Neuroscience · global
Tau-Targeting Drug Clears Biomarker Hurdle but Leaves a “Lower Dose Works Better” Puzzle
Diranersen reduced Tau pathology in the brain in a Phase 2 trial and showed signals of slowing deterioration in early Alzheimer’s disease; however, the trial did not meet its primary endpoint, efficacy was not proportional to dose, and key conclusions still await Phase 3 validation.
Alzheimer’s drug development has long focused on amyloid protein, but another core pathology may now offer an avenue for a breakthrough. In a Phase 2 trial, the experimental drug diranersen not only reduced markers of Tau protein in patients’ brains and cerebrospinal fluid, but also showed signals of slowing deterioration across multiple cognitive assessments. This is a rare instance to date of a Tau-targeting therapy affecting both biomarkers and clinical performance, but the results are not without cracks.
Diranersen, also known as BIIB080, is an antisense oligonucleotide. By interfering with the messenger RNA of the MAPT gene, it reduces the production of various forms of Tau protein at the source, rather than merely clearing proteins that have already formed outside cells. The drug must be injected through a lumbar puncture into the fluid surrounding the spinal cord, making its administration more invasive than that of typical oral drugs or intravenous injections.
The global Phase 2 CELIA trial enrolled 416 patients with mild cognitive impairment or mild dementia due to Alzheimer’s disease, none of whom had previously received anti-amyloid therapy. The randomized, double-blind, placebo-controlled study tested three regimens: 60 milligrams every six months, 115 milligrams every six months, and 115 milligrams every three months, with follow-up lasting 76 weeks.
Regarding biomarkers, mean total Tau in cerebrospinal fluid fell by 50% to 65% from baseline across the dose groups. A positron emission tomography substudy involving 131 participants also showed reductions in Tau pathology across all dose groups in the assessed brain regions. Clinical performance was most notable at the lowest dose: compared with placebo, the 60-milligram-every-six-months group had 26% less worsening on the CDR-SB scale, which integrates cognition and daily function. The reductions in worsening observed on the ADAS-Cog13 and MMSE cognitive scales were 42% and 50%, respectively.
However, CELIA was designed to determine whether higher doses produced greater improvement on CDR-SB, and this relationship did not emerge. The trial therefore did not meet its primary endpoint. Higher doses produced larger reductions in Tau, yet the clinical signal was strongest at the lowest dose. In addition, none of the groups separated from placebo on one measure of daily living function. This mismatch between biomarkers and efficacy means that the current data are closer to a lead worth investigating than to an established treatment effect.
Safety data showed that most adverse events were mild to moderate. Common issues included procedure-related pain, post-lumbar puncture syndrome, and transient confusion; the latter generally occurred within several days of dosing and resolved within one week. Researchers did not observe the brain-imaging abnormalities associated with some anti-amyloid drugs, but the tolerability of long-term, repeated intrathecal administration still requires continued evaluation.
Biogen plans to advance diranersen into a confirmatory Phase 3 trial. The first major challenge will be selecting a dose and reproducing the cognitive and functional benefits in a larger population. At this stage, the data come mainly from analyses presented at academic conferences and released by the company. The drug has not been approved by the U.S. Food and Drug Administration or any other regulator and cannot be used outside clinical trials. If subsequent studies confirm the findings, it could move Alzheimer’s treatment from solely clearing amyloid toward combination strategies that address multiple pathologies simultaneously.