Chemosensitisation of cancer cells by KU-0060648, a potent and selective inhibitor of DNA-PK [abstract]

4080 DNA double strand breaks (DSB) are profoundly cytotoxic unless rapidly repaired. Non-homologous end joining (NHEJ) is the predominant pathway of repair in mammalian cells. DNA-dependent protein kinase (DNA-PK), a member of the PI3-kinase like kinase (PIKK) family, is an essential component of NHEJ. DNA-PK inhibition is therefore an attractive target for radio- and chemosensitisation in cancer therapy.
 We have developed a series of DNA-PK inhibitors of increasing potency culminating in KU-0060648, IC50 = 8.6 nM with 20-1000-fold selectivity for DNA-PK over other PIKKs and a panel of 60 kinases. At non-cytotoxic concentrations KU-0060648 (≤ 1 μM) increased the cytotoxicity of the topoisomerase II poisons, doxorubicin and etoposide, in a panel of human cancer cell lines. Potentiation of 100 nM doxorubicin cytotoxicity ranged from 3-fold in MDA-MB-231 cells to 107-fold in MCF7 cells and potentiation of 1 μM etoposide cytotoxicity ranged from 3-fold in MDA-MB-231 cells to 105-fold in SW620 cells. KU-0060648 increased doxorubicin-dependent H2AX phosphorylation, consistent with increased DNA damage resulting in cytotoxicity. Studies in DNA-PK proficient and deficient cells demonstrated that chemo and radio-sensitisation by KU-0060648 was DNA-PK specific.
 KU-0060648 has been administered i.v. (25 mg/kg), i.p. (25 mg/kg) and p.o (100 mg/kg) to mice without adverse toxicity. Plasma and tissue pharmacokinetic studies show that tumour levels commensurate with sensitisation in vitro can be maintained for at least 4 hours following administration of 10 mg/kg i.p. KU-0060648 showed 100% bioavailability by oral administration at 10mg/kg, and a t1/2 of >2h. Pharmacodynamic assays demonstrate inhibition of DNA-PK activity in tumours. In mice bearing SW620 xenografts the etoposide (10 mg/kg dailyx5)-induced tumour growth delay of 0.5 days was significantly extended to 5 days by KU-0060648 (10 mg/kg 2xdailyx5).
 In conclusion, KU-0060648 is a potent and selective DNA-PK inhibitor. It has acceptable PK and DNA-PK is inhibited in target tissues at tolerable doses. It is a potent chemosensitiser in models of common human malignancies in vitro and in vivo.