J&J stops enrollment in an AC Immune-partnered Alzheimer trial, but the reason is not yet clear

Enrollment in a mid-stage Alzheimer’s study led by Johnson & Johnson has been put on hold, prompting fresh questions about the progress of the program and the broader challenges of running prevention trials in neurodegenerative disease.

The phase 2b Retain trial, launched in 2024, is evaluating the anti-tau immunotherapy ACI-35.030 (also known as JNJ-2056) developed in partnership with Swiss biotech AC Immune. The four-year study aims to enroll 500 individuals with presymptomatic Alzheimer’s disease, randomizing them to receive the vaccine or placebo. In a recent filing with the U.S. Securities and Exchange Commission, AC Immune said enrollment has been “temporarily paused” while J&J’s Janssen division reviews elements of the trial, including recruitment. The company emphasized that dosing and follow-up in already enrolled participants are continuing and that the pause is voluntary, not driven by new safety concerns. It also noted that a predefined interim immunogenicity benchmark required to continue the study has been achieved.

Even so, the market reaction was swift. Shares of AC Immune fell 10% to $2.62 by Wednesday’s close, reflecting investor unease about potential delays in a program that targets tau pathology, one of the key hallmarks of Alzheimer’s disease.

AC Immune not alarmed by the development

According to Fierce Biotech, analysts at Jefferies described the enrollment halt as “unfortunate” but stopped short of viewing it as a fundamental setback. In a note dated Feb. 19, they suggested recruitment difficulties are the most plausible explanation and said the biotech believes J&J remains committed to advancing the asset. However, they cautioned that meeting the interim immunogenicity bar alone is unlikely to justify a move into phase 3, with cognitive outcomes and reductions in tau PET imaging expected to carry greater weight.

AC Immune sought to redirect attention to other upcoming milestones. Interim 12-month data are expected this year from a phase 1b/2 study of ACI-24.060, an anti-amyloid-beta vaccine. In addition, final results from the first part of a phase 2 trial of ACI-7104.056, an anti-alpha-synuclein immunotherapy in Parkinson’s disease, are also due in 2026.

A spokesperson of AC Immune told European Biotech News: “At the outset, we believe it is important to clarify that no new safety signals have emerged and that the pre-specified immunogenicity threshold has been met – indicating that ACI-35.030 is performing as expected. The trial itself is continuing, but new screening and recruitment activities have been paused while J&J reviews all available data. We do not know how long the suspension will last, nor whether our partner may consider adjustments after completing its assessment of all aspects of the study.”

The enrollment pause adds to a period of restructuring for the Lausanne-based company. Five months ago, AC Immune cut roughly 30% of its workforce and streamlined its pipeline, discontinuing select preclinical programs to prioritize later-stage assets. As of the end of September 2025, the company reported cash and equivalents of 108.5 million Swiss francs (around €119 million). According to the latest filing, the trial pause does not alter its financial runway, with existing funds expected to support operations into the third quarter of 2027.

For now, the Retain study remains active but its trajectory will depend on whether recruitment can be restarted smoothly and whether upcoming data across AC Immune’s portfolio can restore investor confidence.