Diranersen, a short RNA based drug from Ionis and Biogen, cut tau in the brain and slowed decline at rates comparable to the approved antibodies that clear amyloid plaques in a mid stage trial.
Diranersen slowed cognitive decline in early Alzheimer's at roughly the same rate as the already-approved anti-amyloid drugs. That match pushes the bottleneck in treating early Alzheimer's downstream of both amyloid and tau.
The Phase 2 CELIA readout, presented Tuesday at the Alzheimer's Association International Conference (AAIC) in London, is the cleanest mid-stage test of the post-amyloid hypothesis: if amyloid is upstream, going after the tau tangles that follow should give patients a larger clinical lift. On the clinical read, diranersen landed at the same level STAT+ describes for lecanemab and donanemab.
Diranersen (BIIB080) is an antisense oligonucleotide, a short strand of modified RNA designed to bind and degrade the messenger RNA that cells use to make tau protein, lowering tau production rather than trying to clear tangles that are already there. It was originated by Ionis and is being developed and commercialized by Biogen. Tau is one of the two hallmark proteins of Alzheimer's pathology; the other is amyloid. Drugs the public has actually heard of, lecanemab (Leqembi) and donanemab (Kisunla), clear amyloid plaques. Diranersen goes after tau tangles, which form inside neurons and track more closely with memory loss.
In the topline from Ionis and Biogen, multiple dose arms showed improvements versus placebo across cognitive and clinical tests, and cerebrospinal fluid tau levels dropped in treated patients. On the clinical side, the magnitude landed at a different level than the biomarker suggested. STAT+ characterized the result as comparable to approved anti-amyloid therapies, not a step past them.
"Comparable" is the most that can be said from a Phase 2 readout without a head-to-head trial, and STAT+'s analysis does not establish equivalence. Specific magnitude figures, CDR-SB deltas, and p-values are not in the publicly hydrated source. The Biogen release and the Ionis release commit to advancing the drug into a Phase 3 pivotal study. Diranersen had already received FDA Fast Track designation earlier in development.
The mechanism-diversity read is the part the wire strips out. A drug with a confirmed biomarker hit on its target, in an early-symptomatic patient population, producing a clinical signal in the same range STAT+ assigns to the anti-amyloid antibodies, points to a rate-limiting step in slowing early Alzheimer's decline that the field hasn't pinned down. Likely a combination of timing, target patient selection, and biology that sits below the level of either protein. The Phase 2 dose-response shape, which the publicly hydrated source does not break out arm by arm, will be one of the few pieces of public evidence available before the Phase 3 protocol is filed: if the highest-dose arm did not show a larger effect than the middle-dose arm, the read is that even a stronger tau push may not break the ceiling.
The next data point is the Phase 3 protocol Biogen files. The dose it picks, the primary cognitive endpoint, and the comparator arm will reveal what the company thinks the comparison against lecanemab and donanemab has to look like, and whether the larger pivotal trial is designed to clear a higher bar than the Phase 2 readout did.