The companies said Friday they saw a statistically significant improvement in patient groups that received their drug, sarilumab, combined with methotrexate compared to those who just received methotrexate and a placebo.
Aside from improving signs and symptoms, the treatment also helped physical function and inhibited joint damage progression, the companies said.
Rheumatoid arthritis is a chronic inflammatory disease that affects the joints and destroys soft tissue, cartilage and bone. It represents a major area of research for drug companies because it is chronic, meaning patients will likely take the drugs regularly for a long time.
Last year, the Food and Drug Administration approved a new treatment for the disease, Pfizer Inc.'s twice-a-day pill Xeljanz.
Shares of Tarrytown, N.Y.-based Regeneron closed at $276.41 on Thursday and have climbed more than 62 percent this year. U.S.-traded shares of Sanofi closed at $52.59 Thursday and have risen 11 percent in 2013.