Beyond the cerebrospinal fluid: Preclinical and clinical evidence on central nervous system penetration and functional activity of oral rapamycin (sirolimus)

Rapamycin, a potent mTORC1 inhibitor, is a leading candidate for treating age-related neurodegenerative disease. Numerous studies demonstrate beneficial effects in a variety of diseases that affect the brain. Yet a persistent belief among researchers is that rapamycin cannot penetrate brain tissue, citing as evidence its absence of detection in cerebrospinal fluid (CSF). This narrative review synthesizes clinical and preclinical evidence challenging this “CSF misconception.” In humans, direct tissue sampling in glioblastoma detects rapamycin in brain at mTORC1-inhibitory concentrations, while functional measures—tumor regression and seizure control in tuberous sclerosis complex, cerebral metabolic preservation on PET in Alzheimer's disease, increased cerebral blood flow in APOE4 carriers, and prolonged antidepressant response—demonstrate incontrovertible activity in the brain. In mice, LC-MS/MS confirms dose-dependent brain accumulation, and rapamycin modulates mTORC1 signaling, autophagy, and synaptic, epigenetic, and inflammatory pathways. Rapamycin cannot be reliably detected in CSF because of four pharmacological factors—high lipophilicity with parenchymal sequestration, extreme red-cell and protein binding, P-glycoprotein efflux at the blood-CSF barrier, and assay insensitivity—none of which supports the contention that rapamycin fails to enter brain tissue. We flag the field's central gap: direct measurement of parent drug in intact human brain parenchyma at therapeutic doses is lacking. CSF concentration is a flawed proxy for rapamycin's CNS penetration. Alzheimer's disease and related dementias (ADRD) trials should instead track surrogate markers of mTORC1 inhibition, functional neuroimaging, clinical outcomes, and ADRD fluid biomarkers. These pharmacodynamics measurements are essential to the rational design and interpretation of prevention trials for the aging brain.