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VRUS Selects PSI-7851 as a Lead HCV Development Candidate

[PSI-7851 is the backup to R7128 from the same class. PSI-6130, which is also referenced in this PR, is the active metabolite of R7128.]

http://biz.yahoo.com/prnews/080519/clm039.html

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Monday May 19, 7:05 am ET

- IND or foreign regulatory equivalent filing anticipated in 1Q 2009 -

PRINCETON, N.J., May 19 /PRNewswire-FirstCall/ -- Pharmasset, Inc. (Nasdaq: VRUS ) has nominated PSI-7851 as a lead development candidate for the treatment of chronic hepatitis C virus (HCV). PSI-7851 is a proprietary nucleotide analogue polymerase inhibitor of HCV that is being advanced into Good Laboratory Practice (GLP) animal toxicity studies required for submission of an Investigational New Drug (IND) application with the FDA or an equivalent foreign regulatory filing. PSI-7851 has demonstrated in vitro potency that is approximately 15- to 20-fold greater than the in vitro potency of R7128, a nucleoside analog polymerase inhibitor of HCV that Pharmasset is developing through its collaboration with Roche.

"Our in-house research team has, once again, discovered a new potential product candidate for the treatment of chronic HCV," stated Dr. Michael Otto, Pharmasset's Chief Scientific Officer. "Our goal continues to be the identification of anti-HCV compounds with improved potency, equivalent or improved safety and oral bioavailability. In addition, we have focused on increasing intra-hepatic triphosphate levels, which may lead to a higher level of active drug in the liver. We anticipate completing the required animal toxicity studies and filing an IND in the first quarter of 2009."

Nucleotide analogs have shown the ability to deliver higher levels of the active triphosphate form of the drug into infected cells. PSI-7851 has demonstrated in vitro anti-HCV activity with EC90 values of 0.31 +/- 0.12 uM, which is approximately 15- to 20-fold more potent than the active metabolite of R7128, PSI-6130. In vitro studies of PSI-7851 have not shown evidence of any mitochondrial or other cellular toxicities that may be associated with some nucleoside analogs. The half-life of the triphosphate in primary human hepatocytes is approximately 38 hours, which suggests the possibility for once-daily dosing. Early animal studies indicate that PSI-7851 can achieve concentrations of active triphosphate in the liver of up to 1,000 times higher than PSI-6130 at equivalent doses.
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