Regeneron Pharmaceuticals saw its stock slip over the weekend after US regulators placed a question mark over the safety of Arcalyst in the gout setting.

In a report posted on the US Food and Drug Administration's website, staff noted that Arcalyst (rilonacept) is a biologic drug with the potential to boost the risk of infections and malignancy via immunosupression, and point to data from a 16-week trial in which "an imbalance in malignancy events" was observed.

Arcalyst was first approved in 2008 for the treatment of a rare genetic condition called Cryopyrin-Associated Periodic Syndromes (CAPS), and Regeneron is seeking to expand its scope for use in patients to prevent gout flare-ups during the initiation of uric acid-lowering therapy.

Determination of the risk-benefit profile for Arcalyst, including whether the 16-week trial provides enough evidence to support safety and efficacy, will be discussed in full at the upcoming arthritis advisory committee meeting later today, FDA staffers said.

The drug is currently not a big earner for Regeneron, pulling in sales of just $20 million in 2011, but this could swell to $209 million in 2015 if the gout indication is approved, according to analysts' estimates compiled by Bloomberg.

The group's stock closed down nearly 2% on the Nasdaq on Friday and continued to inch down over the weekend.