Exploring structure-activity relationships for phosphoinositide 3-kinases (PI3K)- related kinases

Abstract

One challenge of drug design for molecular targets is selectivity for the target enzyme. This is generally true in the development of kinase inhibitors, and specifically a problem in the development of inhibitors of the phosphoinositide 3-kinase related kinases (PIKKs). The catalytic domain of PIKKs are highly homologous with that of phosphatidylinositol 3-kinases (PI3K), hence the name. Several PIKKs participate in the DNA-damage response (DDR) process and therefore are a promising target in oncology. This is because radiation and cytotoxic chemotherapy can trigger the DDR. Although critical for healthy cells, the DDR is a mechanism of resistance in tumour cells targeted in radiation therapy (RT), which induces cell death through irreparable DNA-damage. Therefore, impeding the DDR by inhibiting molecular targets in the pathways, to potentiate the effects of RT, is a promising solution to treat cancers. Of the PIKK family, DNA-PK mediates the repair of double stranded DNA damage, and mTOR is a downstream effector of the PI3K/Akt/mTOR cell signalling pathway, which is highly deregulated in many cancers. Inhibition of DNA-PK, mTOR and PI3K can be useful approaches to therapy, but may be accompanied by normal tissue toxicity. One approach to minimise normal tissue toxicity is to develop very selective inhibitors of each enzyme and combine this approach with specific targeting provided by RT. Development of DNA-PK inhibitors for radiosensitisation is not a new field, but many examples suffered from off-target effects and the current challenge is to design a novel, potent and selective DNA-PK inhibitor. Studying the relationship between structural components of an inhibitor and the binding cavities, of DNA-PK, mTOR and PI3K, will help distinguish which features engender selectivity. These features can be designed and explored using chemical synthesis and enzyme inhibition assays to develop structure-activity relationships (SAR). In this project, elements of an inhibitor scaffold, derived from a known PI3K/mTOR inhibitor in literature (BEZ235), are modified to enhance ligand-protein interactions while determining what features dictate selectivity between DNA-PK, mTOR and PI3Kα. The synthesised compounds are tested in enzyme inhibition assays and cellular radiosensitisation assays to determine their activity and selectivity, and their ability to potentiate γ-radiation, respectively.