On Thursday, biopharmaceutical firm ImClone Systems (NASDAQ:IMCL) released modest first-quarter results. Shares had barely budged since the news that will affect ImClone's future the most came earlier in the quarter, with positive developments for its cancer compound Erbitux and setbacks for its biggest rival, Amgen (NASDAQ:AMGN).

Sales of Erbitux, ImClone's only marketed product, gained 34% year over year this quarter, but revenue was down slightly (once a one-time milestone payment from marketing partner Bristol-Myers Squibb (NYSE:BMY) is excluded from last year's numbers) as a result of decreased sales of the drug to ImClone's partners. Even with the decline in revenue, operating income was up more than 50% once that one-time payment gets backed out.

When Amgen's Vectibix was approved for marketing last year, many, including me, thought it would put a mighty dent in Erbitux sales. This quarter's Erbitux U.S. sales growth was by far the lowest it has been in the past four quarters, partly because of the competitive pressures of Vectibix. But with Vectibix failing to show a survival advantage in one important clinical trial with its use in colorectal cancer, and Erbitux achieving the opposite result in a similar trial, the fortunes for the two compounds are diverging.

Vectibix sales were up 31% sequentially this quarter, but ImClone is fighting back in the marketing battle. It plans to increase its own sales force that details Erbitux by more than 50% in order to drive sales growth higher.

In June, there will be an abundance of new Erbitux clinical trial data to pore over. This data is important because the future of ImClone depends on how well Erbitux can be differentiated from the other cancer compounds on the market. While the first-quarter financial results were nothing to get excited about, the future looks brighter for ImClone after all the positive clinical-stage developments this quarter.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy.