Johnson & Johnson has paused further enrollment in a phase 2b trial of its AC Immune-partnered Alzheimer’s disease candidate, leaving analysts to speculate recruitment challenges may be the culprit.
The four-year-long Retain study kicked off in 2024 with an aim of recruiting 500 participants with presymptomatic Alzheimer’s who would receive either the anti-tau immunotherapy—dubbed ACI-35.030 or JNJ-2056—or placebo.
But in a Securities and Exchange Commission filing yesterday, AC Immune disclosed that “enrollment has been temporarily paused” while J&J’s Janssen unit “evaluates aspects of the trial (including recruitment).”
The Swiss biotech was keen to stress in the filing that the study itself is continuing and that the recruitment pause is “voluntary and not related to new safety findings.” A prespecified interim immunogenicity threshold required for the study to continue had also been met, the company pointed out.
AC Immune focused its attention on various clinical plans that remain unaffected, including interim 12-month results from a phase 1b/2 study of its other Alzheimer’s bet, an anti-amyloid-beta vaccine dubbed ACI-24.060. There are also final data coming down the track from the first part of a phase 2 study of the anti-alpha-synuclein immunotherapy ACI-7104.056 in Parkinson’s disease. Both of those readouts are due this year, according to the biotech’s filing.
Still, those caveats weren’t enough to appease investors, who sent AC Immune’s stock down 10% to $2.62 by close of trading Wednesday.
Jefferies analysts branded the pause in enrollment of the Retain study as “unfortunate” but argued that AC Immune’s “catalysts for key assets” remain “on track.”
The most likely explanation for the pause in enrollment was “recruitment challenges,” said the analysts, adding that AC Immune “understands J&J remains committed to the program.”
“While the pre-specified immunogenicity threshold being passed is a positive, this alone is unlikely to be sufficient to support transition to phase 3, with cognitive scores and Tau PET reductions the key factors,” the analysts explained in a Feb. 19 note.
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Last year, AC Immune published data from a phase 1b/2a study of ACI-35.030 that tied the liposome-based vaccine to an antibody response that was selective to p-Tau species, as well as evidence of altering Alzheimer’s-related plasma biomarkers.
The setback for the Alzheimer’s study comes five months after AC Immune began laying off around 30% of its workforce and scrapped some preclinical candidates as the Lausanne, Switzerland-based biotech focused resources on its more advanced therapies.
AC Immune ended September 2025 with 108.5 million Swiss francs ($140 million) in the bank. The company said in yesterday’s filing that the enrollment pause doesn’t impact its liquidity and that its current cash reserves are still expected to last into the third quarter of 2027.