6-Thio-dG

6-thio-dG is a nucleoside analogue [1], is a telomerase-mediated telomere disrupting compound [2]. It is an anti-cancer inhibitor [1]. Cancer cells were very sensitive to 6-thio-dG with observed IC50 values ranging from 0.7-2.9 μM, depending on cell types [3].

Telomeres are found at the end of eukaryotic linear chromosomes. They are essential for genomic stability and chromosome maintenance [3].

In HCT116 human colon cancer cell line, treatment with 6-thio-dG made progressive telomere shortening independent of telomerase activity inhibition and induced telomere dysfunction. GRN163L is a telomerase inhibitor. In HCT116 cells, treatment with GRN163L and 6-thio-dG together increased telomere shortening. Within 1 week, 6-thio-dG killed most of HCT116 cells and altered cellular morphology. Normal BJ fibroblast cells are telomerase silent. After 1 week, treatment with 6-thio-dG showed no effect on cell morphology. After long-term treatment with 6-thio-dG, no effect on telomere shortening was found [1].

In murine mode with xenograft derived from A549 lung cancer cell line, as compared to controls, intraperitoneal injection with 2 mg/kg of 6-thio-dG every other day completely prevented progressive tumor growth. Ki67 is a biomarker correlating with proliferation levels. Compared to controls, 6-thio-dG decreased Ki67 staining. Treatment with 6-thio-dG through local injection resulted in even more dramatic decrease in the tumor growth rate compared to untreated controls [3].

References:
[1].  Mender I, Gryaznov S, Dikmen ZG, et al. Abstract LB-125: A novel telomerase inhibitor. Cancer Research, 2013, 73(8 Supplement): LB-125-LB-125.
[2].  Mender I, Gryaznov S, Shay JW. A novel telomerase substrate precursor rapidly induces telomere dysfunction in telomerase positive cancer cells but not telomerase silent normal cells. Oncoscience, 2015, 2(8): 693.
[3].  Mender I, Gryaznov S, Dikmen ZG, et al. Induction of telomere dysfunction mediated by the telomerase substrate precursor 6-thio-2-deoxyguanosine. Cancer discovery, 2015, 5(1): 82-95.

Induction of telomere dysfunction mediated by the telomerase substrate precursor 6-thio-2'-deoxyguanosine.[Pubmed:25516420]

Cancer Discov. 2015 Jan;5(1):82-95.

UNLABELLED: The relationships between telomerase and telomeres represent attractive targets for new anticancer agents. Here, we report that the nucleoside analogue 6-thio-2'-deoxyguanosine (6-Thio-dG) is recognized by telomerase and is incorporated into de novo-synthesized telomeres. This results in modified telomeres, leading to telomere dysfunction, but only in cells expressing telomerase. 6-Thio-dG, but not 6-thioguanine, induced telomere dysfunction in telomerase-positive human cancer cells and hTERT-expressing human fibroblasts, but not in telomerase-negative cells. Treatment with 6-Thio-dG resulted in rapid cell death for the vast majority of the cancer cell lines tested, whereas normal human fibroblasts and human colonic epithelial cells were largely unaffected. In A549 lung cancer cell-based mouse xenograft studies, 6-Thio-dG caused a decrease in the tumor growth rate superior to that observed with 6-thioguanine treatment. In addition, 6-Thio-dG increased telomere dysfunction in tumor cells in vivo. These results indicate that 6-Thio-dG may provide a new telomere-addressed telomerase-dependent anticancer approach. SIGNIFICANCE: Telomerase is an almost universal oncology target, yet there are few telomerase-directed therapies in human clinical trials. In the present study, we demonstrate a small-molecule telomerase substrate approach that induces telomerase-mediated targeted "telomere uncapping," but only in telomerase-positive cancer cells, with minimal effects in normal telomerase-negative cells.

Agonists of Toll-like receptor 9 containing synthetic dinucleotide motifs.[Pubmed:17988082]

J Med Chem. 2007 Dec 13;50(25):6411-8.

Oligodeoxynucleotides (ODNs) containing unmethylated CpG motifs activate Toll-like receptor 9 (TLR9). Our previous studies have shown that ODNs containing two 5'-ends are more immunostimulatory than those with one 5'-end. In the present study, to understand the role of functional groups in TLR9 recognition and subsequent immune response, we substituted C or G of a CpG dinucleotide with 5-OH-dC, 5-propyne-dC, furano-dT, 1-(2'-deoxy-beta- d-ribofuranosyl)-2-oxo-7-deaza-8-methyl-purine, dF, 4-thio-dU, N(3)-Me-dC, N (4)-Et-dC, Psi-iso-dC, and arabinoC or 7-deaza-dG, 7-deaza-8-aza-dG, 9-deaza-dG, N(1)-Me-dG, N(2)-Me-dG, 6-Thio-dG, dI, 8-OMe-dG, 8-O-allyl-dG, and arabinoG in ODN containing two 5'-ends. Agonists of TLR9 containing cytosine or guanine modification showed activity in HEK293 cells expressing TLR9, mouse spleen, and human cell-based assays and in vivo in mice. The results presented here provide insight into which specific chemical modifications at C or G of the CpG motif are recognized by TLR9 and the ability to modulate immune responses substituting natural C or G in immune modulatory oligonucleotides.