US drug reviewers on Monday questioned whether the benefits of Pfizer's  experimental treatment for rheumatoid arthritis outweighed its risks of cancerous cells and infections.

The Food and Drug Administration staff said the drug, called tofacitinib, appeared to reduce swollen and tender joints during clinical trials, but questioned the method of analyzing X-rays to prove the drug worked.

The FDA staff review comes ahead of an advisory panel, which will vote on whether to recommend the drug on Wednesday. The FDA will make a final decision in August, taking into account the panel's recommendations.

Analysts say tofacitinib could be a potential blockbuster medicine, reviving faith in Pfizer's research prowess if its benefits are deemed to outweigh its risks.

The FDA staff said the company's method of showing its drug worked, by analyzing and scoring X-rays, did not always mean the drug actually slowed down structural damage in people with rheumatoid arthritis, a potentially crippling condition in which the body's own immune system attacks the joints.

At the same time, tofacitinib was tied to a higher risk of malignancies, or cancerous cells that could spread, especially of lymphoma. The FDA staff said the risk of malignancies may get worse with higher doses or longer exposure to the drug.

"Substantial evidence to support a salutary effect of tofacitinib for structural damage progression becomes even more important in light of the aforementioned malignancy concerns, particularly since other therapies for (rheumatoid arthritis) are available that may not have a similar malignancy risk," the FDA staff said in documents posted online on Monday.

Tofacitinib is one of Pfizer's biggest pipeline hopes and could become one of the few major new medicines to come from its own laboratories since impotence treatment Viagra more than a decade ago. If approved, the likely cheaper pill would compete with widely used injectable medicines, including Abbott Laboratories's $8 billion-a-year Humira. - Reuters