Background: Histone Deacetylases (HDACs) play key roles in tumor cell proliferation, survival and angiogenesis. HDAC inhibitors have been shown to induce cell cycle arrest, terminal differentiation and/or apoptosis in broad spectrum of human tumors and xenograft animal models. JNJ-26481585 is a hydroxamic acid derivative second-generation pan-HDAC inhibitor with nM potency in pre-clinical studies. In animal models, no evidence of cardiac toxicity has been observed, which suggests a better therapeutic index than other HDAC inhibitors.

Aim: We evaluated the anti-leukemia and molecular activity of JNJ-26481585 in human leukemia cell lines (HL60, MOLT4, THP1 and U937) and primary leukemia cells.

Results: JNJ-26481585 potently inhibited proliferation of leukemia cells, with IC50 between 25 nM and 250 nM after 24-hour of treatment. In contrast, analysis of other HDAC inhibitors, including vorinostat and MGCD0103, had IC50 in the μM range. JNJ-26481585 also induced dose-dependent apoptosis (aprox. 25–35% at concentration of 25 nM) of leukemia cell lines by Annexin V staining after 24-hour treatment. A synergistic apoptotic effect was observed by co-treating with hypomethylating agent 5-aza-2′-deoxycytidine (25% with JNJ-26481585 25 nM, 5% with decitabine 1 μM, and 70% for the combination). Acetylation of histones H3, H4 and phosphorylation of H2AX was also observed after exposure to JNJ-26481585 at concentration of 12.5 nM or higher. We also studied fresh primary leukemia cells from 9 patients (ALL N=1, MDS N=1, AML N=7). Six of the 9 patients (67%) had relapsed/refractory disease. Their median white cell counts were 32×103/L (range 6–407×103/L), with 25% to 95% blasts in the peripheral blood. Treatment with JNJ-26481585 for 24-hours induced significant growth arrest and apoptosis (20–50% apoptosis at 25 nM) in a dose-dependent manner (between 12.5–500 nM), but not in mononuclear cells from healthy donors (N=2). It also induced acetylation of histones H3, H4, phosphorylation of H2AX, induction of p21 and HSP70, activation of caspase-3, cleavage of PARP in these primary leukemia cells after 24-hour treatment at concentrations of 12.5–500 nM.

Conclusion: JNJ-26481585 is a potent second generation pan-HDAC inhibitor with activity in human leukemias. It is a promising new agent for future clinical studies either as single agent or combination epigenetic therapy with DNA hypomethylating agents and should be tested in clinical trials.

Disclosures: Arts:Johnson & Johnson: Employment.

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