br Future of DHODH targeted therapy We believe

Future of DHODH-targeted therapy We believe the future of DHODH-targeted therapy in cancer lies in multi-drug combination treatments to produce in vivo synergy. Despite the setbacks of brequinar in clinical trials, DHODH remains a viable anticancer target. The pyrimidine depletion induced by DHODH inhibition may sensitize ISO-1 to better outcomes with current chemotherapy options. In fact, several studies have previously implicated DHODH inhibition as key to overcoming chemotherapy resistance (Brown et al., 2017; He et al., 2014; Shukla et al., 2017). DHODH inhibition sensitizes cancer cells to conventional chemotherapy and overcomes resistance mechanisms by targeting metabolic dependencies. For example, leflunomide is key to overcoming chemotherapy resistance in triple-negative breast cancer cell lines (Brown et al., 2017). Triple-negative breast cancer cell lines exposed to genotoxic agents increased flux through the de novo pyrimidine pathway, resulting in increased nucleotide concentrations to facilitate DNA repair and decreasing their sensitivity to genotoxic agents. Pretreatment with leflunomide induced pyrimidine depletion in triple-negative breast cancer cells and overcame doxorubicin resistance (Brown et al., 2017). Additionally, as previously described, brequinar increased cell sensitivity to TRAIL therapy (He et al., 2014). Beyond doxorubicin and TRAIL, DHODH inhibitors have been shown to sensitize cells to DNA substrate mimics. When leflunomide and gemcitabine were used in combination, the effect was more significant than with single-agent dosing. Similar to combinations of brequinar and 5-FU, leflunomide-induced pyrimidine depletion may have led to a higher incorporation of gemcitabine (Shukla et al., 2017). Similar combinations, such as teriflunomide with 5-azacytidine, were evaluated in resistant cell lines in 5-azacytidine-resistant leukemic cells (Imanishi et al., 2017), and leflunomide with fludarabine in fludarabine-resistant chronic lymphocytic leukemia cells (Dietrich et al., 2012). It is unclear if these combinations would have a similar effect clinically as brequinar and 5-FU. However, preclinical data suggests that DHODH-induced pyrimidine depletion may be used to overcome certain acquired resistance mechanisms. With the recent expanse of technology to evaluate synthetic lethality, new combinations with DHODH inhibitors may be uncovered (O\'Neil, Bailey, & Hieter, 2017). Many oncogenic backgrounds share synthetic lethality with DHODH. Cells with mutant PTEN have increased glutamine metabolism and are sensitive to DHODH inhibition (Mathur et al., 2017). Teriflunomide was synergistic with the BRAF(V600E) inhibitor PLX4720 in melanoma cells (White et al., 2011). KRAS mutant cells were found to be more sensitive to DHODH inhibition over KRAS WT cells (Koundinya et al., 2018). Interestingly, DHODH inhibitors were found to increase p53 synthesis and were synergistic with an MDM2 inhibitor (Ladds et al., 2018). While more studies are needed, these results suggest that targeting pyrimidine biosynthesis with DHODH inhibitors may be a pivotal point of exploitation in specific genomic backgrounds.