Kenyan Orthosiphon schimperi Benth. Essential Oil: Chemical Composition and Cytotoxic Activity on HeLa Cells

The genus Orthosiphon Benth., is a relatively small genus of the Lamiaceae family that includes forty-four accepted species distributed mainly in tropical and sub-tropical areas of Asia, Southern Africa, and Madagascar. The species usually occurs in woodland, grassland, or forest margins. Due to their curative properties, species of this genus have been largely utilized in the popular medicine of several countries. Essential oil from fresh post-flowering aerial parts of Orthosiphon schimperi Benth. (OS), a taxon not previously studied, was collected in Kenyan territory, and it was obtained by hydrodistillation (yield 0.15%); its chemical profile was investigated by GC-MS analysis, using a DB-5ms low-polarity GC column. Oxygenated monoterpenes dominated the OS composition, and eugenol methyl ether (79.5%) was, by far, the main constituent of the sample. Oxygenated sesquiterpenes were the second most abundant class (8.8%), mainly constituted by caryophyllene oxide (8.2%). The biological activity of OS was assessed by using cytotoxicity assays on HeLa cells (human cervical cancer cell line) and HaCaT cells (non-tumorigenic human keratinocytes). OS exhibited selective cytotoxicity toward HeLa cells (IC₅₀ = 29.44 µg/mL after 24 h of treatment), while having minimal or no impact on HaCaT cell viability. Western blot analysis indicates that, indeed, the EO induces selective apoptosis in HeLa cells, thereby suggesting a potential anticancer activity exhibited by O. schimperi EO.