Abstract 822: Drug resistance towards vincristine in acute lymphoblastic leukemia is mediated by the PI3K-Akt pathway
Authors: Koh Grace Shimin , Jiang Nan , Kham Shirley Kow Yin , Chew Fook Tim , Yeoh Allen Eng Juh 
Published: August 2012

Abstract
Contemporary acute lymphoblastic leukemia (ALL) therapy involves the use of powerful anti-mitotic agents such as vincristine (VCR). VCR targets the microtubule cytoskeleton and disrupts important cellular processes, leading to cell death. Nonetheless, around 20% of patients relapse and become resistant to most drugs. At present, mechanisms of resistance toward VCR remain elusive despite its extensive clinical use, warranting a more in-depth study with respect to ALL. Five VCR-resistant (VCR-R) subclones were established by gradually exposing five ALL VCR-sensitive (VCR-S) cell lines (Jurkat, REH, SEM, RS4;11, 697) to incremental doses of VCR. Comparing resistant to sensitive cells, the IC50 at 48h increased 3-6 logs. Both VCR-S and VCR-R cell lines were subjected to cell cycle analysis by flow cytometry using propidium iodide staining. After a 48h exposure to VCR, cell death for VCR-S cells was the result of a typical G2/M phase arrest. In contrast, a G2/M phase arrest did not precede the VCR-induced cell death of VCR-R cells. VCR-R subclones were also observed to be more resistant to vinblastine, an alternative tubulin binding agent. In addition, there was increased resistance towards other chemotherapeutic drugs such as prednisolone, dexamethasone, daunorubicin and doxorubicin, indicating that resistance to VCR may contribute to cross-resistance after a prolonged incubation. The PI3K/Akt/mTOR signaling pathway is known to play important roles in cancer cell proliferation and survival. The combination of inhibitors with other chemotherapeutic agents also has the potential to enhance leukemic cell death. To further investigate if the PI3K/Akt/mTOR signaling pathway is involved in VCR resistance, we screened a panel of twelve kinase inhibitors (Merck) which selectively targets this pathway. Interestingly, PDK1/Akt/Flt Dual Pathway Inhibitor KP372-1, was highly potent toward all cell lines with IC50 less than 100nM. Our results also showed that the administration of PI3Kα Inhibitor VIII, Akt Inhibitor IV and PI-103, as well as pan-selective PI3K inhibitors Wortmannin and LY294002 in combination with VCR was able to promote increased levels of cell death. Cell cycle analysis also revealed that the combination treatment elicits different methods of cell death between VCR-S and VCR-R cells. This is especially evident upon the co-treatment with pan-selective inhibitors. Optimising the sensitivity of ALL cells and/or reversing resistance to VCR promise to maximise the therapeutic value of this important chemotherapy. This study provides evidence that VCR resistance potentially contributes to cross-resistance. Our data also shows that the PI3K-Akt pathway may be involved in VCR resistance in ALL, providing a valid rationale for further studies and the future application of inhibitors in clinical settings.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 822. doi:1538-7445.AM2012-822