Apoptotic pathways induced by ixabepilone in paclitaxel-refractory ovarian carcinoma cells

Abstract

Proc Amer Assoc Cancer Res, Volume 46, 2005 5318 Objectives: Ixabepilone, a novel epothilone B analog that is similar to paclitaxel in mechanism of action, is non-cross resistant with taxanes and superior in anti-tumor efficacy. In this study we compared pathways regulating apoptosis induced by ixabepilone and paclitaxel in two epithelial ovarian cancer cell lines (OC-2 and PAT-7), established from tumor specimens obtained from patients clinically refractory to platinum/paclitaxel. Methods: OC-2 and PAT-7 cells were treated for 1 hr with equimolar doses of ixabepilone or paclitaxel, washed and re-incubated in drug-free medium prior to analysis for apoptosis, caspase activity and survival. Apoptosis was evaluated by fluorescent microscopy following staining with Hoechst 33342 and propidium iodide. Caspase-2 and caspase 9 activity was determined by fluorometry using specific peptide substrates, VDVAD-AFC and LEHD-AFC, respectively. Survival was determined one week after treatment (∼ 2-3 cell population doublings in untreated control). Results: Apoptosis induced by ixabepilone at 24 h following treatment was dose dependent with optimal effects at 0.5 μ M for both OC-2 (39%) and PAT-7 (68%) cells. In PAT-7 cells activation of both caspase 2 and 9 was observed, albeit to different levels, following treatment with ixabepilone (7 to 10-fold) and paclitaxel (2 to 3-fold). In contrast, activation of caspase 2 and 9 by the two drugs differed in OC-2 cells; ixabepilone stimulated only caspase 2 activity (3-fold), whereas paclitaxel enhanced only caspase 9 activity (see [table][1]). Compared to paclitaxel, enhanced apoptosis induced by ixabepilone in both OC-2 and PAT-7 cells strongly correlated with decreased cell survival. Conclusions: These results suggest that unlike taxanes that mediate apoptosis primarily via caspase 9, ixabepilone induced apoptosis is mediated essentially by enhancement of caspase-2 activity. Thus activation of caspase 2 likely represents a novel pathway for ixabepilone induced apoptosis in these ovarian cancer cell lines from patients clinically refractory to paclitaxel. The correlation between apoptosis and survival suggest that apoptosis is a key mechanism of action of ixabepilone. ![Table][2] [1]: #T1 [2]: pending:yes