EXPRESS: Rapamycin increases cerebral blood flow and modulates metabolic, inflammatory, and microbiome profiles in healthy middle-aged APOE4 carriers: a pilot single-arm trial.

Carriers of the apolipoprotein E4 (APOE4) allele often develop cerebrovascular dysfunction and broader systemic alterations decades before the onset of Alzheimer's disease (AD) pathology or clinical symptoms. Early interventions that can improve these functions may help delay or slow AD progression. In this study, we repurposed rapamycin (sirolimus), an FDA-approved medication with anti-aging properties, to target APOE4 associated multisystem dysfunction. We conducted a single arm trial of low dose rapamycin (1 mg/day for 4 weeks) in cognitively normal adults aged 45 to 65 years, prior to detectable AD pathology, stratified by APOE genotype. The primary outcome was cerebral blood flow (CBF), while secondary outcomes included plasma metabolomics, inflammatory cytokines, AD biomarkers, and gut microbiome composition. Twenty-three participants completed the study, including nine APOE4 carriers and fourteen non-carriers. Rapamycin significantly increased CBF in APOE4 carriers, with increases exceeding 15% across multiple brain regions, and improved metabolic and inflammatory profiles with minimal adverse effects. In contrast, non-carriers showed no significant change in CBF and exhibited distinct physiological responses, highlighting genotype dependent effects. These findings suggest that rapamycin may mitigate early cerebrovascular and systemic dysfunction in APOE4 carriers and support a precision medicine approach in which therapeutic response is influenced by genotype.