Interactions between the novel Chk1 inhibitor
MK-8776 and the
histone deacetylase (
HDAC) inhibitor (HDACI)
vorinostat were examined in human
leukemia cells harboring wild-type (wt) or deficient p53.
MK-8776 synergistically potentiated
vorinostat-mediated apoptosis in various p53-wt or -deficient
leukemia cell lines, whereas p53 knockdown by
short hairpin RNA (
shRNA) sensitized p53-wt cells to lethality of this regimen.
Leukemia cell lines carrying FLT3-ITD were also sensitive to the MK-8776/
vorinostat regimen. Synergistic interactions were associated with inhibition of Chk1 activity, interference with the intra-S-phase checkpoint, disruption of DNA replication, and downregulation of
proteins involved in DNA replication (e.g., Cdt1) and repair (e.g., CtIP and BRCA1), resulting in sharp increases in DNA damage, reflected by enhanced γ-H2A.X formation, and apoptosis. Moreover,
leukemia cells expressing
kinase-dead Chk1 (D130A) or Chk1
shRNA were significantly more sensitive to HDACIs compared with their wt counterparts and displayed downregulation of CtIP and BRCA1 phosphorylation following HDACI exposure. Finally, the MK-8776/
vorinostat regimen was active in primary
acute myelogenous leukemia (AML) blasts, particularly against the CD34(+)/CD38(-)/CD123(+) population enriched for
leukemia-initiating cells. In contrast, identical regimens were relatively sparing toward normal cord blood CD34(+) cells. Together, these findings indicate that the novel Chk1 inhibitor
MK-8776 markedly potentiates HDACI lethality in
leukemia cells displaying various genetic backgrounds through mechanisms involving disruption of the intra-S checkpoint, DNA replication, and DNA repair. They also argue that leukemic cells, including those bearing oncogenic mutations associated with poor prognosis, for example, p53 deletion/mutation or FLT3-ITD, may also be susceptible to this strategy.