CD73 (5’-nucleotidase or NT5E) is a cell surface enzyme which catalyzes the extracellular hydrolysis of adenosine monophosphate (AMP) to produce anti-inflammatory and immunosuppressive adenosine. Immunosupressive adenosine contributes to immune evasion in solid tumors by accumulating within the tumor microenvironment and thereby inhibiting immune effector cells (e.g., CD8+ T cells). As a novel checkpoint protein, CD73 is overexpressed in the immune system of various tumors, where adenosine is abundantly enriched. CD73 expression levels correlate with a worse prognosis in specific cancer types, including breast cancer. Adenosine production via CD73 can be therapeutically inhibited by two independent mechanisms: (1) enzyme inhibition, and (2) receptor internalization and subsequent degradation. The reduction of adenosine restores immune function. Once the immunosuppressive effects of adenosine are removed, activation of antitumor immune cells may be restored resulting in tumor cell death.
A large number of monoclonal antibodies (mAbs), nucleotides, and non-nucleotides as potent CD73 inhibitors are being discovered, providing opportunities for novel tumor immunotherapy. CD73 inhibitors in clinical trials show promising results in combination therapy for various solid tumors. The development of CD73-specific companion positron emission tomography imaging ligands holds potential for facilitating diagnosis, patient selection, and treatment efficacy evaluation throughout the entire process of CD73-targeted therapeutic development.

