What happened?

Shares of clinical-stage biotech Adverum Biotechnologies (ADVM -0.51%) are plunging on Thursday after the company announced a clinical update regarding one of its pipeline candidates, ADVM-022. Adverum Biotechnologies' stock was down by 60.2% as of 11:30 a.m. EDT. 

So what 

ADVM-022 is an experimental gene therapy for the treatment of diabetic macular edema (DME). After the close of yesterday's trading session, Adverum Biotechnologies announced a suspected adverse reaction in a phase 2 clinical trial evaluating ADVM-022 in DME patients. Specifically, one patient treated with ADVM-022 during the trial developed hypotony, an eye condition that can lead to decreased vision.

At this point it isn't clear whether treatment with ADVM-022 was responsible for the condition. However, out of an abundance of caution, Adverum Biotechnologies has decided to unmask the study and dig into the data to determine whether other patients who participated in the trial may also be at risk of developing hypotony.

Downward-bound graph on a blackboard.

Image source: Getty Images.

CEO Laurent Fischer said: "The safety of every patient who is participating in our clinical studies with our gene therapy is the utmost priority for us at Adverum. We are fully committed to thoroughly assessing this case and ongoing monitoring of this patient and all patients treated with ADVM-022 with our investigators, data monitoring committee (DMC), scientific advisory board, and healthcare authorities."

Now what

Again, it is worth stressing that treatment with ADVM-022 may not be what caused this adverse reaction. But the market does not like uncertainty, and this news certainly adds a whole lot of uncertainty for Adverum Biotechnologies and its shareholders. That explains why investors are offloading shares of this biotech stock today.