Anti-Amyloid Immunotherapy for Alzheimer Disease
The amyloid cascade hypothesis — which posits that abnormal accumulation of amyloid-beta (Aβ) peptide in the brain initiates a pathologic sequence leading to neurodegeneration and dementia — has driven decades of therapeutic development in Alzheimer disease (AD). Following numerous failed attempts to reduce amyloid through production inhibition (BACE inhibitors) and earlier antibody programs, three monoclonal antibodies targeting aggregated forms of Aβ have achieved FDA approval: aducanumab (2021, subsequently withdrawn), lecanemab (2023), and donanemab (2024). These agents represent the first FDA-approved therapies with demonstrated effects on AD pathology, producing measurable amyloid plaque clearance on PET imaging and modest but statistically significant slowing of clinical decline. Their clinical use requires biomarker-confirmed AD pathology, careful patient selection, APOE genotyping, and serial MRI monitoring for amyloid-related imaging abnormalities (ARIA).
Bottom Line
- Lecanemab (Leqembi): Full FDA approval July 2023; 10 mg/kg IV biweekly for 18 months, then monthly IV or weekly subcutaneous maintenance; 27% slowing of CDR-SB decline in CLARITY AD (N=1,795); 68% of patients became amyloid-negative at 18 months
- Donanemab (Kisunla): FDA approved July 2024; IV monthly with treat-to-clear approach; 35–36% slowing in low/medium tau population (TRAILBLAZER-ALZ 2); 76% achieved amyloid clearance; treatment can be discontinued once clearance is demonstrated
- ARIA is the major safety concern: ARIA-E (edema/effusion) occurs in 12–24% and ARIA-H (hemorrhage/siderosis) in 17–31% of treated patients; risk is highest in APOE4 homozygotes (ARIA-E up to 34–42%); most ARIA is asymptomatic and detected on surveillance MRI
- Patient eligibility: MCI due to AD or mild AD dementia with biomarker-confirmed amyloid pathology (PET, CSF, or plasma p-tau217); APOE genotyping and baseline MRI required before treatment initiation
- Aducanumab: Granted controversial accelerated approval in 2021 based on surrogate biomarker endpoint; limited uptake due to insurance coverage refusal; discontinued by manufacturer after 2024
- Remternetug: Next-generation subcutaneous anti-amyloid antibody in Phase 3 trials (TRAILRUNNER-ALZ 3); higher affinity than donanemab; results expected 2029
- Failed approaches: All BACE inhibitors failed in Phase 3 due to cognitive worsening from off-target substrate effects; solanezumab and gantenerumab also failed to show clinical benefit
Amyloid Cascade Hypothesis and Therapeutic Rationale
The molecular biology advances of the 1980s and 1990s characterized the Aβ peptide that accumulates in neuritic plaques, and transgenic animal models enabled replication of plaque and tangle pathology. The landmark discovery by Schenk and colleagues in 1999 that immunizing transgenic mice against Aβ resulted in clearance of amyloid pathology opened the path for immunotherapeutic approaches. A second critical advance was the development of PET imaging with carbon-11 Pittsburgh Compound B (PiB) and subsequently fluorine-18 tracers (florbetapir, florbetaben, flutemetamol) that allowed accurate in vivo identification and quantification of amyloid plaques using the standardized Centiloid scale. Typical amyloid positivity threshold is approximately 25 Centiloids.
Anti-amyloid antibodies bind to aggregated or fibrillar forms of Aβ and promote plaque clearance through Fc-receptor-mediated microglial phagocytosis. The approved agents differ in their specific epitope targets — lecanemab preferentially binds soluble protofibrils, while donanemab targets pyroglutamated Aβ (N3pG) found in mature plaques — resulting in distinct dosing strategies, clearance kinetics, and adverse event profiles.
Key Concepts in Anti-Amyloid Therapy
- Amyloid clearance vs production inhibition: Antibodies clear existing plaques; BACE inhibitors block amyloid production — fundamentally different mechanisms with dramatically different clinical outcomes
- Biomarker-guided treatment: Amyloid PET quantification enables treatment monitoring, clearance determination, and discontinuation decisions (donanemab treat-to-clear model)
- Tau burden matters: TRAILBLAZER-ALZ 2 demonstrated that baseline tau pathology modulates treatment response; patients with lower tau burden show greater clinical benefit
- Magnitude of effect: 25–36% slowing of decline represents approximately 4.5–7 months of delayed progression over 18 months; clinical meaningfulness at the individual level remains debated
- CMS coverage: Both lecanemab and donanemab are covered under "coverage with evidence determination" requiring enrollment in a CMS-approved outcomes registry
Aducanumab (Aduhelm)
Aducanumab was granted accelerated FDA approval in June 2021 based on amyloid plaque reduction as a surrogate endpoint. The two Phase 3 trials (EMERGE and ENGAGE) followed identical designs but were stopped early after a futility analysis. Subsequent analysis revealed discordant results: EMERGE showed efficacy on clinical outcomes, while ENGAGE did not. The FDA's decision to approve based on the biomarker data was highly controversial. Among participants who underwent sequential PET, 31% in ENGAGE and 48% in EMERGE achieved amyloid-negative status over 78 weeks with high-dose treatment (10 mg/kg monthly).
Appropriate use recommendations were published, but clinical uptake was minimal as Medicare and most insurers declined coverage. Aducanumab was discontinued by Biogen and is no longer available after 2024.
Lecanemab (Leqembi)
Regulatory Timeline
Lecanemab received accelerated approval in January 2023 and full traditional FDA approval in July 2023 based on the CLARITY AD Phase 3 trial. In 2025, the FDA approved both monthly IV maintenance dosing and a subcutaneous formulation (LEQEMBI IQLIK) for maintenance therapy.
CLARITY AD Trial (Phase 3)
| Parameter | Detail |
|---|---|
| Population | N=1,795; early AD (MCI due to AD or mild AD dementia); amyloid-confirmed |
| Treatment | 10 mg/kg IV biweekly for 18 months |
| CDR-SB (primary endpoint) | 27% slowing of decline (−0.45 points; p<0.001) |
| ADAS-Cog14 | Difference of −1.44 (p<0.001) |
| ADCOMS | 24% slowing of decline |
| ADCS-MCI-ADL | 37% slowing of functional decline |
| Amyloid PET reduction | −59.1 centiloids vs placebo; 68% became amyloid-negative |
| Infusion reactions | 26% (most after first dose); fever, flushing, chills, BP changes |
Dosing Regimen
Induction: 10 mg/kg IV every 2 weeks for 18 months. Maintenance (after 18 months): either 10 mg/kg IV every 4 weeks, or subcutaneous 360 mg weekly via autoinjector (LEQEMBI IQLIK, administered in approximately 15 seconds). Infusion reactions are managed with pretreatment using NSAIDs, antihistamines, or glucocorticoids.
Open-Label Extension Data
Over 3 years (core study plus extension), lecanemab reduced CDR-SB decline by −0.95 compared with historical controls. At AAIC 2025, 4-year data demonstrated that cognitive benefits continued to widen over time. Real-world evidence interim data confirmed clinical efficacy consistent with Phase 3 results.
Donanemab (Kisunla)
TRAILBLAZER-ALZ 2 Trial (Phase 3)
Donanemab received FDA approval in July 2024 for early symptomatic AD with confirmed amyloid pathology. The trial uniquely stratified patients by baseline tau burden (tau PET) and employed a treat-to-clear design in which patients achieving amyloid clearance were switched to placebo in a blinded fashion.
| Endpoint | Low/Medium Tau (n=1,182) | Combined Population (n=1,736) |
|---|---|---|
| iADRS | 35% slowing of decline | — |
| CDR-SB | 36% slowing at 76 weeks | 29% slowing |
| No CDR-SB decline at 1 year | 47% vs 29% placebo (p<0.001) | — |
| Amyloid clearance | 84% mean plaque reduction; >75% achieved clearance within 76 weeks | |
| Post-clearance re-accumulation | ~2.8 Centiloids/year | |
Treat-to-Clear Approach (Unique to Donanemab)
Amyloid Clearance and Treatment Discontinuation
- Treatment can be finite: Unlike lecanemab, donanemab may be discontinued once amyloid clearance is demonstrated on PET
- Monitoring PET schedule: Amyloid PET obtained at 24 weeks and 52 weeks during treatment
- Clearance thresholds: Single PET scan <11 Centiloids, OR two consecutive scans showing 11–25 Centiloids
- Slow re-accumulation: After clearance, amyloid re-accumulates at ~2.8 Centiloids/year; patients remain below the positivity threshold (~25 CL) for years
- Dosing: 700 mg IV for first 3 doses, then 1,400 mg every 4 weeks; 2025 updated label includes a new titration schedule to reduce ARIA-E risk
- Tau burden modulates response: Low/medium tau patients showed 35–36% slowing; high tau subgroup showed less benefit (CDR-SB difference −0.69, p=0.006)
Head-to-Head Comparison: Lecanemab vs Donanemab
No direct comparison trials have been conducted. The following table summarizes key differences from their respective Phase 3 trials. Treatment selection involves weighing higher infusion reaction rates and biweekly dosing with lecanemab against higher ARIA rates but monthly dosing with donanemab.
| Feature | Lecanemab (Leqembi) | Donanemab (Kisunla) |
|---|---|---|
| FDA approval | July 2023 (full) | July 2024 |
| Target epitope | Soluble Aβ protofibrils | Pyroglutamated Aβ (N3pG) in mature plaques |
| CDR-SB slowing | 27% (−0.45 points) | 29–36% (−0.67 points, low/med tau) |
| Mean amyloid clearance | 66% (52 CL reduction) | 83% (86 CL reduction) |
| Amyloid-negative at 18 months | 68% | 76% |
| IV dosing frequency | Biweekly (induction); monthly or weekly SC (maintenance) | Monthly; can discontinue after clearance |
| Treatment duration | Indefinite (maintenance phase) | Potentially finite (treat-to-clear) |
| Net ARIA-E (treated − placebo) | 11% | 22% |
| Net ARIA-H (treated − placebo) | 8% | 12% |
| Infusion reactions | 26% | Lower |
| Subcutaneous option | Yes (LEQEMBI IQLIK, approved August 2025) | No (remternetug in development) |
ARIA: Amyloid-Related Imaging Abnormalities
ARIA is the most significant adverse effect of anti-amyloid immunotherapy and encompasses two distinct radiographic findings: ARIA-E (edema or sulcal effusion, seen as FLAIR hyperintensity on MRI) and ARIA-H (microhemorrhages and superficial siderosis, seen on T2*/SWI sequences). ARIA was first reported as a complication of amyloid-lowering therapies in 2010 and typically arises early in treatment or after dose escalation. The majority of ARIA events are asymptomatic and detected only on routine surveillance MRI.
ARIA-E (Edema/Effusion) — Key Facts
- Incidence: 12.6% with lecanemab; 24.4% with donanemab
- Symptomatic cases: 2.8% (lecanemab); 5.8% (donanemab)
- Timing: 71% occur within the first 3 months; 92% within 6 months of treatment initiation
- Resolution: 81% resolve within 4 months of detection
- Symptoms when present: Headache, confusion, dizziness, visual disturbances, nausea
- Management: Pause treatment for moderate-to-severe ARIA-E; resume only after radiographic resolution; most cases are reversible
ARIA-H (Hemorrhage/Siderosis) — Key Facts
- Incidence: 17.3% with lecanemab; 31.3% with donanemab
- Forms: Microhemorrhages (punctate T2* signal loss) and superficial siderosis (linear cortical hemosiderin deposition)
- Grading: Mild (few new microbleeds, may continue treatment with monitoring); moderate (pause treatment, serial MRI); severe (permanent treatment discontinuation)
- Unlike ARIA-E, ARIA-H is not reversible — hemosiderin deposits persist on imaging
- Fatal cases: Three deaths attributed to ARIA-related events in the TRAILBLAZER-ALZ 2 trial
- Anticoagulant risk: Concurrent anticoagulant use is NOT recommended until more data are available; antiplatelet agents (aspirin ≤325 mg/day, clopidogrel) may be considered
ARIA Risk by APOE Genotype
| APOE Genotype | Lecanemab ARIA-E | Donanemab ARIA-E | Lecanemab ARIA-H | Donanemab ARIA-H |
|---|---|---|---|---|
| ε3/ε3 (noncarrier) | 5% | 15% | 12% | 19% |
| ε3/ε4 (heterozygote) | 11% | 21% | 14% | 32% |
| ε4/ε4 (homozygote) | 33% | 36% | 40% | 50% |
APOE Genotyping and Risk Stratification
- APOE genotyping is strongly recommended before initiating anti-amyloid therapy to inform individual ARIA risk
- APOE4 homozygotes: Highest ARIA risk across all agents; ARIA-E rates 2–7× higher than noncarriers; careful risk-benefit discussion is essential; treatment is not contraindicated but requires heightened vigilance
- APOE4 heterozygotes: Moderately increased risk; standard monitoring protocol applies
- APOE4 noncarriers: Lowest ARIA risk; most favorable risk-benefit profile
- Genetic counseling considerations: Results reveal inherited risk for both ARIA and AD itself; patients should be informed of the dual implications before testing
MRI Monitoring Protocol
| Timepoint | Lecanemab | Donanemab |
|---|---|---|
| Baseline | Required before treatment | Required before treatment |
| ~Week 8 | Prior to 5th infusion | Per prescribing information |
| ~Week 12 | Prior to 7th infusion | Per prescribing information |
| ~Week 26 | Prior to 14th infusion | Per prescribing information |
| Week 52 | Especially APOE4 carriers or prior ARIA | Per prescribing information |
| If ARIA detected | Follow-up MRI every 2–4 months until resolution (ARIA-E) or stability (ARIA-H) | |
| Key sequences | FLAIR (for ARIA-E) and T2*/SWI (for ARIA-H); same scanner and parameters across serial exams | |
Patient Eligibility and Selection
Who Qualifies for Anti-Amyloid Therapy?
- Clinical stage: MCI due to AD or mild AD dementia (MMSE generally ≥22); patients with moderate-to-severe dementia are beyond the approved indication
- Biomarker confirmation required: Amyloid PET (positive), OR CSF biomarkers (low Aβ42/40 ratio), OR FDA-cleared plasma p-tau217 blood test (Lumipulse, cleared May 2025)
- APOE genotyping: Strongly recommended (not required for lecanemab) to guide risk-benefit discussion
- Baseline MRI: Must exclude significant cerebrovascular disease; generally exclude if >4 microhemorrhages or any superficial siderosis
- Support person: Adequate caregiver/support for monitoring required
- Ability to undergo serial MRI monitoring throughout treatment course
Relative Contraindications and Cautions
- APOE4 homozygosity: Not an absolute contraindication, but ARIA risk is substantially elevated; detailed risk-benefit discussion is mandatory
- Concurrent anticoagulation: NOT recommended during anti-amyloid therapy; antiplatelet agents at standard doses may be considered
- >4 cerebral microhemorrhages on baseline MRI
- Superficial siderosis on baseline MRI
- Moderate-to-severe dementia: Beyond approved indication; no evidence of benefit at later stages
- Inability to undergo serial MRI (e.g., incompatible implants, severe claustrophobia)
- Significant cerebral amyloid angiopathy on baseline imaging
Remternetug: Next-Generation Subcutaneous Anti-Amyloid Therapy
Remternetug is an IgG1 monoclonal antibody developed by Eli Lilly that targets the same pyroglutamated Aβ epitope as donanemab but with higher binding affinity and is designed for subcutaneous self-administration at home. In the Phase 1 TRAILRUNNER-ALZ 1 trial (N=139), 75% of participants receiving 700–2,800 mg reached amyloid levels below 24.1 Centiloids by day 169.
The Phase 3 TRAILRUNNER-ALZ 3 trial (started October 2024, N=1,400 planned) is evaluating 18 months of subcutaneous home treatment with a primary endpoint of time to progression on CDR. Results are expected by 2029. Remternetug is also being tested in the DIAN-TU Primary Prevention Trial in dominantly inherited AD, enrolling 240 patients as young as age 18.
Failed Anti-Amyloid Approaches
BACE Inhibitors
Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibitors represented a mechanistically rational approach to reducing Aβ production. Despite achieving up to 90% reduction in Aβ levels, all Phase 3 programs failed due to safety and efficacy concerns, and the entire drug class has been effectively abandoned.
| Agent | Sponsor | Outcome | Key Findings |
|---|---|---|---|
| Verubecestat | Merck | Failed (EPOCH) | No benefit in mild-to-moderate AD; increased adverse effects |
| Atabecestat | Janssen/Shionogi | Terminated | Hepatotoxicity |
| Lanabecestat | AstraZeneca/Lilly | Discontinued | Futility |
| Umibecestat | Novartis | Stopped (API Generation) | Cognitive worsening (reversible after washout) |
| Elenbecestat | Eisai/Biogen | Stopped | Unfavorable risk-benefit ratio |
Lessons from BACE Inhibitor Failures
- Off-target effects: BACE1 cleaves substrates beyond APP that are critical for synaptic function (neuregulin, seizure-related gene 6, CHL1), causing mechanism-based cognitive worsening
- Dose-toxicity relationship: Approximately 90% BACE1 inhibition is needed to reduce Aβ by 50% in animal models, but high doses caused the most adverse effects
- Production inhibition ≠ plaque clearance: Reducing amyloid production is not equivalent to clearing existing deposits; once plaques are formed, antibody-mediated clearance is the effective approach
- Timing hypothesis: Inhibiting production may be too late in symptomatic disease when extensive plaque burden has already accumulated
- Reversibility: Cognitive decline from BACE inhibitors was not dose-dependent, did not progress, and resolved after drug washout — suggesting a pharmacologic rather than neurotoxic effect
Other Failed Anti-Amyloid Antibodies
Solanezumab (Eli Lilly) targeted soluble monomeric Aβ rather than aggregated forms and failed to demonstrate clinical benefit in the EXPEDITION 3 trial and the A4 secondary prevention study. Gantenerumab (Roche) targeted aggregated Aβ but failed to meet primary endpoints in the GRADUATE I and II Phase 3 trials, likely due to insufficient amyloid clearance at the doses tested. These failures reinforced the importance of targeting the correct Aβ species and achieving adequate plaque clearance to achieve clinical benefit.
Practical Implementation Challenges
Barriers to Anti-Amyloid Therapy Access
- Infusion center requirements: Lecanemab biweekly IV infusions and donanemab monthly infusions require access to qualified infusion facilities; subcutaneous lecanemab (maintenance only) may reduce this burden
- MRI monitoring burden: Multiple surveillance MRIs during the first year (4–6 scans minimum) increase cost and scheduling complexity; same scanner and sequences required for valid comparison
- Cost: Annual drug cost approximately $26,500 (lecanemab) and $32,000 (donanemab) before infusion, monitoring, and biomarker testing costs
- CMS registry requirements: Medicare coverage requires prescriber enrollment in a CMS-approved registry with cognitive and functional data submission every 6 months
- Health equity concerns: Black and Hispanic patients have reduced access to memory clinics, PET scans, and specialized diagnostics; later-stage diagnosis in underserved populations reduces eligibility for treatment indicated only in early disease
- Biomarker access: Amyloid PET costs $3,000–$6,000; FDA clearance of plasma p-tau217 (May 2025) and availability through Labcorp and Quest Diagnostics may partially address this barrier
- Specialist workforce: Most anti-amyloid therapy is managed through expert memory clinics, creating bottlenecks in patient access
Emerging Directions
Prevention trials represent the next frontier for anti-amyloid therapy. The AHEAD Study is testing lecanemab in asymptomatic amyloid-positive individuals, while TRAILBLAZER-ALZ 3 is evaluating donanemab for preventive treatment in preclinical AD (stages 1 and 2). If successful, these studies could shift the treatment paradigm toward intervention before symptom onset.
Combination approaches pairing anti-amyloid with anti-tau therapy are under investigation. The Eisai Phase 2 Study 202 is testing etalanetug (an anti-MTBR tau antibody) combined with lecanemab as standard of care in early sporadic AD. Roche is developing trontinemab, an anti-amyloid antibody using Brainshuttle technology to enhance blood-brain barrier crossing, with three Phase 3 trials planned.
Blood-based biomarker screening with plasma p-tau217 is expected to dramatically expand access to diagnosis and treatment eligibility assessment, potentially shifting amyloid detection from specialized PET centers to primary care settings.
References
- Schenk D, Barbour R, Dunn W, et al. Immunization with amyloid-beta attenuates Alzheimer-disease-like pathology in the PDAPP mouse. Nature 1999;400(6740):173–177.
- Budd Haeberlein S, Aisen PS, Barkhof F, et al. Two randomized phase 3 studies of aducanumab in early Alzheimer's disease. J Prev Alzheimers Dis 2022;9(2):197–210.
- Cummings J, Rabinovici GD, Atri A, et al. Aducanumab: appropriate use recommendations update. J Prev Alzheimers Dis 2022;9(2):221–230.
- van Dyck CH, Swanson CJ, Aisen P, et al. Lecanemab in early Alzheimer's disease. N Engl J Med 2023;388(1):9–21.
- Cummings J, Apostolova L, Rabinovici GD, et al. Lecanemab: appropriate use recommendations. J Prev Alzheimers Dis 2023;10(3):362–377.
- Mintun MA, Lo AC, Duggan Evans C, et al. Donanemab in early Alzheimer's disease. N Engl J Med 2021;384(18):1691–1704.
- Sims JR, Zimmer JA, Evans CD, et al. Donanemab in early symptomatic Alzheimer disease: the TRAILBLAZER-ALZ 2 randomized clinical trial. JAMA 2023;330(6):512–527.
- Geldmacher DS. Treatment of Alzheimer disease. Continuum (Minneap Minn) 2024;30(6, Dementia):1823–1844.
- Morris E, Chalkidou A, Hammers A, et al. Diagnostic accuracy of (18)F amyloid PET tracers for the diagnosis of Alzheimer's disease: a systematic review and meta-analysis. Eur J Nucl Med Mol Imaging 2016;43(2):374–385.
- Jack CR Jr, Andrews JS, Beach TG, et al. Revised criteria for diagnosis and staging of Alzheimer's disease: Alzheimer's Association Workgroup. Alzheimers Dement 2024;20:5143–5169.
- Cummings J, Zhou Y, Lee G, et al. Alzheimer's disease drug development pipeline: 2023. Alzheimers Dement (N Y) 2023;9(2):e12385.
- Murchison CF, Kennedy RE, McConathy JE, Roberson ED. Racial differences in Alzheimer's disease specialist encounters are associated with usage of molecular imaging and dementia medications. J Alzheimers Dis 2021;79(2):543–557.