Synergistic Anticancer Effects of Cinnamon Bark Oil and Cinnamaldehyde with Endocrine Therapy in Hormone Receptor-Positive Breast Cancer: An In Vitro Study

This study evaluated the combinatorial effects of Cinnamomum zeylanicum essential oil (CEO) and cinnamaldehyde (CA) with tamoxifen (TAM) and anastrozole (ANA) in estrogen receptor-positive (ER⁺) breast cancer. CEO was obtained by hydro-distillation and chemically characterized using GC–MS, HPLC, and FTIR. Its antioxidant capacity was assessed by the DPPH assay. MCF-7 (cancer) and Vero (normal) cells were treated with CEO, CA, TAM, ANA, and their combinations, and cytotoxicity was evaluated by MTT assay, with IC₅₀, selectivity index (SI), and combination index (CI) calculated. Apoptosis, mitochondrial integrity, and cell cycle distribution were examined by flow cytometry, and ERα/ERβ expression was quantified by RT-PCR. Genotoxicity and mutagenicity were assessed by comet and Ames assays. Hydro-distillation yielded 1.03% CEO, with cinnamaldehyde (69.57%) identified as the major constituent. CEO exhibited strong antioxidant activity (IC₅₀ = 58.06 µg/mL) and selective cytotoxicity against MCF-7 cells (IC₅₀ = 47.28 µg/mL; SI = 2.56). CEO + TAM demonstrated the most potent synergistic effect (CI = 0.205), inducing 58.9% cell death, disrupting mitochondrial integrity, and causing G0/G1 arrest (71.2%). RT-PCR analysis confirmed significant downregulation of ERα and ERβ with CEO + TAM treatment. Comet and Ames assays revealed minimal DNA damage and mutagenicity, supporting a preliminary safety profile. Overall, CEO, particularly in combination with tamoxifen, demonstrated strong synergistic anticancer effects against ER⁺ breast cancer through apoptosis induction, G0/G1 cell cycle arrest, and estrogen receptor suppression, without significant genotoxicity. These results highlight CEO as a promising natural adjunct to endocrine therapy, warranting further in vivo and clinical evaluation.