Synergistic Antitumor Effects of Rosmarinic Acid and Cisplatin in Retinoblastoma: Evidence from 2D and 3D Tumor Models

<b>Background/Objectives</b>: Retinoblastoma (RB) is the most common primary intraocular malignancy in children, with treatment limited by chemoresistance and therapy-related toxicity. Enhancing the efficacy of conventional chemotherapeutics while reducing dose-related adverse effects is crucial. This study investigates the chemosensitizing potential of rosmarinic acid (RA), a natural polyphenolic compound, in combination with cisplatin (Cis) in RB models. <b>Methods</b>: The antiproliferative and synergistic effects of RA and Cis were evaluated in Y79 and WERI-Rb1 RB cell lines using MTT assays and Combination Index (CI) analysis. Apoptosis and oxidative stress were assessed by Annexin V-FITC/PI flow cytometry and intracellular reactive oxygen species (ROS) measurements, respectively. Three-dimensional (3D) tumor spheroids were generated from Y79 cells for in vitro validation using spheroid size analysis, ATP-based viability assays, and live/dead fluorescence staining. The ROS dependency of cytotoxicity was further examined using N-acetylcysteine (NAC) pretreatment. Cytokine secretion was analyzed by ELISA, and apoptosis-related gene expression was assessed by qRT-PCR. <b>Results</b>: RA and Cis reduced cell viability in a dose- and time-dependent manner, while their combination induced significantly enhanced cytotoxicity, confirmed by CI values < 1. Combined treatment increased apoptotic populations, elevated intracellular ROS, and upregulated Caspase-3 and Caspase-9. These effects were maintained in 3D spheroids, with reduced spheroid size and impaired integrity. NAC pretreatment attenuated ROS generation and partially rescued cell viability, indicating a ROS-dependent, but not exclusive, contribution to cytotoxicity. <b>Conclusions</b>: RA synergistically enhances cisplatin-induced anticancer effects in RB through oxidative stress, engagement of intrinsic (mitochondria-associated) apoptotic signaling, and reduction of tumor cell-derived inflammatory and angiogenic mediators. These findings highlight the potential of RA and Cis combination as a chemosensitizing strategy for RB therapy, warranting further in vivo evaluation.