SAN DIEGO ( TheStreet) -- Arena Pharmaceuticals ( ARNA - Get Report) CEO Jack Lief told investors Monday that U.S. regulators provided a "path forward" for the eventual approval of the company's weight-loss drug lorcaserin but Lief refused to say how long the process would take.

Arena shares were essentially unchanged in early Monday trading, up 6 cents to $1.69, reflecting uncertainty over lorcaserin's future.

In a widely expected decision, the U.S. Food and Drug Administration rejected Arena's lorcaserin Friday, citing the drug's "marginal" efficacy and asking for additional rat cancer data. On a conference call Monday, Arena's Lief said the company hopes to meet with the FDA in November or early December about next steps necessary to resubmit lorcaserin for another review.

"We just received the complete response letter on Friday and have not met with the FDA, so it's too early to provide clarity on timing," said Lief.

Lief did downplay at least some of the expectations about results from a study of lorcaserin in patients with Type II diabetes that the company expects to release in the next few weeks. The FDA, in its complete response letter, asked the company to submit data from the so-called Bloom DM study for review.

Previous studies show that meaningful weight loss in diabetes patients can be difficult to achieve, Lief said.

That may be more problematic for lorcaserin given the relatively small, 3% (placebo-adjusted) weight loss attributed to the drug in already-completed studies enrolling obese but not diabetic patients.

Competing weight-loss drugs have also had a tough time helping diabetics lose weight. Diabetics treated with Vivus' ( VVUS - Get Report) Qnexa, for instance, lost about 7% of their weight, adjusted for placebo, compared to 9% weight loss, also placebo adjusted, for non-diabetic, obese patients. Likewise, the weight loss seen with Orexigen Therapeutics' ( OREX) Contrave was also lower in diabetics than it was in patients with the disease, according to data compiled by JMP Securities analyst Jason Butler.

Arena's Lief stressed that other efficacy measures, such as helping diabetics better control their blood sugar levels, will be an important consideration when reviewing lorcaserin results from the Bloom DM study.

Lief, however, did not say whether efficacy measures other than weight loss would be enough to convince the FDA to approve lorcaserin. He also sidestepped questions about the potential impact of a possible approval that restricted lorcaserin use to only obese diabetics.

Lief also had no response to a question about whether Arena's partner Eisai could or would divorce itself of lorcaserin if the drug's approval was somehow restricted.

On the subject of lorcaserin and cancer risk, Lief said the company would likely have to conduct additional "short term" animal studies to shed more light on the possible ways in which lorcaserin could be contributing to the findings of breast and brain cancer in rats. The company does not believe, however, that longer, two-year animal cancer studies will be necessary.

--Written by Adam Feuerstein in Boston.

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