SCR7

Scr7 is a DNA ligase IV inhibitor, initially identified as an anti-cancer agent [1].
Scr7 targets the DNA binding domain of DNA ligase IV, reducing its affinity for double strand breaks (DSBs) and inhibiting its function. Scr7 also inhibits DNA ligase III (but not DNA ligase I), albeit less efficiently. Cells were treated with doxycycline to induce Cas9 expression, with various concentrations of Scr7 for 24 h. Scr7 maintained cells capable of entering S/G2 phase, which is necessary for HDR. [1] Treatment of mice with Scr7 affects lymphocyte development, as DNA ligase IV plays a key role in the joining of coding ends during V(D)J recombination by means of C-NHEJ16. The defects in lymphocyte development upon Scr7 treatment are transient and reversible, due to the noncovalent mode of binding of Scr7. Scr7 enhanced the frequency of HDR by transiently blocking NHEJ (with the exception of DNA ligase I–dependent alt-NHEJ), resulting in precise genome editing by CRISPR-Cas9 in both cultured cells and in mice. [2]
References:
[1]. Srivastava M, Nambiar M, Sharma S et al. An inhibitor of nonhomologous end-joining abrogates double-strand break repair and impedes cancer progression. Cell. 2012 Dec 21;151(7):1474-87. doi: 10.1016/j.cell.2012.11.054.
[2]. Maruyama T, Dougan SK, Truttmann MC et al.Increasing the efficiency of precise genome editing with CRISPR-Cas9 by inhibition of nonhomologous end joining. Nat Biotechnol. 2015 Mar 23. doi: 10.1038/nbt.3190. [Epub ahead of print]

SCR7 is neither a selective nor a potent inhibitor of human DNA ligase IV.[Pubmed:27235626]

DNA Repair (Amst). 2016 Jul;43:18-23.

DNA ligases are attractive therapeutics because of their involvement in completing the repair of almost all types of DNA damage. A series of DNA ligase inhibitors with differing selectivity for the three human DNA ligases were identified using a structure-based approach with one of these inhibitors being used to inhibit abnormal DNA ligase IIIalpha-dependent repair of DNA double-strand breaks (DSB)s in breast cancer, neuroblastoma and leukemia cell lines. Raghavan and colleagues reported the characterization of a derivative of one of the previously identified DNA ligase inhibitors, which they called SCR7 (designated SCR7-R in our experiments using SCR7). SCR7 appeared to show increased selectivity for DNA ligase IV, inhibit the repair of DSBs by the DNA ligase IV-dependent non-homologous end-joining (NHEJ) pathway, reduce tumor growth, and increase the efficacy of DSB-inducing therapeutic modalities in mouse xenografts. In attempting to synthesize SCR7, we encountered problems with the synthesis procedures and discovered discrepancies in its reported structure. We determined the structure of a sample of SCR7 and a related compound, SCR7-G, that is the major product generated by the published synthesis procedure for SCR7. We also found that SCR7-G has the same structure as the compound (SCR7-X) available from a commercial vendor (XcessBio). The various SCR7 preparations had similar activity in DNA ligation assay assays, exhibiting greater activity against DNA ligases I and III than DNA ligase IV. Furthermore, SCR7-R failed to inhibit DNA ligase IV-dependent V(D)J recombination in a cell-based assay. Based on our results, we conclude that SCR7 and the SCR7 derivatives are neither selective nor potent inhibitors of DNA ligase IV.

Enhanced efficacy of pluronic copolymer micelle encapsulated SCR7 against cancer cell proliferation.[Pubmed:25515310]

Macromol Biosci. 2015 Apr;15(4):521-34.

5,6-Bis(benzylideneamino)-2-mercaptopyrimidin-4-ol (SCR7) is a new anti cancer molecule having capability to selectively inhibit non-homologous end joining (NHEJ), one of the DNA double strand break (DSB) repair pathways inside the cells. In spite of the promising potential as an anticancer agent, hydrophobicity of SCR7 decreases its bioavailability. Herein the entrapment of SCR7 in Pluronic copolymer is reported. The size of the aggregates was determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS) which yields an average diameter of 23 nm. SCR7 encapsulated micelles (ES) were also characterized by small-angle neutron scattering (SANS). Evaluation of its biological properties by using a variety of techniques, including Trypan blue, MTT and Live-dead cell assays, reveal that encapsulated SCR7 can induce cytotoxicity in cancer cell lines, being more effective in breast cancer cell line. Encapsulated SCR7 treatment resulted in accumulation of DNA breaks within the cells, resulting in cell cycle arrest at G1 phase and activation of apoptosis. More importantly, we found approximately 5 fold increase in cell death, when encapsulated SCR7 was used in comparison with SCR7 alone.

DNA repair of myeloma plasma cells correlates with clinical outcome: the effect of the nonhomologous end-joining inhibitor SCR7.[Pubmed:27443291]

Blood. 2016 Sep 1;128(9):1214-25.

DNA repair activity of malignant cells seems to influence therapeutic outcome and patients' survival. Herein, we investigated the mechanistic basis for the link between DNA repair efficiency and response to antimyeloma therapy. Nucleotide excision repair (NER), interstrand cross-links repair (ICL/R), double-strand breaks repair (DSB/R), and chromatin structure were evaluated in multiple myeloma (MM) cell lines (melphalan-sensitive RPMI8226; melphalan-resistant LR5) and bone marrow plasma cells (BMPCs) from MM patients who responded (n = 17) or did not respond (n = 9) to subsequent melphalan therapy. The effect of DSB/R inhibition was also evaluated. Responders' BMPCs showed slower rates of NER and DSB/R (P <0022), similar rates of ICL/R, and more condensed chromatin structure compared with nonresponders. Moreover, apoptosis rates of BMPCs were inversely correlated with individual DNA repair efficiency and were higher in responders' cells compared with those of nonresponders (P = .0011). Similarly, RPMI8226 cells showed slower rates of NER and DSB/R, comparable rates of ICL/R, more condensed chromatin structure, and higher sensitivity than LR5 cells. Interestingly, cotreatment of BMPCs or cell lines with DSB/R inhibitors significantly reduced the rates of DSB/R and increased melphalan sensitivity of the cells, with the nonhomologous end-joining inhibitor SCR7 showing the strongest effect. Together, responders' BMPCs are characterized by lower efficiencies of NER and DSB/R mechanisms, resulting in higher accumulation of the extremely cytotoxic ICLs and DSBs lesions, which in turn triggers the induction of the apoptotic pathway. Moreover, the enhancement of melphalan cytotoxicity by DSB/R inhibition offers a promising strategy toward improvement of existing antimyeloma regimens.

Pluronic copolymer encapsulated SCR7 as a potential anticancer agent.[Pubmed:25608025]

Faraday Discuss. 2015;177:155-61.

Nonhomologous end joining (NHEJ) of DNA double strand breaks (DSBs) inside cells can be selectively inhibited by 5,6-bis-(benzylideneamino)-2-mercaptopyrimidin-4-ol (SCR7) which possesses anticancer properties. The hydrophobicity of SCR7 decreases its bioavailability which is a major setback in the utilization of this compound as a therapeutic agent. In order to circumvent the drawback of SCR7, we prepared a polymer encapsulated form of SCR7. The physical interaction of SCR7 and Pluronic(R) copolymer is evident from different analytical techniques. The in vitro cytotoxicity of the drug formulations is established using the MTT assay.

SCR7 is a DNA Ligase IV inhibitor with anticancer activity and is also a CRISPR HDR enhancer which increases the efficiency of Cas9-mediated HDR.

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