Abstract 308: Melatonin activity in uterine leiomyoma and leiomyosarcoma cell lines: A preliminary study of its inhibitory potential in mesenchymal tumors.

Abstract Background: Uterine smooth muscle tumors (USMT) encompass benign leiomyomas (LM) and highly aggressive leiomyosarcomas (LMS). Although melatonin has been reported to exert antiproliferative and oncostatic effects in several tumor models, its impact on the growth dynamics of LM and LMS cells remains poorly understood. Here, we evaluated the dose- and time-dependent effects of melatonin on the proliferation and migration of LM and LMS cell lines to investigate the responsiveness of both benign and malignant USMT cells. Methods: LM and LMS cells were plated at a density of 1×104 cells per well in 96-well plates. Cell proliferation was assessed at 0, 24, 48, 72, and 96 hours following treatment with melatonin at 3 mM, 5 mM, or 10 mM. Each condition included six technical replicates and two experimental replicates. Control groups consisted of untreated cells (medium only) and vehicle controls (medium plus ethanol). Fluorescence-based viability/proliferation readings (Presto Blue reagent®) were used as a measure of cell growth over time. Scratch assays were used to assess the effects on the cell migration using the same time and doses of melatonin, followed by Image J® software analyses of the acquired pictures. Results: Melatonin significantly inhibited proliferation in both LM and LMS cells in a dose-dependent manner. In LM cells, melatonin induced a marked and sustained reduction in proliferation at all-time points, with the strongest suppression observed at 10 mM, which maintained minimal proliferative activity throughout the experiment. LMS cells also exhibited reduced proliferation in response to melatonin, although the inhibitory effect was less pronounced than in LM cells, particularly at earlier time points. At 72 hours, control LMS groups displayed a sharp proliferative peak, whereas melatonin-treated cells - especially at 10 mM - showed substantial attenuation of this growth increase. Across both tumor types, 10 mM consistently showed the most robust antiproliferative effect. Very similar results were obtained for the migration analyses, both to LM and LMS cells. Conclusion: Melatonin exerts clear inhibitory activity in USMT cells, with a stronger suppressive effect in LM cells compared with LMS cells. These findings suggest intrinsic differences in melatonin responsiveness between LM and LMS, potentially reflective of altered growth-regulatory pathways associated with malignant transformation. Melatonin may represent a biologically relevant modulator of uterine smooth muscle cells tumorigenesis, warranting further mechanistic studies and exploration as an adjunct therapeutic strategy. Citation Format: Tatielly T. Miranda, Edmund C. Baracat, Roseli S. Soares, Katia C. Carvalho. Melatonin activity in uterine leiomyoma and leiomyosarcoma cell lines: A preliminary study of its inhibitory potential in mesenchymal tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 308.