br Acknowledgements The authors are supported by National

Acknowledgements The authors are supported by National Institutes of Health grants (CA162804, CA92584, and CA13499 to DJC and CA166677 to BPPC) and Cancer Prevention and Research Institute of Texas (RP110465 to DJC).
The cellular response to DNA double-strand break (DSB) formation is an essential component of normal cell survival, following exposure to DNA-damaging chemicals and ionising radiation. The DNA-dependent protein kinase (DNA-PK), a member of the phosphatidylinositol (PI) 3-kinase-related kinase (PIKK) family, plays an important role in DNA DSB repair via the non-homologous end-joining (NHEJ) pathway., , The ability of DNA-PK to detect and signal the repair of DNA damage may also protect cancer Protease Inhibitor Cocktail (EDTA-Free, 200X in DMSO) synthesis from the cytotoxic effects of DNA-damaging cancer therapies. Accordingly, inhibition of DNA-PK has been demonstrated to potentiate the cytotoxicity of ionising radiation and a number of DSB-inducing antitumour agents in vitro., A major objective of our research is the development of potent and selective DNA-PK inhibitors, suitable for clinical evaluation as chemo- and radio-sensitisers in the treatment of cancer. In the absence of suitable structural biology information for DNA-PK, inhibitor design has been guided by a combination of homology modelling, utilising the known crystal structure of PI 3-kinase, and pharmacophore mapping based on the non-selective DNA-PK inhibitor LY294002 (). These initial studies enabled the elucidation of structure–activity relationships (SARs) for DNA-PK inhibition, and the discovery of potent and selective chromenone inhibitors, exemplified by NU7026 (). Encouraged by the potency and kinase-selectivity of and related compounds, a systematic variation of the substitution pattern on the chromenone pharmacophore was undertaken, employing a solution-phase multiple-parallel synthesis approach for the preparation of focused compound libraries., NU7441 () was prominent amongst several hits emanating from the library screen, with an independent resynthesis confirming the high potency (IC=12nM) and DNA-PK selectivity of this chromenone. Preclinical antitumour studies with have demonstrated that this DNA-PK inhibitor potentiates the in vivo cytotoxicity of topoisomerase inhibitors in a human tumour model. Although less potent than NU7441 as a DNA-PK inhibitor, the 8-(biphenyl-3-yl)chromen-4-one derivative (; IC=180nM) was also identified from the library as a potentially interesting structural lead. In particular, the non-planar biphenyl motif of offered the opportunity to probe alternative regions of the ATP-binding domain of the kinase. In this letter, we report the results of studies designed to elucidate preliminary SARs for the 8-biarylchromenone pharmacophore varied with respect to three parameters as shown in , namely the substituent R on the 3′-phenyl group, and the nature of the ‘proximal’ and ‘distal’ aryl groups. For this purpose, the core chromen-4-one scaffold and the 2-morpholin-4-yl substituent were retained. A solution-phase multiple-parallel synthesis approach was employed for the preparation of libraries of the required 8-biarylchromenones, several members of which were found to exhibit potency comparable with the benchmark DNA-PK inhibitor NU7441 (). Our previous studies have utilised the chromenone triflate derivative as a key reagent for the preparation of 8-substituted chromenone libraries, employing Suzuki–Miyaura palladium-catalysed cross-coupling reactions., , , This strategy was also amenable for the synthesis of the target substituted 8-biaryl-3-ylchromenones () through analogous reactions on the triflate building block , which was readily accessible from (). For ease of manipulation, and with a view to preparing on a convenient scale, initial studies utilised 3-benzyloxyphenylboronic acid (), which was available from 3-bromophenol by standard methods. Coupling of with gave the chromenone in high yield, and subsequent removal of the benzyl protecting group by hydrogenation afforded the required phenol .