This study was terminated based on a business decision by the Sponsor.

This study has two parts: dose escalation and dose expansion.

The primary objectives are:

• For Dose Escalation, to assess the safety and tolerability of DS-1205c when combined with osimertinib in the study population and to determine the recommended dose for expansion of DS-1205c when combined with osimertinib in the study population
• For Dose Expansion, to assess the safety and tolerability of DS-1205c when combined with osimertinib in the study population

In Dose Escalation, after a 7-day run in period (Cycle 0), there will be 21-day cycles (Cycle 1 onward). In Dose Expansion, there will be 21-day cycles.

The number of treatment cycles is not fixed in this study. Participants will continue study treatment until they decide not to (withdraw consent), their disease gets worse [progressive disease (PD)], or side effects become unacceptable (unacceptable toxicity).

Inclusion Criteria:

1. Has histologically or cytologically documented adenocarcinoma NSCLC.

2. Has locally advanced or metastatic NSCLC, not amenable to curative surgery or radiation.

3. Has acquired resistance to EGFR TKI according to the Jackman criteria (PMID: 19949011):

   1. Historical confirmation that the tumor harbors an EGFR mutation known to be associated with EGFR TKI sensitivity (including G719X, exon 19 deletion, L858R, L861Q) or
   2. Has experienced clinical benefit from an EGFR TKI, followed by systemic progression (Response Evaluation Criteria in Solid Tumors [RECIST version 1.1] or World Health Organization [WHO]) while on continuous treatment with an EGFR TKI.

4. Is currently receiving, and is able to discontinue erlotinib, gefinitib, or afatinib; or is currently receiving osimertinib at the prescribed 80 mg dose and is able to interrupt osimertinib.

5. Has been receiving erlotinib, gefitinib, or afatinib for at least 6 weeks with well-controlled related toxicities less than Grade 3 in severity at the time of screening period.

6. Has radiological documentation of disease progression while receiving continuous treatment with erlotinib, gefitinib, afatinib, or osimertinib.

7. Has at least one measurable lesion per RECIST version 1.1.

8. Is willing to provide archival tumor tissue from a biopsy performed after progression during treatment with erlotinib, gefitinib, afatinib , or osimertinib OR has at least one lesion not previously irradiated, amenable to core biopsy, and is willing to undergo screening tumor biopsy.

9. Demonstrates absence of EGFR T790M. No EGFR mutation testing is required if treated with osimertinib.

10. Has Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1, with no deterioration over the previous 2 weeks.

Exclusion Criteria:

1. Has any evidence of small cell histology, or combined small cell and non-small cell histology, in original tumor biopsy or in screening biopsy performed since progression.

2. Has previously documented evidence of anaplastic lymphoma kinase (ALK) fusion, ROS proto-oncogene 1 (ROS1) fusion, BRAF V600E mutation, rearranged during transfection (RET) rearrangement, human epidermal growth factor receptor 2 (HER2) mutation, or MET exon 14 skipping mutation. No new testing for these genomic alterations is required for Screening.

3. Has received treatment with any of the following:

   1. Any cytotoxic chemotherapy, immune checkpoint inhibitor therapy, investigational agent or other anticancer drug(s) from a previous cancer treatment regimen or clinical study (other than EGFR TKI), within 14 days of the first dose of study treatment.
   2. Immune checkpoint inhibitor therapy within 30 days of first dose of study treatment.
   3. Major surgery (excluding placement of vascular access) within 4 weeks of the first dose of study treatment.
   4. Radiotherapy treatment to more than 30% of the bone marrow or with a wide field of radiation within 4 weeks, or palliative radiation therapy within 2 weeks of the first dose of study drug treatment.

4. Has history of other active malignancy within 3 years prior to enrollment, except:

   1. Adequately treated non-melanoma skin cancer OR
   2. Superficial bladder tumors (tumor stage "a" [Ta], tumor stage "is" [Tis], tumor stage "1" [T1]) OR
   3. Curatively treated in situ disease OR
   4. Low risk non-metastatic prostate cancer (with Gleason score < 7, and following local treatment or undergoing active surveillance)

5. Has spinal cord compression or clinically active brain metastases, defined as untreated and symptomatic, or requiring therapy with corticosteroids or anticonvulsants to control associated symptoms. Subjects with clinically inactive brain metastases may be included in the study. Subjects with treated brain metastases that are no longer symptomatic and who require no treatment with corticosteroids or anticonvulsants may be included in the study if they have recovered from the acute toxic effect of radiotherapy. A minimum of 2 weeks must have elapsed between the end of whole brain radiotherapy and study enrollment (1 week for stereotactic radiotherapy).

6. Presence of retinal disease in the eye that is not due to neovascular age-related macular degeneration (nAMD; eg, significant diabetic retinopathy, glaucomatous retinal atrophy, retinal detachment).

7. Has history of myocardial infarction within the past 6 months.

8. Has symptomatic congestive heart failure (New York Heart Association [NYHA] Classes II-IV), unstable angina, or cardiac arrhythmia requiring antiarrhythmic treatment.

9. Has left ventricular ejection fraction (LVEF) <45% by either echocardiogram (ECHO) or multigated acquisition (MUGA) scan.

10. Has any clinically important abnormalities in rhythm, conduction or morphology of resting ECG, eg, complete left bundle branch block, third-degree heart block, second-degree heart block, or PR interval >250 milliseconds (ms).

11. Has a mean QT interval corrected using Fridericia's correction (QTcF) prolongation >470 ms for females and >450 ms for males in three successive Screening measurements.

12. Unable or unwilling to discontinue concomitant use of drugs that are known to prolong the QT interval.

13. Has any factors that increase the risk of QTc prolongation or risk of arrhythmic events, such as congenital long QT syndrome, family history of long QT syndrome or unexplained sudden death under 40 years of age in first degree relatives.

14. Has any history of interstitial lung disease (pulmonary fibrosis or severe radiation pneumonitis) or is suspected to have such disease by imaging during screening.

15. Has history of pancreatitis within the past 6 months.

De-identified individual participant data (IPD) and applicable supporting clinical trial documents may be available upon request at https://vivli.org/. In cases where clinical trial data and supporting documents are provided pursuant to our company policies and procedures, Daiichi Sankyo will continue to protect the privacy of our clinical trial participants. Details on data sharing criteria and the procedure for requesting access can be found at this web address: https://vivli.org/ourmember/daiichi-sankyo/

Studies for which the medicine and indication have received European Union (EU) and United States (US), and/or Japan (JP) marketing approval on or after 01 January 2014 or by the US or EU or JP Health Authorities when regulatory submissions in all regions are not planned and after the primary study results have been accepted for publication.

Formal request from qualified scientific and medical researchers on IPD and clinical study documents from clinical trials supporting products submitted and licensed in the United States, the European Union and/or Japan from 01 January 2014 and beyond for the purpose of conducting legitimate research. This must be consistent with the principle of safeguarding study participants' privacy and consistent with provision of informed consent.