Selective Inhibition of HDAC1 and HDAC2 Is a Potential Therapeutic Option for B-All

Abstract 2900

Histone deacetylase inhibitors (HDACi) have emerged as potent anticancer agents, and could open the door for future epigenetic therapies. As our understanding of the importance of epigenetic histone modifications in B-cell acute lymphoblastic leukemia (B-ALL) increases, we hypothesized that HDACi could potentially be a useful therapeutic option. The pan-HDAC inhibitor LAQ824 (Novartis) was toxic to B-ALLs in low nM concentrations in vitro, and treated cells had increased p21 and DNA damage response as indicated by increased γH2A.X protein levels. Additionally, the related compound panobinostat (Novartis) reduced leukemic burden from B-ALL patient samples in primary xenograft models, indicating that pan-HDAC inhibition is a putative B-ALL therapeutic option. To determine HDAC isoform-specific effects, we used a high throughput assay that exposed B-ALL cell lines to a panel of HDAC inhibitors. This screen indicated that tubacin, an HDAC6 specific inhibitor, cannot inhibit B-ALL cell growth within a dose range where HDAC6 is the only HDAC targeted. This finding was further validated using another HDAC6 specific inhibitor, WT-161. The screen also indicated that benzimide compounds such as MGCD-0103 (MethylGene) and MS-275 (Entinostat, Syndax) which only target class I HDACs (HDAC1-3) effectively inhibited growth in the cell lines. These data indicate that inhibiting the class I HDACs is sufficient to suppress B-ALL cell line growth. To determine which HDACs are necessary for cell viability, we lentivirally introduced isoform-specific shRNAs into our ALL cell lines. Knockdown of HDAC1 or HDAC2 resulted in p21 induction, slowed growth rate and resulted in a modest increase in apoptosis. Knockdown of HDAC3 lead to increased p21 and γH2A.X protein levels, along with induction of apoptosis, closely mimicking the results of pan-HDAC inhibitor treatment of the cells. Although depletion of HDAC3 had a more immediate impact on B-ALL viability by comparison to HDAC1/2, concerns about the contribution of HDAC3 inhibition to toxicity led us to further investigate whether specific inhibition of HDAC1/2 might be efficacious in B-ALL. Treatment of B-ALL cells with Merck 60, a tool compound with selectivity for HDAC1/2, was efficacious against was effective against B-ALL lines in the low to mid nM range. The kinetics of growth suppression were slower with this compound than with the pan-HDAC inhibitors. Using this compound, the ALL lines required 72 hours of exposure before cell growth was diminished, and apoptosis ensued. This may be due to the increased time necessary to accumulate acetylated histone marks as observable by western blot (18 hours for Merck 60 vs. 2–4 hours for LAQ824). Increased levels of p21 and γH2A.X were also observed. Interestingly, AML cell lines were much less sensitive to the HDAC1/2 specific inhibitor than were the B-ALL lines (roughly 5–10 fold), whereas pan-HDAC inhibitors were equally effective against AML and ALL. Additionally, non-hematopoietic tumor derived cell lines were insensitive to Merck 60, with EC₅₀ values exceeding 20μM. Our findings indicate that pan-HDAC and class I specific HDAC inhibitors are possible therapeutic options for B-ALL. In contrast to most other cancer cell types studied, selective inhibition of HDAC1 and HDAC2 was sufficient to induce apoptosis in B-ALL lines. Together, these results suggest that small molecules specifically targeting HDAC1/2 may have therapeutic utility in B-ALL, and may provide improved therapeutic index by comparison to pan-HDAC or class I HDAC inhibitors that also target HDAC3.