Advances of molecular imaging of immune checkpoint targeting PD-1/PD-L1 in tumors

Programmed cell death-1 (PD-1) and its ligand (PD-L1) inhibitors can inhibit the PD-1 and PD-L1 combination to block negative control signals. This condition leadings to the activity of T lymphocytes and the enhancement of immune responses. Clinical studies have shown that PD-1/PD-L1 inhibitors have a definite effect on a broad set of malignancies, but a substantial number of patients do not respond. Predicting the immunotherapy response by evaluating the expression of PD-1 and PD-L1 is urgently needed. Molecular imaging modalities, such as PET, SPECT, optical imaging, and MRI, can noninvasively and dynamically detect the overall expression of PD-1/PD-L1 in patients. They can help in selecting patients who are suitable for immunotherapy and monitor the tumor response. This review describes the current status and progress of molecular imaging targeting PD-1/PD-L1 in tumors.