Pfizer Inc. said Monday that its new rheumatoid arthritis drug, Xeljanz, has been approved for use in Japan.

The Japanese Ministry of Health, Labor and Welfare approved the twice-a-day pill for use in patients whose symptoms have not improved after using at least one other disease-modifying drug. Those drugs are meant to slow worsening of the disabling autoimmune disorder, rather than just relieve joint pain and inflammation.

In rheumatoid arthritis, the immune system mistakenly attacks tissue in the body, primarily the joints, limiting mobility and sometimes leaving fingers bent at uncomfortable angles. It can also attack the lungs, heart, eyes, skin, blood and nerves. It's a chronic disease that generally worsens over time, but that progression can be slowed down by early, aggressive treatment.

Xeljanz, known chemically as tofacitinib, is the first approved pill in a new class of medicines called Janus kinase inhibitors, or JAK inhibitors for short.

The drug from New York-based Pfizer works inside cells, inhibiting the Janus kinase enzyme's signaling pathways. Those pathways are involved in the inflammation caused by rheumatoid arthritis.

Injectable biologic drugs such as Enbrel and Remicade, grown in living cells from genetically engineered proteins, work outside the body's cells to slow progression of the disorder by broadly suppressing the immune system. They are expensive, must be injected or infused, and can cause significant side effects. Those range from persistent fevers and flu-like illness to susceptibility to infections and flare-ups of dormant diseases such as tuberculosis.

Pfizer, the world's largest drugmaker by revenue, said that initially, Xeljanz will be made available in Japan to medical institutions participating in a program monitoring the progress of all patients.

Xeljanz, which was approved in the U.S. in November, is viewed as a potential big seller for Pfizer.

"RA is a serious and disabling disease and there is a need for new treatment options, as a significant number of patients do not adequately respond to current therapies," Mark Swindell, head of Pfizer's specialty care business in Japan, said in a statement.

Rheumatoid arthritis affects about 750,000 people in Japan and 23.7 million people worldwide. Up to one-third of patients don't respond adequately to current therapies, and about half stop using a particular disease-modifying treatment within five years, according to Pfizer.


Follow Linda A. Johnson on Twitter at