Pfizer Inc. (NYSE: PFE) has announced that the U.S. Food and Drug Administration approved its drug candidate called XELJANZ. The treatment here is for rheumatoid arthritis and this is the 5 mg twice daily dosing. The exact treatment is for adults with moderately to severely active rheumatoid arthritis who have had an inadequate response or intolerance to methotrexate.

Pfizer said that the approval will allow for XELJANZ to be used as monotherapy (alone) or in combination with methotrexate or other non-biologic disease-modifying antirheumatic drugs.

What may interest investors is that this was the first approved RA treatment in a new class of medicines called Janus kinase (JAK) inhibitors, It is also the first of class to be approved to treat RA in over 10 years. XELJANZ is approved as a second-line medicine for RA, which means treatment with a biologic is not required before taking XELJANZ.

The study evaluated approximately 5,000 patients from a broad cross-section of the RA patient population.

There are some issues here that will require ongoing safety measurements. Today’s approval said that XELJANZ should not be used in combination with biologic DMARDs or potent immunosuppressives, such as azathioprine and cyclosporine. Safety findings observed in the overall XELJANZ RA program include serious and other important infections, including tuberculosis and herpes zoster; malignancies, including lymphoma; gastrointestinal perforations; decreased neutrophil and lymphocyte counts; decreased hemoglobin; liver enzyme elevations; and lipid elevations. The most common serious adverse events were serious infections. The most commonly reported adverse events were upper respiratory tract infections, headache, diarrhea and nasopharyngitis.

On the two dosages Pfizer said, “XELJANZ was studied in both a 5 mg and 10 mg twice-daily dosing regimen. The FDA has approved the 5 mg twice-daily dose in the second-line setting and has indicated that further data are required to assess the benefit: risk profile of the 10 mg twice-daily dose. Pfizer will continue to generate additional clinical data on the 10 mg twice-daily dose and work with the FDA to understand the additional data needed for further assessment of the 10 mg twice-daily dose.”

We will not proclaim this the next blockbuster drug (over $1 billion in annual sales) for Pfizer until we see formal pricing for the drug and the actual size of the real target population. Pfizer said that RA affects approximately 1.6 million Americans and 23.7 million people worldwide. The Arthritis Foundation’s figures show that there are an estimated 1.3 million people in the United States who have RA. The actual target market will be smaller than either the 1.3 or 1.6 million counted in the total market because of exclusions from other medications and conditions.

Pfizer closed up 0.5% at $24.71 and shares are up 1.2% at $25.01 in the after-hours session. With a market cap of $184 billion, it generally takes a lot to influence a move one way or the other here.

JON C. OGG

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