A little jittery, are we, Seattle Genetics (Nasdaq: SGEN) investors? The company gets unanimous recommendations to approve Adcetris, aka brentuximab vedotin, for two different types of lymphomas, yet the stock trades down as much as 12%.

The Food and Drug Administration advisory committee recommendations were for approvals of the accelerated variety, but as I said on Wednesday, a full approval was a long shot at best. There just wasn't enough data to give Seattle Genetics a full approval, with would free the company from ever having to check back in with the FDA.

An accelerated approval requires companies to run a post-marketing trial to confirm that the initial data is correct. Considering how well Adcetris works, I don't think investors need to be concerned that a later trial will fail to justify the drug's presence on market, as we have seen with Roche's Avastin and AstraZeneca's (NYSE: AZN) Iressa.

The bigger concern is that the PDUFA date for Adcetris is a little more than a month away, and the two sides will have to reach an agreement on the confirmatory trial design before the FDA grants accelerated approval.

The FDA's and Seattle Genetics' motivations in designing a trial aren't completely aligned. The agency primarily wants to get data from enough patients to fully convince it of the drug's effectiveness. But if it's going to have to pay for a trial, Seattle Genetics would like the trial to be useful from a marketing standpoint.

There will be some push and pull in the negotiations, but this isn't like the NFL labor negotiations, where either side holding out can hurt the other side financially. Seattle Genetics has very little bargaining power here, and it'll ultimately have to do what the agency wants.

It's possible the FDA might propose a trial that's virtually impossible to run. For example, Adcetris seems to work so well that it may be hard to enroll patients into a trial where it's is being compared to another drug. With Adcetris on the market, there's little incentive for patients to risk being randomized into the arm of the trial that doesn't get Adcetris. But I don't see that as a huge stumbling block. Like Novartis (NYSE: NVS), Eli Lilly (NYSE: LLY), Bristol-Myers Squibb (NYSE: BMY), and others that have been slow to complete their confirmatory trials, Seattle Genetics can just nod, say it'll run the trial, and deal with the consequences later.