Semorinemab Phase II Trial Failure in Early Alzheimer's Disease: Implications for Tau-Targeted Therapeutics

Today, Roche and its partner AC Immune announced that their tau antibody semorinemab failed in the Phase II TAURIEL clinical trial. The trial enrolled 457 patients with early-stage (asymptomatic or mildly symptomatic) Alzheimer’s disease to compare the effects of 79 weeks of treatment with semorinemab versus placebo on cognitive function. The primary endpoint was the Clinical Dementia Rating–Sum of Boxes (CDR-SB) score, while secondary endpoints included scores on the Alzheimer’s Disease Assessment Scale–Cognitive Subscale (ADAS-Cog13) and the Alzheimer’s Disease Cooperative Study–Activities of Daily Living (ADCS-ADL). Semorinemab missed all these efficacy endpoints, although its safety profile was favorable. The drug is still under investigation in the ongoing Phase II LAURIET trial involving patients with moderate Alzheimer’s disease. AC Immune’s stock (ACIU) dropped 48% today.

Alzheimer’s disease (AD) is a multifactorial condition, with symptoms extending beyond dementia to include psychiatric manifestations and insomnia. Amyloid-beta is the most prominent pathological feature of AD and has been the primary focus of pharmaceutical investment. However, the amyloid hypothesis is losing momentum following the failure of more than a dozen late-stage clinical trials targeting various mechanisms and stages of the disease. Tau protein represents another readily observable pathological hallmark. Although certain dementias are associated with Tau mutations, such mutations have not been identified in patients with AD. In AD, Tau undergoes hyperphosphorylation, leading to its aggregation with other proteins and the formation of neurofibrillary tangles, a process that remains questionable. The progression of Tau tangle formation is insidious, typically originating in a specific brain region and subsequently spreading to others. Consequently, drugs aimed at blocking Tau propagation are currently under clinical investigation. There is also a notable interplay between Tau and amyloid-beta; Tau acts as an essential cofactor mediating amyloid-beta toxicity. Furthermore, both proteins are implicated in central nervous system iron metabolism.

Most tau-targeted drug development efforts remain in the preclinical stage. However, inhibiting tau aggregation differs substantially from traditional enzyme inhibition or receptor modulation. Since tau has no specific function directly linked to Alzheimer’s disease (AD), its aggregation is primarily problematic due to its propensity to form clusters. Inducing tau degradation is a promising strategy, but proteolysis-targeting chimeras (PROTACs) face significant challenges in crossing the blood–brain barrier to reach the central nervous system (CNS). Although other degradation-inducing technologies exist, they currently lack sufficient technical support and selectivity. Four years ago, TauRx’s methylthioninium derivative, TRx0237, failed in Phase III trials; however, whether this compound truly inhibits tau remains questionable. The current trial lacks a critical biomarker endpoint: whether semorinemab downregulates tau levels in the CNS. If tau levels are not reduced, efficacy would be unattainable even if the underlying hypothesis were correct, implying that the failure stems from flaws in the drug itself. Conversely, if tau levels are indeed lowered, the implications would be far more profound. It would suggest either that tau is not causally linked to AD symptoms, or that the timing of intervention was inappropriate. The current consensus holds that earlier intervention yields higher success rates. Given that this trial enrolled patients at the earliest disease stages, the likelihood of an incorrect study population is low.

If Tau is not a validated therapeutic target for Alzheimer’s disease (AD), the path forward becomes even more challenging. Tau and amyloid-beta (Aβ) are merely the most conspicuous players at the “crime scene,” yet their exact degree of involvement in the disease process remains uncertain. The true culprit could be an inconspicuous bystander, or may have long since departed from the scene. Central nervous system (CNS) inflammation is another hypothesis garnering significant attention. Many elderly individuals exhibit abnormal levels of Tau and Aβ without manifesting AD symptoms; although Tau and Aβ are akin to flammable and explosive hazardous materials, they cause limited damage in the absence of CNS inflammation to trigger the disease. Other hypotheses exist as well, such as the neuronal regeneration hypothesis discussed previously. However, identifying appropriate drugs and suitable patient populations to validate these hypotheses is highly difficult, and achieving success is even more formidable. This is a war against disease; much like the D-Day landings, encountering heavy fire and sniper attacks is to be expected, and one cannot anticipate being welcomed with food and drink by the local populace. Nevertheless, this is a battle that must be fought. The illustrations accompanying this article are heart-wrenching.

ACIU is a smaller-scale Biogen, with its entire product portfolio focused on highly complex and poorly understood neurodegenerative diseases of aging. ACIU began an in-depth collaboration with Roche in Alzheimer’s disease (AD) in 2006. In addition to its tau antibody, the company has a tau vaccine (ACI-35.030) and a small-molecule tau inhibitor (ACI-3024), as well as another tau-targeting agent currently in clinical trials. ACIU is also the inventor of the anti-amyloid-beta antibody crenezumab; however, Roche terminated late-stage development last year after interim analyses of the two Phase III trials, CREAD 1 and CREAD 2, showed futility. Today, ACIU’s CEO stated that the outcome was unexpected given the strong association between tau pathology and AD symptoms. Nevertheless, successful cases in AD drug development are rare. As one industry leader remarked today, we often do not know why clinical trials fail, nor why they succeed. This is an industry heavily dependent on chance, so there is little room for surprise.