Immunotherapy targeting PD‑1/PD‑L1: A potential approach for the treatment of cancer bone metastases (Review).

Immune checkpoint molecules, such as programmed cell death 1 (PD‑1) and programmed cell death ligand 1 (PD‑L1), have a critical role in regulating immune responses, including in tumor tissues. Monoclonal antibodies against these molecules, known as immune checkpoint inhibitors (ICIs), have been shown to be effective against a variety of cancers; however, significant patient populations are resistant to such treatment. Clinical studies to date have shown that ICIs are less effective in cancer patients with bone metastases. The effect of anti‑PD‑1/PD‑L1 antibodies on bone metastases, as assessed by the bone metastasis‑specific response classification criteria, was relatively low. In addition, the presence of bone metastases showed a trend toward worse progression‑free survival and overall survival in cancer patients treated with ICIs. To improve the efficacy of ICIs in bone metastases, several combination therapies are under investigation and certain studies have reported better responses. The present review summarizes the current understanding of the effects of anti‑PD‑1/PD‑L1 antibodies on bone metastases based on the reported clinical and preclinical studies.