6-thio-2'-deoxyguanosine in GBM: Pre-clinical Evaluation of Mechanism of action, Efficacy and Biomarker identification

PROJECT SUMMARY – Project 1
Gliomas are the most common primary malignant brain tumor in adults and account for over 14,000 deaths
annually in the United States. The most common type of glioma, glioblastoma (GBM) has a median overall
survival of less than 21 months in spite of aggressive therapy. GBMs, like other human cancers, have activated
an enzyme called telomerase that rebuilds the ends of the chromosomes – regions known as telomeres – to
enable the cell’s replicative immortality. Indeed, roughly 90% of GBM cases harbor genetic alterations in the
TERT gene that activate telomerase. Unfortunately, efforts to directly target telomerase activity to date have
been hindered by lack of effective small molecules that cross the blood-brain-barrier, demonstrate on-target
effects, and show efficacy and specificity in GBMs. Therefore, there is a critical need to develop safe and
efficacious telomerase-targeted therapies for patients with GBM whose tumors harbor telomerase activating
genetic alterations.
We previously used the purine analog pro-drug 6-thio-2’-deoxyguanosine (6-thio-dG), which was used in
human clinical trials in the 1970s, to develop a strategy for rapidly inducing telomerase-mediated cytotoxic DNA
damage at telomeres. Rather than inhibiting telomerase and allowing telomeres to get progressively shorter, 6-
thio-dG is taken up by cancer cells and converted into 6-thio-dGTP, which is then incorporated into newly
synthesized telomeric repeats. Once these modified segments accumulate in the telomeres, telomeric DNA
damage rapidly results, ultimately leading to cell death. In pre-clinical models of lung, colon, and melanoma,
treatment with 6-thio-dG led to rapid killing of the cancer cells with little toxicity to normal cells and tissues.
Importantly, telomeric DNA damage induced by 6-thio-dG also enhanced anti-tumor innate immunity. Building
on these data, we have extended our pre-clinical analysis to GBMs and obtained evidence that 6-thio-dG crosses
the blood-brain-barrier. The overall objective for Project 1 is to advance 6-thio-dG toward a clinical trial to be
conducted by Project 2. We propose the following Specific Aims: 1) Characterize the pre-clinical efficacy and
pharmacodynamic biomarkers of 6-thio-dG treatment alone or in combination with Temozolomide (TMZ) in an
extended panel of patient-derived cell lines, PDX and organoid models; 2) Test the anti-tumor efficacy and
inflammatory potential of 6-thio-dG alone and in combination with TMZ or immune checkpoint blockade (ICB)
therapies in immune competent murine models of GBM; and 3) Define cell toxicity and innate inflammatory
potential of 6-thio-dG in an ex vivo glioma tissue framework and patient-derived organoids. These studies will
determine the pre-clinical efficacy of 6-thio-dG in GBM and confirm biomarkers of efficacy that will guide the
design of clinical trials including enrollment criteria. This Project will work closely with the proposed Administrative
Core, Molecular Biomarker Core Resource, and Project 2 to achieve our shared goal of advancing 6-thio-dG
toward clinical application in GBM.

Grant Number: 5U19CA264385-05
NIH Institute/Center: NIH
Principal Investigator: David Ashley