In vitro activity: Compared with Gentamicin which is only active at much higher concentrations, PTC124 is a more potent nonsense-suppressing agent and exhibits 4- to 15-fold stimulation of read-through relative to controls. PTC124 (0.01-3 μM) promotes dose-dependent read-through of all three nonsense codons in HEK293 cells harboring LUC-190 nonsense alleles with the highest read-through at UGA, followed by UAG and then UAA, but it does not suppress multiple proximal nonsense codons. Like Gentamicin, PTC124 is most active when a pyrimidine (in particular cytosine, C) follows the nonsense codon. Consistent with the stable cell line reporter assay, PTC124 (17 μM) promotes significant production of dystrophin in primary muscle cells from Duchenne muscular dystrophy (DMD) patients or MDXMDX mice expressing dystrophin nonsense alleles. PTC124 selectively promotes ribosomal read-through of premature termination but not normal termination codons, even at concentrations substantially greater than the values achieving maximal activity.

Cell Assay: Cultured HEK293 cells harbouring UAA, UAG or UGA LUC-190 nonsense alleles were treated with increasing concentrations of PTC124 for 16 h, and assayed for luciferase activity. PTC124 promoted dose-dependent readthrough of all three nonsense codons. Levels of suppression correlated inversely with established termination efficiencies, with the highest readthrough at UGA, followed by UAG and then UAA. The minimal concentration of PTC124 showing discernable readthrough was 0.01–0.1 μM, whereas the concentration promoting maximal activity was approximately 3 μM.