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.