Warning: Don't Sell on This News

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Seattle Genetics (NAS: SGEN) fell quite a bit after the company announced a change to the warnings section of the label for its blood cancer drug Adcetris. The label warns of a possible link to a brain infection called progressive multifocal leukoencephalopathy, or PML, and also warns that Adcetris shouldn't be used with bleomycin.

None of this is new news, though.

Seattle Genetics discovered the bleomycin contraindication while testing its drug in combination with a chemotherapy regimen called ABVD; bleomycin is the B. While ABVD is the current standard of care, doctors don't really like bleomycin because it has pulmonary side effects. Adcetris exacerbated that effect, so the company decided to test AVD plus Adcetris. If that works better than AVBD with less severe side effects, the warning about concomitant use won't be a problem. No one's going to bleomycin's funeral.

The PML side effect is more worrisome, but not particularly surprising. One patient in the clinical trials for Adcetris came down with the potentially life-threatening disease, and a second case occurred after the drug made it to market. A third suspected, but unconfirmed, case has also been reported.

Adcetris could be contributing to the PML, but we won't know for sure until a substantial number of patients have been treated with the drug. Patients with lymphomas have weakened immune systems that can give the JC virus, which causes PML, the opportunity to infect the brain.

At this point, the rate in patients taking Adcetris appears higher than the published rate -- although keep in mind the rate can vary widely when there are only a few cases. But even if it becomes clear that Adcetris is causing some of the PML cases, it's unlikely to affect current sales much as long as the rate isn't substantially higher. The drug is approved to treat patients that have failed other treatments. With no other options and a clear efficacy benefit, most patients will take the gamble of an approximately 0.15% chance of getting PML for the potential to put their lymphoma into remission.

Seattle Genetics could run into more problems as it goes after patients earlier in their disease progression because it'll have additional competition, but even that might not be a major problem. Biogen Idec (NAS: BIIB) and Elan's (NYS: ELN) Tysabri, which also increases the rate of PML, has done just fine in the crowded multiple sclerosis market. Many patients have decided that the increased chance of getting PML is worth the risk for the increased efficacy over drugs such as Teva Pharmaceuticals' Copaxone and Rebif, sold by Merck KGaA and Pfizer.

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At the time this article was published Fool contributorBrian Orelliholds no position in any company mentioned. Check out hisholdings and a short bio. The Motley Fool owns shares of Teva Pharmaceutical Industries.Motley Fool newsletter serviceshave recommended buying shares of Teva Pharmaceutical Industries, Pfizer, and Elan. Try any of our Foolish newsletter servicesfree for 30 days. We Fools don't all hold the same opinions, but we all believe thatconsidering a diverse range of insightsmakes us better investors. The Motley Fool has adisclosure policy.

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