Endogenous replication stress marks melanomas sensitive to CHK1 inhibitors in vivo

Purpose: CHEK1 inhibitors (CHEK1i) have single agent activity in vitro and in vivo. Here we have investigated the molecular basis of this activity. Experimental Design: We have assessed a panel of melanoma cell lines for their sensitivity to the CHEK1i GNE-323 and GDC-0575 in vitro and in vivo. The effects of these compounds on responses to DNA replication stress were analyzed in the hypersensitive cell lines. Results: A subset of melanoma cell lines are hypersensitive to CHEK1i-induced cell death in vitro, and the drug effectively inhibits tumour growth in vivo. In the hypersensitive cell lines, GNE-323 triggers cell death without cells entering mitosis. CHEK1i treatment triggers strong RPA2 hyper-phosphorylation and increased DNA damage in only hypersensitive cells. The increased replication stress was associated with a defective S phase cell cycle checkpoint. The number and intensity of pRPA2 Ser4/8 foci in untreated tumours appeared to be a marker of elevated replication stress correlated with sensitivity to CHEK1i. Conclusions: CHEK1i have single-agent activity in a subset of melanomas with elevated endogenous replication stress. CHEK1i treatment strongly increased this replication stress and DNA damage, and this correlated to increased cell death. The level of endogenous replication is marked by the pRPA2Ser4/8 foci in the untreated tumours, and may be useful marker of replication stress in vivo. 

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