Amylin Pharmaceuticals (NASDAQ:AMLN) announced results yesterday for its 24-week phase 2 trial of a new weight-loss treatment. While the drug combination looks good, the subsequent phase 3 trial will truly determine its marketability.

The trial tested a combination of pramlintide, the active ingredient in Amylin's diabetes drug SYMLIN, with an analog of leptin, a signaling molecule that regulates energy metabolism and body weight. The combination product reduced body weight on average by 12.7%. That's significantly more than the 8.4% reduction subjects experience from pramlintide alone.

In a third arm of the study, the subjects received only leptin, but Amylin didn't report the weight loss they experienced. That's important to know, because the weight loss is more impressive if the two drugs have a synergistic effect, rather than acting independently and merely having an additive effect. Investors will have to wait until Amylin's Research and Development Day at the end of the month to get further details.

Perhaps the most promising part of the data was that subjects receiving the combination of pramlintide and leptin had continuous weight loss all the way to the end of the study. Those treated with pramlintide alone had weight loss that leveled off toward the end. This is important, because in the past, some previous potential anti-obesity medicines lost their effectiveness after six months, a problem the combination drug might ultimately avoid. A pivotal phase 3 trial will almost certainly measure the drugs' effects over a longer period of time.

In addition, if weight loss continues beyond six months for the combo drug, the weight loss total could be more substantial than the 12.7% decrease (an average of 25 pounds) reported. That could determine whether the pramlintide-leptin combination is a blockbuster, or just a niche product. I'm not sure that patients will view 25 pounds of weight loss as a good enough trade-off for twice-a-day injections. With GlaxoSmithKline's (NYSE:GSK) over-the-counter product, alli, and prescription medicines from Abbott (NYSE:ABT) (MERIDIA) and Sanofi-Aventis (NYSE:SNY) (Acomplia) all being oral compounds, Amylin will need to show superior results to convince patients to use its product instead.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Glaxo is a selection of the Income Investor newsletter. The Fool has a disclosure policy.