CD73 and CD39 are cell surface enzymes that catabolize the breakdown of extracellular ATP into adenosine. As ectonucleo-sidases they play important roles in maintaining tissue and immune homeostasis through interfering with the extracellular purigeneric pathway. Since several labs independently have demonstrated an immunosuppressive role of CD73-adenosine in cancer, the CD73-adenosine axis has emerged as one of the most promising therapeutic targets in immuno-oncology.

Adenosine triphosphate (ATP) is the major source of energy for the cell. Malignant cells can release high levels of ATP (e.g. after damage by radiotherapy or chemotherapy). Extracellular ATP provokes inflammation by driving "purinergic signals&rsquo and plays a significant role in promoting anti-tumor responses. Many tumors express the ectonucleotidases CD39 and CD73 to scarve such proinflammatory mediators and generate immunosuppressive adenosine nucleosides.