Objective: Hippo signaling pathway is known to regulate organ development. In Hippo signaling pathway, YAP or TAZ works as a transcriptional co-activator and forms a transcriptional complex with TEAD. In several cancers, upstream factors in Hippo pathway are inactivated by genetic alterations. When the upstream factors are inactivated, TEAD is activated and forms a complex with YAP/TAZ resulting in enhancement of cell proliferation, drug resistance and so on. In the activation process, S-palmitoylation of TEAD is necessary for binding to YAP/TAZ. Malignant pleural mesothelioma (MPM) is one of cancer types which have genetic alterations in Hippo pathway genes. Although YAP/TAZ-TEAD inhibitor should be an ideal drug for MPM therapy, there are only a few reports about YAP/TAZ-TEAD inhibitor and the efficacy and selectivity are not sufficient. In this study, we succeeded to synthesize a small molecule TEAD inhibitor, K-975, and evaluated its mechanism of action and anti-tumor effect against MPM. Materials/methods: Inhibitory activity of K-975 on YAP/TAZ-TEAD protein-protein interaction (PPI) was evaluated in surface plasmon resonance (SPR) and co-immunoprecipitation assay. The effect of K-975 on palmitoylation status of TEAD was also evaluated. The three-dimensional structure of YAP-binding domain of TEAD1 in complex with K-975 was determined by X-ray crystallography. Anti-tumor effect of K-975 was evaluated by using MPM cell lines. Furthermore, using a derivative of K-975, 2 week-toxicity studies in rats and monkeys were performed. Results: K-975 inhibited YAP-TEAD and TAZ-TEAD PPI in NCI-H226 cells, a human MPM cell line. Also, K-975 inhibited palmitoylation of TEAD. The crystal structure revealed that K-975 directly bound to cysteine residue in YAP-binding domain of TEAD1. This cysteine residue is highly conserved in TEAD family and known as a site of S-palmitoylation. K-975 inhibited the cell proliferation of NCI-H226 with GI50 of about 20 nmol/L. K-975 also induced a change of gene expressions similar to that induced by YAP knockdown. In vivo experiments, K-975 strongly suppressed the tumor growth in several s.c. xenograft models and showed a significant survival benefit in an orthotopic xenograft model. However, 2 week-toxicity studies of a K-975 derivative with optimized bioavailability showed some pathological findings which suggested the renal toxicity. Conclusion: We synthesized a first-in-class drug which directly binds to TEAD protein and inhibits YAP/TAZ-TEAD PPI. K-975 showed a strong anti-tumor effect in pre-clinical MPM models. Although the renal toxicity might cause some difficulty in clinical use, we believe that a K-975 derivative has a possibility to become an effective drug candidate for MPM therapy. Citation Format: Ayumi Kaneda, Toshihiro Seike, Takeshi Uemori, Kensuke Myojo, Kensuke Aida, Tomohiro Danjo, Takahiro Nakajima, Daisuke Yamaguchi, Tomoko Hamada, Yoshiro Tsuji, Kaori Hamaguchi, Mai Yasunaga, Nobumasa Otsubo, Hideyuki Onodera, Yoichi Nishiya, Michihiko Suzuki, Junichi Saito, Toshihiko Ishii, Ryuichiro Nakai. Discovery of a first-in-class TEAD inhibitor which directly inhibits YAP/TAZ-TEAD protein-protein interaction and shows a potent anti-tumor effect in malignant pleural mesothelioma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3086.

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Objective EWS‐FLI1 is the most common oncogenic fusion protein in Ewing's sarcoma family tumors (ESFTs). DAX1, an orphan member of the nuclear receptor superfamily, is up‐regulated by EWS‐FLI1 and plays a key role in the transformed phenotype of ESFTs. Methods To discover a functional inhibitor of DAX1 and EWS‐FLI1, we screened small‐molecular inhibitors using a DAX1 reporter assay system. Results K‐234 and its derivatives, which were dihydroorotate dehydrogenase (DHODH) inhibitors, showed inhibitory effects in the reporter assay. K‐234 inhibited the growth of Ewing's sarcoma with various fusion types, and K‐234 derivatives altered the expression of EWS‐FLI1‐regulated genes. The DAX1 expression had no effect on the growth inhibitory effect of the K‐234 derivatives, while DHODH overexpression or uridine treatment attenuated their inhibitory effects, suggesting that inhibition by K‐234 derivatives occurs through DHODH inhibition. An in vivo study showed that a K‐234 derivative clearly inhibited tumor growth in an Ewing's sarcoma xenograft mouse model. Conclusion Taken together, the present results suggest that DHODH inhibitors can inhibit the function of DAX1/EWS‐FLI1 in ESFTs and might be a therapeutic agent with potent anti‐tumor activity for Ewing's sarcoma patients.