In the mountains of northwestern Colombia, thousands of members of extended families carry a single genetic mutation that all but guarantees they will develop Alzheimer’s disease before age 50. For decades, these families have been at the center of some of the most important Alzheimer’s research in the world. Now they are helping scientists answer a new question: Can pairing two experimental therapies strip amyloid plaques from the brain faster while taming the most dangerous side effect dogging every approved anti-amyloid drug?
A clinical trial registered in early 2025 as NCT06996730 is combining the antibody donanemab with a gamma-secretase modulator called RG6289 in carriers of the PSEN1 E280A mutation, the variant responsible for the familial Alzheimer’s clusters in Colombia’s Antioquia region. If the combination works, it could change how doctors weigh the benefits and risks of a drug class that has finally begun to show real, if modest, results against the disease.
Two drugs, two angles of attack
Donanemab is already on the market. The FDA approved it in July 2024 under the brand name Kisunla after the TRAILBLAZER-ALZ 2 trial showed it slowed cognitive decline in people with early symptomatic Alzheimer’s compared to placebo. But the approval came with a boxed warning for amyloid-related imaging abnormalities, known as ARIA, a catch-all term for brain swelling (ARIA-E) and tiny brain bleeds (ARIA-H) that show up on MRI scans. ARIA has been the persistent shadow over every anti-amyloid antibody, including lecanemab (Leqembi), the other drug in this class with full FDA approval. The side effect has restricted which patients doctors feel comfortable treating and forced the FDA to mandate intensive MRI monitoring schedules.
RG6289 takes a fundamentally different approach. Instead of binding to amyloid plaques after they have already formed, it modifies the activity of gamma-secretase, the enzyme that snips amyloid precursor protein into the smaller peptide fragments that clump into plaques. The goal is to shift the enzyme’s cut site so it produces shorter, less toxic fragments rather than the longer, stickier ones that seed deposits. This is not the same as the gamma-secretase inhibitors that failed in earlier Alzheimer’s trials and caused serious side effects by shutting the enzyme down entirely. Modulators aim to fine-tune the enzyme rather than silence it.
The logic of combining the two is straightforward: donanemab clears plaques that already exist while RG6289 slows the production of new ones. According to the trial’s registry entry, the study compares three active arms head to head: donanemab alone, RG6289 alone, and the combination. Amyloid PET scans measured in centiloids, a standardized scale of plaque burden, serve as the primary readout.
Why this population matters
The PSEN1 E280A mutation carriers chosen for this trial are not a random convenience sample. Researchers have studied these Colombian families for more than three decades, building one of the most detailed natural histories of any genetic disease. Because carriers develop heavy amyloid deposits on a predictable timeline, often visible on PET scans a full decade before symptoms appear, they offer an unusually clean window into whether a drug is actually moving the biology. The Alzheimer’s Prevention Initiative, led by the Banner Alzheimer’s Institute in partnership with Colombian investigators, has used this population to test earlier interventions, including a previous trial of the anti-amyloid antibody crenezumab.
That genetic uniformity is a strength for detecting a drug signal, but it also limits how far the results can travel. Late-onset sporadic Alzheimer’s, which accounts for the vast majority of cases worldwide, involves a messier mix of vascular disease, inflammation, tau pathology, and genetic risk factors like APOE4. Findings in young mutation carriers may not predict what happens in a 75-year-old with multiple comorbidities. Any positive result from this trial would need to be tested again in broader populations before it could change standard care.
The ARIA problem, and early clues it can be managed
ARIA remains the central safety concern for anti-amyloid antibodies. The swelling and microbleeds are thought to arise when antibodies latch onto amyloid deposits lining blood vessel walls, triggering local inflammation and weakening vessel integrity. The National Institute of Neurological Disorders and Stroke convened a workshop specifically on ARIA and anti-amyloid immunotherapy, where experts emphasized that the side effect likely involves several overlapping processes: vascular fragility, complement-driven immune activation, and rapid shifts in plaque burden along vessel walls. Different antibodies, dosing schedules, and patient risk profiles can produce distinct ARIA patterns, making it difficult to generalize safety data from one drug to another.
There are, however, early signs that ARIA can be dialed down without sacrificing efficacy. The TRAILBLAZER-ALZ 6 study, a Phase 3 trial that tested a modified donanemab dosing schedule, found that a gentler ramp-up reduced the rate of ARIA-E while preserving amyloid clearance through 76 weeks of follow-up. That result, published in a peer-reviewed journal, established a practical principle: how aggressively the drug is introduced matters as much as the drug itself.
Preclinical work has explored other angles. A mouse study published in PubMed Central investigated whether blocking C1q, a protein that initiates the complement cascade, could reduce the vascular damage associated with anti-amyloid antibodies. The researchers reported less microhemorrhage and red blood cell leakage in treated animals, both hallmarks of ARIA-H. But the authors themselves described the experiment as a pilot with a small sample, and mouse models of amyloid pathology have a mixed track record of predicting human outcomes. (Note: the PubMed Central identifier PMC12724820 cited in earlier coverage of this study could not be independently verified as of May 2026; readers should confirm the link resolves to the intended paper before relying on it.) The complement pathway is one of several suspected contributors to ARIA, not a confirmed therapeutic target in people.
What we do not know yet
No data from the NCT06996730 combination trial have been publicly released as of May 2026. The registry entry provides the study design, arms, and planned outcome measures, but enrollment figures, specific dosing protocols, and early safety signals remain undisclosed. Whether combining donanemab and RG6289 will produce additive plaque clearance or introduce new toxicities is an open question.
Cost is another blank space. Existing anti-amyloid monoclonal antibodies already carry high list prices and require repeated MRI monitoring that adds logistical and financial burden. Layering a second branded therapy on top raises obvious questions about affordability and insurance coverage. Neither the trial sponsors nor federal agencies have released projections for what a two-drug Alzheimer’s regimen might cost, and payers have already pushed back on covering single-agent anti-amyloid treatments for some patient groups.
There is also no guarantee that reducing amyloid production with a gamma-secretase modulator will translate into less ARIA. The biological rationale is plausible: if fewer new plaques form along blood vessels while existing ones are being cleared, the inflammatory reaction might be less intense. But that hypothesis has not been tested in humans, and the relationship between plaque removal speed, vascular amyloid burden, and ARIA severity is not fully mapped.
Why the combination trial could sharpen the ARIA debate
The donanemab-RG6289 trial is best understood as a carefully targeted experiment, not a preview of tomorrow’s prescriptions. It draws on converging but incomplete evidence: regulatory experience with donanemab’s benefits and risks, clinical proof from TRAILBLAZER-ALZ 6 that dosing strategy influences ARIA rates, mechanistic hints from complement-focused animal work, and expert consensus that attacking amyloid from multiple directions might yield a better balance of benefit and harm.
For the families in Antioquia who have watched generation after generation lose memory, language, and independence to a disease written into their DNA, the trial represents another chapter in a research partnership that has already lasted decades. For the broader Alzheimer’s field, it is a test of whether combination therapy can solve the problem that has limited every anti-amyloid drug so far: clearing plaques without damaging the brain in the process. The first results, whenever they arrive, will not end that debate, but they could sharpen it considerably.
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*This article was researched with the help of AI, with human editors creating the final content.
