Background Small selleck screening library molecule inhibitors against checkpoint kinases con stitute a promising class of targeted cancer therapeutics and many are currently under preclinical or even clinical evalu ation. The role of checkpoint kinases is to respond to stress, typically damaged DNA or aberrant chromosomal struc ture, and stop the cell division process long enough for the damage to be repaired. These checkpoints prevent cells from dividing and perpetuating mutations or chromosomal anomalies that would otherwise lead to cellular lethality. The rationale for inhibiting checkpoint kinases is to accu mulate irreparable and fatal genetic lesions by compromis ing the DNA damage response and forcing premature or untimely cell division. Notable examples in clude the mitotic checkpoint kinases Aurora A and B, checkpoint kinase 1, CHK2, ATR, and WEE1.

Several CHK1 inhibitors have been employed in early stage clinical trials. Notably, MK 8776, a CHK1 selective inhibitor, is under evalu ation in phase I studies in combination with gemcitabine or cytarabine. Only one inhibitor of WEE1 has been explored clinically. MK 1775, a potent and selective inhibi tor of WEE1, achieved favorable phase I pharmacokinetic and pharmacodynamic endpoints in combination with car boplatin, cisplatin, and gemcitabine, and is under further in vestigation as a chemosensitizer in a phase II trial. CHK1 is an essential serine threonine kinase involved in S and G2 M phase checkpoints, replica tion initiation and fork stability, homologous recombination repair, and entry into mitosis in normal cycling cells.

Importantly, CHK1 is neces sary for unperturbed DNA replication and cell cycle co ordination even in the absence of any exogenous insult. The cytotoxic nature of CHK1 knockdown or in hibition, either alone or in combination with DNA damaging therapeutics, has been described extensively. WEE1 is an essential tyrosine kinase that is also involved in S and G2 M checkpoints. WEE1 directly phosphorylates and inhibits CDK1 and CDK2 at the conserved tyrosine 15 residue, affecting entry into mi tosis as well as coordination of DNA replication events. WEE1 is therefore critical for properly timing cell div ision in unperturbed cells, and loss of WEE1 results in chromosomal aneuploidy and accumulated DNA damage. Additionally, WEE1 is critical to S and G2 M phase checkpoint responses following DNA dam age as well as in unperturbed cells.

Interfering with WEE1 has been shown to repress cancer cell prolif eration and sensitize theme to the anti tumor growth effects of DNA damaging chemotherapeutics or radi ation therapy. Considering the overlapping roles of WEE1 and CHK1 in mitotic entry, DNA replication, and the DDR, we sought to determine whether Drug_discovery inhibition of these two kinases was redundant or complimentary.