The Targeted Histone Deacetylase Inhibitor Tefinostat (CHR-2845) Shows Selective In Vitro Efficacy In Monocytoid-Lineage Acute Myeloid Leukaemia (AML)

Tefinostat (CHR-2845) is a novel monocyte/macrophage-targeted histone deacetylase inhibitor (HDACi) that is cleaved to an active acid, CHR-2847, by an intracellular esterase (human carboxylesterase-1, hCE-1), found only in cells of monocytoid lineage and hepatocytes. The clinical uptake of HDAC inhibition to date has been restricted by systemic toxicities including gastrointestinal disturbance, thrombocytopenia and fatigue. Accumulation of CHR-2847 in hCE-1-expressing cells results in a 20-100-fold increase in targeted anti-proliferative potency, considerably widening the potential therapeutic window in malignancies involving cells of monocytoid lineage (AML-M4, AML-M5 and CMML) by sparing the systemic toxicological effects associated with non-selective HDAC inhibition.

The in vitro efficacy of tefinostat was assessed in primary AMLs using stored mononuclear cells obtained at diagnosis from 70 AML patients. Dose-dependent induction of apoptosis and significant growth inhibitory effects were seen in M4 /M5 AMLs (median IC50; 1.1µM+/-1.8) compared to non-M4/M5 FAB types (median IC50 5.1µM +/-4.7) (p=0.007). This potency and monocytoid specificity was not reproduced when using an alternative HDACi, tefinostat analogue CHR-8185 which is not cleaved by hCE-1.

hCE-1 protein expression in patient samples was measured by both intracellular flow cytometry and immunoblotting, with highest levels seen in M4/M5 patients. This observation was validated by microarray analysis of hCE-1 mRNA in a further 130 AML samples with M4/M5 AMLs showing significant overexpression compared to normal bone marrow CD34⁺ cells (p=0.009). High levels of hCE-1 expression were found to drive a significant increase in tefinostat efficacy as measured by growth inhibition assays (p=0.001), and also strongly correlated with expression of the mature monocytoid marker CD14⁺.

Sub-population analysis by flow cytometry revealed variable sensitivity to tefinostat within AML blasts, with CD14⁺ expressing cells showing maximum growth inhibition. This CD14⁺ response was accompanied by an induction of intracellular protein acetylation at nanomolar concentrations in tefinostat-responsive samples. Tefinostat-sensitive samples also showed strong induction of the cell cycle arrest and DNA damage sensor protein pH2AX, which is a potential biomarker of patient responsiveness.

Importantly, no growth inhibitory effects were seen in normal bone marrow cells (n=5) exposed to AML-toxic doses of tefinostat while, in comparison, equivalent concentrations of the non-hCE-1-dependent analogue CHR-8185 caused considerable cytotoxicity, again emphasising the potential for expansion of the clinical therapeutic window using an hCE-1-dependent agent.

In vitro synergy was demonstrated in combination experiments with tefinostat and cytarabine (median Combination Index value=0.68) which is likely to be a logical combination for future clinical evaluation.

In summary, monocytoid targeting of HDACi activity was achieved using tefinostat in primary AML samples of monocytoid lineage, with minimal toxicity to normal bone marrow cells at equimolar concentrations. Given the absence of significant toxicity seen in a recently-published phase 1 study of tefinostat in patients with advanced haematological malignancies, further larger scale clinical evaluation of this compound is warranted in haematological malignancies involving cells of monocytoid lineage.