Rigel Pharmaceuticals, Inc announced positive clinical safety data from a Phase I trial for R803, an experimental drug to treat Hepatitis C Virus (HCV), the blood-borne virus that affects nearly 170 million people worldwide. Clinical data indicates that R803 is well tolerated with no notable adverse effects reported in the dose levels that Rigel plans to use moving forward. Rigel plans to launch a Phase I/II efficacy clinical trial in the US during the second quarter of 2004 in HCV-infected patients. This trial will monitor viral clearance and safety over numerous days of drug administration.

In the Phase I trial, an escalating dose regimen of R803 was studied in 42 volunteers and was compared with placebo controls. The safety data collected indicated that subjects treated with R803 were indistinguishable from the placebo controls across a wide range of clinical and laboratory safety tests, including clinical signs and symptoms, serial electrocardiography, and clinical chemistry and hematology studies. Pharmacokinetic data, which will aid in planning dosing in the next clinical study, was also collected. The trial was conducted in the UK and the results will be part of the US IND package that Rigel expects to file with the FDA later in the first quarter of 2004.

"A leading issue for the millions of Americans infected with chronic HCV are the side effects of current therapies and their relatively limited efficacy," noted Jules Dienstag, professor of Medicine and associate dean for Academic and Clinical Programs at Harvard Medical School and a steering committee member of the upcoming study. "R803 continues to show promise as a unique first-line anti-HCV therapeutic, directly targeting HCV by interfering with the viral polymerase protein that is needed for replication."

"The successful completion of Phase I safety trials represents another major step in Rigel's efforts to advance the clinical development of R803," said Elliott Grossbard MD, senior vice president, Medical Development. "The availability of a new oral therapy that is convenient, safe and effective would be an important addition to currently available treatment options for patients with HCV."

Rigel's R803, a non-nucleoside HCV polymerase inhibitor, is an oral, small-molecule compound. To date, R803 has demonstrated potent efficacy in inhibiting viral replication in cell-based assay systems and in live virus assays. R803 has been shown to be efficacious against various genotypes of HCV, including genotype 1, the most common in North America and Europe. In various assays, R803 appears to act within days to reduce viral levels significantly. In addition, as a result of R803's novel viral binding site, resistance may be slow to develop.