The Histone Deacetylase-8 (HDAC8) Selective Inhibitor PCI-34051 Decreases Interleukin-1 Beta Secretion in Vitro and Reduces Inflammation in Vivo

Inhibitors of histone deacetylases (HDACs) which are currently in clinical testing for treating various cancers typically inhibit multiple isoforms of the 11-member HDAC family. We have developed an isoform-selective HDAC inhibitor, PCI-34051, that inhibits HDAC8 with a K_i of 10 nM and greater than 200-fold selectivity over other HDAC isoforms (Balasubramanian et al. (2008) Leukemia,22:1026–34). We have shown that PCI-34051 selectively induced apoptosis in cell lines derived from T-cell lymphomas and leukemias, but not in other tumor or normal cell types. Here we show that it potently inhibits the secretion of the pro-inflammatory cytokine interleukin-1 beta (IL-1b) in lipopolysaccharide (LPS)-stimulated peripheral mononuclear blood cells (PBMC) and isolated monocytes. PCI-34051 inhibited IL-1b secretion (by 80% compared to control) from LPS-stimulated human PBMC with an IC₅₀ of 0.6 uM, which is much lower than the growth inhibitory concentrations of 2.4–4 uM required in T-cell lymphomas. We found that PCI-34051 also inhibited the secretion of interleukin-18 (IL-18) to a similar extent as IL-1b, but secretion of other pro-inflammatory cytokines, including MIP-1b, MCP-1, TNFa, and IL-6, was inhibited to a smaller extent. Interestingly, IL-18, like IL-1b, is synthesized without a signal peptide, and also utilizes the same non-classical endosomal secretory pathway as IL-1b including cleavage of the pro-form by caspase-1. Thus, we theorized that the modulatory effect of PCI-34051 is likely to involve modulation of the post-translational secretory process. In accordance, we found that the IL-1b mRNA levels were reduced by only 20% compared to control, but the intracellular protein levels of the pro-form was increased by >50% in primary monocytes after treatment with PCI-34051, indicating that the mechanism was due to inhibition of the processing from the pro- to the mature form of the cytokine. We showed that this was not due to a direct inhibition of caspase-1 or TACE (TNF-alpha converting enzyme), but is likely due to an as-yet unidentified substrate of HDAC8. In vivo, PCI-34051 inhibited ear swelling induced by oxazolone in a model of contact hypersensitivity in mice, and we showed that this was accompanied by a reduction in IL-1b at both the protein and mRNA levels. Based on this result, we examined the effect of PCI-34051 on IL-1b secretion in human keratinocytes, as well as in PBMC from psoriasis patients, and found that it could reduce IL-1b secretion in both. We found that PCI-34051 decreased IL-1b by 60% in LPS-stimulated PBMC from rheumatoid arthritis (RA) patients, but the pan-HDAC inhibitors which were only weakly inhibitory to HDAC8 did not have this effect, indicating a specific role for HDAC8 in the secretory process. Finally, we found that in unstimulated PBMC from RA patients that had basal production of IL-1b that this could be decreased by 90% by treatment with PCI-34051. Taken together, these findings indicate that PCI-34051 is an active drug that could be useful for the treatment of T-cell lymphoma as well as for autoinflammatory diseases such as RA and psoriasis.