reduction in glecaprevir susceptibility, while Q80 substitutions in genotypes 1a and 1b (including genotype 1a Q80K) did not reduce glecaprevir susceptibility. Individual amino acid substitutions associated with resistance to other HCV protease inhibitors at positions 36, 43, 54, 55, 56, 155, 166, or 170 in NS3 generally did not reduce susceptibility to glecaprevir.
Selection of HCV genotype 1a, 2a or 3a replicons for reduced susceptibility to pibrentasvir resulted in the emergence of amino acid substitutions at known NS5A inhibitor resistance-associated positions, including Q30D/deletion, Y93D/H/N or H58D +Y93H in genotype 1a replicons, F28S + M31I or P29S + K30G in genotype 2a replicons, and Y93H in genotype 3a replicons. The majority of individual amino acid substitutions associated with resistance to other HCV NS5A inhibitors at positions 24, 28, 30, 31, 58, 92, or 93 in NS5A did not reduce susceptibility to pibrentasvir. Individual NS5A amino acid substitutions that reduced susceptibility to pibrentasvir include M28G or Q30D in a genotype 1a replicon (244- and 94-fold, respectively), and P32-deletion in a genotype 1b replicon (1,036-fold). Some combinations of two or more NS5A inhibitor resistance-associated amino acid substitutions may result in greater reductions in pibrentasvir susceptibility. In a genotype 3b replicon, the presence of naturally occurring polymorphisms K30 and M31 in NS5A reduced susceptibility to pibrentasvir by 24-fold relative to the activity of pibrentasvir in a genotype 3a replicon. Introduction of an NS5A Y93H substitution into a genotype 3b replicon further reduced susceptibility to pibrentasvir by 6336-fold.
In Clinical Studies
Studies in Treatment-Naïve and (peg)Interferon, Ribavirin and/or Sofosbuvir Treatment-Experienced Subjects with or without Cirrhosis
In pooled analyses of NS3/4A PI- and NS5A inhibitor-naïve subjects who received MAVYRET for 8, 12, or 16 weeks in the registrational Phase 2 and 3 clinical studies (including EXPEDITION-2 and MAGELLAN-2), treatment-emergent resistance analyses were conducted for 24 subjects who experienced virologic failure (2 with genotype 1, 2 with genotype 2, 20 with genotype 3 infection). No subjects with HCV genotype 4, 5 or 6 infection experienced virologic failure.
Among the two genotype 1-infected subjects who experienced virologic failure, both subjects had a subtype 1a infection. One subject had treatment-emergent substitutions A156V in NS3, and Q30R, L31M and H58D in NS5A (Q30R and L31M were also detected at a low frequency at baseline). One subject had treatment-emergent Q30R and H58D (while Y93N was present at baseline and post-treatment) in NS5A.
Among the two genotype 2-infected subjects who experienced virologic failure, both subjects had a subtype 2a infection, and no treatment-emergent substitutions were observed in NS3 or NS5A.
Among the 20 genotype 3-infected subjects who experienced virologic failure, treatment-emergent NS3 substitutions Y56H/N, Q80K/R, A156G, or Q168L/R were observed in 13 subjects. A166S or Q168R were present at baseline and post-treatment in 5 subjects. Treatment-emergent NS5A substitutions S24F, M28G/K, A30G/K, L31F, P58T, or Y93H were observed in 17 subjects, and 14 subjects had A30K (n=9) or Y93H (n=6) at baseline and post-treatment.
Studies in Subjects with or without Cirrhosis Who Were Treatment-Experienced to NS3/4A Protease and/or NS5A Inhibitors
Treatment-emergent resistance analyses were conducted for 11 HCV genotype 1-infected subjects (10 genotype 1a, 1 genotype 1b) with prior NS3/4A PI or NS5A inhibitor treatment experience who experienced virologic failure with MAVYRET with or without ribavirin in the MAGELLAN-1 study. Treatment-emergent NS3 substitutions V36A/M, Y56H, R155K/T, A156G/T/V, or D168A/T were observed in 73% (8/11) of subjects. Nine of 10 subjects (90%, not including one subject missing NS5A data at failure) had treatment-emergent NS5A substitutions M28A/G (or L28M for genotype 1b), P29Q/R, Q30K/R, H58D or Y93H/N. All 11 subjects also had NS5A inhibitor resistance-associated substitutions detected at baseline, and 7/11 had NS3 PI resistance-associated substitutions detected at baseline (see Cross-Resistance for the effect of baseline resistance-associated substitutions on treatment response for NS3/4A PI or NS5A inhibitor treatment-experienced patients).
Effect of Baseline HCV Amino Acid Polymorphisms on Treatment Response (NS3/4A PI- and NS5A Inhibitor-Naïve Subjects)
A pooled analysis of NS3/4A PI- and NS5A inhibitor-naïve subjects who received MAVYRET in the Phase 2 and Phase 3 clinical studies was conducted to identify the HCV subtypes represented and explore the association between baseline amino acid polymorphisms and treatment outcome. Baseline polymorphisms relative to a subtype-specific reference sequence at resistance-associated amino acid positions 155, 156, and 168 in NS3, and 24, 28, 30, 31, 58, 92, and 93 in NS5A were evaluated at a 15% detection threshold by next-generation sequencing. Among subjects who received MAVYRET for 8-, 12-, or 16 weeks, baseline polymorphisms in NS3 were detected in 1% (9/845), 1% (3/398), 2% (10/613), 1% (2/164), 42% (13/31), and 3% (1/34) of subjects with HCV genotype 1, 2, 3, 4, 5, and 6 infection, respectively. No baseline polymorphisms were detected at NS3 amino acid position 156 across all genotypes. Baseline polymorphisms in NS5A were detected in 27% (225/841), 80% (331/415), 22% (136/615), 50% (80/161), 13% (4/31), and 54% (20/37) of subjects with HCV genotype 1, 2, 3, 4, 5, and 6 infection, respectively.
Genotype 1, 2, 4, 5, and 6: Baseline HCV polymorphisms in genotypes 1, 2, 4, 5 and 6 had no impact on treatment outcome.
Genotype 3: In registrational trials, HCV subtype 3a was the predominant genotype 3 subtype overall and was detected in >99% of U.S. genotype 3-infected subjects. Among treatment-naïve, genotype 3a-infected subjects without cirrhosis who received MAVYRET for 8 weeks, an NS5A A30K polymorphism was detected in 10% (18/179) of subjects, of whom 78% (14/18) achieved SVR12. Limited data are available to characterize the impact of the A30K polymorphism in genotype 3a-infected subjects with cirrhosis (n=3 who received MAVYRET for 8 weeks, all achieved SVR12) or prior treatment experience (n=1 who received MAVYRET for 16 weeks, relapse). In the pooled Phase 2 and Phase 3 trials, including post-registrational trials EXPEDITION-8, VOYAGE-1 and VOYAGE-2, 94% (15/16) of genotype 3a-infected subjects with Y93H in NS5A at baseline who received the recommended MAVYRET regimens achieved SVR12. In MAGELLAN-2 (post-transplant subjects), SVR12 was achieved in 2 of 3 (67%) genotype 3-infected subjects with the NS5A Y93H baseline polymorphism. In the VOYAGE-1 and VOYAGE-2 trials conducted in China, Singapore, and South Korea, 50% (20/40) of genotype 3-infected subjects had subtype 3b, of whom 14 (70%) achieved SVR12 with MAVYRET durations of 8, 12 or 16 weeks [see Clinical Studies (14.4)]. The naturally occurring NS5A K30 and M31 polymorphisms were detected in 95% (19/20) and 100% (20/20) of genotype 3b-infected subjects.
Cross-resistance
Based on resistance patterns observed in cell culture replicon studies and HCV-infected subjects, cross-resistance is possible between glecaprevir and other HCV NS3/4A PIs, and between pibrentasvir and other HCV NS5A inhibitors. Cross-resistance is not expected between MAVYRET and sofosbuvir, (peg)interferon or ribavirin.
In the MAGELLAN-1 study, HCV genotype 1-infected subjects who had failed prior treatment with NS3/4A protease and/or NS5A inhibitors were treated with MAVYRET for 12 or 16 weeks. Baseline sequences were analyzed by next generation sequencing at a 15% detection threshold.
Among 23 NS3/4A PI-experienced/NS5A inhibitor-naïve subjects who received MAVYRET for 12 weeks in MAGELLAN-1 (excluding 2 non-virologic failure subjects), 2 subjects each had baseline NS3 R155K or D168E/V substitutions; all 23 subjects achieved SVR12.
Among NS5A inhibitor-experienced/PI-naïve subjects who received MAVYRET for 16 weeks, baseline NS5A resistance-associated substitutions [R30Q (n=1), Y93H/N (n=5), M28A+Q30R (n=1), Q30H+Y93H (n=1), Q30R+L31M (n=2), L31M+H58P (n=1)], were detected in 73% (11/15) of subjects with available data, of whom 91% (10/11) achieved SVR12. The non-SVR12 subject experienced on-treatment virologic failure and had a genotype 1a infection with baseline NS5A Q30R and L31M substitutions.
Persistence of Resistance-Associated Substitutions
Data on the persistence of glecaprevir and pibrentasvir resistance-associated substitutions are not available. NS5A resistance-associated substitutions observed in patients treated with other NS5A inhibitors have been found to persist for longer than 1 year. In patients treated with other NS3/4A PI, viral populations with NS3 resistance-associated substitutions have been found to decline in some patients through post-treatment weeks 24 and 48. The long-term clinical impact of the emergence or persistence of virus containing glecaprevir or pibrentasvir resistance-associated substitutions is unknown.