Drug developer Trimeris (NASDAQ:TRMS) needs to push another drug out of its pipeline because sales of its only drug, Fuzeon, appear to be hitting the wall. When a company has only one product, it just doesn't take too long until sales stagnate as the product reaches market saturation.

While global sales of the company's HIV fusion inhibitor in the second quarter increased 8% over the same quarter last year, sales were down 3.6% over last quarter. That marks the second quarter in a row with decreased sales. Sales in the U.S. and Canada -- where Trimeris receives a larger portion of the profits from its partner Roche -- were actually up 12% over last quarter, so it's possible that the drop in overall sales won't hurt the company that much. We'll have to wait a few weeks until Trimeris releases earnings for the quarter to see just how much the decreased sales spoiled the bottom line.

Roche and Trimeris are trying to increase sales by expanding the labeling indications that Fuzeon is approved for. Earlier this week, the companies announced the start of a trial to test Fuzeon's efficacy in combination with an investigational integrase inhibitor. Perhaps more importantly, the trial will also test whether the drug can be injected once a day instead of twice a day. Part of the lower-than-expected sales of Fuzeon is due to the fact that no one likes to give themselves shots, so changing the dosing to once a day might help sales.

While Roche and Trimeris have had the HIV fusion inhibitor market all to themselves for the past two years, there are a few drugs on the horizon that Fuzeon will need to compete with. Pfizer (NYSE:PFE) is awaiting marketing approval for maraviroc in the U.S. and the European Union. It recently received an approvable letter from the FDA; the problem may be as simple as a labeling issue and approval could be imminent. Schering-Plough (NYSE: SGP) and Tanox (Nasdaq: TNOX), which is being acquired by Genentech (NYSE: DNA), also have drugs in the clinic that inhibit the infection of healthy T-cells.

Trimeris' next generation fusion inhibitor, TRI-1144, is in a state of flux after Roche decided to stop funding the preclinical program earlier this year. The company could try to find another partner for the project, but with $54.7 million in the bank at the end of last quarter, and free cash flow for 2007 expected to approach $20 million, it should be able to fund most of the studies on its own. It expects to start clinical trials in the first half of 2008.

One-drug wonders can fade into oblivion as fast as Right Said Fred became un-sexy. Trimeris needs to expand the labeling of Fuzeon to increase sales or push another drug out of its pipeline. If it doesn't it will begin to lose value until it gets swallowed up by one of the big fish for less than it's truly worth.

Want to know the latest drug stock we've picked for the Fool's market-beating Rule Breakers newsletter? You can take a look at all our recommendations, as well as get access to our message boards and exclusive content, with a 30-day free trial.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is an Inside Value recommendation. The Fool's disclosure policy is rock solid.