Exposure of SMN reporter cells to constant ratios of 4, 27, and 5. (A) SMN2, SV40 reporter cells, and SH5Y5Y were analyzed in duplicate or triplicate, with indicated concentrations of 27 for 24 or 48 h, respectively. SMN-luc (■; n = 3), Renilla luc (◆; n = 3), and SV40 luc activities (▲, n = 2) and cell viability (●; n = 3) were measured. Compound 27 was incubated with recombinant luciferase (▼, n = 2) at indicated doses, and luc activity was measured. (B) SMN reporter cells were treated with the indicated concentrations of 4 (blue) and 27 (red), individually or in combination (purple) for 24 h, and analyzed for SMN-luc activity. (C) SMN reporter cells were treated with the indicated concentrations of 5 (green) and 27 (red), individually or in combination (purple) for 24 h, and analyzed for SMN-luc activity. The theoretical additive was calculated by addition of the observed experimental activities of the individual drug exposures. Data are expressed as SEM (n ≥ 3).