Document Type


Publication Date

Fall 2010


Terri Hunter

Advisor Email


Small molecule BH3 mimetic obatoclax (GX15-070) enhances the chemotherapeutic action of paclitaxel by increasing in vitro occurrence of Type I cell death (apoptosis) in selected small cell lung cancer (SCLC) lines. SCLC is historically resistant to paclitaxel treatment, yielding only single-digit response rates in second line chemotherapy. This resistance is attributed to a paclitaxel-induced upregulation of mitochondrial pro-survival protein bcl-xL. To increase SCLC response rates to taxol treatment, it is important to find a therapeutic modality that disables the bcl-xL line of defense against apoptosis. It was hypothesized that a combination of paclitaxel and obatoclax would synergize to enhance in vitro SCLC killing through a consistent mechanism of action. Obatoclax inhibits bcl-xL, triggering an apoptosome-independent apoptotic cascade via pro-apoptotic Bak to initiate programmed cell death via unsequestration of Bak/Bax. This unsequestration is confirmed by immunoprecipitation. High throughput data for a CT Blue viability assay suggest moderate synergy in an obatoclax-paclitaxel combination against SCLC line 86M1 at optimal concentration ratios. Annexin V staining and flow cytometry further indicate synergistic SCLC apoptosis. Additional Western blot analyses confirm synergy between the two drugs and clarify signaling events upstream or downstream of Bak/Bax unsequestration in selected SCLC lines. Subcellular fractionation reveals critical signaling translocation events as well. By combining the two drugs, it is thereby possible to maximize SCLC apoptosis while minimizing single agent toxicity. There is also clear evidence of involvement of the mTOR signaling pathway and the possibility of autophagic induction. Elucidation of these pathways can offer further possibilities of enhancing this bivalent treatment. In treating SCLC, the obatoclax-paclitaxel combination is thus a promising therapeutic modality that has clinical potential.