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Regulatory T cells (Treg) critically maintain immuno-suppression in the tumor microenvironment, representing an attractive immuno-oncology target. The Treg lineage is defined by expression of the FOXP3 transcription factor, which controls immune-suppressive functions. We have developed the clinical candidate AZD8701, a next-generation antisense oligonucleotide inhibitor of FOXP3 (utilizing the Ionis Gen 2.5 cEt-modified ASO platform). AZD8701 treatment knocked down FOXP3 in primary human Tregs via free uptake (IC50 65nM), which was also associated with modulation of known FOXP3 target genes including 25-50% reduction in CTLA4, ICOS, CCR8 and GITR. Tregs treated with AZD8701 further exhibited reduced suppressive functions in in vitro suppression assays, which confirmed the functional effects of FOXP3 modulation. Finally, AZD8701 promoted dose-dependent knockdown of FOXP3 in humanized mice, including >50% FOXP3 knockdown at doses that can be feasibly achieved with the Gen 2.5 ASO platform in the clinic. To support the importance of FOXP3 in immuno-oncological settings, we characterized murine surrogate FOXP3 ASOs in the context of syngeneic tumour bearing mice. Murine FOXP3 ASOs similarly promoted >50% FOXP3 knockdown in mice and were well tolerated with no overt toxicological findings at high doses, over a maximum of 12 weeks of treatment. Murine FOXP3 ASOs significantly attenuated tumour growth in A20 and ID8-VEGF syngeneic models, which was associated with some complete tumour regressions. Moreover, we found that mouse surrogate FOXP3 ASOs promoted additive/enhanced therapeutic effects when combined with immune checkpoint blockade. Collectively, FOXP3 ASOs represent a first-in-class strategy to target Tregs in cancer in a highly selective manner. The clinical application of AZD8701 may provide therapeutic benefit to patients either as a monotherapy or in combination with immune checkpoint blocking agents. Citation Format: Charles Sinclair, Alexey S. Revenko, R B. Johnson, Alison Peter, Molly A. Taylor, Lisa A. Hettrick, Stephanie Klein, Anisha Solanki, Melissa Chapman, James Yates, Helen K. Angell, Andrew Watt, Danielle Gattis, Brett P. Monia, Simon T. Barry, Paul Lyne, Mark Edbrooke, Frederick Goldberg, A R. Macleod. Discovery and characterization of AZD8701, a high affinity antisense oligonucleotide targeting FOXP3 to relieve immunosuppression in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2713.