An experimental psoriasis drug from Johnson & Johnson may raise the risk of cancer, U.S. drug reviewers said in documents recently released.

"Long-term use of ustekinumab may lead to increased risk of tumor development in psoriasis patients," Food and Drug Administration reviewers wrote in an analysis prepared for an advisory panel that is set to discuss the drug next week.

A spokesman for J&J's Centocor unit said the risk of cancer could be just theoretical and the company planned to monitor long-term safety after approval.

The FDA will ask the panel of outside advisers if it would be adequate to note animal data suggesting a cancer risk on the prescribing instructions for ustekinumab, or if the company should do more studies, the reviewers said.

An estimated 7.5 million Americans have psoriasis, a skin disorder that causes scaly red patches.

Two company-sponsored trials showed ustekinumab injections were effective at reducing the severity of psoriasis symptoms, the FDA reviewers said.

If approved, ustekinumab would compete with other leading injectable drugs -- Amgen Inc and Wyeth's Enbrel and Abbott Laboratories Inc's Humira -- as well as J&J's Remicade, which is given by intravenous infusion.

Those drugs work by blocking an inflammation-causing protein called tumor necrosis factor (TNF).

Ustekinumab blocks interleukin-12 and interleukin-23, two other immune-system proteins linked to inflammation.

The FDA reviewers said animal studies showed blocking those proteins raised the chances of developing cancer, which was enough to justify including those findings on the drug label "to inform prescribers about the potential carcinogenic risk."

Centocor spokesman Michael Parks said "although the occurrence of malignancies were comparable to placebo in clinical trials for ustekinumab, the risk of malignancies with immunosuppressive therapy is theoretically possible."

Prescribing information for the TNF-blocking drugs warn about a possible risk of cancer. The FDA said earlier this month it was investigating if those medicines were linked to cancer cases reported in children and young adults.

The FDA is expected to make a decision on whether to approve ustekinumab by September. The agency usually follows panel recommendations.