Reactive oxygen species (ROS) promote tumor cell proliferation and survival by directly modulating growthregulatory molecules and key transcription factors. The signal transducer and activator of transcription 3 (STAT3) is constitutively active in a variety of tumor cell types, where the effect of ROS on the Janus kinase/ STAT pathway has been examined. We report here that STAT3 is directly sensitive to intracellular oxidants. Oxidation of conserved cysteines by peroxide decreased STAT3 binding to consensus serum-inducible elements (SIE) in vitro and in vivo and diminished interleukin (IL)-6-mediated reporter expression. Inhibitory effects produced by cysteine oxidation in STAT3 were negated in redox-insensitive STAT3 mutants. In contrast, ROS had no effect on IL-6-induced STAT3 recruitment to the c-myc P2 promoter. Expression of a redoxinsensitive STAT3 in breast carcinoma cells accelerated their proliferation while reducing resistance to oxidative stress. Our results implicate STAT3 in coupling intracellular redox homeostasis to cell proliferation and survival. Cancer Res; 70(20); 8222-32. ©2010 AACR.

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PIP4K2A is an insufficiently studied type II lipid kinase that catalyzes the conversion of phosphatidylinositol-5-phosphate (PI5P) into phosphatidylinositol 4,5-bisphosphate (PI4,5P2). The involvement of PIP4K2A/B in cancer has been suggested, particularly in the context of p53 mutant/null tumors. PIP4K2A/B depletion has been shown to induce tumor growth inhibition, possibly due to hyperactivation of AKT and reactive oxygen species-mediated apoptosis. Herein, we report the identification of the novel potent and highly selective inhibitors BAY-091 and BAY-297 of the kinase PIP4K2A by high-throughput screening and subsequent structure-based optimization. Cellular target engagement of BAY-091 and BAY-297 was demonstrated using cellular thermal shift assay technology. However, inhibition of PIP4K2A with BAY-091 or BAY-297 did not translate into the hypothesized mode of action and antiproliferative activity in p53-deficient tumor cells. Therefore, BAY-091 and BAY-297 serve as valuable chemical probes to study PIP4K2A signaling and its involvement in pathophysiological conditions such as cancer.

The mutation of K-RAS represents one of the most frequent genetic alterations in cancer. Targeting of downstream effectors of RAS, including of MEK and ERK, has limited clinical success in cancer patients with K-RAS mutations. The reduced sensitivity of K-RAS-mutated cells to certain MEK inhibitors (MEKi) is associated with the feedback phosphorylation of MEK by C-RAF and with the reactivation of mitogen-activated protein kinase (MAPK) signaling. Here, we report that the RAF dimer inhibitors lifirafenib (BGB-283) and compound C show a strong synergistic effect with MEKi, including mirdametinib (PD-0325901) and selumetinib, in suppressing the proliferation of K-RAS-mutated non-small-cell lung cancer and colorectal cancer (CRC) cell lines. This synergistic effect was not observed with the B-RAF V600E selective inhibitor vemurafenib. Our mechanistic analysis revealed that RAF dimer inhibition suppresses RAF-dependent MEK reactivation and leads to the sustained inhibition of MAPK signaling in K-RAS-mutated cells. This synergistic effect was also observed in several K-RAS mutant mouse xenograft models. A pharmacodynamic analysis supported a role for the synergistic phospho-ERK blockade in enhancing the antitumor activity observed in the K-RAS mutant models. These findings support a vertical inhibition strategy in which RAF dimer and MEKi are combined to target K-RAS-mutated cancers, and have led to a Phase 1b/2 combination therapy study of lifirafenib and mirdametinib in solid tumor patients with K-RAS mutations and other MAPK pathway aberrations.Abbreviations CR, complete regression; CRC, colorectal cancer; EOHSA, excess over highest single agent; MAPK, mitogen-activated protein kinase; MEKi, MEK inhibitors; NFA, negative feedback amplified; NSCLC, non-small-cell lung cancer; PR, partial regression; TGI, tumor growth inhibition; WT, wild-type.

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Oncogenic B-RAF, which drives cell transformation and proliferation, has been detected in approximately 70% of human malignant melanomas and 5-15% of colorectal cancers (CRC). B-RAFV600E mutation, which gives rise to constitutive MAPK signaling, accounts for at least 90% of oncogenic B-RAF mutations. Despite the remarkable clinical activities achieved by vemurafenib and dabrafenib in treating B-RAFV600E metastatic melanoma, their clinical efficacy in B-RAFV600E CRC is far less impressive. Prior studies suggested that feedback activation of EGFR and MAPK signaling upon B-RAF inhibition contributed to the intrinsic resistance of CRC to the first generation B-RAF inhibitors. This report represents the first characterization of a dual RAF kinase/EGFR inhibitor, BGB-283, which is currently under clinical investigations. BGB-283 is a potent and selective pan-RAF and EGFR inhibitor with IC50 ranging from 5 to 47 nM on RAF and EGFR kinases. In vitro, BGB-283 potently inhibits B-RAFV600E-activated ERK phosphorylation and cell proliferation. It demonstrates selective cytotoxicity and preferentially inhibits proliferation of cancer cells harbouring B-RAFV600E and EGFR mutation/amplification. In B-RAFV600E CRC cell lines, BGB-283 effectively inhibits the reactivation of EGFR and achieved sustained inhibition of MAPK pathway. In vivo, BGB-283 is highly efficacious in inhibiting tumor growth accompanied by partial and complete tumor regressions in both BRAFV600E mutant cell derived CRC xenograft models, including HT29, Colo205, WiDr, as well as two primary human CRC xenograft models. In particular, BGB-283 shows compelling efficacy and potent inhibition of EGFR/MAPK signaling in WiDr xenograft model where EGFR reactivation occurs upon B-RAF inhibition. These findings support BGB-283 as a potent antitumor drug candidate with clinical potential for treating CRC harboring B-RAFV600E mutation. Citation Format: Zhiyu Tang, Xi Yuan, Rong Du, Shing-Hu Cheung, Guoliang Zhang, Jing Wei, Yuan Zhao, Yingcai Feng, Yi Zhang, Yunguang Du, Xiaoxia Hu, Wenfeng Gong, Yong Liu, Yajuan Gao, Rui Hao, Jiafu Ji, Lianhai Zhang, Shuangxi Li, David Sutton, Min Wei, Changyou Zhou, Lai Wang, Lusong Luo. BGB-283, a novel RAF kinase and EGFR dual inhibitor, displays potent antitumor activity in B-RAF mutated colorectal cancers. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4692. doi:10.1158/1538-7445.AM2015-4692

K-RAS mutation represents one of the most frequent genetic alterations in cancers. Monotherapy approaches targeting RAS downstream effectors such as MEK and ERK have had limited clinical success in patients with K-RAS-mutated cancers. The reduced sensitivity of K-RAS-mutated cells to certain MEK inhibitors is associated with feedback phosphorylation of MEK and reactivation of mitogen-activated protein kinase (MAPK) signaling. Herein, we report that RAF dimer inhibitor lifirafenib (BGB-283) shows strong synergistic effect with MEK inhibitor mirdametinib (PD-0325901) in suppressing proliferation of K-RAS-mutated cancer cell lines. This synergistic effect was not observed using vemurafenib, a first-generation B-RAFV600E selective inhibitor. Mechanistic analysis revealed that RAF dimer inhibition could suppress RAF-dependent MEK reactivation and led to sustained inhibition of MAPK signaling in K-RAS-mutated cells. Furthermore, combination synergy was observed in K-RAS mutant xenograft models. Pharmacodynamic analysis supported the role of synergistic phospho-ERK blockade in enhancing the antitumor activity in the K-RAS mutant models. These findings support the rationale to combine a RAF dimer inhibitor and a MEK inhibitor to treat K-RAS-mutated cancers and led to the ongoing a Phase Ib/II clinical trial of lifirafenib and mirdametinib in patients with K-RAS mutations or other MAPK pathway aberrations (Clinical Trial ID: NCT03905148). Citation Format: Xi Yuan, Xinwen Zhang, Rong Du, Shing-Hu Cheung, Min Wei, Changyou Zhou, Lai Wang, Todd Shearer, Badreddin Edris, Mary Smith, Zhan Yao, Claire N. Thant, Neal Rosen, Lusong Luo. RAF dimer inhibitor lifirafenib enhances the antitumor activity of MEK inhibitor mirdametinib in RAS mutant tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6415.