The ability of a series of substituted kynurenic acids, thienopyridinonecarboxylic acids, and related compounds to inhibit the binding of nerve growth factor (NGF) to the p75 NGF receptor (NGFR) was evaluated in a radioligand binding assay that utilized a biotinylated derivative of the extracellular domain of p75 NGFR (p75ext) fixed to streptavidin-coated plastic wells. Two compounds, 6-aminokynurenic acid (5h) and the 3-methyl ester of 4,7-dihydro-2-methyl-7-oxothieno[3,2-b]pyridine-3,5-dicarboxylic acid (16), were found to inhibit the binding of [125I]NGF to p75ext with IC50 values in the low micromolar range. Other amino-substituted kynurenic acids also possessed activity at slightly higher concentrations. Several structural features seem to be essential, including the carboxylic acid, a polar group on the benzene ring (or thiophene ring, in the case of analogues of 16), and the C-4 carbonyl group in the pyridinone ring. These compounds were also found to inhibit the binding of [125I]NGF to its receptors in membranes from PC12 cells (which express p75 as well as trka receptors for NGF) and DG44-CHO cells (transfected with full length p75 NGFR). The available data for 5h and 16 do not allow the determination of whether the effects of these compounds are mediated by their interaction with NGF or the NGF receptors.