Ultragenyx, Mereo Sink After Hours As Bone Drug Trial Continues Without Early Stop; Retail Traders Feel Selloff Is Overdone

Shares of Ultragenyx and Mereo BioPharma slipped in after-hours trading Wednesday after the companies said they will continue their Phase 3 trial of UX143 (setrusumab) as initially planned, rather than ending it early as they had hoped.

In after-hours trading on Wednesday, Mereo BioPharma dropped 38.8% to $1.80, while Ultragenyx fell 25.8% to $30.76.

UX143 is a monoclonal antibody that inhibits sclerostin, a negative regulator of bone formation. It is being jointly developed by Ultragenyx and Mereo BioPharma.

The Data Monitoring Committee (DMC) reviewed the Orbit study and found that the treatment’s safety profile appeared acceptable, recommending continuation to the final analysis.

Final results from both the Orbit and Cosmic studies are expected to be released around the end of the year.

The Orbit trial is testing UX143 in patients aged 5 to 25 with osteogenesis imperfecta, a condition characterized by brittle bones. 

Participants are receiving either the experimental treatment or a placebo, with the primary focus on the frequency of fractures. 

After the primary data is collected, all patients will move to open-label treatment.

The Cosmic trial is running in parallel for children aged 2 to 7, comparing UX143 to standard bisphosphonate therapy. 

The study is also ongoing, with final results expected after patients have completed at least 18 months of treatment.

On Stocktwits, retail sentiment for both Mereo BioPharma and Ultragenyx was tagged as ‘extremely bullish’ with ‘extremely high’ message volume.

Some users suggested that the market was overreacting to the update from Mereo BioPharma and Ultragenyx, with one describing Mereo as a “good medium to long-term candidate” and another attributing the drop to a retail misunderstanding of standard clinical trial practices.

One comment noted that the final analysis will use a conventional 0.05 p-value threshold and expressed optimism about the chances of success, while an Ultragenyx watcher speculated the market was prematurely pricing in study failure.

While Ultragenyx’s stock has risen 0.7% so far in 2025, Mereo’s stock has declined 14% over the same period. 

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