c Oxidative DNA damage in knockdown cells: knockdown cells were cultured with the indicated concentrations (0C20?M) of THC for 24?h. is a highly bioavailable curcumin dispersed with colloidal submicron particles. Methods We examined antitumor effects of THC on ESCC cells by cell viability assay, colony and spheroid formation assay, and xenograft models. To reveal its mechanisms, we investigated the levels of reactive oxygen species (ROS) and performed microarray gene expression analysis. According to those analyses, we focused on NQO1, which KX2-391 2HCl involved in the removal of ROS, and examined the effects of NQO1-knockdown or overexpression on THC treatment. Moreover, the therapeutic effect of THC and NQO1 inhibitor on ESCC patient-derived xenografts (PDX) was investigated. Results THC caused cytotoxicity in ESCC cells, and suppressed the growth of xenografted tumors more efficiently than curcumin. THC increased ROS levels and activated the NRF2CNMRAL2PCNQO1 expressions. Inhibition of NQO1 in ESCC cells by shRNA or NQO1 inhibitor resulted in an increased sensitivity of cells to THC, whereas overexpression of NQO1 antagonized it. Notably, NQO1 inhibitor significantly enhanced the antitumor effects of THC in ESCC PDX tumors. Conclusions These findings suggest the potential usefulness of THC and its combination with NQO1 inhibitor as a therapeutic option for ESCC. Electronic supplementary material The online version of this article (10.1007/s00535-019-01549-x) contains supplementary material, which is available to authorized users. that is recognized as a generally safe compound by the Food and Drug Administration [7, 8]. Curcumin demonstrates various biological benefits including antimicrobial and anti-inflammatory actions, and is RGS17 involved in the regulation of programmed cell death and survival pathways by modulating transcription factors such as nuclear factor-B, growth factors, inflammatory cytokines, and receptors . Curcumin has been shown to have antitumor effects on several types of cancer cells including lung cancer , glioblastoma , colon cancer , pancreatic cancer , prostate cancer , and ESCC [15C17]. Despite the demonstration of the promising antitumor effects of curcumin in preclinical studies, its clinical use is currently limited because of its poor bioavailability in humans . Curcumin is not easily soluble in water , and oral administration of curcumin does not achieve sufficient blood concentrations to exert therapeutic efficacy [20C22]. To overcome this limitation, various strategies of drug development have been attempted to improve the bioavailability of curcumin [23C27]. Theracurmin? (THC, curcumin content 30%?w/w) is an effective preparation of curcumin dispersed with colloidal submicron particles, making it easily disperse in water . Consequently, the bioavailability of curcumin in THC is much improved, and the area under the blood concentrationCtime curve (AUC) after the oral administration of THC is more than 40-fold higher than that of curcumin in rats and 27-fold higher than that of curcumin in humans . In fact, THC has been reported to be clinically useful for treating osteoarthritis , muscle damage , and atherosclerotic hyperlipidemia . With regard to experimental cancer research, the cytotoxicity KX2-391 2HCl or antitumor effects of THC have been reported using several cancer cell lines [31, 32], but the effectiveness of THC against ESCC has not been fully clarified. The purposes of our study were to investigate the antitumor effects of THC on ESCC cells and to compare the effects of curcumin and THC in vivo. Here, we found that induction of NAD(P)H quinone dehydrogenase 1 (NQO1), which is the enzyme that scavenge reactive oxygen species (ROS) , plays an antagonistic role in THC-induced antitumor effects, and we, therefore, examined the effects on ESCC of a combination treatment with THC and NQO1 inhibitor. Materials and methods In vitro assay and analysis Methods for cell culture, WST-1 cell viability assay, Caspase-Glo? 3/7 assay, spheroid assay, soft agar colony formation assay, microarray hybridization, real-time reverse transcriptionCpolymerase chain reaction (RTCPCR), western blotting, chromatin protein isolation, measurement of intracellular and/or mitochondrial ROS levels, immunofluorescent staining for 8-hydroxy-2-deoxyguanosine (8-OHdG), cell cycle assay, senescence-associated -galactosidase (SABG) assay, viral infections, and metabolite analysis are described in Supplementary materials and methods. Assessment of bioavailability and antitumor effects of curcumin and THC in vivo All animal experiments conformed to the relevant regulatory standards and KX2-391 2HCl were approved by.