Inhibitors of cyclin-dependent kinases (Cdks) have been reported to have activities in chronic lymphocytic leukemia cells by inhibiting Cdk7 and Cdk9, which control transcription. Here we studied the novel Cdk inhibitor SNS-032, which exhibits potent and selective inhibitory activity against Cdk2, Cdk7, and Cdk9. We hypoth-

Abstract2V -C -cyano-2V -deoxy-1-B-D-arabino -pentofuranosylcytosine (CNDAC) is a nucleoside analogue with a novel mechanism of action that is currently being evaluated in clinical trials. Incorporation of CNDAC triphosphate into DNA and extension during replication leads to single-strand breaks directly caused by B-elimination. These breaks, or the lesions that arise from further processing, cause cells to arrest in G 2 . The purpose of this investigation was to define the molecular basis for G 2 checkpoint activation and to delineate the sequelae of its abrogation. Cell lines derived from diverse human tissues underwent G 2 arrest after CNDAC treatment, suggesting a common mechanism of response to the damage created. CNDAC-induced G 2 arrest was instituted by activation of the Chk1-Cdc25C-Cdk1/cyclin B checkpoint pathway. Neither Chk2, p38, nor p53 was required for checkpoint activation. Inhibition of Chk1 kinase with 7-hydroxystaurosporine (UCN-01) abrogated the checkpoint pathway as indicated by dephosphorylation of checkpoint proteins and progression of cells through mitosis and into G 1 . Cell death was first evident in hematologic cell lines after G 1 entry. As indicated by histone H2AX phosphorylation, DNA damage initiated by CNDAC incorporation was transformed into double-strand breaks when ML-1 cells arrested in G 2 . Some breaks were manifested as chromosomal aberrations when the G 2 checkpoint of CNDAC-arrested cells was abrogated by UCN-01 but also in a minor population of cells that escaped to mitosis during treatment with CNDAC alone. These findings provide a mechanistic rationale for the design of new strategies, combining CNDAC with inhibitors of cell cycle checkpoint regulation in the therapy of hematologic malignancies. (Cancer Res 2005; 65(15): 6874-81)

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2-Chloro-2′-deoxyadenosine (CldAdo) and 9-beta-D-arabinosyl-2- fluoroadenine (F-ara-A) have shown marked activity in the treatment of indolent lymphoid malignancies. Based on the susceptibility of various lymphocyte populations to apoptosis, we investigated whether CldAdo or F-ara-A would induce this process in lymphocytes from patients with chronic lymphocytic leukemia (CLL). In vitro exposure of leukemic lymphocytes to CldAdo or F-ara-A for 24 to 72 hours elicited features of apoptosis visible by light and electron microscopy. Analysis of DNA integrity showed DNA cleavage into nucleosomal-sized multimers. Using a quantitative assay, drug-induced DNA fragmentation was both time and dose dependent. Inhibition of active macromolecular synthesis did not prevent drug-induced fragmentation; however, both drug-induced and spontaneous DNA fragmentation were prevented by intracellular calcium chelation. In vitro culture with phorbol ester generally decreased drug- induced DNA cleavage. After prolonged incubation, CLL cells exhibited spontaneous cleavage; albeit, at significantly lower rates than drug- treated cells. Heterogeneity was observed for spontaneous and drug- induced DNA fragmentation and was significantly lower in B-leukemic cells obtained from patients with high-risk and refractory disease. We conclude that CldAdo and F-ara-A are potent inducers of apoptotic death in CLL and that this feature correlates with the disease status.

SNS-032 is a potent inhibitor of cyclin-dependent kinases (Cdk) 2, 7, and 9 that regulate the cell cycle and transcription. Our studies in indolent primary chronic lymphocytic leukemia cells showed that SNS-032 inhibited transcription, diminished the antiapoptotic protein Mcl-1, and induced apoptosis. The present study focuses on evaluating this compound in four proliferating mantle cell lymphoma lines (Jeko-1, Granta 519, Mino, and SP-53). Consistent with its action against Cdk9 and Cdk7, SNS-032 inhibited the phosphorylation of RNA pol II in all four lines and blocked RNA synthesis. The transcripts and protein levels of short-lived proteins decreased, including cyclin D1 and Mcl-1. Cell growth was inhibited in a concentration-dependent manner in all lines. Apoptosis was induced in JeKo-1, Mino, and SP-53 cells without disrupting cell cycle distribution. However, apoptosis was limited in Granta cells; rather, there was a significant reduction of clonogenic survival. Small interfering RNA was used to specifically knock down Mcl-1 and cyclin D1 in JeKo-1 and Granta cells. Knocking down Mcl-1 induced significant apoptosis in Jeko-1 cells but not Granta cells. Reducing cyclin D1, rather than Mcl-1, was associated with loss of clonogenic survival in Granta cells. Thus, these results indicated that mantle cell lymphoma cell lines have distinct mechanisms sustaining their survival, and the mechanism of action of SNS-032 is dependent on the biological context of an individual line.