Characterization of drugs that inactivate VKR for vitamin K reduction or affect vitamin K availability within the cells. Inhibition of VKD carboxylation by idebenone (A), clofazimine (D), and itraconazole (G). FIXgla-PC/HEK293 and DGKO cells were incubated with increasing concentrations of the test drug in cell culture medium containing 5 µM KO (KO_HEK293) and vitamin K (K_DGKO), respectively, for the cell-based activity assay and cell viability assay (cell viability). Inhibition of KO reduction by idebenone (B), clofazimine (E), and itraconazole (H) in GGCX-knockout HEK293 cells. The KO reduction activity was evaluated by the production of vitamin K (marked with vertical dashed lines) in the chromatogram, as described in the legend to Figure 2D. Inhibition of vitamin K reduction by idebenone (C), clofazimine (F), and itraconazole (I) in DGKO cells. Vitamin K reduction activity was evaluated by the production of KO (marked with vertical dashed lines) in the chromatogram, as described in the legend to Figure 2E.