Antioxidant capacity, cytotoxicity and post-treatment effect of Myrtus communis L. leaf extract in a 6-hydroxydopamine-induced SH-SY5Y cell model

Purpose: Oxidative stress drives the loss of dopaminergic neurons in Parkinson's disease, which is usually diagnosed after many of these neurons are lost. Agents acting after injury are therefore of interest. This study measured the phenolic content and antioxidant capacity of an ethanolic Myrtus communis L. leaf extract, defined its non-cytotoxic concentration range, and tested whether the extract, applied after 6-hydroxydopamine (6-OHDA) injury, improves the viability of human SH-SY5Y neuroblastoma cells.Method: Leaves of M. communis were collected in Hatay Province (Türkiye), dried, powdered and macerated in ethanol for 3 days. Total phenolic (Folin–Ciocalteu) and total flavonoid (aluminum chloride) contents, DPPH and ABTS radical scavenging, and ferric reducing antioxidant power (FRAP) were measured. Cytotoxicity was assessed in L929 fibroblasts and SH-SY5Y cells by the MTT assay after 24 and 48 h. SH-SY5Y cells were exposed to 200 µM 6-OHDA for 24 h and then treated with the extract (10–1000 µg/mL) for 24 h. Groups were compared by ANOVA and Dunnett's test.Findings: The extract (yield 16.87%) contained 256.00 ± 6.16 mg GAE/g total phenolics and 48.00 ± 11.41 mg QE/g total flavonoids, and its FRAP value was 285.66 ± 21.92 mg GAE/g extract. DPPH and ABTS IC50 values were 75.97 and 106.7 µg/mL (gallic acid: 45.25 and 64.41 µg/mL). By the ISO 10993-5 criterion (≥70% viability), L929 cells tolerated up to 31.25 µg/mL for 48 h. SH-SY5Y cells were about five times more sensitive than L929 cells (48-h IC50 31.16 vs 158.4 µg/mL). 6-OHDA reduced SH-SY5Y viability slightly at 24 h (87.5% at 200 µM) and by about half at 48 h (IC50 227.7 µM). Post-treatment increased the MTT signal only at 75 µg/mL (116.0% of 6-OHDA-only cells, p = 0.016), whereas concentrations ≥200 µg/mL lowered it to 54.0–73.1%.Conclusion: The extract showed moderate radical-scavenging activity but was tolerated only in a narrow range, and SH-SY5Y cells were more sensitive to it than fibroblasts. Evidence for a post-injury benefit was limited to a higher MTT signal at a single concentration. This exploratory finding needs confirmation with independent viability assays, appropriate controls, stronger injury and mechanistic endpoints.