Why Isis Pharmaceuticals' Shares Fell Off a Cliff

Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of rare-disease biotechnology company Isis Pharmaceuticals (Nasdaq: ISIS  ) shed as much as 20% in intraday trading Tuesday following the release of briefing documents from an Food and Drug Administration panel. Concerns were raised about its lead drug, Kynamro (aka mipomersen), which is being developed in collaboration with Sanofi (NYSE: SNY  ) to treat homozygous familial hypercholesterolemia, or HoFH.

So what: Out of the two primary concerns brought to light, one wasn't much of a surprise. According to the FDA panel's briefing notes, it shares the same concerns as Aegerion Pharmaceuticals' (Nasdaq: AEGR  ) lomitapide in that both drugs negatively impact liver function and create a fat buildup. Just like Monday's wording, the panel recommended Kynamro be used at targeted patient populations.

The really damaging, and surprising, news comes from the panel's concern over abnormal growths, known as neoplasms, in 3.1% of all tested patients. Of the 23 neoplasms tested, nine turned out to be malignant compared to just two total neoplasms in the placebo group (although keep in mind there were 749 patients taking the drug and just 221 taking the placebo). The panel suggested that the imbalance between neoplasm occurrence from taking Kynamro and the placebo should be investigated further, but left the door open for post-approval studies.

Now what: As the Fool's Brian Orelli pointed out last week, patients suffering with HoFH have very few treatment options at this point, so today's disappointing briefing news isn't a complete wash-out for Kynamro, but it does likely give Aegerion's lomitapide a safety edge. Isis will have its say before the panel on Thursday and will ultimately receive a verdict from the FDA by the end of January. At this point, I'm concerned about the side effects of both HoFH drugs in spite of the unmet needs and would just assume back off both at the moment.

Craving more input? Start by adding Isis Pharmaceuticals to your free and personalized Watchlist so you can keep up on the latest news with the company.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

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