System X_c^- Antiporter Inhibitors: Azo-Linked Amino-Naphthyl-Sulfonate Analogues of Sulfasalazine

The cystine/glutamate antiporter system X_c^- (SX_c^-) mediates the exchange of intracellular L-glutamate (L-Glu) with extracellular L-cystine (L-Cys₂). Both the import of L-Cys₂ and the export of L-Glu take on added significance in CNS cells, especially astrocytes. When the relative activity of SX_c^- overwhelms the regulatory capacity of the EAATs, the efflux of L-Glu through the antiporter can be significant enough to trigger excitotoxic pathology, as is thought to occur in glioblastoma. This has prompted considerable interest in the pharmacological specificity of SX_c^- and the development of inhibitors. The present study explores a series of analogues that are structurally related to sulfasalazine, a widely employed inhibitor of SX_c^-. We identify a number of novel aryl-substituted amino-naphthylsulfonate analogues that inhibit SX_c^- more potently than sulfasalazine. Interestingly, the inhibitors switch from a competitive to noncompetitive mechanism with increased length and lipophilic substitutions, a structure-activity relationship that was previously observed with aryl-substituted isoxazole. These results suggest that the two classes of inhibitors may interact with some of the same domains on the antiporter protein and that the substrate and inhibitor binding sites may be in close proximity to one another. Molecular modeling is used to explore this possibility.