Background: Osimertinib is a third-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI) that is an established standard treatment option for chemotherapy-naive patients with EGFR mutation-positive non-small cell lung cancer (NSCLC). However, of such patients who have received prior treatment with a first- or second-generation EGFR TKI, only approximately half are eligible for osimertinib therapy because its indication as second-line treatment and beyond is limited to metastatic NSCLC that is positive for the T790M resistance mutation of the EGFR gene. This study was initiated at the request of a dedicated network for patients with lung cancer in Japan., Methods: We conducted a phase II study to assess the efficacy of osimertinib in patients with EGFR mutation-positive NSCLC in whom systemic disease (T790M-negative) progressed after treatment with first- or second-generation EGFR TKIs and platinum-based chemotherapy. The primary end point was response rate (assessed by a central imaging reviewer)., Results: From August 2020 to February 2021, 55 patients from 15 institutions were enrolled in the study. The overall response for primary analysis was achieved in 16 patients (29.1 %; 95 % CI, 17.6-42.9), which exceeded the threshold response rate necessary for analysis. Stable disease was found in 16 patients (29.1 %), and progressive disease, in 18 (32.7 %). The median length of progression-free survival (PFS) was 4.07 months (95 % CI 2.10-4.30), and the rate of 12-month PFS was 17.3 %., Conclusions: Osimertinib demonstrated modest antitumor activity against progressive EGFR T790M-negative disease., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Dr. Takeda reports personal fees from Chugai Pharmaceutical Co., LTD., AstraZeneca K.K., Bristol-Myers Squibb Company, Novartis Pharma K.K., and Ono Pharmaceutical Co.,LTD. Dr Shimokawa has no conflicts of interest. Dr. Nakamura reports personal fees from MSD, AstraZeneca, Chugai Pharma, Kyowa Hakko Kirin, Nippon Boehringer Ingelheim, and Taiho Pharmaceutical. Dr. Nosaki reports personal fees from AstraZeneca K.K. Dr Watanabe has no conflicts of interest. Dr. Kato reports grants from Abbvie, Amgen, AstraZeneca, Blueprint, Chugai, Eli Lilly, Haihe, Merck Biopharma, MSD, Novartis, Pfizer, Regeneron, and Takeda, and personal fees Abbvie, Amgen, AstraZeneca, Beigene, Boehringer Ingelheim, Bristol-Myers Squibb, Chugai, Daiichi Sankyo, Eli Lilly, Glaxo, Merck Biopharma, MSD, Nippon Kayaku, Novartis, Ono, Pfizer, Roche, Taiho, and Takeda. Dr. Hayakawa has no conflicts of interest. Dr. Tanaka reports grants and personal fees from Astra Zeneca, Chugai Pharmaceutical, MSD, Ono Pharmaceutical, Bristol-Myers Squibb, Daiichi Sankyo Pharmaceutical, Eli Lilly, Takeda Pharmaceutical, Taiho Pharmaceutical, Merck, and Boehringer Ingelheim; grants from Amgen and Abbvie; and personal fees from Sun Pharmaceutical, Eisai, and Novartis. Dr. Takahashi reports grants from AstraZeneca KK, Chugai Pharmaceutical Co., LTD., Eli Lilly Japan K.K., Ono Pharmaceutical Co., LTD., MSD K.K., Pfizer Japan Inc., Amgen Inc., Boehringer Ingelheim Japan, and Merck Biopharma Co., LTD, and personal fees from AstraZeneca KK, Chugai Pharmaceutical Co., LTD., Eli Lilly Japan K.K., Ono Pharmaceutical Co., LTD., MSD K.K., Pfizer Japan Inc., Boehringer Ingelheim Japan, INC, Roche Diagnostics K.K., Takeda Pharmaceutical Co LTD., and Yakult Honsha CO. LTD. Dr. Oki reports research grants from MSD K.K., PAREXEL International Corp, Sanofi K.K., AstraZeneca K.K., and Janssen Pharmaceutical K.K. Dr. Tachihara reports grants and personal fees from AstraZeneca and personal fees from Taiho Pharmaceutical, MSD, Eli Lilly Japan K.K, Nippon Boehringer Ingelheim, Chugai Pharmaceutical, and Ono Pharmaceutical. Dr. Fujimoto reports personal fees from AstraZeneca KK, Ono Pharmaceutical Co Ltd, Bristol-Myers Squibb Co Ltd, Taiho Pharmaceutical Co Ltd, Chugai Pharmaceutical Co Ltd, MSD KK, Boehringer Ingelheim Japan Inc, Eli Lilly Japan KK, and Novartis Pharma K.K. Dr. Hayashi reports grants from AstraZeneca K.K., Astellas Pharma Inc., MSD K.K., Ono Pharmaceutical Co., Ltd., Nippon Boehringer Ingelheim Co., Ltd., Novartis Pharma K.K., grants, Pfizer Japan Inc., Bristol-Myers Squibb Company, Eli Lilly Japan K.K., Chugai Pharmaceutical Co., Ltd., Daiichi Sankyo Co., Ltd., Merck Serono Co., Ltd./ Merck Biopharma Co., Ltd., Takeda Pharmaceutical Co., Ltd., Taiho Pharmaceutical Co., Ltd., SymBio Pharmaceuticals Limited., AbbVie Inc, inVentiv Health Japan, ICON Japan K.K., GRITSONE ONCOLOGY.INC, PAREXEL International Corp., Kissei Pharmaceutical Co., Ltd., EPS Corporation., Syneos Health., Pfizer R&D Japan G.K., A2 Healthcare Corp., Quintiles Inc./IQVIA Services JAPAN K.K., EP-CRSU CO., LTD., Linical Co., Ltd., Eisai Co., Ltd., CMIC Shift Zero K.K., and Kyowa Hakko Kirin Co., Ltd, and personal fees from Bayer Yakuhin, Ltd, EPS International Co., Ltd., Otsuka Pharmaceutical Co., Ltd., Amgen K.K, AstraZeneca K.K., Boehringer Ingelheim Japan Inc., Bristol-Myers Squibb Co. Ltd., Chugai Pharmaceutical Co. Ltd., Daiichi Sankyo Co. Ltd., Eli Lilly Japan K.K., Guardant healthcare, Kyorin Pharmaceutical Co. Ltd., Merck Biopharma Co. Ltd., MSD K.K., Novartis Pharmaceuticals K.K., Ono Pharmaceutical Co. Ltd., Shanghai Haihe Biopharm, Taiho Pharmaceutical Co. Ltd., Pfizer, and Takeda Pharmaceutical Co. Ltd. Dr. Yamaguchi has no conflicts of interest. Dr. S. Yamamoto have no conflicts of interest. Dr. Iwama reports personal fees from AstraZeneca. Dr. Azuma reports personal fees from AstraZeneca, grants and personal fees from Boehringer Ingelheim, Ono Pharmaceutical, MSD Oncology, Bristol-Myers Squibb, Chugai Pharma. Dr. N. Yamamoto reports grants and personal fees from MSD, personal fees from AstraZeneca, grants and personal fees from Ono Pharmaceutical Co., Ltd., personal fees from Thermo Fisher Scientific, grants and personal fees from Daiichi Sankyo, grants and personal fees from Taiho Pharmaceutical Co., Ltd., grants and personal fees from Takeda Pharmaceutical Co., Ltd., grants and personal fees from Chugai Pharmaceutical Co., Ltd., grants and personal fees from Eli Lilly Japan K.K., grants and personal fees from Boehringer Ingelheim, personal fees from Novartis, grants and personal fees from Pfizer, personal fees from Bristol-Myers Squibb, personal fees from Life Technologies Japan Ltd., personal fees from Nippon Kayaku, grants from Astellas Pharma Inc., grants from Shionogi, grants from Abbvie, grants from MEDICAL RESEARCH, grants from Kyorin Pharmaceutical Co., Ltd., grants from Eisai Co., Ltd., grants from TERUMO CORPORATION, grants from TOPPAN PRINTING Co., Ltd., and grants from TOSOH CORPORATION. Dr. Nakagawa reports grants and personal fees from AstraZeneca K.K.; grants and personal fees from Astellas Pharma Inc.; grants and personal fees from MSD K.K.; grants, personal fees, and other financial assistance from Ono Pharmaceutical Co., Ltd.; grants and personal fees from Nippon Boehringer Ingelheim Co., Ltd.; grants and personal fees from Novartis Pharma K.K.; grants, personal fees, and other financial assistance from Pfizer Japan Inc.; grants and personal fees from Bristol-Myers Squibb Company; grants, personal fees, and other financial assistance from Eli Lilly Japan K.K.; grants and personal fees from Chugai Pharmaceutical Co., Ltd.; grants and personal fees from Daiichi Sankyo Co., Ltd.; grants and personal fees from Merck Serono Co., Ltd./Merck Biopharma Co., Ltd., during the study; personal fees from Clinical Trial Co., Ltd.; personal fees from MEDICUS SHUPPAN, Publishers Co., Ltd.; personal fees from Care Net, Inc.; personal fees from Reno Medical K.K.; personal fees and other financial assistance from Kyorin Pharmaceutical Co., Ltd.; personal fees from Medical Review Co., Ltd.; personal fees from Roche Diagnostics K.K.; personal fees from Bayer Yakuhin, Ltd.; personal fees from Medical Mobile Communications Co., Ltd.; personal fees from 3H Clinical Trial Inc.; personal fees from Nichi-Iko Pharmaceutical Co., Ltd.; grants, personal fees, and other financial assistance from Takeda Pharmaceutical Co., Ltd.; grants and personal fees from Taiho Pharmaceutical Co., Ltd.; grants and personal fees from SymBio Pharmaceuticals Limited; personal fees from NANZANDO Co., Ltd.; personal fees from YODOSHA CO., LTD.; personal fees from Nikkei Business Publications, Inc.; personal fees from Thermo Fisher Scientific K.K.; personal fees from YOMIURI TELECASTING CORPORATION; personal fees from Nippon Kayaku Co., Ltd.; grants and personal fees from AbbVie Inc.; grants from inVentiv Health Japan; grants from ICON Japan K.K.; grants from GRITSONE ONCOLOGY INC.; grants from PAREXEL International Corp.; grants from Kissei Pharmaceutical Co., Ltd.; grants from EPS Corporation; grants from Syneos Health; grants from Pfizer R&D Japan G.K.; grants from A2 Healthcare Corp.; grants from Quintiles Inc./IQVIA Services JAPAN K.K.; grants from EP-CRSU CO., LTD.; grants from Linical Co., Ltd.; grants from Eisai Co., Ltd.; grants from CMIC Shift Zero K.K.; grants from Kyowa Hakko Kirin Co., Ltd; grants from Bayer Yakuhin, Ltd; grants from EPS International Co., Ltd.; and grants from Otsuka Pharmaceutical Co., Ltd., (Copyright © 2023 The Author(s). 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