A PHASE 1 OPEN-LABEL STUDY OF HER3-DXd (PATRITUMAB DERUXTECAN; U3-1402) IN COMBINATION WITH OSIMERTINIB IN SUBJECTS WITH LOCALLY ADVANCED OR METASTATIC EGFR-MUTATED NON-SMALL CELL LUNG CANCER (NSCLC)

This study includes a Dose Escalation Part to identify the recommended combination dose (RCD)
and a Dose Expansion Part to further evaluate efficacy and safety.

The primary objectives:

Dose Escalation: To assess the safety and tolerability of HER3-DXd (patritumab deruxtecan;
U3-1402) and osimertinib in subjects with locally advanced or metastatic non-small cell lung
cancer (NSCLC) with an EGFR exon 19 deletion or L858R mutation with tumor progression after
treatment with osimertinib, and to determine the recommended combination dose (RCD).

Second-Line Dose Expansion Arm 1 and Arm 1b: To assess the preliminary antitumor activity of
HER3-DXd and osimertinib in subjects with locally advanced or metastatic NSCLC with an EGFR
exon 19 deletion or L858R mutation with tumor progression after treatment with osimertinib.
Note: One or both of the study arms may open with one or two distinct dosing schedules.

Second-Line Dose Expansion Arm 2: To assess the preliminary antitumor activity of HER3-DXd
monotherapy in subjects with locally advanced or metastatic NSCLC with an EGFR exon 19
deletion or L858R mutation with tumor progression after treatment with osimertinib.

First-Line Dose Expansion Cohorts 3, 4a, and 4b: To assess the preliminary antitumor activity
of HER3-DXd and osimertinib in subjects with locally advanced or metastatic NSCLC with an
EGFR exon 19 deletion or L858R mutation without prior systemic treatment for locally advanced
or metastatic disease.

Inclusion Criteria:

Inclusion Criteria Specific to Dose Escalation and Second-line Dose Expansion:

- Documentation of EGFR exon 19 deletion or L858R mutation detected from tumor tissue

- Must have received osimertinib for locally advanced or metastatic disease at a dose of
80 mg once daily (QD) for at least 6 weeks and must not miss more than two doses
during the 2 weeks prior to the first day of study treatment (Cycle 1, Day 1)

- Must not have received any other prior systemic cancer therapies in the locally
advanced/metastatic setting

- Has documentation of radiological disease progression following first-line treatment
with osimertinib in the locally advanced or metastatic setting

Inclusion Criteria Specific to First-line Dose Expansion:

- The tumor tissue harbors one of the 2 common EGFR mutations occurring in NSCLC known
to be associated with EGFR-TKI sensitivity (exon 19 deletion or L858R) as assessed by
Clinical Laboratory Improvement Amendments (CLIA)-certified (United States [US]
sites), accredited (outside of the US), local laboratory or central laboratory. Only
tissue-based testing will be accepted.

- Participants must have previously untreated locally advanced or metastatic NSCLC and
must be eligible to receive first-line treatment with osimertinib, according to the
judgment of the investigator. Prior adjuvant or neo-adjuvant therapy (chemotherapy,
radiotherapy, investigational agents; except with osimertinib) is permitted.

All Participants:

Participants must meet all criteria to be eligible for inclusion in this study:

- Histologically or cytologically documented locally advanced or metastatic NSCLC not
amenable to curative surgery or radiation.

- At least 1 measurable lesion as assessed by Investigator as per Response Evaluation
Criteria in Solid Tumors (RECIST v1.1)

- Tissue requirements

- For Dose Escalation (all cohorts): provide an optional pre-treatment tumor tissue
of sufficient quantity, as defined in the laboratory manual. The optional
pre-treatment tumor tissue can be provided as either:

- Pre-treatment tumor biopsy from at least 1 lesion not previously irradiated
and amenable to core biopsy OR

- Archival tissue collected from a biopsy performed prior to signing of the
tissue consent, and since progression while on treatment with the most
recent cancer therapy

- For First-line Dose Expansion (Cohorts 3, 4a, and 4b): provide an optional
pre-treatment and optional on-treatment tumor tissues of sufficient quantity, as
defined in the laboratory manual. The optional pre-treatment tumor tissue can be
provided as either:

- Pre-treatment tumor biopsy from at least 1 lesion not previously irradiated
and amenable to core biopsy OR

- Archival tissue collected from a biopsy performed at the time of initial
diagnosis or later

- For Second-line Dose Expansion (Arm 1, Arm 2, and Arm 1b): provide a required
pre-treatment and required on-treatment tumor tissues of sufficient quantity, as
defined in the laboratory manual. The required pre-treatment tumor tissue can be
provided as either:

- Pre-treatment tumor biopsy from at least 1 lesion not previously irradiated
and amenable to core biopsy OR

- Archival tissue collected from a biopsy performed prior to signing of the
tissue consent, and since progression while on treatment with the most
recent cancer therapy regimen

- Note: For all participants who consent to a pre-treatment biopsy, in
order to ensure an adequate amount of tissue is available, a core
needle biopsy is required.

- Has adequate bone marrow reserve and organ function based on local laboratory data
within 14 days prior to Cycle 1, Day 1 (Dose Escalation and First-line Dose Expansion)
or within 14 days prior to randomization (Second-line Dose Expansion):

- Platelet count: ≥100 000/mm^3 or ≥100 × 10^9/L (platelet transfusions are not
allowed within 14 days prior to the assessment of platelets during the screening
period in order to meet the study inclusion criterion)

- Hemoglobin: ≥9.0 g/dL (transfusion of red blood cells and/or growth factor
support is not allowed within 14 days prior to the assessment of hemoglobin
during the screening period in order to meet the study inclusion criterion)

- Absolute neutrophil count (ANC): 1500/mm^3 or ≥1.5 × 10^9/L (granulocyte colony
stimulating factor support is not allowed within 14 days prior to the assessment
of ANC during the screening period in order to meet the study inclusion
criterion)

- Creatinine clearance (CrCl): CrCl ≥30 mL/min as calculated using the
Cockcroft-Gault equation or measured CrCl

- Aspartate aminotransferase/ alanine aminotransferase: ≤3 × ULN (if liver
metastases are present, ≤5 × ULN)

- Total bilirubin: ≤1.5 × ULN if no liver metastases (<3 × ULN in the presence of
documented Gilbert's syndrome [unconjugated hyperbilirubinemia] or liver
metastases)

- Serum albumin: ≥2.5 g/dL

- Alkaline phosphatase (ALP) and Gamma-glutamyl transferase (GGT): ≤2.5 × ULN of
both ALP and GGT

- Prothrombin time (PT) or PT-international normalized ratio (INR) and Activated
partial thromboplastin time (aPTT) / Partial thromboplastin time (PTT): ≤1.5 ×
ULN, except for subjects on coumarin-derivative anticoagulants or other similar
anticoagulant therapy, who must have PT-INR within therapeutic range as deemed
appropriate by the Investigator.

Exclusion Criteria:

- Any previously documented histologic or cytologic evidence of small cell OR combined
small cell/non-small cell disease

- Has a history of (non-infectious) interstitial lung disease (ILD)/pneumonitis that
required corticosteroid therapy, has current ILD, or is suspected to have such disease
by imaging during screening.

- Clinically severe pulmonary compromise (based on Investigator's assessment) resulting
from intercurrent pulmonary illnesses including, but not limited to:

- Any underlying pulmonary disorder (eg, pulmonary emboli within three months of
the study enrollment or randomization, severe asthma, severe chronic obstructive
pulmonary disease, restrictive lung disease, pleural effusion);

- Any autoimmune, connective tissue or inflammatory disorders with pulmonary
involvement (eg, rheumatoid arthritis, Sjogren's syndrome, sarcoidosis); OR prior
complete pneumonectomy.

- Is receiving chronic systemic corticosteroids dosed at >10 mg prednisone or equivalent
anti-inflammatory activity or any form of immunosuppressive therapy prior to Cycle 1,
Day 1. Participants who require use of bronchodilators, inhaled or topical steroids,
or local steroid injections may be included in the study.

- Evidence of any leptomeningeal disease.

- Has spinal cord compression or clinically active central nervous system metastases,
defined as symptomatic, or requiring therapy with corticosteroids or anticonvulsants
to control associated symptoms. Participants with clinically inactive brain metastases
may be included in the study. Participants with treated brain metastases who are no
longer symptomatic and who require no treatment with corticosteroids or
anticonvulsants may be included in the study, but must have a stable neurologic status
for at least 4 weeks prior to Cycle 1, Day 1. Participants with asymptomatic brain
metastases and treated with anticonvulsants as prophylaxis are able to enroll
following a 14-day washout period. Note: A CT or MRI scan of the brain at baseline is
required for all participants.

- Inadequate washout period prior to Cycle 1, Day 1 defined as:

- Whole brain radiation therapy <28 days or stereotactic brain radiation therapy <7
days;

- Any systemic anticancer therapy (excluding osimertinib in all Dose Escalation
Cohorts and in Second-line Dose Expansion [Arm 1, Arm 2, and Arm 1b]), including
investigational agents, <14 days or 5 half-lives, whichever is longer

- Immune checkpoint inhibitor therapy ≤21 days

- Major surgery (excluding placement of vascular access) <4 weeks

- Radiotherapy treatment to more than 30% of the bone marrow or with a wide field
of radiation < 28 days or palliative radiation therapy <7 days

- Chloroquine or hydroxychloroquine ≤14 days

- Medications or herbal supplemented known to be strong inducers of cytochrome P450
(CYP) 3A4 <21 days.

- Has unresolved toxicities from previous anticancer therapy, defined as toxicities
(other than alopecia) not yet resolved to National Cancer Institute Common Terminology
Criteria for Adverse Events (NCI-CTCAE) version 5.0, Grade ≤1 or baseline. Subjects
with chronic Grade 2 toxicities may be eligible per the discretion of the Investigator
after consultation with the Sponsor Medical Monitor or designee.

- Refractory nausea and vomiting, chronic gastrointestinal diseases, inability to
swallow osimertinib, or previous significant bowel resection that would preclude
adequate absorption of osimertinib.

- Has any primary malignancy other than locally advanced or metastatic NSCLC within 3
years prior to Cycle 1, Day 1 (Dose Escalation and First-line Dose Expansion) or
within 3 years prior to randomization (Second-Line Dose Expansion), except adequately
resected non-melanoma skin cancer, curatively treated in situ disease, or other solid
tumors curatively treated.

- Uncontrolled or significant cardiovascular disease prior to Cycle 1, Day 1 including:

- Mean corrected QT interval using Fridericia's formula (QTcF) interval of >450 ms
in 3 successive central screening measurements

- Left ventricular ejection fraction (LVEF) ≤45% by either echocardiogram (ECHO) or
multigated acquisition (MUGA) scan

- Resting systolic blood pressure >180 mmHg or diastolic blood pressure >110 mmHg

- Myocardial infarction within 6 months

- New York Heart Association (NYHA) Classes 3 or 4 congestive heart failure within
28 days

- Uncontrolled angina pectoris within 6 months

- Has cardiac arrhythmia requiring antiarrhythmic treatment

- Complete left or right bundle branch block within 6 months

- History of second- or third-degree heart block or PR interval >250 ms within 6
months

- History of clinically relevant ventricular arrhythmias, such as ventricular
tachycardia, ventricular fibrillation, or Torsade de Pointes

- Has any factors that increase the risk of corrected QT (QTc) prolongation or risk
of arrhythmic events, such as heart failure, hypokalemia, congenital long QT
syndrome, family history of long QT syndrome, or unexplained sudden death under
40 years of age in first-degree relatives, or any concomitant medication known to
prolong the QT interval

- Has clinically significant corneal disease

- Any evidence of severe or uncontrolled diseases including active bleeding diatheses,
active infection, psychiatric illness/social situations, geographical factors,
substance abuse, or other factors which in the Investigator's opinion makes it
undesirable for the subject to participate in the study or which would jeopardize
compliance with the protocol. Screening for chronic conditions is not required.

- Has a known human immunodeficiency virus (HIV) infection that is not well controlled.