Screening of Onosma Species for Cytotoxic Activity ^†

Onosma L. genus (Boraginaceae) comprises of 230 species that is represented by 102 species in the flora of Turkey with almost 50% endemism ratio (48 species), implying Anatolia as the gene center of Onosma species. In this study, we evaluated the petroleum ether, dichlorometane and methanol root extracts of 5 Onosma species (O. mite, O. mollis, O. taurica var. taurica, O. frutescens, and an unidentified Onosma member) collected from Southwestern Turkey for their cytotoxic activities. The extracts were screened versus seven human cancer cell lines, namely DU145, Capan-1, HCC-1937, MCF-7, HeLa, HEPG2, A-459 and a normal cell line, MRC-5, using MTT assay. As a result, the petroleum ether and dichlorometane extracts of Onosma species, rich in naphthoquinones, were more active than the methanol extracts in a dose range of 8 to 32 μg/mL. The petroleum ether and dichlorometane extracts of O. taurica and the unidentified species along with the petroleum ether extract of O. mollis showed strong cytotoxicity versus all the cell lines with IC₅₀ values lower than 8 μg/mL (the lowest test dose). Thus, O. taurica var. taurica and O. mollis were taken into cytotoxic activity-guided fractionation studies to isolate the bioactive compounds.