The Novel Small Molecule Chk1/Chk2 Inhibitor PF-0477736 (Pfizer) Is Highly Active As Single Agent in Philadelphia-Positive Acute Lymphoblastic Leukemia (Ph+ ALL)

Checkpoint kinase 1 (Chk1) and 2 (Chk2) are serine/threonine kinases regulated by Ataxia-Telangiectasia and Rad3-related (ATR) kinases and involved in the DNA damage response and in the regulation of cell cycle progression at S-G2 phase. Deregulation of these pathways has been previously described in BCR-ABL positive cells and involved in chemoresistance. Based on the potential utility of DNA checkpoint inhibition in enhancing tumor cell death, in this study we aimed to investigate the preclinical activity of PF-0477736 (Pfizer), a potent and selective Chk1/2 inhibitor, in Ph+ ALL and to determine potential biomarkers of functional inhibition. We first examined Chk1 and Chk2 mRNA expression levels in 45 newly diagnosed Ph+ ALL patients, in their paired remission samples, in 14 relapsed cases and in 3 Ph+ cell lines (BV-173, SUPB-15 and K562) by Fluidigm Dynamic Array real-time qPCR assay (Fluidigm Corporation). Higher transcript levels of Chk1 but not Chk2 were found in newly diagnosed patients compared to remission samples (p = 0.0009 for Chk1 and p= 0.8183 for Chk2). Chk1 transcript levels were comparable between diagnosis and relapsed cases (p = 0.5728), suggesting that the ATR-Chk1 pathway is strongly activated in Bcr-Abl-positive cells. This was confirmed by phosphorylation of Chk1 Ser317 by western blotting analysis in Ph+ cell lines. We then evaluated the effect of PF-0477736 as single agent on cell viability using the Cell Proliferation Reagent WST-1 (Roche). BV-173, SUPB-15 and K562 cell lines were incubated with increasing concentration of PF-0477736 (0.005-2 μM) for 24, 48 and 72 hours. PF-0477736 inhibition of Chk1 resulted in dose and time-dependent cytotoxicity with IC50 at 24 hours of 0.1–0.5 μM, with BV-173 being the most sensitive, while K562 the most resistant. These results were confirmed in primary blasts cells from a Ph+ ALL patient with wild-type Bcr-Abl and from 3 cases harboring the T315I Bcr-Abl mutation found to be insensitive to the available TKIs (IC50 ranged from 0.1–0.5 μM at 48 hours). Consistent with the WST-1 results, Annexin V/Propidium Iodide staining analysis showed a significant increase of apoptosis at 24 and 48 hours in both cell lines and primary cells. To test whether increased apoptosis resulted from Chk1 inhibition, we assessed the changes in phosphorylation of Cdc25c phosphatase, which is inactivated by Chk1 to prevent mitotic entry, and of {gamma}H2AX, which is increased in response to DNA damage. Western blot analysis showed that PF-0477736 decreases the inhibitory phosphorylation of Cdc25c Ser216 and increases levels of {gamma}H2AX. Since multiple studies reported a higher activity of PF-0477736 against p53-defective cancer cells, we performed a mutational screening by amplification and subsequent sequencing of all coding exons of p53. All cell lines except for K562 and primary leukemia cells lacked mutations in the p53 gene, demonstrating that in Ph+ ALL PF-0477736 is highly effective also on p53 wild-type tumor cells. Finally, in order to elucidate the mechanisms of action of PF-0477736 and to determine biomarkers of response, gene expression profiling analysis (Affymetrix GeneChip Human Gene 1.0 ST) was performed on 3 treated Ph+ cell lines and their untreated counterparts. Consistent with a specific Chk1-mechanism of action, treatment resulted in differential expression (p < 0.05) of 211 genes including those involved in apoptosis and cell cycle (CEBPB, CUL1, Histone H1-H2A, 2B family clusters, Histone H4, DHX15, SNCB, FOS) and DNA damage, such as DNA-damage-inducible transcript 3 (DDIT3) and growth-arrest and DNA damage-inducible proteins GADD34 and GADD45a, suggesting that PF-0477736 contributes to a checkpoint abrogation and to an activation of DNA damage response in Ph+ ALL cells. In conclusion, for the first time we demonstrate in a large cohort of Ph+ ALL patients an over-expression of Chk1, providing a strong rational for its inhibition. In vitro treatment of Ph+ ALL cells with PF-0477736 alone resulted in reduction of inhibitory phosphorylation of Cdc25c, inhibition of proliferation and induction of biomarkers of DNA damage and apoptosis, suggesting that single-agent Chk1/2 inhibition may be an effective treatment strategy for Ph+ ALL.