Targeting collateral deletion of housekeeping genes caused by the loss of tumor suppressor genes is a potential strategy to identify context-specific, molecular targeted therapies in cancer. In mammals, phosphatidylserine (PS) synthesis depends on two redundant phosphatidylserine synthetases, PTDSS1 and PTDSS2, and PTDSS2 is located at a tumor-suppressive locus, 11p15.5. Here, we sought to determine whether PTDSS2 loss would confer vulnerability to disruption of PTDSS1 function. PTDSS2 was lost in a wide range of cancer types, and PTDSS1 depletion specifically suppressed growth in PTDSS2-deficient cancer cell lines. Potent and selective PTDSS1 inhibitors were developed whose activity phenocopied the effect of PTDSS1 depletion, and in vivo treatment of PTDSS2-deleted tumors with these inhibitors led to tumor regression. Whole-transcriptome analysis revealed that inhibition of PTDSS1 in PTDSS2-depleted cells activated the endoplasmic reticulum (ER) stress response downstream of PS depletion. ER stress-mediated cell death induced by PTDSS1 inhibitors activated tumor immunity through the secretion of HMGB1 protein followed by activation of bone marrow-derived dendritic cells. PTDSS2 loss showed intratumoral heterogeneity in clinical samples, raising concerns about resistance to PTDSS1 inhibition. However, the PTDSS1 inhibitor effectively suppressed the growth of tumor containing both PTDSS2 wild-type and knockout cells in immunocompetent mice, showing potency for overcoming tumor heterogeneity by modulating the tumor immune microenvironment. Thus, these newly developed PTDSS1 inhibitors provide a therapeutic option for treating cancer with PTDSS2 loss, harnessing the synthetic lethality of PTDSS1/2.

No abstract

Supplementary Data from Potent and Selective PTDSS1 Inhibitors Induce Collateral Lethality in Cancers with PTDSS2 Deletion

Supplementary Data from Potent and Selective PTDSS1 Inhibitors Induce Collateral Lethality in Cancers with PTDSS2 Deletion

Background: RET gene rearrangement has been detected in several cancers including 1-2% and 50% of non-small cell lung cancer (NSCLC) and papillary thyroid carcinoma, respectively, and it is known as a driver mutation. Several US Food and Drug Administration (FDA)-approved multi-tyrosine kinase inhibitors (MTKIs) have shown inhibitory effects on RET kinase activity and have been tested in several clinical studies. However, MTKIs do not appear potent enough to show clinical benefits. The FDA-approved MTKIs have been reported to exhibit dose-limiting toxicity (DLT) at doses below those suppressing RET kinase activity. Kinase insert domain receptor (KDR) inhibition especially leads to anti-angiogenesis-related DLT such as hypertension. Thus, the development of highly potent and selective second-generation RET inhibitors is desired. DS-5010 is an orally available small-molecule RET inhibitor that shows a specific and highly potent activity against RET and gatekeeper-mutated RET (RET-GKm) and slight KDR activity. In this study, we characterized the in vitro and in vivo activities of DS-5010. Results: In biochemical assays of 106 kinases, RET and platelet-derived growth factor receptor (PDGFR) alpha/beta were inhibited more than 80% by 193 nM DS-5010. The half-maximal inhibitory concentration (IC50) values of DS-5010 against RET, RET-GKm (V804L) were single digit nano-molar even under a condition of high concentration of ATP; besides it against KDR was more than 1000 nM. In a Ba/F3-RET subcutaneous tumor model, DS-5010 dosing at 10 mg/kg twice daily (bid) induced tumor regression. Moreover, DS-5010 exhibited a similar antitumor effect in a Ba/F3-RET-GKm (V804L) subcutaneous tumor model. In contrast, the FDA-approved MTKIs (cabozantinib, vandetanib, and alectinib) showed no significant antitumor effect on a Ba/F3-RET-GKm (V804L) subcutaneous tumor model. In an LC2/ad NSCLC xenograft model, which has the RET-CCDC6 fusion gene, DS-5010 dosing at 1 mg/kg thrice daily (tid) induced tumor regression. To predict acquired mutations against FDA-approved MTKIs, resistant clones were established by prolonged incubation of Ba/F3-RET cells with cabozantinib. A sequence analysis revealed that all the resistant clones possessed V804E mutation in the RET kinase domain and DS-5010 inhibited cell proliferation of Ba/F3-RET (V804E) mutation in the low nano-molar range. However, the FDA-approved MTKIs failed to show strong growth inhibitory effects (half-maximal growth inhibition [GI50]: 1584-5381 nM). Conclusion: These results indicate that DS-5010 has potent in vitro and in vivo activities against RET and RET-GKm, suggesting the potential usefulness of the compound for targeted therapy of cancers with RET gene rearrangements and mutations. Moreover, its potential effectiveness against acquired MTKI-resistant cells was also demonstrated. We are currently performing investigational new drug-enabling studies of DS-5010. Citation Format: Yasuyuki Kaneta, Takahiro Komatsu, Masashi Miyamoto, Megumi Goto, Hidenori Namiki, Yoshihiro Shibata, Hideaki Kageji, Hiroaki Inagaki, Kiyoshi Nakayama, Yuichi Tominaga, Takeshi Isoyama. Preclinical characterization and antitumor efficacy of DS-5010, a highly potent and selective RET inhibitor [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B173.