Girinimbine produced in <i>Bergera koenigii</i> callus cultures with antiproliferative activity against glioblastoma stem cells

Callus cultures derived from medicinal plants offer useful platforms for producing valuable secondary metabolites. We established a callus culture system from Bergera koenigii leaves and examined the effects of phytohormone composition on callus growth and carbazole alkaloid accumulation. To the best of our knowledge, this is the first report of girinimbine (1), a carbazole alkaloid, isolated from leaf-derived callus, and we further synthesized 1. Compound 1 displayed potent antiproliferative activity against cancer stem cells (IC₅₀ = 4.1 µm) and non-cancer stem cells (IC₅₀ = 14.0 µm) derived from human glioblastoma U-251 MG cells. High-performance liquid chromatography revealed that alkaloid content in the callus varied with culture conditions. Callus cultured with 0.20 mg/L IAA and 0.40 mg/L BA tended to produce more 1 than the roots. These findings demonstrate the potential of optimized callus cultures as useful sources of bioactive carbazole alkaloids, especially 1, from B. koenigii.