By Liz Highleyman

As background, researchers from ImQuest Biosciences noted that the experimental agents have antiviral activity that results in a significant reduction of viral RNA synthesis not related to known mechanisms including inhibition of viral entry into cells, initiation of IRES translation, inhibition of the HCV NS2/3 or NS3/4A HCV protease enzyme, or interference with the HCV NS5B viral RNA-dependent RNA polymerase.

In previous laboratory studies, ATI-0810 was 100- to 200-fold less toxic than ribavirin, and in fact appeared to reduce the toxicity of ribavirin when the drugs were administered together. ATI-0810 was shown to be non-toxic to fresh human hepatocytes (liver cells) at the highest concentration tested (1.33 mM). Furthermore, serial passage of cells infected with a virus related to HCV (bovine viral diarrhea virus) in the presence of escalating doses of ATI-0810 failed to select for drug-resistant virus, suggesting a high genetic barrier to resistance.

In the present study, an HCV replicon system was used to analyze the activity of ATI-0810 and 10 chemically related compounds (PG204057, PG702253, PG702273, PG702306, PG702307, PG702379, PG702532, PG702548, PG702617, and PG703010), and to generate ATI-0810 resistant HCV replicons. The investigators examined cellular gene expression in the presence and absence of ATI-0810.

Results

	In vitro antiviral activity of ATI-0810 and the related compounds ranged from 0.21 to 3.4 mcM.
	Selection of ATI-0810 resistant replicons revealed potential resistance-conferring mutations in the HCV NS3 and NS5A enzymes.
	A single NS5A mutation, C446R, conferred a 21-fold decrease in sensitivity to ATI-0810.
	This mutation is the P2 proximal amino acid at the NS5A/NS5B junction.
	Sensitivity testing of ATI-0810 in replicons cells expanded after selection for this mutation revealed an approximately 17- to 20-fold increase in the drug's EC50 value (50% effective concentration).
	The C446R mutation and 3 others -- NS3 D168N, NS5A L199F, and V296 -- emerged in replicons cultured with ATI-0810, but not those grown without the agent.
	Differential gene expression revealed no significant changes in cellular RNA accumulation in the presence of ATI-0810.

Based on these data, the researchers hypothesized that ATI-0810 inhibits HCV replication through a novel mechanism of action.

"ATI-0810 does not inhibit viral protease or polymerase activity and does not appear to inhibit IRES-mediated translation or NS2 protease activity," they stated.

They added that in tests combining ATI-0810 with other anti-HCV agents, the new agent "reduces the toxicity of ribavirin and interferon and is additive with interferon and additive/slightly synergistic with ribavirin."

ImQuest BioSci., Frederick, MD.

9/15/09

Reference
TB Parsley, l Yang, and RW Buckheit. ATI-0810 is a Novel Late Stage Inhibitor of HCV Replicon Replication. 49th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC 2009). San Francisco. September 12-15, 2009. Abstract H-215.