The adenosine pathway in immuno-oncology

Signalling induced by extracellular adenosine (eADO) can suppress antitumour immunity through multiple mechanisms. Herein, the authors review the pathophysiological functions of eADO in cancer and the related prognostic implications. They discuss the associated opportunities for eADO pathway-targeted immunotherapy, highlighting potential limitations and the scope for combination and biomarker-based strategies. The data emerging from oncology clinical trials of the diverse range of therapies that have been developed to target the eADO signalling pathway are also described. Cancer immunotherapy based on immune-checkpoint inhibition or adoptive cell therapy has revolutionized cancer care. Nevertheless, a large proportion of patients do not benefit from such treatments. Over the past decade, remarkable progress has been made in the development of 'next-generation' therapeutics in immuno-oncology, with inhibitors of extracellular adenosine (eADO) signalling constituting an expanding class of agents. Induced by tissue hypoxia, inflammation, tissue repair and specific oncogenic pathways, the adenosinergic axis is a broadly immunosuppressive pathway that regulates both innate and adaptive immune responses. Inhibition of eADO-generating enzymes and/or eADO receptors can promote antitumour immunity through multiple mechanisms, including enhancement of T cell and natural killer cell function, suppression of the pro-tumourigenic effects of myeloid cells and other immunoregulatory cells, and promotion of antigen presentation. With several clinical trials currently evaluating inhibitors of the eADO pathway in patients with cancer, we herein review the pathophysiological function of eADO with a focus on effects on antitumour immunity. We also discuss the treatment opportunities, potential limitations and biomarker-based strategies related to adenosine-targeted therapy in oncology.