In July, Pfizer disclosed that the FDA sought a "routine" analysis of clinical trial data for the drug that could delay a decision by three months or more beyond the agency's August 21 deadline.

If approved, the pill could offer a more convenient alternative to Abbott Laboratories Inc's $8 billion-a-year injectable medication Humira.

The FDA now views the additional analysis as a major amendment to Pfizer's application for approval, and set a new November 21 deadline for reviewing the drug, the company said on Tuesday. It did not give more details of the data analysis it provided.

Pfizer said it still believes trial results favor the drug's use, and is working with the FDA, as well as regulators in Europe, Japan and other countries, to bring it to market.

Rheumatoid arthritis, a chronic inflammatory autoimmune disease, affects about 1.6 million Americans and 23.7 million people worldwide, the company said.

Tofacitinib blocks signals that activate immune and inflammatory responses in the body, and is the first in a new class of drugs called Janus kinase (JAK) inhibitors.

A panel of outside advisers to the FDA recommended the agency approve the drug during a May meeting, though many said it should be used only after patients had tried other treatments. Safety concerns included possible infections, such as pneumonia, or malignancies.

Wall Street analysts estimate that tofacitinib could reap up to $3 billion in annual sales if approved.

Pfizer shares rose 2 cents to $23.91 in premarket trading.

(Reporting by Michele Gershberg; Editing by Maureen Bavdek and Jeffrey Benkoe)

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