When Regeneron Pharmaceuticals (Nasdaq: REGN) and sanofi-aventis' (NYSE: SNY) Zaltrap failed to help lung cancer patients last month, it was a disappointment, but perhaps not unexpected given how difficult it is to treat lung cancer.

Another phase 3 clinical trial in colon cancer was still pending, offering investors upside with perhaps less downside risk given the earlier failure.

The drug blocks vascular endothelial growth factor, or VEGF, a molecule that promotes blood vessel formation, growth, and survival. Since tumors need nutrients in blood to survive, if you inhibit the blood vessels, you can theoretically inhibit the growth of the tumor.

There's evidence that targeting VEGF slows colorectal tumors. VEGF-targeting Avastin from Roche is approved to treat colon cancer, but a host of VEGF inhibitors including Pfizer's (NYSE: PFE) Sutent, Bristol-Myers Squibb's (NYSE: BMY) brivanib, and GlaxoSmithKline's (NYSE: GSK) Votrient have failed to show an effect on colon cancer. Zaltrap's working in colon cancer was far from a sure thing.

Thus the near-30% increase in Regeneron today after the company said that combining Zaltrap with chemotherapy improved overall survival in colon cancer patients that fail an initial treatment. Considering the biotech is now a $6 billion company, that's a substantial increase in valuation.

The duo plan to submit a marketing application to the Food and Drug Administration and EU regulators in the second half of the year, which would put a decision from regulators sometime next year.

Without the full data -- how much did Zaltrap increase survival -- it's a little difficult to know how much the drug could bring in. But any improvement is likely enough to get the drug on the market, considering these patients have already failed a prior treatment.

Interested in keeping track of Regeneron as it tries to get Zaltrap approved? Click here to add it to My Watchlist, which will help you keep track of all our Foolish analysis on Regeneron.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Motley Fool has a disclosure policy.