Neotrofin (AIT-082; leteprinim potassium) is transported out of brain by a saturable mechanism and in this study the mechanisms mediating this efflux were evaluated. Intracerebroventricular coadministration of [(14)C]Neotrofin with verapamil, a P-glycoprotein inhibitor, probenecid, an organic anion transporter inhibitor, 3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid (MK571), a multidrug resistance-associated protein inhibitor, and salicylate or benzoate, both monocarboxylic acid transporter substrates, inhibited the efflux of [(14)C]Neotrofin. Additionally, Neotrofin inhibited the efflux of [(3)H]quinidine from brain. Compounds can diffuse from cerebrospinal fluid (CSF) into extracellular fluid of brain parenchyma and thus, efflux of [(14)C]Neotrofin after intracerebroventricular administration may indicate active transport across choroid plexus epithelium, brain capillary endothelium, or both. To determine whether [(14)C]Neotrofin efflux occurs at the brain capillary endothelium, experiments were performed in which [(14)C]Neotrofin was administered intraparenchymally. The t(1/2) for [(14)C]Neotrofin disappearance from brain after intraparenchymal administration was significantly lower than that for [(3)H]sucrose and the efflux of Neotrofin was inhibited by 600-fold excess of unlabeled Neotrofin, verapamil, MK571, and salicylate. Together, these data suggest that a saturable mechanism for the efflux of Neotrofin is located at the blood-brain barrier and possibly the blood-CSF barrier. It is likely that multiple transporters are involved in the efflux of Neotrofin and these may include multidrug resistance and monocarboxylic acid transporters. These data are discussed in detail with respect to the site of transporter expression, the recent identification of numerous multidrug resistance-associated protein and monocarboxylic acid transporter homologs, the existence of other potential brain efflux transporters, and the availability of specific pharmacological agents with which to distinguish these transporters.