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Our previous study reported that stemofoline (STF) exhibited a synergistic effect with chemotherapeutic drugs in human multidrug-resistant (MDR) leukemic cells (K526/Adr) by inhibiting the function of P-glycoprotein, which is a membrane transporter that is overexpressed in several types of MDR cancers. This study further investigated the effects of a combination treatment of STF and doxorubicin (DOX) in vitro and in vivo. The combination treatment of 50 mg/kg of STF strongly enhanced the anti-tumor activity of DOX in SCID-beige mice bearing K562/Adr xenografts without additional toxicity when compared to the single treatment groups. Additionally, an examination of the proliferation markers (Ki67) and the apoptotic marker (TUNEL) in tumor tissues in each group revealed that the combination therapy significantly reduced Ki67 positive cells and increased apoptotic cells. From the in vitro experiments we also found that this combination treatment dramatically induced G1 and G2M arrest in K562/Adr when compared to a single treatment of DOX. STF treatment alone did not show any cytotoxic effect to the cells. These results suggest that the accumulation of DOX enhanced by STF was sufficient to induce cell cycle arrest in K562/Adr. These findings support our previous in vitro data and indicate the possibility of developing STF as an adjuvant therapy in cancer treatments.

This report is an overview of our research on phytochemical, synthetic and biological studies of the Stemona and Stichoneuron species of plants.

Background: Multidrug resistance (MDR) is a major reason for the failure of chemotherapy in the treatment of cancer patients. P-gp over-expression in MDR cancer cells is a multifactorial phenomenon with biochemical resistance mechanisms. Stemofoline (STF), isolated from Stemona bukillii, has been reported to be an MDR reversing compound. Purpose: This study investigated whether other Stemona alkaloids that had been purified from Stemonaceae plants exerted MDR modulation activity. Methods: MTT assay was performed to determine the MDR reversing property of the alkaloids. Modulation of P-gp function by these compounds was investigated using cell cycle analysis and P-gp fluorescent substrate accumulation assays. P-gp expression was determined by Western blot analysis. We preliminarily examined the safety of these compounds in normal human fibroblasts and human peripheral blood mononuclear cells (PBMCs) using the MTT assay, and in red blood cells (human and rat) through in vitro hemolysis assays. Results: Three of the eight alkaloids tested, isostemofoline (ISTF), 11Z -didehydrostemofoline (11Z-DSTF) and 11E-didehydrostemofoline (11E-DSTF), enhanced the chemotherapeutic sensitivity of MDR leukemic K562/Adr cells, which overexpressed P-gp. The P-gp functional studies showed that these three alkaloids increased the accumulation of P-gp substrates, calcein-AM (C-AM) and rhodamine123 (Rho 123) in K562/Adr cells, while this effect was not seen in drug sensitive parental K562 cells. Whereas, the alkaloids did not alter P-gp expression as was determined by Western blotting analysis. Conclusion: The alkaloids reversed MDR via the inhibition of P-gp function. For pharmaceutical safety testing, the alkaloids were found to be not toxic to normal human fibroblasts and PBMCs. Moreover, the effective compounds did not induce hemolysis in either human or rat erythrocytes. These compounds may be introduced as potential candidate molecules for treating cancers exhibiting P-gp-mediated MD

Resistance to chemotherapy in cancer patients has been correlated to the overexpression of the ATP-binding cassette (ABC) drug transporters including P-glycoprotein (P-gp) that actively efflux chemotherapeutic drugs from cancer cells. We examined the mutidrug resistance reversing property of stemofoline derivatives in drug-resistance human cervical carcinoma (KB-V1) and human leukemic (K562/Adr) cell lines that overexpress P-gp. Didehydrostemofoline and eleven of its derivatives were synthesized and the cytotoxicity and their effect on doxorubicin, vinblastine and paclitaxel sensitivity in drug resistant (KB-V1 and K562/Adr) and drug sensitive (KB-3-1 and K562) cell lines by a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay were determined. We found that three out of the twelve stemofoline derivatives including OH-A1, NH-B6 and NH-D6 showed commitment efficiency to increase sensitivity to doxorubicin, vinblastine and paclitaxel in KB-V1 cells and increase sensitivity to doxorubicin, and paclitaxel in K562/Adr cells whereas the effects have not been seen in their parental sensitive cancer cell lines (KB-3-1 and K562). These results indicate that stemofoline derivatives reversed P-gp-mediated multidrug resistance in vitro, and thus could be developed as effective chemosensitizers to treat multidrug-resistant cancers. The molecular mechanism of modulation of P-gp would be further determined.