Epigenetic modifiers of the histone deacetylase (HDAC) family contribute to autoimmunity, cancer, HIV infection, inflammation, and neurodegeneration. Hence, histone deacetylase inhibitors (HDACi), which alter protein acetylation, gene expression patterns, and cell fate decisions, represent promising new drugs for the therapy of these diseases. Whereas pan-HDACi inhibit all 11 Zn-dependent histone deacetylases (HDACs) and cause a broad spectrum of side effects, specific inhibitors of histone deacetylase 6 (HDAC6i) are supposed to have less side effects. We present the synthesis and biological evaluation of Marbostats, novel HDAC6i that contain the hydroxamic acid moiety linked to tetrahydro-β-carboline derivatives. Our lead compound Marbostat-100 is a more potent and more selective HDAC6i than previously established well-characterized compounds in vitro as well as in cells. Moreover, Marbostat-100 is well tolerated by mice and effective against collagen type II induced arthritis. Thus, Marbostat-100 represents a most selective known HDAC6i and the possibility for clinical evaluation of a HDAC isoform-specific drug.

Aberrant epigenetic changes in DNA methylation and histone modification by acetylation or deacetylation regulate the pathogenesis of many diseases. Especially selective inhibitors are getting more and more attention. We recently reported on a new class of potent and selective anti-inflammatory and antirheumatic histone deacetylase 6 (HDAC6) inhibitors (e.g., Marbostat-100). The attachment of a morpholinoethoxy part to the head group dramatically enhances the solubility, in particular the solubility in aqueous solutions, of the lead compound Marbostat-100.Here, we present the enantioselective synthesis of small-molecule compounds based on the tetrahydro-β-carboline core system with improved solubility, and the influence of the stereochemistry on the biological activity. The enantiomers were synthesized in good enantiomeric excess (ee) purity and were potent and selective HDAC6 inhibitors, whereas the S-derivative S-21 is clearly the eutomer. The potency of our selective HDAC6 inhibitors is demonstrated by K i values in the range of 0.5-2 nM toward HDAC6, and the selectivity was proved in cellular assays by Western blot analysis taking ac-tubulin as surrogate parameter.