Ultragenyx Pharmaceutical Inc. Class Action Lawsuit – RARE

Introduction to Ultragenyx Pharmaceutical Inc. (RARE) Securities Class Action Lawsuit

A federal securities fraud class action, under the Securities Exchange Act of 1934, including Section 10(b) and Rule 10b-5 has been filed against Ultragenyx Pharmaceutical Inc. (NASDAQ: RARE) in the Northern District of California. The proposed class covers investors who acquired Ultragenyx common stock, traded on the NASDAQ under ticker RARE, between August 3, 2023 and December 26, 2025, inclusive. 

According to the complaint, investors allege the company and senior executives repeatedly touted setrusumab (UX143) for Osteogenesis Imperfecta, claiming strong bone mineral density gains, a secondary endpoint, would translate into fewer fractures and that the Phase III Orbit and Cosmic randomized, double-blind, placebo-controlled study designs would reliably show that effect. The truth emerged when the company later disclosed that the Phase III studies failed to achieve statistical significance on their primary fracture-reduction endpoints (annualized clinical fracture rate reduction), and management acknowledged uncertainty about the data. Investors experienced sharp stock declines following the corrective disclosures.

“Most RARE shareholders never file or join the class action, which means they miss out on potential recovery funds,” said Attorney Joseph Levi.

Ultragenyx Pharmaceutical Inc. (RARE) Securities Lawsuit Case Details

Case Name: Bailey v. Ultragenyx Pharmaceutical Inc. et al.
Case No.: 3:26-cv-01097
Jurisdiction: U.S. District Court, Northern District of California
Filed on: February 4, 2026

Ultragenyx Pharmaceutical Inc. (RARE) Company Profile

Ultragenyx is a biopharmaceutical company, a Delaware corporation headquartered in California focused on rare and ultrarare genetic disorders (orphan diseases), with product candidates typically in-licensed from partners or academic institutions, and it develops gene therapies using a proprietary HeLa cell manufacturing platform. One key program during the period was setrusumab (UX143), a drug that targets bone metabolism for Osteogenesis Imperfecta.

Ultragenyx Pharmaceutical Inc. (RARE) Securities Lawsuit Class Period

August 3, 2023-December 26, 2025, inclusive.

All investors who purchased or otherwise acquired Ultragenyx common stock, ticker RARE, during the Class Period may be eligible to join the Ultragenyx Pharmaceutical Inc. (RARE) class action lawsuit.

Allegations in the Ultragenyx Pharmaceutical Inc. (RARE) Securities Class Action Lawsuit

The complaint names Ultragenyx, Founder/President/CEO/Director Emil D. Kakkis, and Chief Medical Officer/EVP Eric Crombez. Investors allege these defendants promoted setrusumab’s prospects in the Phase III Orbit and Cosmic studies by linking early bone mineral density gains to expected reductions in fracture rates and by expressing confidence in study design and control of variability, in violation of the Securities Exchange Act of 1934, including Section 10(b) and Rule 10b-5. 

The narrative begins on August 3, 2023, when CEO Emil Kakkis told investors on an earnings call that the bone mineral density increases, a secondary endpoint seen at three months were “sufficient enough to improve the strength of bones” and would “translate into fracture improvements.” He followed on May 2, 2024, stating on another earnings call, “I really don’t see any uncontrolled factors,” signaling strong control over study variables. On August 1, 2024, Kakkis said the effect was “very large” and that he felt “pretty confident” stronger bones would compensate for activity changes in patients. 

That tone continued into November 5, 2024, when Kakkis discussed powering assumptions, citing an assumed 50% reduction in the annualized clinical fracture rate and suggesting results closer to 67%, concluding they felt “pretty comfortable” with the study design. On August 5, 2025, he again assured investors that, based on Phase II data, even though the Phase II predecessor studies lacked a placebo control, UX143 would be a “transformational treatment” for pediatric and adult patients with Osteogenesis Imperfecta. 

Meanwhile, the complaint alleges that these positive statements concealed material adverse facts: while setrusumab increased bone mineral density, that increase did not correlate to a reduction in annualized fracture rates with statistical significance, and the Phase III Orbit and Cosmic studies, including the placebo-controlled Orbit study and the bisphosphonate-comparator Cosmic study were far less likely to demonstrate such a link than management claimed. Investors allege defendants created the false impression they had reliable information pointing to Phase III success and minimized risks from study variability and from the clinical endpoint’s sensitivity to low placebo-group fracture rates, artificially inflating the stock price during the class period.

The Truth Emerges

The story turned on July 9, 2025, when Ultragenyx issued a press release stating the randomized, placebo-controlled, double-blind Phase 3 portion of the Orbit study (with a primary endpoint of annualized clinical fracture rate reduction) was progressing toward a final analysis around year-end. According to the complaint, this revealed the study had not achieved a second interim analysis that prior confidence had led investors to expect. 

The full picture arrived on December 29, 2025, when Ultragenyx filed a Form 8-K with the Securities and Exchange Commission announcing that both Phase III Orbit and pediatric Cosmic studies failed to achieve statistical significance on their primary endpoints of reducing annualized clinical fracture rates versus placebo or bisphosphonates. Management attributed the outcome in part to a low fracture rate in the Orbit placebo group and conceded that bone mineral density changes, a secondary endpoint, were not accompanied by corresponding fracture reductions. On January 12, 2026, CEO Emil Kakkis added that the company needed to understand “why it is the way it is,” noting long bone results did not appear better to a statistically significant degree and questioning whether increased activity or other factors explained the fractures.

Market Reaction

The market reacted swiftly as disclosures unfolded. From a closing price of $41.44 per share on July 9, 2025, Ultragenyx fell to $31.03 on July 10, 2025, a one-day decline of $10.41 or about 25.12% after investors learned the Phase 3 Orbit study had not achieved the expected second interim analysis and was moving to a year-end final analysis, as the market digested a corrective disclosure and artificial inflation dissipated. The selloff deepened when the company disclosed the Phase III failures. 

From a closing price of $34.19 per share on December 26, 2025, Ultragenyx fell to $19.72 on December 29, 2025, a one-day drop of $14.47 or approximately 42.32% as the market absorbed that neither Orbit nor Cosmic met the primary fracture-reduction endpoints, and investors suffered substantial losses.

Next Steps

• The Court will issue its order for lead plaintiff and counsel in the weeks after submissions are due.

• The Court will then consider motion for class certification.

• The Court will later consider a motion to dismiss.

Disclaimer: This shareholder alert is for informational purposes only and does not constitutelegal advice. Consult a qualified attorney for personalized guidance. No specific outcomes are guaranteed.

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