A sub-study of patients taking part in the trial of the drug, baricitinib, also showed that the two highest doses tested helped to reduce joint damage, based on Magnetic Resonance Imaging (MRI) tests.

The companies in June released positive data from the 301-subject Phase II study after 12 weeks of treatment in patients with mild to moderate RA who had an inadequate response to methotrexate. Data from the ongoing extension study, presented Tuesday at the American College of Rheumatology meeting in Washington, measured baricitinib treatment through the 24 weeks.

Based on the data collected from its Phase II programs, Lilly said it has moved into late-stage testing of the drug.

Four Phase III RA studies of baricitinib using the 2 milligram and 4 mg doses are planned for patients who have not previously been treated with methotrexate or injectable biotech drugs and also in patients who did have prior treatment with biologics, or drugs made from living organisms or their products, the companies said. Those studies will form the basis of the data package used to seek approval of the drug.

After 24 weeks, 73 percent of patients who received 8 mg of the Lilly drug once daily achieved the ACR20 goal, or a 20 percent improvement in rheumatoid arthritis symptoms. That compared with 78 percent who hit ACR20 at 12 weeks.

For the 4 mg dose, 78 percent of patients hit ACR 20 at 24 weeks, up from 75 percent at week 12.

The 2 mg dose that failed to show statistical significance compared with a placebo at 12 weeks had 63 percent of patients achieve ACR20 by week 24 of treatment, the data showed.

The study also measured ACR50 and ACR70 rates, or 50 percent and 70 percent improvement. All three doses showed improvement at 24 weeks from measurements taken at 12 weeks.

"These data are important because collectively they show patients experienced improvement with baricitinib as early as week two that was sustained through week 24," Dr. Mark Genovese, co-chief of the division of immunology and rheumatology at Stanford University School of Medicine, said in a statement.

"Also of note is that the percentage of patients achieving ACR50 and ACR70 increased over time and no unexpected safety findings emerged with continued dosing," said Genovese, a member of the steering committee for the study.

Baricitinib belongs to a hot new class of oral medicines called Jak inhibitors that aim to compete with the injected rheumatoid arthritis drugs that currently dominate the market with billions in sales. Pfizer Inc last week became the first company to bring one of the new drugs to market with the U.S. approval of tofacitinib, which will be sold under the brand name Xeljanz.

Jak inhibitors block enzymes believed to be involved in the inflammatory process.

In the sub-study of 154 patients who underwent MRI testing, there was a statistically significant improvement in measures of inflammation and joint damage at the 4 mg and 8 mg doses after 12 weeks compared with placebo, the companies said. The effects persisted through 24 weeks, they said.

In order to compete with the biologic blockbuster injected drugs, such as Abbott Laboratories' $8 billion a year Humira, the Jak inhibitors must show that they can prevent or delay joint deterioration as well as alleviate symptoms.

(Reporting by Bill Berkrot in New York; editing by Matthew Lewis and Prudence Crowther)

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