What happened

Shares of Kura Oncology (NASDAQ:KURA), a clinical-stage biopharmaceutical company, are down sharply in response to a clinical hold for the company's experimental leukemia treatment. The stock was down 15.6% as of 11:20 a.m. ET on Wednesday.

So what 

The Food and Drug Administration placed a partial clinical hold on a phase 1 trial with KO-539 after the death of a patient. The adverse event was potentially associated with differentiation syndrome (DS).

Disappointed investor with face in hands.

Image source: Getty Images.

KO-539 is supposed to treat patients with acute myeloid leukemia by inhibiting a protein called menin. Unfortunately, this isn't the first time a menin inhibitor has been associated with DS, a life-threatening complication of therapy with differentiating agents like KO-539. 

Now what

This is bad news, but not necessarily catastrophic for the company. The FDA halted enrollment of new patients into Kura's dose determination study, but participants who were already enrolled will continue to receive KO-539.

The FDA takes patient safety seriously, even when those patients have an aggressive form of leukemia that relapsed after they received other treatment options. That said, some level of DS risk doesn't necessarily mean KO-539 is all washed up. 

In 2018, the FDA approved Tibsovo, an oral treatment for acute myeloid leukemia that acts differently but is associated with DS. Despite roughly one-fifth of patients exhibiting signs of DS in a pivotal trial, the FDA approved Tibsovo in 2018 with a black box warning for oncologists to monitor for symptoms of the potentially fatal complication. 

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