Selective radiosensitization of hypoxic cells using BCCA621C: a novel hypoxia activated prodrug targeting DNA-dependent protein kinase

Abstract

Tumour hypoxia presents a barrier to conventional chemotherapy and radiation therapy. To combat hypoxic cells a number of hypoxia modifying treatments are currently in development. In this study we assessed the potential for inhibiting DNA double strand break repair in hypoxic cells by targeting DNA-dependent protein kinase (DNA-PK) and we report the synthesis and in vitro efficacy of BCCA621C (1), a hypoxia activated inhibitor of DNA-PK. We found that DNA-PK deficient hypoxic cells are radiosensitive compared to hypoxic DNA-PK proficient cells and that this effect can be observed using both a small molecule inhibitor of DNA-PK, IC86621 (2), as well as with a genetically deficient model cell line. BCCA621C, which is designed to selectively release a DNA-PK inhibitor in hypoxic cells was synthesized and assessed for bioreduction using mouse liver microsomes and NCI-H460 cells. BCCA621C is activated by bioreduction in severely hypoxic NCI-H460 cells and was able to radiosensitize hypoxic NCI-H460 cells with a sensitizer enhancement ratio (SER) of 1.85. No enhancement of radiosensitivity was found to occur with BCCA621C treatment in oxygenated NCI-H460 cells in a range of clinically relevant ionizing radiation doses.

Overcoming hypoxia-induced tumor radioresistance in non-small cell lung cancer by targeting DNA-dependent protein kinase in combination with carbon ion irradiation

“…However, only little is known about the effect of DNA-PKc inhibition on the radiosensitivity of hypoxic cells. Lindquist and colleagues described radiosensitivity effects with photon irradiation of DNA-PK deficient mouse embryonic fibroblast cells (SCID/st) under normoxic and hypoxic conditions compared to isogenic DNA-PK wildtype cells (CB.17) and that DNA-PK inhibition in combination with photon irradiation decreases the clonogenic survival of both oxic and hypoxic cells [ 31 ]. They suggested that impairment of DNA-PK radiosensitizes independently of cellular oxygen status.…”

Targeting the hypoxic fraction of tumours using hypoxia-activated prodrugs

“…Whilst HAPs designed to release DNA-damaging species are still being developed, other HAPs have been designed to release inhibitors of DNA damage response pathways [ 123 , 124 ]. Release of the Chk1/Aurora A inhibitor CH-01 (Fig.…”

“…Similarly, attachment of a DNA-PK inhibitor to a nitroimidazole hypoxia trigger group led to inactivation of the complex (BCCA621C, Fig. 9 ) and no cytotoxic activity under aerobic conditions was reported [ 124 ]. Under severe hypoxia, however, the inhibitor was released and was able to radiosensitise hypoxic H460 cells in vitro with a sensitiser enhancement ratio of 1.85.…”

Hypoxia and Predicting Radiation Response

“…Future studies of modifying the radiation response of hypoxic cells in tumors with existing drugs or with newer approaches such as improving oxygen availability in tumors with metformin treatment to reduce oxygen consumption 134 or hypoxia-activated drugs which target DNA damage response pathways 135,136 , should be conducted with prospective hypoxia measurements and include measurements made during the course of therapy. Using techniques for hypoxia measurement that effectively address issues of heterogeneity are required for such studies.…”