SOUTH SAN FRANCISCO, Calif., Nov. 22 /PRNewswire-FirstCall/ -- Rigel
Pharmaceuticals, Inc. RIGL announced today the results from its Phase
I/II clinical study of R803, a novel oral hepatitis C RNA polymerase
inhibitor. The two-center, double-blind, dose-escalation study
examined the safety and pharmacokinetics of R803 and its effectiveness
in reducing viral levels in 32 patients infected with Hepatitis C
Virus (HCV). The results demonstrated that R803 was well-tolerated and
that there were no significant adverse effects. The study showed that
the drug was only present in the blood stream at sufficient levels for
a limited number of hours during the course of each dosing day. There
was a decline in viral levels over the course of treatment and viral
levels rose after dosing was discontinued. However, the decline in
viral levels was not statistically significant or clinically
meaningful. Rigel has continued to develop better delivery approaches
of R803, such as a pro-drug of R803, and derivatives of R803 that it
believes exhibit superior bioavailability. These potential
improvements and next steps in the Rigel HCV clinical program will be
discussed at Rigel's analyst day meeting in New York City on December
1, 2004.

These results, although disappointing, have provided us with very
useful clinical information on the improvements necessary to achieve
significant results in HCV infection." said James M. Gower, chairman
and CEO, Rigel Pharmaceuticals. "We have learned a great deal about
how R803 is metabolized and its bioavailability. While viral loads
were not meaningfully reduced, we believe that with an R803 pro-drug
or derivatives the pharmacokinetics and drug exposure could be
improved. We believe we have identified drug candidates with improved
bioavailability and good anti-viral activity for our HCV program. We
will be studying these issues in detail over the next several weeks
before deciding on our future clinical direction."
R803 Phase I/II Clinical Trial Design

The study was a 32 patient Phase I/II randomized, placebo-controlled,
double-blind multiple, dose escalation study, which took place in
Miami and New Orleans. The objectives of the study were to evaluate
the safety and pharmacokinetics of R803 in patients with HCV and to
explore the effectiveness of R803 in reducing viral levels. Patients
were divided into eight groups, with each group receiving an
increasing dose or increasing number of days of treatment. Subjects
were dosed for two to four days, plus the morning dose on the
following day. HCV viral RNA levels were measured multiple time levels
following the administration of R803