While there were plenty of winners last week, two losers stand out. The first failure belongs to GlaxoSmithKline (NYSE:GSK). This giant is leading the industry with 20 new molecular entity approvals over the past six years. But its recent failure shows how the complexities of human biology can bring down even the best.

The largest miss last week wasn't necessarily a failure at all. The FDA gave Geron (NASDAQ:GERN) another lesson in regulator unpredictability.

Strike 2
Last fall, cancer vaccine MAGE-A3 from GlaxoSmithKline failed a late-stage trial by not significantly extending disease-free survival in melanoma patients. Last week, the vaccine failed similar endpoints for sufferers of non-small cell lung cancer.

Shares of GlaxoSmithKline finished the week down almost 3% following the news, but its partner for the vaccine really took it on the chin. Agenus (NASDAQ:AGEN) closed out the week down more than 16% following the announcement. The company's tree-bark derived QS-21 Stimulon adjuvant is a component of Glaxo's vaccine. It also comprises a one-third Agenus' core technology portfolio.

GlaxoSmithKline isn't giving up on the vaccine yet. Investigators will continue both trials in hopes of finding a genetic subset of the patient population that shows a stronger response.

Hold on
The ballad of imetelstat hit a sour note earlier this month when the FDA slapped a full clinical hold on trials sponsored by Geron. The company's shares sank more than 60% following the announcement. Last week the agency stepped in on an investigator-sponsored trial at the Mayo Clinic. In both trials the regulator cited signs of liver damage as the reason for intervention.

There are rays of hope for Geron and imetelstat. The regulator placed a probationary partial-hold on the investigator-sponsored trial with myelofibrosis patients. Those that clearly display clinical benefit will be allowed to continue with the treatment. More importantly, the FDA wants proof that liver enzyme elevations are reversible.

In the conference call following the first hold, Geron CEO, John Scarlett tried to assure analysts that wouldn't be a problem. The company noticed elevations in the past and adjusted dosing temporarily, which led to a reversal.

A lack of effective myelofibrosis therapies might soften the FDA's stance. At the moment, Incyte's (NASDAQ:INCY) Janus kinase, or JAK inhibitor, Jakafi is the only approved treatment, but several companies have similar drugs in late-stage development.

Last year, Gilead Sciences (NASDAQ:GILD) acquired momelotinib, along with YM Biosciences. Gilead has since advanced the JAK inhibitor into late stage trials. While these drugs alleviate symptoms, their ability to slow progression of the disease is unproven.

Imetelstat is a first-in-class telomerase inhibitor that actually caused a complete response in some myelofibrosis patients. Given that imetelstat is the only therapy proven to slow progression of a potentially fatal disease, I would be surprised if the FDA let minor, potentially reversible liver damage keep it off the market.

Unfortunately, the FDA can be full of surprises. If the FDA doesn't lift the hold, Geron is in serious trouble, as imetelstat is the company's sole product candidate.

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Cory Renauer has no position in any stocks mentioned. The Motley Fool recommends Gilead Sciences. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.

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