Beyond Amyloid: Systemic and Brain Frailty as Determinants of Response to Anti-Amyloid Therapy in Alzheimer's Disease-A Conceptual Review.

Anti-amyloid therapy (AAT) with monoclonal antibodies (mAbs) modestly slow cognitive and functional decline in early Alzheimer's disease (AD). However, both the magnitude of clinical benefit and the risk of treatment-related complications vary substantially even among patients with similar biomarker profiles. Frailty, both brain and systemic, is highly prevalent in older adults with AD and affects a large proportion of those considered for AAT. Despite this, it has been largely absent from current decision frameworks. Brain frailty, defined by structural and microvascular damage (e.g., small-vessel disease, microbleeds, and atrophy), limits the clinical benefit of amyloid clearance and increases susceptibility to amyloid-related imaging abnormalities. In contrast, systemic frailty, reflecting reduced physiological reserve, mainly affects treatment tolerance and recovery from adverse events. In this narrative, conceptual review, we synthesize evidence that both forms of frailty act as biologically grounded modifiers of AAT efficacy and safety and may limit the clinical benefit while increasing susceptibility to complications and decompensation. Importantly, the precise empirical thresholds at which frailty begins to exert harmful effects remain unknown. We further outline how MRI-based markers of brain frailty, combined with brief systemic frailty measures, could support risk stratification, patient selection, monitoring intensity, and shared decision-making, including deferring treatment when the benefit-risk balance is unfavorable, while avoiding exclusion of patients who may still benefit. Taken together, we propose that future studies should incorporate frailty measures and perform precise assessments of both brain and systemic frailty, as this may improve patient stratification and better characterize the effects of AAT.