What happened

Seattle Genetics (NASDAQ:SGEN) is down 5.9% as of this writing, having been down as much as 10.5% Monday following a weekend of presentations at the American Society of Hematology (ASH) meeting.

So what

The biotech has issued four press releases containing data from the meeting so far today, including a midmorning release, which might be helping reverse the weak opening.

Two of the data reports over the weekend probably aren't moving the stock. One was five-year survival data from a phase 1 trial testing Adcetris in newly diagnosed patients with mature T-cell lymphoma. The trial only had 26 patients, and there were no progression events or deaths in the trial since the three-year follow-up was reported. Essentially that's the best investors could hope for.

The other non-event came from updated data from the phase 3 ALCANZA trial testing Adcetris in patients with cutaneous T-cell lymphoma (CTCL). The data from the trial was originally presented at ASH last year and has already been submitted to the FDA to expand Adcetris' label. The extra data shouldn't affect its approvability, which appeared likely based on the initial data presentation.

Doctor talking to a patient

Image source: Getty Images.

What appears to be driving shares lower is the presentation of the phase 3 ECHELON-1 trial in newly diagnosed patients with advanced classical Hodgkin lymphoma. Investors already knew the trial succeeded, with Adcetris plus older chemotherapy drugs -- adriamycin, vinblastine, dacarbazine (AVD) -- working better than the control, AVD plus bleomycin (ABVD).

The full data presented at ASH were also published in the New England Journal of Medicine. They showed that while Adcetris+AVD passed its primary endpoint, increasing modified progression-free survival -- defined as progression, death, or patients getting an additional anticancer therapy because they didn't have a complete response -- compared to the control treatment, Adcetris+AVD wasn't positive on a couple of key secondary endpoints, although they were trending in the right direction. Adcetris+AVD reduced the risk of death by 28%, but that wasn't statistically significant. The complete response rate was 73% versus 70% for patients treated with the control drug, and the objective response rate was also only slightly better at 86% for Adcetris+AVD compared to 83% for the control.

Now what

Bleomycin is pretty toxic, so doctors would be happy to swap it out for something that works only slightly better with less toxicity. Unfortunately, while Adcetris+AVD had less pulmonary toxicity than ABVD, Adcetris causes problems with lowering white blood cells, which led to more infections. Seattle Genetics has some data showing treatments with drugs that boost the immune system can counteract that, so the toxicities for Adcetris+AVD may be more manageable than AVBD, but it isn't going to be a cakewalk for Seattle Genetics to get doctors to change their current standard practice.

The conversion of newly diagnosed Hodgkin lymphoma patients to Adcetris+AVD may not matter much in the long term because Seattle Genetics and Bristol-Myers Squibb (NYSE:BMY) presented data today from a phase 1/2 trial showing that Adcetris plus Bristol-Myers' Opdivo produced an 83% objective response rate with a 62% complete response rate. The companies are testing the combination in the phase 3 CHECKMATE 812 trial, which is in relapsed patients, but if it works there, the companies would presumably test it in newly diagnosed patients, potentially replacing Adcetris+AVD.

Brian Orelli has no position in any of the stocks mentioned. The Motley Fool recommends Seattle Genetics. The Motley Fool has a disclosure policy.