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Departments of ¹ Medicine and ² Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia; ³ Department of Oncology, Novartis Institutes for Biomedical Research, East Hanover, New Jersey; and ⁴ Department of Pharmacology, Dartmouth Medical Center, Hanover, New Hampshire

Requests for reprints: Steven Grant, Division of Hematology/Oncology, Department of Medicine, Virginia Commonwealth University/Medical College of Virginia, MCV Station Box 230, Richmond, VA 23298. Phone: 804-828-5211; Fax: 804-828-8079. E-mail: stgrant{at}hsc.vcu.edu

Interactions between the novel histone deacetylase inhibitor LAQ824 and the cyclin-dependent kinase inhibitor roscovitine were examined in human leukemia cells. Pretreatment (24 hours) with a subtoxic concentration of LAQ824 (30 nmol/L) followed by a minimally toxic concentration of roscovitine (10 µmol/L; 24 hours) resulted in greater than additive effects on apoptosis in U937, Jurkat, and HL-60 human leukemia cells and blasts from three patients with acute myelogenous leukemia. These events were associated with enhanced conformational changes in Bax; mitochondrial release of cytochrome c, Smac/DIABLO, and apoptosis-inducing factor; and a marked increase in caspase activation. LAQ824/roscovitine–treated cells displayed caspase-dependent down-regulation of p21^CIP1 and Mcl-1 and a pronounced caspase-independent reduction in X-linked inhibitor of apoptosis (XIAP) expression. The lethality of this regimen was significantly attenuated by ectopic expression of XIAP, a nuclear localization signal–defective p21^CIP1 mutant, Mcl-1, and Bcl-2. Combined exposure to LAQ824 and roscovitine resulted in a significant reduction in XIAP mRNA levels and diminished phosphorylation of the carboxyl-terminal domain of RNA polymerase II. Notably, roscovitine blocked LAQ824-mediated differentiation. Finally, LAQ824 and roscovitine individually and in combination triggered an increase in generation of reactive oxygen species; moreover, coadministration of the free radical scavenger N-acetylcysteine prevented LAQ824/roscovitine–mediated mitochondrial injury and apoptosis. Collectively, these findings suggest that combined treatment of human leukemia cells with LAQ824 and roscovitine disrupts maturation and synergistically induces apoptosis, lending further support for an antileukemic strategy combining novel histone deacetylase and cyclin-dependent kinase inhibitors.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

⁵ Supplementary material for this article is available at Molecular Cancer Therapeutics Online (http://mct.aacrjournals.org).

⁶ Rosato and Grant, unpublished observations.

Received 5/17/05; revised 8/10/05; accepted 8/30/05.