Anti-proliferative effects of simocyclinone D8 (SD8), a novel catalytic inhibitor of topoisomerase II

Introduction: Simocyclinone D-8 (SD8), a semi-synthetic compound derived from yeast, has been shown to decrease the proliferation of MCF-7 breast cancer cells. It has been shown to be a potent bacterial DNA gyrase inhibitor, a homologue of human topoisomerase II (hTopoII). We tested SD8 activity alone and in combination with cisplatin against malignant mesothelioma (MM) and non-small cell cancer (NSCLC) cell lines. Methods: Inhibition of hTopoII supercoiling function by SD8 and a known hTopoII poison, etoposide, were done by in vitro assay using purified hTopoII and kinetoplast DNA as the substrate. The DNA products were analyzed by agarose gel electrophoresis after treatment with increasing concentrations of each drug. Mesothelioma cell lines (H2373, H2461 and H2596) and NSCLC cell lines (H2030, H460, and H2009) grown in RPMI with 10% calf serum were used. Non-malignant mesothelial cells, LP9, were grown in 1:1 ratio of MCDB:199E medium supplemented with 15% calf serum, 0.4 μg/mL hydrocortisone, and 15 ng/mL epidermal grown factor. Cell proliferation assays were performed in 96-well plates using the CCK-8 kit (Dojindo inc.). Cells were treated for 72 h with various SD8 concentrations and controls containing equal volume of the vehicle, DMSO. Treated cells were assayed for the induction of apoptosis with poly ADP-ribose polymerase-1 (PARP) cleavage assay. Results: Biochemical assays revealed that the IC₅₀ for hTopoII inhibition was 100 μM for SD8 and 400 μM for etoposide. SD8 inhibited hTopoII function without inducing DNA cleavage events. SD8 inhibited the growth of NSCLC and Mesothelioma cells with IC₅₀ ranging from 75-125 μM. Furthermore, SD8 was not toxic to non-transformed primary mesothelial cell line, LP9 at the IC₅₀ doses. SD8 induced apoptosis in all cell lines tested. Conclusions: SD8 inhibits hTopoII in vitro without inducing DNA strands breaks and has significant activity against NSCLC and MM cell lines. While doses required for SD8 anticancer activity are unlikely to be achieved in vivo, chemical modifications to SD8 to increase its potency could lead to improved therapies for these diseases.