Nucleoside Analog HCV Polymerase Inhibitors May Have a Higher
Barrier to Resistance than Other Directly Targeted Anti-HCV Agents
Directly
targeted anti-HCV agents that inhibit various stages of the viral lifecycle are
the current focus of hepatitis C therapy but -- as with antiretroviral drugs for
HIV -- these HCV can develop resistance to these, suggesting that combination
therapy and drug sequencing will become an increasingly important aspect of hepatitis
C treatment.
Novel
inhibitors of HCV replication currently in development include:
•
R1479: NS5B
nucleoside analog HCV polymerase inhibitor (active moiety of prodrug R1626);
•
PSI-6130:
NS5B nucleoside analog HCV polymerase inhibitor (active moiety of prodrug R7128);
•
NM107: NS5B
nucleoside analog HCV polymerase (active moiety of prodrug NM283, or valopicitabine);
All
these agents have demonstrated anti-HCV activity in preclinical laboratory studies
and in chronically infected HCV patients. However, development of drug resistance
remains a barrier to long-term treatment success.
Two
studies of HCV resistance to targeted antiviral agents were presented at the 14th
International Symposium on Hepatitis C Virus and Related Viruses, taking place
this week in Glasgow, UK (September 9-13, 2007).
In
the first study, researchers from Roche analyzed the in vitro development
of resistance to the NS5B nucleoside polymerase inhibitors R1479, PSI-6130, and
NM107, in comparison with the non-nucleoside HCV-796 and the protease inhibitor
telaprevir, both individually and in combination.
The
potential for an HCV mutation to confer cross-resistance among the inhibitors
was examined by introducing the mutations observed after selection with each compound,
as well as previously reported mutations, into a transient HCV replicon. The effect
of the mutations on the potency of the inhibitors was then determined.
Results
•
R1479,
NM107, and PSI-6130 at 10 x and 15 x the IC50 (50% inhibitory concentration) resulted
in clearance of the HCV replicon.
•
No
NS5B mutations were observed using a 1 x IC50 concentration. Distinct colonies
were observed at 10 x and 15 x the IC50 of HCV-796; the C316Y and S365S/A mutations
were identified in NS5B from 10 x or 15 x treated cells.
•
Distinct
colonies were observed at 10 x and 15 x the IC50 for telaprevir; the A156T/S and
T54T/A mutations were identified in NS3 from 10 x or 15 x treated cells.
•
The
number of observed colonies was reduced with a combination of 1 x R1479 or PSI-6130
plus either HCV-796 or telaprevir.
•
The
replicon was cleared using a combination of 10 x or 15 x R1479 or PSI-6130 plus
either HCV-796 or telaprevir.
•
R1479
and PSI-6130 maintained potency against mutant replicons with reduced sensitivity
to VX-950 or telaprevir.
•
HCV-796
and telaprevir maintained potency against mutant replicons with reduced sensitivity
to R1479 or PSI-6130.
Conclusions
The
study investigators concluded:
•
The
non-nucleoside polymerase inhibitor HCV-796 selected for the amino acid substitutions
C316Y and S365S/A in the NS5B coding region (as previously reported) after a 3-three
week selection at 10 x and 15 x the IC50.
•
The
protease inhibitor telaprevir selected for the amino acid substitutions A156T/S
and T54T/A in the NS3 coding region (as previously reported) after a 3-week selection
at 10 x and 15 x the IC50.
•
Under
the same conditions, the nucleoside analog polymerase inhibitors R1479, PSI-6130,
and NM107) did not select for resistance, and the replicon was cleared.
•
The
in vitro selected amino acid substitutions that confer resistance to telaprevir
(A156T/S or T54T/A) or HCV-796 (C316Y) correlate with the resistance mutations
identified in clinical studies.
•
In
combination with R1479 or PSI-6130, the number of telaprevir and HCV-796 resistant
colonies was decreased.
•
1479
and PSI-6130 maintained potency against clinically relevant telaprevir and HCV-796
resistance mutations.
•
Telaprevir
and HCV-796 maintained potency against mutants with reduced sensitivity to R1479
and PSI-6130.
These
findings indicate that the in vitro genetic barrier to resistance may be higher
for nucleoside analog inhibitors compared with non-nucleoside polymerase inhibitors
or protease inhibitors. The data also showed that combining telaprevir or HCV-796
with a nucleoside analog inhibitor may decrease the frequency of drug resistance
selection.
"These
data support further investigations into the potential clinical benefit of combination
therapies," the researchers concluded.
Roche
Palo Alto, Viral Diseases Therapy Area, Palo Alto, CA, USA.
Resistance
to PSI-6130
R7128
is a prodrug of PSI-6130, a potent and selective inhibitor of HCV NS5B polymerase.
PSI-6130 has demonstrated potent activity against HCV replicons containing NS5B
genes derived from multiple genotype 1a and 1b clinical isolates.
The
primary goal of the present study was to determine and characterize the in
vitro selected mutations that confer reduced sensitivity to PSI-6130.
Results
•
Treatment
of cells harboring a subgenomic replicon for 1 month with a 10 x EC50 concentration
of PSI-6130 cleared viral RNA from the cells without selecting resistant variants.
•
Long-term
(6-8 months) sequential passage of cells harboring the GT-1b wild-type replicon
with PSI-6130 or R7128 resulted in the selection of a number of amino acid substitutions.
•
All
amino acid substitutions reduced replication capacity with the exception of I239L.
•
The
S282T substitution conferred 3.1-fold reduced sensitivity to PSI-6130 alone and
3- to 8-fold reduced sensitivity in combination with other selected amino acid
substitutions.
•
S282T
conferred 22-fold reduced sensitivity to NM107.
•
The
S282T substitution conferred a 2- to 5-fold reduced sensitivity of the replicon
to PSI-6130 compared to a 17- to 22-fold loss to NM107 (in transient and stable
replicon systems, respectively), and no loss in sensitivity to R1479.
•
The
S282T substitution reduces the sensitivity of recombinant NS5B by 5.9-fold to
PSI-6130 compared to 111.4-fold to NM107.
Conclusions
•
Selection
of Huh-7 replicon cells with PSI-6130 resulted in the selection of the S282T substitution
within 6-8 months.
•
The
S282T substitution, either alone or in combination with other observed substitutions,
conferred a 3- to 8-fold reduced sensitivity of the replicon to PSI-6130.
•
PSI-6130
demonstrated potent and highly consistent activity against GT-1a and GT-1b clinical
isolates.
•
In
the replicon clearance assay, treatment with 10 x EC50 concentration of PSI-6130
resulted in clearance of the replicon within 1 month.
•
PSI-6130
and R1479 select for different mutations in NS5B.
These
results indicate that the development of resistance to some directly targeted
anti-HCV agents is likely to confer cross-resistance to other similar agents.
As
shown by the previous study, however, combination therapy with multiple agents
may be able to prevent or overcome drug resistance.
M
McCown, S Rajyaguru, J Symons, and others. The Nucleoside Inhibitors R1479, PSI-6130,
and NM107 have a Higher Genetic Barrier to Resistance than the Non-Nucleoside
Inhibitor HCV-796 and the Protease Inhibitor VX-950. 14th International Symposium
on Hepatitis C Virus and Related Viruses. September 9-13, 2007. Glasgow, UK. Abstract
(poster) P-265.
S
Ali, V Leveque, S Le Pogam, and others. In Vitro Selection and Characterization
of HCV Replicons with Reduced Sensitivity to PSI-6130. 14th International Symposium
on Hepatitis C Virus and Related Viruses. September 9-13, 2007. Glasgow, UK. Abstract
(poster) P-263.
