Objectives: The dosing pattern of pembrolizumab is based on population pharmacokinetic (Pop-PK) analysis of clinical trials. Data for Japanese patients or patient populations with poor conditions such as cachexia are scarce. In this study, we performed a Pop-PK analysis of Japanese non-small cell lung cancer patients and analyzed the relationship between exposure, treatment effect, and survival., Materials and Methods: A total of 270 blood samples from 76 patients who received 200 mg pembrolizumab every 3 weeks between March 2017 and December 2018 were included. Blood concentrations of pembrolizumab were measured using mass spectrometry, and Pop-PK analysis was conducted using the Phoenix NLME software with a one-compartment model., Results: The estimated median of clearance (CL) in this analysis population was 0.104 L/day, about half of the historical data for Western data. Overall, pembrolizumab CL decreased over time, with some populations showing increased CL early in the treatment and others showing decreased CL over time. When the time-varying CL was stratified by quartile, the group with decreasing CL showed significantly better treatment response and survival than the group with increasing CL, even though the group included more patients with cachexia. Detailed analysis suggested that the patient population that responded to pembrolizumab treatment had an improved general condition and reduced protein catabolism, further decreasing CL., Conclusion: In populations that benefit from pembrolizumab treatment, CL may be reduced early in their treatment, which may be a predictive and prognostic factor. However, further prospective validation of our findings is needed., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Dr. Yagishita has received grants from Nippon Boehringer Ingelheim. Dr. Masuda received honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events from ONO, Astra Zeneca, Chugai, Bristol Myers Squibb, and Healios. Dr Shinno received grants from ONO PHARMACEUTICAL, Janssen Pharmaceutical, Japan Clinical Research Operations, TAIHO PHARMACEUTICAL and honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events from AstraZeneca, Bristol Myers Squibb, CHUGAI PHARMACEUTICAL, ONO PHARMACEUTICAL, and Eli Lilly Japan. Dr. Yoshida has received grants and personal fees from AstraZeneca, Bristol-Myers Squibb, grants from Abbvie, MSD, Ono Pharmaceutical, Takeda Pharmaceutical, and personal fees from Chugai, Novartis. Dr. Okuma has received grants from Chugai, AbbVie, Ono and honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events from AstraZeneca, Chugai, Eli-Lilly. Dr. Goto has received grants AstraZeneca, Pfizer, IQVIA, MSD, Astrazeneca, Abbvie, Amgen, CReS Kyushu, Ono, Cmic, Daiichi Sankyo, Takeda, Chugai, Eli Lilly, Novartis, Pfizer, Mebix, BMS, Medpace, Merck BioPharma, Janssen, Preferred network, and Payment or honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events from Chugai,Taiho, Boehringer Ingelheim, Ono Pharmaceutical, Bristol Myers Squibb, Pfizer, MSD, Novartis. Merck, Thermo Fischer, Astrazeneca, Kyowa Kirin, MSD, Creative therapeutical, GSK, Taiho, Chugai, Novartis, Eli Lilly, Johnson & Johnson, Guardant Health, Nippon Kayaku, Daiichi Sankyo, BMS, Janssen, Towa, Takeda and participation on a Data Safety Monitoring Board or Advisory Board from AstraZeneca, Chugai, Boehringer Ingelheim, Eli Lilly, Taiho, Pfizer, Novartis, Guardant Health Inc., Illumina, DaiichiSankyo, Ono Pharmaceutical, Bristol Myers Squibb, MSD and Leadership or fiduciary role from Cancer Net Japan, JAMT. Dr. Horinouchi has received grants and personal fees from AstraZeneca, BMS, Chugai, Eli Lilly, MSD, Taiho Pharmaceutical, Ono Pharmaceutical, and grants from Astellas, Genomic Health, Merck Serono. Dr. Yamamoto has received grants and personal fees from BMS, Boehringer Ingelheim, Chugai, Eisai, Eli Lilly, Ono Pharmaceutical, Pfizer, Takeda Pharmaceutical, grants from Astellas, Bayer, Chiome Bioscience Inc., Daiichi-Sankyo, GSK, Janssen Pharma, Kyowa-Hakko kirin, MSD, Merck, Novartis, Otsuka, Taiho Pharmaceutical, Quintiles, Sumitomo Dainippon, and personal fees from AstraZeneca, Otsuka, Cimic, Sysmex. Dr. Ohe received grants from AstraZeneca, Chugai, Lilly, ONO, BMS, Kyorin, Dainippon- Sumitomo, Pfizer, Taiho, Novartis, Ignyta, Takeda, Kissei, Daiichi-Sankyo, Janssen, LOXO and consulting fee from AstraZeneca, Chugai, ONO, BMS, Kyorin, Celltrion, Amgen, Nippon Kayaku and honoraria for lectures, presentations, speakers bureaus, manuscript writing or educational events from AstraZeneca, Chugai, Eli Lilly, ONO, BMS,Boehringer Ingelheim, Bayer, Pfizer, MSD, Taiho, Nippon Kayaku, Kyowa Hakko Kirin and board member of JSMO, JLCS, and JCOG. Dr. Hamada received grants from Eisai, Chordia Therapeutics, Daiichi-sankyo, Tosoh, Konica Minolta, Healios, Eli Lilly, Sysmex, ThermoFisherScientific, and Chugai. The remaining authors declare no competing interests., (Copyright © 2022 Elsevier B.V. All rights reserved.)