Fast-growing transgenic root cultures of Leonurus sibiricus transformed with the AtPAP1 transcriptional factor may be used as a base for potential large-scale phenolic acid production. The present study investigates the effect of different volumes of medium (300 mL, 1 L, 3 L, 5L flasks) and a 5 L bioreactor on biomass increase and phenolic acid production by transgenic roots of L. sibiricus. Of these cultures, those from the 5 L bioreactor demonstrated the greatest increase in dry weight (20.83g/L) and highest yields of phenolic acids (chlorogenic acid 448 mg/L and caffeic acid 302 mg/L). Additionally, they also displayed a cytotoxic effect on melanoma cells across the range of tested concentrations, as well as antioxidant activity on human blood plasma. This approach may serve as an alternative to conventional field crops for enabling large-scale production of the active constituents of AtPAP1 root extract, which would be of great value for pharmaceutical production.