Recent
data show that the oral HCV protease inhibitor telaprevir
(VX-950) demonstrates antiviral activity alone or in combination with pegylated
interferon.
Three presentations at the recent 57th Annual Meeting of the
American Association for the Study of Liver Diseases (AALSD) looked at emergence
of resistance mutations in laboratory studies.
Study
1
As
previously reported, T. Keiffer of Vertex reported that among patients who
received telaprevir plus pegylated interferon for 14 days, wild-type (non-mutated)
HCV was detected at Day 4, and either wild-type or resistant HCV was observed
at Day 8 in 4 out of 8 patients with detectable HCV RNA. Among patients who received
telaprevir monotherapy for 14 days, 4 experienced virological breakthrough or
a plateau response; in these individuals, viral variants with the R155 and A156V/T
mutations were detectable by Day 8. By Day 14, a variant with V36(M/A)/R155(K/T)
mutations predominated by Day 14. Of the 4 remaining patients who experienced
continuous HCV RNA decline, 2 had variants with A156V/T by Day 8, while the other
2 had undetectable viral load.
The researchers concluded that, "The
rapid and dramatic antiviral response to telaprevir reflects a sharp reduction
in wild-type virus. Viral variants were uncovered as wild-type virus was cleared."
They added that, "HCV RNA levels can decline in patients receiving telaprevir
monotherapy, even in those with detectable A156V/T variant," and that, "viral
rebound with monotherapy appears to result from the presence of V36(M/A)/R155(K/T)."
They also noted that the combination of telaprevir and pegylated interferon suppressed
resistant variants.
Study 2
In
the second study, conducted by C. Lin and colleagues from Vertex, variant HCV
containing substitutions at positions 36, 54, 155, or 156 of the HCV NS3 protease
domain were expressed and purified, and the potency of telaprevir against these
variant enzymes was determined. The variants were also introduced into an HCV
replicon model, and cell lines containing the variant proteases were selected.
The researchers then determined the anti-HCV activity of telaprevir and interferon
alpha against these variant replicon cell lines.
Results
Single variants, including V36M/A, T54A, or R155K/M, conferred low-level (<
10-fold change in sensitivity) in vitro resistance to telaprevir.
The A156V/T single variants or V36M/A+R155K/T double variants had high-level (>
40-fold) in vitro resistance to telaprevir.
Computational modeling analysis suggests that there are 3 distinct mechanisms
of resistance for these variants:
- an increase in flexibility of the
binding pocket; - a loss of interaction (van der Waals contact, hydrogen bond,
or salt bridge); or - an increase in steric hindrance.
However, the interferon alpha IC50s (50% inhibitory concentrations) of all these
variants were similar to that of wild-type replicon cells.
"These
in vitro studies confirm that NS3 variants observed during telaprevir dosing have
reduced sensitivity to telaprevir, but retain full sensitivity to interferon alpha,"
the researchers concluded. "Thus, they provide a rationale for combination
therapy with telaprevir + pegylated interferon and are consistent with the results
of two clinical studies of telaprevir + pegylated interferon +/- ribavirin, in
which continuous decline of HCV RNA was observed in all patients without evidence
of viral breakthrough."
Study
3
The third study, also by Vertex researchers, looked at viral
factors responsible for HCV persistence and immune evasion.
The HCV NS3/4A
protease is implicated in viral evasion of TLR3 or Rig-I mediated innate immunity
by cleaving the TRIF and interferon-b promoter stimulator 1 (IPS-1) protein adaptors,
respectively, the researchers wrote.
Cleavage of IPS-1 is observed in
HCV infections both in vitro and in vivo. The purpose of this study
was to compare the cleavage of viral (5A/B) and innate immunity (TRIF and IPS-1)
substrates by HCV proteases from different HCV genotypes (1b, 2a, and 3a), to
demonstrate the inhibition of this cleavage by telaprevir, and to test the ability
of telaprevir-resistant mutants to cleave these substrates.
Results
NS3/4A protease from HCV genotypes 1, 2, and 3 were able to cleave TRIF and IPS-1.
Telaprevir,
which inhibits cleavage of the natural HCV substrate (5A/B), was also able to
inhibit the genotype 1 HCV protease-mediated cleavage of TRIF in a similar manner.
A156V/T
variants with reduced sensitivity to telaprevir and impaired viral fitness compared
to wild-type virus have been reported in vitro and in vivo.
These in vitro telaprevir-resistant mutants were found to have significantly reduced
ability to cleave HCV 5A/B consistent with their poor replicative fitness.
Cleavage of TRIF and IPS-1 substrates by these mutants was also impaired.
Computational modeling suggested that steric hindrance might decrease binding
of TRIF to the A156T/V mutant protease, resulting in inefficient cleavage.
The A156V/T variants exhibit impaired cleavage of TRIF and IPS-1, suggesting that
the emergence of these telaprevir resistance mutants may not compromise the potential
restoration of innate immunity by telaprevir," the researchers wrote.
They
also noted that the fact that telaprevir both inhibits cleavage of the natural
HCV substrate (5A/B) and also inhibited the genotype 1 HCV protease-mediated cleavage
of TRIF supports the hypothesis that the new drug "might play a dual role
in the treatment of HCV infection by acting as a direct antiviral agent as well
as restoring innate immunity."
12/05/06
References
T
Kieffer, C Sarrazin, J Miller, and others. Combination of Telaprevir (VX-950)
and Peg-IFN-alfa Suppresses Both Wild-type Virus and Resistance Variants in HCV
Genotype 1-Infected Patients in a 14-day Phase 1b study. 57th AASLD. October 27-31,
2006. Boston, MA. Abstract 92.
C Lin, Y Zhou, U Muh, and others. In vitro
Characterization of Telaprevir (VX-950) NS3 Protease Variants. 57th AASLD. Abstract
89.
G Kalkeri, B Roa, D Brennan, and others. Telaprevir (TVR, VX-950) resistance
variants A156T/V exhibit reduced cleavage of innate immunity signaling proteins,
TRIF and IPS-1. 57th AASLD. Abstract 129.
Single variants, including V36M/A, T54A, or R155K/M, conferred low-level (<
10-fold change in sensitivity) in vitro resistance to telaprevir. 
