Abstract 722: Acquired resistance to vorinostat in acute myeloid leukemia

Abstract Histone deacetylase inhibitors (HDACi) have recently emerged as promising anticancer agents. Some HDACi have advanced into Phase II and III clinical trials, both as a single agent and in combination with cytotoxics for a variety of malignancies. Encouraging results have been obtained in acute myeloid leukemia (AML). However, HDACi target many pathways and the full mechanism responsible for their anti-neoplastic activity is still far from clear. An understanding of the molecular mechanisms underlying resistance to HDACi may help to elucidate their mechanism of action and may be of relevance in an attempt to design more effective combination strategies. The purpose of this study is to understand the molecular alterations associated with resistance to vorinostat (Zolinza®). A vorinostat resistant clone (U937-VR) was derived from the AML cell line U937 using a dose escalation protocol. The vorinostat-resistant cells (U937-VR) are able to grow in 2 µM vorinostat without induction of apoptosis. U937-VR cells are cross-resistant to the cytotoxic effects of other HDACi, as measured by propidium iodide staining and caspase 3/7 activation. Using western blot, we observed that vorinostat exposure increases tubulin acetylation in U937-VR cells while being incapable of inducing histone acetylation. These data demonstrate effective drug import into the cells, an event that is otherwise commonly altered in drug resistant cells. Furthermore, western blotting reveals a drastic difference in the protein level of the histone acetyltransferase CBP (CREB binding protein) between VR-U937 and parental U937 cells, whereas no alterations in the protein levels of the main HDACs targeted by vorinostat is seen. This indicates that the balance between HDAC and HAT activity might be perturbed. Finally, the resistance index (the ratio between the LD50 of resistant and sensitive cells) of different drugs was evaluated by measuring apoptosis using propidium iodide staining. U937 parental and U937-VR cells have equivalent LD50 for the DNA damaging agent cisplatin and for the microtubule stabilizing agent taxol. In contrast, we observed that U937-VR cells exhibit increased sensitivity toward the proteasome inhibitors MG132 and Bortezomib. This increased sensitivity correlates with a higher level of ubiquitinated protein accumulation in U937-VR. Cumulatively, our data suggest that the U937-VR cells are not multidrug resistant and that the apoptotic cascade remains functional. The importance of the alteration in HAT level and the acquired sensitivity to proteasome inhibitor by chronic exposure to HDACi need further investigation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 722. doi:10.1158/1538-7445.AM2011-722