Introduction Isocitrate dehydrogenase 1 ( IDH1 ) is mutated in various types of human cancer and the presence of a mutation is associated with improved responses to irradiation and chemotherapy in solid tumor cells. Mutated IDH1 (IDH1 MUT ) enzymes consume NADPH to produce D ‐2‐hydroxyglutarate ( D ‐2HG) resulting in a decreased reducing power needed for detoxification of e.g . reactive oxygen species (ROS). The objective of the current study was to investigate the mechanism behind the chemosensitivity of the widely‐used anti‐cancer agent cisplatin in IDH1 MUT cancer cells. Methods Oxidative stress, DNA damage and mitochondrial dysfunction caused by cisplatin treatment were monitored in IDH1 MUT HCT116 cells and U251 glioma cells. Results By performing histo‐ and cytochemical experiments, we found that exposure to cisplatin induced higher levels of ROS, DNA double‐strand breaks (DSBs), and cell death in IDH1 MUT cancer cells as compared to IDH1 wild‐type ( IDH1 WT ) cells. Mechanistic investigations as in situ histochemical enzyme activity assays revealed that cisplatin treatment dose‐dependently reduced oxidative respiration in lDH1 MUT cells, which was accompanied by disturbed mitochondrial proteostasis, indicative for impaired mitochondrial activity. These effects were abolished by the IDH1 MUT inhibitor AGI‐5198 and were recapitulated by treatment with D ‐2HG. Conclusions Our study shows that altered oxidative stress responses and a vulnerable oxidative metabolism underlie the sensitivity of IDH1 MUT cancer cells to cisplatin. Our study may have clinical implications and imply that administration of IDH1 MUT inhibitors to patients with IDH1 MUT cancer abolishes the therapeutic effect of cisplatin. Our in vitro results suggest that concomitant administration of IDH1 MUT inhibitors and cisplatin may result in an unfavorable clinical outcome. Support or Funding Information This work was supported by Dutch Cancer Society Grants KWF‐UVA 2014‐6839 (to M.K., V.V.V.H., R.J.M., and C.J.F.V.N.) and AMC2016.1‐10460 (to M.K., R.J.M., J.W.W., and C.J.F.V.N.). A) Proliferation assay using HCT116 cells after cisplatin exposure and in the presence or absence of ROS‐scavenging NAC.B) Colony‐forming assay after cisplatin exposure with HCT116 cells in the presence or absence of AGI‐5198 or of D‐2HG.C) Representative photomicrographs and plots of cells in the presence or absence of AGI‐5198, and cisplatin treatment. Y‐H2AX was stained immunocytochemically (red) to demonstrate DNA DSBs and with DAPI (blue) to demonstrate DNA nucleus content. image A) Proliferation assay using HCT116 cells after cisplatin exposure and in the presence or absence of ROS‐scavenging NAC.B) Colony‐forming assay after cisplatin exposure with HCT116 cells in the presence or absence of AGI‐5198 or of D‐2HG.C) Representative photomicrographs and plots of cells in the presence or absence of AGI‐5198, and cisplatin treatment. Y‐H2AX was stained immunocytochemically (red) to demonstrate DNA DSBs and with DAPI (blue) to demonstrate DNA nucleus content. D) The basal oxygen consumption rate of HCT116 cells shown in the presence or absence of cisplatin. E) Western blot and mitonuclear protein imbalance of MTCO1 and nuclear DNA–encoded SDHA expression in HCT116 cells after exposure to cisplatin or carboplatin. F) Representative monochromatic light photomicrographs and quantification of histochemical enzyme activity of HCT116 cells treated with cisplatin or left untreated after staining for SDH activity in the presence or absence of sodium azide. image D) The basal oxygen consumption rate of HCT116 cells shown in the presence or absence of cisplatin. E) Western blot and mitonuclear protein imbalance of MTCO1 and nuclear DNA–encoded SDHA expression in HCT116 cells after exposure to cisplatin or carboplatin. F) Representative monochromatic light photomicrographs and quantification of histochemical enzyme activity of HCT116 cells treated with cisplatin or left untreated after staining for SDH activity in the presence or absence of sodium azide. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .