The amyloid hypothesis remains **clinically relevant but no longer dominant**. The 2023 approvals of lecanemab (Leqembi, Eisai/Biogen) and donanemab (Kisunla, Eli Lilly) validate that clearing amyloid plaques can slow cognitive decline—but only modestly (27-35% slowing on CDR-SB). This shifts the question from *does amyloid matter?* to *is amyloid sufficient?*
**Druggability reality**: Anti-amyloid mAbs are now proven-class drugs, but face major limitations: (1) ARIA-E occurs in 12-17% (21-37% in APOE4 carriers), (2) requires biweekly IV infusion, (3) $26K-32K/year cost, (4) only works in early disease. The competitive landscape is consolidating—Roche abandoned gantenerumab, Biogen shelved aducanumab expansion, while Eli Lilly and Eisai are doubling down.
**The field is hedging**: Most pharma has **dual-mechanism** programs now. Examples: (1) Alector's AL001 (TREM2 agonist) + anti-tau, (2) Denali's DNL593 (RIPK1 inhibitor) targeting neuroinflammation, (3) AC Immune's semorinemab (anti-tau) after pivoting from anti-amyloid. The ~$2B investment in anti-tau trials (NCT03828747, NCT04619420) signals the field expects amyloid-alone is insufficient.
**Safety red flags**: Long-term ARIA monitoring is unresolved—the Clarity-AD extension (NCT05269394) and TRAILBLAZER-ALZ 2 extension are ongoing. We don't know 5-year microhemorrhage burden or whether serial amyloid removal causes chronic inflammation.
**Bottom line for drug development**: Amyloid is a **validated but narrow target**. Estimate $800M-1.2B and 8-10 years to bring a novel anti-amyloid to Phase 3—but you're entering a crowded, commoditizing space with modest effect sizes. The innovation opportunity is in *combination* therapies (amyloid + tau, amyloid + neuroinflammation) or *earlier intervention* (preclinical AD). For a neurodegen portfolio, I'd allocate 30% to amyloid-combinations, 40% to non-amyloid mechanisms, 30% to platform plays (BBB penetration, biomarkers).