Alzheimer's disease is a progressive neurodegenerative disorder characterized by the deposit of amyloid fibrils in the brain that result from the self-aggregative polymerization of the beta-amyloid peptide (A beta). Evidence of a direct correlation between the ability of A beta to form stable aggregates in aqueous solution and its neurotoxicity has been reported. The cytotoxic effects of A beta have been attributed to the aggregation properties of a domain corresponding to the peptide fragment A beta 25-35. In an effort to generate novel inhibitors of A beta neurotoxicity and/or aggregation, a mixture-based synthetic combinatorial library composed of 23 375 imidazopyridoindoles was generated and screened for inhibition of A beta 25-35 neurotoxicity toward the rat pheochromocytoma PC-12 cell line. The effect of the identified lead compounds on A beta 25-35 aggregation was then evaluated by means of circular dichroism (CD) and thioflavin-T fluorescence spectroscopy. Their activity against A beta 1-42 neurotoxicity toward the PC-12 cell line was also determined. The most active imidazopyridoindoles inhibited both A beta 25-35 and A beta 1-42 neurotoxicity in the low- to mid-micromolar range. Furthermore, inhibition of the random coil to beta-sheet transition and self-aggregation of A beta 25-35 was observed by CD and fluorescence spectroscopy, supporting the relationship between inhibition of the A beta aggregation process and neurotoxicity.