Shares of Agios Pharmaceuticals (NASDAQ:AGIO) spiked over 20% this morning after the company announced that its early stage blood cancer drug, AG-221, is showing unexpectedly strong efficacy signals. Specifically, six of the seven evaluable patients exhibited objective responses, with three showing complete remissions. The drug's safety profile also appears favorable, with only a couple of adverse events occurring so far, although the trial is still in its early days. These data are being presented today at a Clinical Trials Symposium at the American Association for Cancer Research. Agios is developing AG-221 in collaboration with Celgene, which owns the worldwide rights to the drug. 

Thus far, the ongoing study has enrolled 22 patients with acute myeloid leukemia (AML) or myelodysplastic syndrome that harbor the IDH2 mutation. Cells with the IDH2 mutation produce increased levels of an oncometabolite believed to prevent normal bone marrow cells from maturing, whereby causing cancer. Agios' orally available drug functions by inhibiting the expression of this mutated protein, thus lowering the levels of the undesirable oncometabolite.

What's key to understand is that the overall survival rate for patients with these specific types of blood cancers is generally less than 10%. Put simply, the standard therapies are essentially ineffective and new treatment modalities are needed.

Is Agios' early stage drug worth keeping tabs on?
Despite these initially strong results, I believe it's important to keep our enthusiasm in check going forward. There is a good reason why the treatment options for these types of blood cancers are limited. For instance, Geron's (NASDAQ:GERN) telomerase-inhibitor imetelstat also showed strong early signals of efficacy in a handful of blood cancers, including AML. However, the U.S. Food and Drug Administration has since placed a clinical hold on the therapy over toxicity concerns.

Additionally, Sunesis Pharmaceuticals (NASDAQ:SNSS) recently reported that it is delaying a late-stage data readout for its AML candidate vosaroxin after patients were reported to be surviving longer than expected. Although this delay could be meaningless, there is a chance that Sunesis may have one of the first effective treatments for AML, which would likely affect AG-221's commercial aspirations. Overall, there are still a lot of moving parts to this story. So I recommend investors exercise caution with these early stage, albeit promising, trial results.      

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George Budwell has no position in any stocks mentioned. The Motley Fool recommends Celgene. 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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