Abstract 1622: Ecubectedin is a novel transcriptional inhibitor that displays potent antitumor effects in vitro and in vivo

Abstract Background - Ecubectedin (PM14) is a novel transcriptional inhibitor related to ecteinascidins family. In this work, we present its antiproliferative activity and mechanism of action. The in vitro antitumor activity obtained in cellular models has been also demonstrated in in vivo models. Methods and results - The antineoplastic activity of ecubectedin was evaluated in a panel of cell lines representative of different solid human cancers, by MTT assays. Ecubectedin exhibits potent antitumor in vitro activity with mean GI50 values in the very low nanomolar range. Electrophoretic Mobility Shift assays demonstrate that ecubectedin binds to DNA and experiments of 3H-uridine incorporation show that the drug efficiently inhibits mRNA synthesis (85% reduction after 90 min). Moreover, ecubectedin specifically inhibits transactivated transcription, as evaluated in a reporter system of the transcriptional activity mediated by NF-κB after activation with TNFα. In the presence of ecubectedin, transactivation was almost completely abolished. The mechanism of transcriptional inhibition involves the stalling and irreversible proteasomal degradation of elongating RNA Pol II. As a final event, ecubectedin induces double strand breaks in the DNA, inducing cell cycle arrest in the S-phase that can be followed through flow cytometry and triggers apoptotic death of the tumor cell as seen through Annexin V flow cytometry assays. The in vivo antitumor activity of ecubectedin was characterized in xenografted tumors of human origin in athymic nu/nu mice, namely breast (MDA-MB-231), soft tissue sarcoma (HT-1080), SCLC (H526 and H82) and prostate (22Rv1). Ecubectedin was intravenously administered (at 1.25 mg/kg) on days 0, 7 and 14. Compared to placebo, ecubectedin treatment of mice bearing xenografts resulted in a statistically significant reduction of volume in MDA-MB-231 (p≤0.0005 on days 7-35), HT-1080 (p≤0.0004 on days 2-14), H82 (p≤0.0001 on days 5-14), H526 (p≤0.0001 from day 4 to the end of the experimental period), and 22Rv1 (p≤0.0001 on days 3-14) tumors. In mice bearing MDA-MB-231, H526 and 22Rv1 tumors, complete tumor remissions were seen in 1/10 (lasting 103 days), 9/10 (lasting 220 days) and 7/10 (lasting 13 days) ecubectedin-treated animals, respectively. Conclusions - Ecubectedin is a novel transcriptional inhibitor that displays potent antiproliferative activity in different human solid cancer models in vitro and in vivo. The drug is currently in clinical development in Phase II trials. Citation Format: Gema Santamaría Nuñez, Marta Martínez Diez, María José Guillén, Eva Maria Garrido-Martin, Pablo Avilés, Carmen Cuevas. Ecubectedin is a novel transcriptional inhibitor that displays potent antitumor effects in vitro and in vivo [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 1622.