title: A critical evaluation of the brain efflux index method as applied to the nitric oxide synthase inhibitor, aminoguanidine.
abstract: The Brain Efflux Index (BEI) method is an in vivo procedure designed to quantitate saturable efflux mechanisms resident at the blood--brain barrier (BBB). The present work utilized the BEI method to assess the BBB efflux mechanisms of [(14)C]aminoguanidine, a nitric oxide synthase inhibitor. The BEI for [(14)C]aminoguanidine was >100% (relative to [(3)H]inulin diffusion) over a range of 41-184 pmol after 40 min. The unusually high retention (>100%) of [(14)C]aminoguanidine suggested brain parenchymal sequestration, either by neuronal uptake or tissue protein binding. The uptake of [(14)C]aminoguanidine in dendritic neuronal endings (synaptosomes) showed a saturable concentration dependency, consistent with a carrier-mediated process. Nonlinear least-squares regression yielded the following Michaelis--Menten and diffusional (k(ns)) parameters for synaptosomal [(14)C]aminoguanidine uptake: V(max)=118.50 +/- 28.77 pmol x mg protein(-1)/3 min; K(m)=58.34 +/- 8.33 muM; k(ns)=0.15 +/- 0.029 pmol x mg protein(-1)/3 min/muM; mean +/- SEM; n=3 concentration profiles). Protein binding studies using brain tissue showed negligible binding. In summary, this work identified three principle findings: (1) An apparent lack of quantifiable aminoguanidine BBB efflux; (2) a previously undescribed synaptosomal accumulation process for aminoguanidine; and (3) an interesting limitation of the BEI technique where unusual brain parenchymal sequestration yields values >100%.
authors.affiliation: Department of Pharmaceutical Sciences, School of Pharmacy and Pharmaceutical Sciences, University at Buffalo, Buffalo, NY 14260, USA.
authors.forename: J J
authors.initials: JJ
authors.lastname: Raybon
authors.forename: K M
authors.initials: KM
authors.lastname: Boje
mesh_terms: D000818:Animals; D001692:Biological Transport; D001812:Blood-Brain Barrier / Q000502:physiology*; D001921:Brain / Q000201:enzymology / Q000378:metabolism*; D002250:Carbon Radioisotopes / Q000493:pharmacokinetics; D004353:Drug Evaluation, Preclinical / Q000379:methods*; D004791:Enzyme Inhibitors / Q000493:pharmacokinetics*; D006146:Guanidines / Q000493:pharmacokinetics*; D007276:Injections, Intraventricular; D008297:Male; D009004:Monosaccharide Transport Proteins / Q000493:pharmacokinetics; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D011135:Polysaccharides, Bacterial / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013574:Synaptosomes / Q000201:enzymology / Q000378:metabolism
chemical_list: D002250:Carbon Radioisotopes; D004791:Enzyme Inhibitors; D006146:Guanidines; D009004:Monosaccharide Transport Proteins; C057520:O-methylglucose polysaccharide; D011135:Polysaccharides, Bacterial; D019001:Nitric Oxide Synthase; C004479:pimagedine