Thiopurine 6TG treatment increases tumor immunogenicity and response to immune checkpoint blockade

ABSTRACT

Immune-checkpoint inhibitors (ICI) are highly effective in reinvigorating T cells to attack cancer. Nevertheless, a large subset of patients fails to benefit from ICI, partly due to lack of the cancer neoepitopes necessary to trigger an immune response. In this study, we used the thiopurine 6-thioguanine (6TG) to induce random mutations and thus increase the level of neoepitopes presented by tumor cells. Thiopurines are prodrugs which are converted into thioguanine nucleotides that are incorporated into DNA (DNA-TG), where they can induce mutation through single nucleotide mismatching. In a pre-clinical mouse model of a mutation-low melanoma cell line, we demonstrated that 6TG induced clinical-grade DNA-TG integration resulting in an improved tumor control that was strongly T cell dependent. 6TG exposure increased the tumor mutational burden, without affecting tumor cell proliferation and cell death. Moreover, 6TG treatment re-shaped the tumor microenvironment by increasing T and NK immune cells, making the tumors more responsive to immune-checkpoint blockade. We further validated that 6TG exposure improved tumor control in additional mouse models of melanoma. These findings have paved the way for a phase I/II clinical trial that explores whether treatment with thiopurines can increase the proportion of otherwise treatment-resistant cancer patients who may benefit from ICI therapy (NCT05276284).

KEYWORDS:

• Thiopurine
• 6TG
• immune checkpoint inhibitors
• melanoma
• mouse model

Acknowledgments

The authors acknowledge the staff of the Animal Facility at the Danish Cancer Society, the Histocore Facility at Copenhagen University, the Center for Genomic Medicine at Copenhagen University Hospital, and Laila Fischer for help with secretarial work. This project has received financial support from The Danish Ministry of Health (Sundhedsstyrelsen Grant no: 05-0400-45), The Danish Childhood Cancer Foundation (Børnecancerfonden 2019-5963, 2019-5934, 2020-5769). The work is part of the Danish nation-wide research program Childhood Oncology Network Targeting Research, Organisation & Life expectancy (CONTROL) and supported by the Danish Cancer Society (R204-A12424 to D.D.Z.; R-257-A14720 to K.S.). D.D.Z. is supported by the Melanoma Research Alliance (MRA 620385) and the LEO Foundation (LF-OC-19-000004). The Melanoma Research Team is part of the Center of Excellence for Autophagy, Recycling and Disease (CARD), funded by Danmarks Grundforskningsfond (DNRF125).

Disclosure statement

The authors declare no potential conflicts of interests.

Data availability statement

The WGS and RNAseq datasets generated and analysed during the current study are currently deposited on Computerome, the Danish National Computer for Life Sciences, and will be available from the corresponding author on request. https://computerome.dtu.dk

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/2162402X.2022.2158610

Additional information

Funding

This work was supported by the Børnecancerfonden [2019-5963]; Børnecancerfonden [2019-5934]; Børnecancerfonden [2020-5769]; Danmarks Grundforskningsfond [DNRF125]; LEO Fondet [LF-OC-19-000004]; Melanoma Research Alliance young investigator [MRA 620385]; Sundhedsstyrelsen [05-0400-45]; the Danish Cancer Society [R204-A12424]; the Danish Cancer Society [R-257-A14720].