Hypoxia-activated prodrugs of phenolic olaparib analogues for tumour-selective chemosensitisation

Poly(ADP-ribose)polymerase inhibitors (PARPi) are used for treatment of tumours with a defect in homologous recombination (HR) repair. Combination with radio- or chemotherapy could broaden their applicability but a major hurdle is enhancement of normal tissue toxicity. Development of hypoxia-activated prodrugs (HAPs) of PARPi has potential to restrict PARP inhibition to tumours thereby avoiding off-target toxicity. We have designed and synthesised phenolic derivatives of olaparib (termed phenolaparibs) and corresponding ether-linked HAPs. Phenolaparib cytotoxicity in HR-proficient and deficient cell lines was consistent with inhibition of PARP-1. Prodrugs were deactivated relative to phenolaparibs in biochemical PARP-1 inhibition assays, and cell culture. Prodrug 7 was selectively converted to phenolaparib 4 under hypoxia and demonstrated hypoxia-selective cytotoxicity, including chemosensitisation of HR-proficient cells in combination with temozolomide. This work demonstrates the feasibility of a HAP approach to PARPi for use in combination therapies. Hypoxia-activated prodrugs of phenolic olaparib analogues are deactivated in oxic cell culture and cytotoxicity is restored under hypoxia. Temozolomide combination studies suggest a feasible route to PARP inhibitor use beyond synthetic lethality.