Cancer drugs typically don't work on all patients. Most work on less than half of the patients they treat, and as you get farther along in the disease progression, the number of patients that are helped go down substantially.

That's what makes Seattle Genetics (Nasdaq: SGEN) data released today so amazing. Brentuximab vedotin (SGN-35) was able to elicit a complete or partial response in 75% of Hodgkin lymphoma patients treated, with more than half of responses lasting longer than six months. These are patients that had already failed or couldn't take other drugs, making the robust effect even more impressive.

Seattle Genetics is up more than 18% on the news, even though positive results were already expected. In previous trials, the drug showed a response rate in the 50% to 60% range.

Seattle Genetics and marketing partner Takeda Pharmaceutical expect to use the data to file for Food and Drug Administration approval in the first half of next year. Given the unmet need, I'd expect a priority review, which would result in a six-month review and an approval before the end of next year.

Everything isn't ideal with the brentuximab vedotin results, though. The trial was relatively small -- just 102 patients -- and there was no placebo control to compare the results with. The FDA usually wants two well-controlled trials to approve a drug, but does have a mechanism -- called an accelerated approval -- for drugs that seem to be working on diseases with unmet needs.

Given the poor prognosis for these patients, the company had previously said that a 25% to 30% response rate would likely be good enough for approval. Brentuximab vedotin blew past that, so a thumbs-up seems likely.

Investors may be a little skittish after Roche and ImmunoGen (Nasdaq: IMGN) weren't even able to get the FDA to review trastuzumab-DM1, their breast cancer drug that was up for accelerated approval. But brentuximab vedotin isn't likely to be shot down for the same reason. According to the FDA, Roche tested trastuzumab-DM1 in the wrong population, but Seattle Genetics has a Special Protocol Assessment, so the FDA has already signed off on the trial design.

The bigger risk for Seattle Genetics investors is that brentuximab vedotin might fail follow-up trials that the FDA requires -- like Pfizer's (NYSE: PFE) Mylotarg and AstraZeneca's (NYSE: AZN) Iressa -- and risk being pulled from the market. But after clearing initial hurdles by so much, that risk seems fairly small at this point.

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