Abstract 1765: Creating novel translation inhibitors to target pro-survival proteins in chronic lymphocytic leukemia
Authors: Chen Rong , Zhu Mingzhao , Chen Yuling , Skillern Wesley , Wierda William G. , Hull Ken , Romo Daniel , Plunkett William 
Published: August 2015

Abstract
Many aspects of malignant phenotypes are associated with dysregulation of translation control. Due to the fact that the great majority of oncoproteins turnover rapidly, and have an acute need for mRNA translation compared to housekeeping proteins, protein synthesis is a promising avenue to explore for targeted cancer therapy. We propose to target the biological rewiring that is characteristic of B cell malignancies, namely the overexpression of short-lived pro-survival proteins that keep these cells from undergoing apoptosis. We hypothesize that transient inhibition of translation will cause a lethal decrease in those labile pro-survival proteins in CLL cells initiating apoptosis. Further, this will provide selectivity toward the malignancy over normal cells and may prove to be a more general strategy for overcoming drug resistance. The marine natural product pateamine A (PatA) inhibits cap-dependent translation initiation by binding to the eukaryotic initiation factor 4A (eIF4A) and stalls the initiation complex on mRNA. We have synthesized a family of PatA-based small molecules to test their potency for inducing apoptosis in primary CLL cells. Our lead compound, DMDAPatA, is more stable and easier to synthesize than the parent natural product, and demonstrated anti-leukemia activity in primary CLL cells with an IC50 of 2.5 μM after 24 h of incubation. This derivative was equally active in samples from patients with poor prognostic characteristics such as advanced Rai stage, the unmutated status of the IgVH gene, or the over-expression of the ZAP70 protein, suggesting its ability to bypass resistance to conventional CLL therapy. Tritium labeled leucine incorporation measurements confirmed inhibition of translation by DMDAPatA that occurred as soon as 4 h, preceding the full activation of apoptosis. Reduction of the short-lived anti-apoptotic proteins Mcl-1 and XIAP were observed by 4 h, and persisted over 24 h, while the levels of the more stable proteins Bcl-2 and Bcl-XL were not affected. This is consistent with the selective activity of translation inhibitors toward proteins with rapid turnover rates. Diminished levels of Mcl-1 and XIAP reduced the anti-apoptotic capacity such that the pro-apoptotic proteins became more prevalent to initiate cell death. Since DMDAPatA depleted Mcl-1 without affecting Bcl-2, and ABT-199 inhibits Bcl-2 activity but spares Mcl-1, the combination of DMDAPatA and ABT-199 targeted both arms of apoptosis control and killed CLL cells synergistically. Dose reduction index analysis demonstrated mutual potentiation of ABT-199 and DMDAPatA. Thus, these studies provide the first proof-of-mechanism investigations of PatA analogs for CLL therapeutics. DMDAPatA appeared to be highly plasma protein bound compared to PatA. New derivatives are being synthesized to reduce plasma protein binding and increase potency.
Citation Format: Rong Chen, Mingzhao Zhu, Yuling Chen, Wesley Skillern, William G. Wierda, Ken Hull, Daniel Romo, William Plunkett. Creating novel translation inhibitors to target pro-survival proteins in chronic lymphocytic leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1765. doi:10.1158/1538-7445.AM2015-1765