Induction of Human Fetal Hemoglobin Expression by Selective Inhibitors of Histone Deacetylase 1 and 2 (HDAC1/2)

Fetal hemoglobin (HbF) induction is an established therapeutic strategy in sickle cell disease (SCD), and could also be effective in treating beta-thalassemia (bT). The only drug with proven efficacy in SCD is hydroxyurea, which is cytotoxic, poorly tolerated, and only reduces the frequency and severity of SCD crises in a subset of patients. For bT there are no approved drug treatments and, for the most severe forms of bT, patients require repeated blood transfusions for life and chelation therapy to reduce iron overload. Fetal (γ) globin expression is silenced in adults partly through the action of a complex containing BCL11A and HDAC1/2. Genetic ablation and chemical inhibition of HDAC1 or HDAC2, but not HDAC3, resulted in the induction of γ-globin synthesis in adult bone marrow derived CD34+ erythroid cells (Bradner, Proc Natl Acad Sci 2010). While a variety of non-specific HDAC inhibitors have been used successfully to induce HbF, further clinical development has been limited by their variable efficacy and concerns over off-target side-effects observed in small clinical trials, potentially due to inhibition of HDAC3. Therefore, development of selective and potent HDAC1/2 inhibitors leading to HbF induction represents a refined and targeted therapeutic approach for the treatment of SCD and bT.

Recently, selective inhibitors for HDAC1 and 2 have been described for compounds containing a benzamide zinc chelating group (Witter, Bioorg Med Chem Lett 2008). Acetylon Pharmaceuticals has generated a library of structurally distinct compounds containing a modified benzamide biasing element. We have screened these molecules in an HDAC biochemical assay platform and found IC50 values for HDAC1 and 2 ranging from 5 to 10 nM and 10 to 30 nM, respectively, with approximately 10- to 100-fold selectivity over inhibition of HDAC3. These compounds had good oral bioavailability in rat, with exposure (AUC) exceeding 5000 hr*ng/mL following an oral dose of 5 mg/kg. Furthermore, half-life was found to range between 4 to 8 hours. In cultured human CD34+ bone marrow cells undergoing erythroid differentiation, these compounds induced a dose dependent increase in fetal hemoglobin expression, with a 5-fold induction observed at 1 μM. The γ-globin induction we observe is comparable to MS-275, a non-selective HDAC1/2/3 inhibitor, and decitabine, a DNA methyltransferase inhibitor which has been in clinical trials to induce HbF in SCD. The absolute levels of adult β-globin remained unchanged. These results were confirmed using flow cytometry, where a 2- to 4-fold increase in the number of cells expressing HbF protein was observed.

These results suggest that inhibition of HDAC1 and 2 is sufficient to induce fetal globin expression. Our selective HDAC1/2 inhibitors have highly favorable oral pharmacokinetic profiles suitable for further development towards the treatment of SCD and bT.

Shearstone:Acetylon Pharmaceuticals, Inc.: Employment. van Duzer:Acetylon Pharmaceuticals, Inc.: Employment. Bradner:Acetylon Pharmaceuticals, Inc.: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Mazitschek:Acetylon Pharmaceuticals, Inc. : Consultancy, Equity Ownership. Jones:Acetylon Pharmaceuticals, Inc.: Employment. Jarpe:Acetylon Pharmaceuticals, Inc.: Employment.