June 26 (HealthDay News) -- A new drug to treat rheumatoid arthritis reduces joint inflammation in severe cases while causing only mild to moderate side effects, according to a report from the first clinical trial of the drug on humans.

Masitinib, which is being developed by AB Science pharmaceuticals, is supposed to halt the activity of mast cells, a part of the immune system believed to be involved in the start and progression of rheumatoid arthritis.

The results of the French trial, involving 43 people with arthritis that other treatments had failed to help, appears online in Arthritis Research and Therapy.

"We are encouraged from this study that masitinib not only appears to be effective, but that within the first three months of treatment, the worst of its side effects were over, possibly making it suitable for long-term treatment regimens," one of the researchers, Olivier Hermine, said in a news release from the journal's publisher. The next step will be placebo-controlled trials, he added.

SOURCE: BioMed Central, news release, June 26, 2009