Gemcitabine irreversibly inhibits
ribonucleotide reductase and induces S phase arrest but whether this occurs in
tumors in mice or patients has not been established.
Tumor cells in culture were incubated with
gemcitabine for 6 h to approximate the administration schedule in a patient. Concentrations that induced persistent S phase arrest thereafter correlated with cell killing. Administration of
gemcitabine to mice also demonstrated a persistent S phase arrest in their
tumor. The minimum dose that induced almost complete S phase arrest after 24 h (40 mg/kg) was well below the maximum tolerated dose in mice. S phase arrest was also observed in
tumors of
bladder cancer patients receiving
gemcitabine. The Chk1 inhibitor
MK-8776 sensitized cells to
gemcitabine with the greatest cell killing when added 18 h after
gemcitabine. In mice, the administration of
MK-8776 18 h after
gemcitabine elicited positivity for the DNA damage marker γH2AX; this also occurred at relatively low dose (40 mg/kg)
gemcitabine. Hence, in both cell culture and xenografts,
MK-8776 can markedly enhance cell killing of cells reversibly arrested in S phase by
gemcitabine. Some cell lines are hypersensitive to
MK-8776 as monotherapy, but this was not observed in xenograft models. Effective monotherapy requires a higher dose of Chk1 inhibitor, and target inhibition over a longer time period as compared to its use in combination. These results have important implications for combining Chk1 inhibitors with
gemcitabine and suggest that Chk1 inhibitors with increased bioavailability may have improved efficacy both in combination and as monotherapy.