[18F]AzaFol PET captures folic acid dynamics at folate receptors in the brain.

Cerebral folate receptor α (FRα) is essential for transporting folate into the central nervous system, supporting normal biological functions and fetal neurodevelopment. We aimed to provide in vivo measurements of folic acid dynamics at FRα in non-human primate brains using positron emission tomography (PET) and [18F]AzaFol, a high-affinity FRα radioligand. A 0.01 mg/kg intravenous dose of folic acid, comparable to pregnancy supplementation doses (400-800 µg), dramatically reduced FRα availability in the choroid plexus (CP), based on uptake profiles and exploratory kinetic modeling. Following this dose, [18F]AzaFol uptake increased, potentially capturing (1) FRα-mediated transport of folic acid and (2) real-time FRα recycling, which may restore binding sites as serum folic acid levels decline, reducing receptor competition. Experiments with constant infusion of folic acid during PET imaging aimed to identify the possible folic acid transport rate and serum concentrations at which this process reaches a steady state. Pharmacokinetic analysis suggests full FRα blockage with serum folic acid concentrations within the range of 63-90 ng/mL and cumulative dose slightly above 0.01 mg/kg. Findings enhance the understanding of folic acid's in vivo pharmacology at cerebral FRα and may inform future studies on disorders in which impaired folic acid transport is suspected, either at the CP or at the placenta.