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  • Compounds that interact with MDR can do so by


    Compounds that interact with MDR1 can do so by different mechanisms. Verapamil is known to modulate drug resistance by acting as a competitive MDR1 substrate [36]. Interestingly, NU7441 has similar growth inhibitory activity in the sensitive and resistant Zebularine and there was no observed reduction in intracellular levels of NU7441 over time (Fig. 2, Fig. 6). This result suggests that NU7441 may not act as a competitive substrate but rather as an inhibitor of MDR1. However, further investigations would be necessary to definitively prove this interesting finding. Although the clinical relevance of MDR1 expression in solid tumours remains controversial, MDR1 has been shown to be relevant to acquired drug resistance in haematological cancers [37]. MDR is a biomarker for poor prognosis in patients with haematological cancers who can be cross-resistant to standard chemotherapeutic agents that are MDR1 substrates. The MDR substrates doxorubicin, etoposide and vincristine are used clinically in the treatment of haematological malignancies, such as acute myeloid leukaemia (AML) and acute lymphoblastic leukaemia (ALL) [38], [39], [40]. NU7441, in its capacity as a DNA-PK inhibitor, has been shown to sensitise cells to mitoxantrone, doxorubicin (in vitro) and etoposide (in vitro and in vivo) [7], [9], and this study has demonstrated that NU7441 can sensitise CCRF-CEM cells to vincristine.
    Acknowledgements These studies were supported by a Cancer Research UK PhD Studentship and Cancer Research UK Centre and Programme Awards. The authors would like to thank Professor Iain McNeish (University of Glasgow) for kindly providing the SKOV3 and SKOV3-TR, and the A2780 and A2780-TX1000, paired cell lines for use in this study.
    Introduction DNA-PK is a serine/threonine nuclear kinase machinery consists of a 70/80-kDa regulatory heterodimer protein called Ku, and a 470-kDa catalytic subunit, termed as DNA-PKcs [1]. The carboxy-terminal of the catalytic subunit (∼500 residues) belongs to the phosphatidylinositol-3 (PI-3) kinase-like kinase (PIKK) family [2], [3]. One of the most severe lesions that may affect the human genome is the DNA double strand break (DSB) that emerges as a consequence of exposure to different clastogens [4], [5]. The eukaryotic cells responded to such kind of lesions by initiating one of two major repair mechanisms namely, homologous recombination (HR) and non-homologous DNA end joining (NHEJ). It is estimated that around ten double-strand breaks occur per each cell per day, where some of them (such as replication across the neck, chromatid breaks during the S-phase) are fixed by the action of homologous recombination mechanism [6]. On the other hand, in the absence of nearby DNA homology donor, the NHEJ DNA-repair mechanism will be in effect. Moreover, improper response to DSBs can lead to genomic instability and carcinogenic events, while, unrepaired breaks usually induce apoptosis and cell death [7]. DNA-PK plays an essential role in protecting genome stability during the events of double-strand break (DSB), and considered crucial component in NHEJ repair pathway. Where, in such pathway, DNA-PKcs is recruited at the site of DSB by the Ku protein, which in turn recruits and phosphorylates a nuclease called Artemis that cleans the DNA ends before ligation with XRCC4/ligase IV complex [8], [9]. Anticancer agents such as ionizing radiation and chemotherapy eradicate cancer cells by inducing different types of DNA lesions. Among these lesions, DSB is the most important in terms of cell death. Experimental evidences demonstrated that elevated DSB repair activity is correlated with the resistance shown by different cancer cells toward these agents [10], [11], [12]. On the other hand, hypersensitivity to ionizing radiation and enhancement of chemotherapeutics effect were seen in mutant cell lines known to be DNA-PKcs deficient [13]. A number of small molecule inhibitors of the DNA-PK were identified over the years (Fig. 1). The potent sterol fungal metabolite wortmannin was found to exhibit irreversible inhibition pattern against members of the PIKK family including; DNA-PK, ATM and ATR [14], [15]. The naturally-inspired chromone derivative LY294002 and the product of three-substituted indolin-2-one library screening SU11752 were found to have higher selectivity toward the DNA-PK than other members of the family showing reversible competitive inhibition pattern within the ATP-binding site [16], [17]. Structural investigations based on the earlier compounds led to discovery of a chromenone derivative named NU7026, which is considered as the first selective/reversible inhibitor of DNA-PK with an IC50 of 0.23μM (Fig. 2). Cancer cells exposed to the effect of NU7026 showed G2/M arrest and impairment in their double-strand break repair mechanism [18].