One of the greatest challenges in the treatment of pancreatic cancer remains its inherent lack of beneficial response to cytotoxic chemotherapy. Chinese herbal extracts have been widely used for the treatment of various cancers, but objective information on their efficacy in pancreatic cancer is lacking. Eight human pancreatic cancer cell lines (MIA, Panc-1, BxPC, ASPC, HS-766T, CaPan-2, CFPAC, and HTB-147) were studied for in vitro susceptibility to ethanol extracts of SPES and PC-SPES, two quality-controlled, dried, encapsulated supplements of 15 and eight Chinese herbs, respectively. Resulting toxicities, alone and in combination with doxorubicin or gemcitabine, were analyzed by [(3)H]thymidine incorporation or sulforhodamine B staining, colony formation, and TUNEL flow cytometry assays. Combination toxicity mechanisms were calculated by the combination index method of Chou and Talalay. In all cell lines, there was dose-dependent inhibition of proliferation. By [(3)H]thymidine incorporation assay, 50% growth inhibition after 48 h continuous exposure (IC(50)) occurred at concentrations of 0.2-0.8 microl/ml SPES and 0.4-1.3 microl/ml PC-SPES. Growth inhibition was accompanied by a significant enhancement of the TUNEL-positive apoptotic fraction of all cell lines after treatment with either extract. After treatment with PC-SPES, the cell lines consistently displayed a G2 cell cycle block; SPES induced an increase in S phase, with a smaller impact on G2. When added at a concentration of 0.2 microl/ml (approximately IC(20)), both extracts enhanced Panc-1 cell killing mediated by doxorubicin, with an average decrease in the corresponding IC(50) of 33% (range 11-62%). Combination effects with either extract appeared to be antagonistic in the case of gemcitabine and additive to mildly synergistic in the case of doxorubicin. Both SPES and PC-SPES exhibited significant toxicity in pancreatic cancer cells, mediated via induction of apoptosis. Both mixtures should be evaluated for their in vivo and clinical therapeutic utility as monotherapy agents against pancreatic cancer. SPES could possibly be combined with cell cycle-independent cytotoxic drugs. Due to a consistent G2 blocking pattern, PC-SPES may prove useful as a radiation sensitizer.