FDA's Review of Arena Pharma's Obesity Drug Sunnier Than Expected

SAN DIEGO (TheStreet) -- The FDA's review of Arena Pharmaceuticals'(ARNA) weight-loss drug lorcaserin is... and it's more positive than I expected.

The outcome of Thursday's FDA advisory panel is still unknown, of course, but Arena bulls (the "Areniacs") should feel more confident in a positive vote recommending lorcaserin's approval.

Arena shares are up 41 cents, or 15%, to $3.14 in recent trading.

Arena's lorcaserin still doesn't help people lose much weight. A 300-pound person is going to lose about 9 pounds after a year of treatment. But the efficacy hurdle for approval of an obesity drug in the U.S. is low, so lorcaserin gets a pass. More important to the outcome of Thursday's FDA advisory panel, the agency appears to be relatively satisfied with lorcaserin's safety. Several of the issues that dogged the drug and caused it to be rejected in 2010 have been addressed, according to my interpretation of FDA's briefing documents released today.

FDA seems less concerned about lorcaserin-fueled rat tumors and the potential risk for breast cancers and other tumors in humans.

On the subject of mammry adenocarcinomas in rats, FDA reviewers state:

"Readjudication by the PWG allowed identification of a 24-fold safety margin for mammary adenocarcinoma relative to the clinical dose of 10mg bid lorcaserin. In general, the Agency interprets a 24-fold safety margin to a non-genotoxic carcinogen in rodents as indicative of negligible risk to human subjects."

FDA did conclude that lorcaserin causes significant increases in benign fibroadenomas in rats with no acceptable safety margin relative to human doses, but the agency also downplays the concerns.