**Keisuke Onoi, Yusuke Chihara, Junji Uchino \*, Takayuki Shimamoto, Yoshie Morimoto, Masahiro Iwasaku, Yoshiko Kaneko, Tadaaki Yamada and Koichi Takayama**

Department of Pulmonary Medicine, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kyoto 602-8566, Japan; onoi@koto.kpu-m.ac.jp (K.O.); c1981311@koto.kpu-m.ac.jp (Y.C.); m04035ts@koto.kpu-m.ac.jp (T.S.); yoshie-m@koto.kpu-m.ac.jp (Y.M.); miwasaku@koto.kpu-m.ac.jp (M.I.); kaneko-y@koto.kpu-m.ac.jp (Y.K.); tayamada@koto.kpu-m.ac.jp (T.Y.); takayama@koto.kpu-m.ac.jp (K.T.) **\*** Correspondence: uchino@koto.kpu-m.ac.jp; Tel.: +81-75-251-5513

Received: 26 March 2020; Accepted: 4 May 2020; Published: 6 May 2020

**Abstract:** The treatment of lung cancer has changed drastically in recent years owing to the advent of immune checkpoint inhibitors (ICIs). A 1992 study reported that programmed cell death-1 (PD-1), an immune checkpoint molecule, is upregulated during the induction of T cell death. Since then, various immunoregulatory mechanisms involving PD-1 have been clarified, and the successful use of PD-1 blockers in anticancer therapy eventually led to the development of the current generation of ICIs. Nivolumab was the first ICI approved for treating lung cancer in 2014. Since then, various ICIs such as pembrolizumab, atezolizumab, and durvalumab have been successively introduced into clinical medicine and have shown remarkable efficacy. The introduction of ICIs constituted a major advancement in lung cancer treatment, but disease prognosis continues to remain low. Therefore, new molecular-targeted therapies coupled with existing anticancer drugs and radiotherapy have recently been explored. This review encompasses the current status, challenges, and future perspectives of ICI treatment in lung cancer.

**Keywords:** immune checkpoint inhibitors; non-small-cell lung cancer; PD-1; biomarker
