Radiotracer retention in the brain was significantly higher with low MA [ 18 F]2 compared to high MA, shown through the area under the time activity curve (AUC; 117.1 ± 31.3% ID•h/g and 35.4 ± 6.0% ID•h/g, respectively; p = 0.006; n = 3-7; Fig. ). High brain uptake at low MA is likely a product of increased blood concentrations of a blood brain barrier (BBB)-permeable [ 18 F]1, or a consequence of efflux transporter (e.g. P-glycoprotein) saturation on the BBB, facilitating transient accumulation in the brain. Hepatobiliary excretion was evident by ~20 min p.i., suggesting possible metabolism of the parent compound at this time point (Fig. ).