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RB and E2F genes are frequently altered in cancer. Cyclin/Cdk (cyclin-dependent kinase) complexes regulate the activity of RB and E2F to drive cell cycle progression and ensure fidelity of DNA replication. A critical subset of their substrates (Rb, E2F, Cdc6, etc.) requires a short linear motif (RxL) that interacts with the hydrophobic patch on cyclins. Peptides that disrupt the cyclin/RxL interaction were found to be synthetically lethal in cancer cell lines displaying Rb-E2F dysregulation (Chen, 1999). Using structure-guided design, cell-permeable macrocycles were synthesized that selectively inhibit RxL-mediated binding to cyclins A and B. Preclinical studies with an orally bioavailable cyclin A/B RxL inhibitor show activity in cell lines with Rb-E2F pathway dysregulation, and inhibition of tumor growth and regression in xenograft models of small cell lung carcinoma (SCLC). RB-null mutations occur with high frequency in SCLC tumors (Febres-Aldana 2022). Previously we reported (AACR April 2022 Abs.#5379) sensitivity of a large proportion of SCLC cell lines to cyclin RxL inhibitors. Further studies have shown that such compounds have growth inhibitory activity in cell lines representing multiple types of solid tumors and hematological malignancies. Sensitivity of the cell lines to these macrocyclic inhibitors is associated with increased expression of genes involved in the E2F pathway, consistent with the proposed Rb/cyclin/E2F pathway synthetic lethality mechanism. Disruption of RxL binding to both cyclin A and B is required for sensitivity. Cyclin A/B RxL inhibitors disrupt the interactions of E2F and Cdc6 with cyclin A/Cdk complexes in co-immunoprecipitation studies and have EC50 values consistent with biochemical and cellular activity. Cyclin A/B RxL inhibition results in accumulation of cells in the G2/M phase of the cell cycle which leads to apoptosis both in vitro and in vivo. A separate study (Singh et al. AACR 2023) utilizing a CRISPR screen identified that genes of the spindle assembly checkpoint (SAC) are required for the synthetic lethality induced by the cyclin A/B RxL inhibitors. The inhibitors assessed in vitro for off-target activity showed negligible activity in a panel of 468 kinases and in safety panels that include GPCRs, ion channels, and transporters. In a 14-day tolerability study, neither neutropenia nor significant effects in other blood cell counts were noted. Cyclin A/B RxL inhibitors cause significant tumor regression via oral administration in SCLC xenograft models. Given their compelling characteristics, we are progressing the development of these orally bioavailable macrocyclic cyclin A/B RxL inhibitors to the clinic. Citation Format: Catherine E. Gleason, Pablo D. Garcia, Ranya Odeh, Frances Hamkins-Indik, Daphne He, Meisam Nosrati, Gavin Situ, Roberta Sala, Bernard Levin, Li-Fen Liu, Evelyn W. Wang, Siegfried Leung, Breena Fraga, Andrew T. Bockus, Justin Shapiro, Nathan Dupper, Chinmay Bhatt, Kai Yang, Megan DeMart, Sammy Metobo, James Aggen, Peadar Cremin, Ramesh Bambal, Constantine Kreatsoulas, David J. Earp, Rajinder Singh. Orally bioavailable macrocycles that target cyclins A and B RxL motifs cause tumor regression in xenograft models and in vitro show activity across multiple cancer types [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1560. |
