The Novel Histone Deacetylase Inhibitor OSU-HDAC42 Has Class I and II Histone Deacetylase (HDAC) Inhibitory Activity and Represents a Novel Therapy for Chronic Lymphocytic Leukemia.

Inhibitors of histone deacetylase (HDAC) have generated major interest for the treatment of multiple cancers including B-cell Chronic Lymphocytic Leukemia (CLL). To date, HDAC inhibitors introduced for clinical development in CLL have been associated either with suboptimal activity relative to concentrations required to mediate cytotoxicity in vitro (Valproic Acid, MS-275, SAHA), or demonstrate unacceptable acute or long-term toxicities (depsipeptide) that limit their clinical potential. Fortunately, several alternative HDAC inhibitors are in pre-clinical or early clinical development. One such agent currently undergoing pre-clinical testing by the National Cancer Institute-sponsored RAID program is OSU-HDAC42 (s-HDAC-42), a novel, orally bioavailable phenylbutyrate-derived HDAC inhibitor with both in vitro and in vivo efficacy against prostate cancer cells. We therefore tested OSU-HDAC42 against CD19-positive cells obtained from patients with CLL to determine its potential in this disease. The LC₅₀ of OSU-HDAC42 in CLL cells was 0.46 uM at 48 hours of continuous incubation by MTT assay, which was corroborated by annexin V-FITC/propidium iodide flow cytometry. To determine the minimum amount of time that OSU-HDAC42 must be present to induce cell death, cells were incubated for various times, washed, resuspended in fresh media without drug, then assessed by MTT at a total of 48 hours incubation. The effects of OSU-HDAC42 were eliminated in CLL cells when drug was removed after 4 or 6 hours. However, there was a gradual increase in effect over time, and by 16 hours, approximately 60% of the cytotoxicity achieved with continuous incubation was retained. OSU-HDAC42 induced acetylation of histone proteins H3 and H4 as early as 4 hours that was dose and time dependent. LC/MS interrogation of OSU-HDAC42-treated CLL cells is currently underway to determine specific post-translational modification changes of all histone proteins and variants. OSU-HDAC42 also was able to sensitize CLL cells to TNF-Related Apoptosis Inducing Ligand (TRAIL) at 24 hours in a dose-dependent manner, supporting its class I HDAC inhibitory activity as recently reported by Inoue and colleagues (