A Study of Patritumab Deruxtecan in Pediatric Participants With Relapsed or Refractory Solid Tumors (MK-9999-01C/LIGHTBEAM-U01)

May 8, 2026 updated by: Merck Sharp & Dohme LLC

LIGHTBEAM-U01 Substudy 01C: A Phase 1/2 Substudy to Evaluate the Safety and Efficacy of Patritumab Deruxtecan in Pediatric Participants With Relapsed or Refractory Solid Tumors

Researchers are looking for new ways to treat children with hepatoblastoma or rhabdomyosarcoma (RMS) that has relapsed or is refractory:

  • Hepatoblastoma is a common liver cancer in babies and very young children
  • RMS is a cancer that starts in muscle cells, often in a child's head and neck, bladder, arms, or legs
  • Relapsed means the cancer came back after treatment
  • Refractory means the cancer did not respond (get smaller or go away) to treatment

The study treatment HER3-DXd (also known as MK-1022 or patritumab deruxtecan) is an antibody-drug conjugate (ADC). An ADC attaches to a protein on cancer cells and delivers treatment to destroy those cells. The goals of this study are to learn:

  • About the safety of HER3-DXd in children and if they tolerate it
  • What happens to HER3-DXd in children's bodies over time
  • If children who receive HER3-DXd have the cancer get smaller or go away

Study Overview

Status

Recruiting

Conditions

Intervention / Treatment

Detailed Description

This study will have 2 parts: a safety lead-in to demonstrate a tolerable safety profile and confirm a preliminary recommended phase 2 dose (RP2D) (Part 1) followed by an efficacy evaluation (Part 2)

Study Type

Interventional

Enrollment (Estimated)

50

Phase

  • Phase 2
  • Phase 1

Contacts and Locations

This section provides the contact details for those conducting the study, and information on where this study is being conducted.

Study Contact

Study Locations

  • Australia
    • New South Wales
      • Sydney, New South Wales, Australia, 2031
        • Recruiting
        • Sydney Children's Hospital-Kids Cancer Centre ( Site 3997)
        • Contact:
          • Study Coordinator
          • Phone Number: 61293821111
    • Queensland
      • Brisbane, Queensland, Australia, 4101
        • Recruiting
        • Queensland Children's Hospital ( Site 3996)
        • Contact:
          • Study Coordinator
          • Phone Number: 61730681111
  • Belgium
    • Oost-Vlaanderen
      • Ghent, Oost-Vlaanderen, Belgium, 9000
        • Recruiting
        • UZ Gent ( Site 3428)
        • Contact:
          • Study Coordinator
          • Phone Number: +3293324812
  • Brazil
    • Rio Grande do Sul
      • Porto Alegre, Rio Grande do Sul, Brazil, 90035-903
        • Recruiting
        • Hospital de Clinicas de Porto Alegre ( Site 3265)
        • Contact:
          • Study Coordinator
          • Phone Number: +5551980139616
    • São Paulo
      • Barretos, São Paulo, Brazil, 14784-400
        • Recruiting
        • Fundação Pio XII - Hospital de Câncer de Barretos ( Site 3264)
        • Contact:
          • Study Coordinator
          • Phone Number: +551733216638
      • São José do Rio Preto, São Paulo, Brazil, 15090-000
        • Recruiting
        • Fundação Faculdade Regional de Medicina de São José do Rio Preto ( Site 3267)
        • Contact:
          • Study Coordinator
          • Phone Number: +55 17 99625-3919
  • Canada
    • Ontario
      • Toronto, Ontario, Canada, M5G 1X8
        • Recruiting
        • The Hospital for Sick Children ( Site 3225)
        • Contact:
          • Study Coordinator
          • Phone Number: 4168131500
  • Chile
    • Valparaiso
      • Valparaíso, Valparaiso, Chile, 2341131
        • Recruiting
        • Hospital Carlos Van Buren ( Site 3880)
        • Contact:
          • Study Coordinator
          • Phone Number: 56978546125
  • Colombia
    • Antioquia
      • Medellín, Antioquia, Colombia, 05034
        • Recruiting
        • Hospital Pablo Tobon Uribe ( Site 3923)
        • Contact:
          • Study Coordinator
          • Phone Number: +57 3006523572
    • Atlántico
      • Barranquilla, Atlántico, Colombia, 080020
        • Recruiting
        • Clinica de la Costa S.A.S. ( Site 3924)
        • Contact:
          • Study Coordinator
          • Phone Number: +573008096054
    • Departamento de Córdoba
      • Montería, Departamento de Córdoba, Colombia, 230002
        • Recruiting
        • IMAT S.A.S ( Site 3921)
        • Contact:
          • Study Coordinator
          • Phone Number: +576047862333
  • Czechia
    • Brno-mesto
      • Brno, Brno-mesto, Czechia, 613 00
        • Recruiting
        • Detska nemocnice FN Brno ( Site 3388)
        • Contact:
          • Study Coordinator
          • Phone Number: +420532234755
    • Praha 5
      • Prague, Praha 5, Czechia, 150 00
        • Recruiting
        • Fakultni nemocnice v Motole-Klinika detske hematologie a onkologie ( Site 3387)
        • Contact:
          • Study Coordinator
          • Phone Number: +420224436475
  • Denmark
    • Capital Region
      • Copenhagen, Capital Region, Denmark, DK-2100
        • Recruiting
        • Rigshospitalet-Department of paediatrics and adolescent medicine, Section of Paed haem-onc ( Site 3467)
        • Contact:
          • Study Coordinator
          • Phone Number: +4535452462
  • France
    • Aquitaine
      • Bordeaux, Aquitaine, France, 33076
        • Recruiting
        • Bordeaux University Hospital - Pellegrin ( Site 3105)
        • Contact:
          • Study Coordinator
          • Phone Number: +33556795679
    • Bouches-du-Rhone
      • Marseille, Bouches-du-Rhone, France, 13005
        • Recruiting
        • Assistance Publique Hôpitaux de Marseille - Hôpital de la Timone ( Site 3102)
        • Contact:
          • Study Coordinator
          • Phone Number: +33491385238
    • Loire-Atlantique
      • Nantes, Loire-Atlantique, France, 44093
        • Recruiting
        • Centre Hospitalier Universitaire de Nantes - Hôpital Femme-Enfant-Adolescent Chu De Nantes ( Site 3104)
        • Contact:
          • Study Coordinator
          • Phone Number: +33240083610
    • Rhone
      • Lyon, Rhone, France, 69373
        • Recruiting
        • Centre Leon-Berard ( Site 3100)
        • Contact:
          • Study Coordinator
          • Phone Number: 33469166572
    • Île-de-France Region
      • Paris, Île-de-France Region, France, 75248
        • Recruiting
        • Institut Curie ( Site 3101)
        • Contact:
          • Study Coordinator
          • Phone Number: +33144324015
  • Germany
    • North Rhine-Westphalia
      • Münster, North Rhine-Westphalia, Germany, 48149
        • Recruiting
        • Universitätsklinikum Münster - Albert Schweitzer Campus ( Site 3141)
        • Contact:
          • Study Coordinator
          • Phone Number: +492518347742
  • Greece
    • Attica
      • Athens, Attica, Greece, 11527
        • Recruiting
        • Aghia Sophia Children's Hospital-First Department of Pediatrics, National and Kapodistrian Universi ( Site 3797)
        • Contact:
          • Study Coordinator
          • Phone Number: +302107452125
  • Hungary
    • Pest County
      • Budapest, Pest County, Hungary, 1094
        • Recruiting
        • Semmelweis University ( Site 3838)
        • Contact:
          • Study Coordinator
          • Phone Number: +3612151380
  • Israel
      • Haifa, Israel, 3109601
        • Recruiting
        • Rambam Health Care Campus ( Site 3674)
        • Contact:
          • Study Coordinator
          • Phone Number: 97247774718
      • Ramat Gan, Israel, 5265601
        • Recruiting
        • Sheba Medical Center ( Site 3675)
        • Contact:
          • Study Coordinator
          • Phone Number: +97235302996
  • Italy
      • Milan, Italy, 20133
        • Recruiting
        • Fondazione IRCCS Istituto Nazionale dei Tumori ( Site 3552)
        • Contact:
          • Study Coordinator
          • Phone Number: 00390223902593
      • Roma, Italy, 00165
        • Recruiting
        • Ospedale Pediatrico Bambino Gesù IRCCS ( Site 3553)
        • Contact:
          • Study Coordinator
          • Phone Number: 00390668593697
      • Torino, Italy, 10126
        • Recruiting
        • Ospedale Infantile Regina Margherita ( Site 3551)
        • Contact:
          • Study Coordinator
          • Phone Number: 00390113135230
  • Netherlands
      • Utrecht, Netherlands, 3584 CS
        • Recruiting
        • Prinses Maxima Centrum voor Kinderoncologie ( Site 3510)
        • Contact:
          • Study Coordinator
          • Phone Number: +31889727272
  • Slovakia
    • Bratislava Region
      • Bratislava, Bratislava Region, Slovakia, 831 01
        • Recruiting
        • Narodny ustav detskych chorob ( Site 3592)
        • Contact:
          • Study Coordinator
          • Phone Number: +421259371205
  • South Korea
      • Seoul, South Korea, 03080
        • Recruiting
        • Seoul National University Hospital-Pediatrics ( Site 3972)
        • Contact:
          • Study Coordinator
          • Phone Number: +82220723304
      • Seoul, South Korea, 05505
        • Recruiting
        • Asan Medical Center-Pediatrics - Pedicatric Oncology ( Site 3973)
        • Contact:
          • Study Coordinator
          • Phone Number: +82230105994
  • Spain
      • Barcelona, Spain, 08035
        • Recruiting
        • Hospital Universitari Vall d''Hebron ( Site 3716)
        • Contact:
          • Study Coordinator
          • Phone Number: +34915035900x662
      • Madrid, Spain, 28009
        • Recruiting
        • Hospital Infantil Universitario Nino Jesus ( Site 3715)
        • Contact:
          • Study Coordinator
          • Phone Number: +34915035900
    • Barcelona
      • Esplugues de Llobregat, Barcelona, Spain, 08950
        • Recruiting
        • Hospital Sant Joan de Déu ( Site 3717)
        • Contact:
          • Study Coordinator
          • Phone Number: +34671600093
  • Sweden
    • Västra Götaland County
      • Gothenburg, Västra Götaland County, Sweden, 416 85
        • Recruiting
        • Sahlgrenska Universitetssjukhuset ( Site 3634)
        • Contact:
          • Study Coordinator
          • Phone Number: +46313435865
  • Taiwan
    • Taipei
      • Taiwan, Taipei, Taiwan, 10002
        • Recruiting
        • National Taiwan University Hospital ( Site 3983)
        • Contact:
          • Study Coordinator
          • Phone Number: 8862-23123456#70559
  • Turkey (Türkiye)
      • Ankara, Turkey (Türkiye), 06230
        • Recruiting
        • Hacettepe Universitesi Tip Fakultesi Hastanesi ( Site 3961)
        • Contact:
          • Study Coordinator
          • Phone Number: +90 312 305 50 00
      • Ankara, Turkey (Türkiye), 6800
        • Recruiting
        • Ankara Bilkent Şehir Hastanesi. ( Site 3962)
        • Contact:
          • Study Coordinator
          • Phone Number: +90 312 552 60 00
      • Izmir, Turkey (Türkiye), 35100
        • Recruiting
        • Ege Universitesi Hastanesi ( Site 3963)
        • Contact:
          • Study Coordinator
          • Phone Number: +905301469380
  • United Kingdom
      • Cardiff, United Kingdom, CF14 4XW
        • Recruiting
        • University Hospital of Wales ( Site 3346)
        • Contact:
          • Study Coordinator
          • Phone Number: +4402921842107
      • Sutton, United Kingdom, SM2 5PT
        • Recruiting
        • Royal Marsden Hospital ( Site 3347)
        • Contact:
          • Study Coordinator
          • Phone Number: 020 8642 6011
    • England
      • Birmingham, England, United Kingdom, B4 6NH
        • Recruiting
        • Birmingham Children's Hospital ( Site 3349)
        • Contact:
          • Study Coordinator
          • Phone Number: +441213338233
      • Newcastle upon Tyne, England, United Kingdom, NE1 4PL
        • Recruiting
        • Royal Victoria Infirmary ( Site 3348)
        • Contact:
          • Study Coordinator
          • Phone Number: 0191 282 1014
  • United States
    • California
      • Los Angeles, California, United States, 90027
        • Recruiting
        • Childrens Hospital Los Angeles ( Site 3006)
        • Contact:
          • Study Coordinator
          • Phone Number: 323-361-2121
    • Colorado
      • Aurora, Colorado, United States, 80045
        • Recruiting
        • Children's Hospital Colorado-Center for Cancer and Blood Disorders ( Site 3016)
        • Contact:
          • Study Coordinator
          • Phone Number: 720-777-1234
    • Connecticut
      • New Haven, Connecticut, United States, 06510
        • Recruiting
        • Yale New Haven Hospital ( Site 3012)
        • Contact:
          • Study Coordinator
          • Phone Number: 203-785-4640
    • Florida
      • St. Petersburg, Florida, United States, 33701
        • Recruiting
        • Johns Hopkins All Children's Hospital ( Site 3025)
        • Contact:
          • Study Coordinator
          • Phone Number: 727-767-4176
    • Iowa
      • Iowa City, Iowa, United States, 52242
        • Recruiting
        • University of Iowa Health Care Holden Comprehensive Cancer Center ( Site 3017)
        • Contact:
          • Study Coordinator
          • Phone Number: 319-356-2296
    • Massachusetts
      • Boston, Massachusetts, United States, 02215
        • Recruiting
        • Dana-Farber Cancer Institute ( Site 3013)
        • Contact:
          • Study Coordinator
          • Phone Number: 617-632-4580
    • Michigan
      • Grand Rapids, Michigan, United States, 49503
        • Recruiting
        • Corewell Health ( Site 3001)
        • Contact:
          • Study Coordinator
          • Phone Number: 616-486-0746
    • Missouri
      • Kansas City, Missouri, United States, 64108
        • Recruiting
        • Children's Mercy Hospital ( Site 3024)
        • Contact:
          • Study Coordinator
          • Phone Number: 816-302-6808
    • New Jersey
      • New Brunswick, New Jersey, United States, 08901
        • Recruiting
        • Rutgers Cancer Institute of New Jersey ( Site 3008)
        • Contact:
          • Study Coordinator
          • Phone Number: 732-235-2465
    • New York
      • New York, New York, United States, 10065
        • Recruiting
        • Memorial Sloan Kettering Cancer Center ( Site 3010)
        • Contact:
          • Study Coordinator
          • Phone Number: 888-492-8401
      • Valhalla, New York, United States, 10595
        • Recruiting
        • New York Medical College ( Site 3023)
        • Contact:
          • Study Coordinator
          • Phone Number: 914-614-4270
    • North Dakota
      • Fargo, North Dakota, United States, 58102
        • Recruiting
        • Sanford Fargo Medical Center-Roger Maris Cancer Center ( Site 3003)
        • Contact:
          • Study Coordinator
          • Phone Number: 701-234-2000
    • Oregon
      • Portland, Oregon, United States, 97239
        • Recruiting
        • Oregon Health and Science University ( Site 3004)
        • Contact:
          • Study Coordinator
          • Phone Number: 503-494-8311
    • Pennsylvania
      • Philadelphia, Pennsylvania, United States, 19104
        • Recruiting
        • Children's Hospital of Philadelphia (CHOP) ( Site 3021)
        • Contact:
          • Study Coordinator
          • Phone Number: 267-425-5544
    • South Dakota
      • Sioux Falls, South Dakota, United States, 57117
        • Recruiting
        • Sanford Children's Hospital ( Site 3015)
        • Contact:
          • Study Coordinator
          • Phone Number: 605-312-1000
    • Texas
      • Houston, Texas, United States, 77030
        • Recruiting
        • University of Texas-MD Anderson Cancer Center ( Site 3007)
        • Contact:
          • Study Coordinator
          • Phone Number: 713-792-5410
    • Utah
      • Salt Lake City, Utah, United States, 84113
        • Recruiting
        • Intermountain - Primary Children's Hospital ( Site 3014)
        • Contact:
          • Study Coordinator
          • Phone Number: 801-662-4700

Participation Criteria

Researchers look for people who fit a certain description, called eligibility criteria. Some examples of these criteria are a person's general health condition or prior treatments.

Eligibility Criteria

Ages Eligible for Study

  • Child

Accepts Healthy Volunteers

No

Description

The main inclusion criteria include but are not limited to the following:

  • Has one of the following histologically confirmed advanced or metastatic solid tumors: Rhabdomyosarcoma (RMS), or Hepatoblastoma
  • Has progressed after at least 1 prior systemic treatment for RMS or hepatoblastoma and who has no satisfactory alternative treatment option (ie, is ineligible for other standard treatment regimens)
  • Participants who have adverse events (AEs) due to previous anticancer therapies must have recovered to Grade ≤1 or baseline. Participants with endocrine-related AEs who are adequately treated with hormone replacement or participants who have Grade ≤2 neuropathy are eligible. Participants with Grade ≤2 alopecia are also eligible
  • Hepatitis B surface antigen (HBsAg) positive participants are eligible if they have received hepatitis B virus (HBV) antiviral therapy and have undetectable HBV viral load
  • Participants with a history of hepatitis C virus (HCV) infection are eligible if HCV viral load is undetectable

The main exclusion criteria include but are not limited to the following:

  • Has a history of (noninfectious) interstitial lung disease (ILD)/pneumonitis that required steroids or has current ILD/pneumonitis, and/or suspected ILD/pneumonitis that cannot be ruled out by standard diagnostic assessments
  • Has clinically severe respiratory compromise resulting from intercurrent pulmonary illness
  • Has a history of solid organ transplant
  • Has a history of allogeneic stem cell transplant
  • Has clinically significant corneal disease
  • Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis/leptomeningeal disease; participants with previously treated brain metastases may participate provided they are radiologically stable (ie, without evidence of progression) for at least 4 weeks
  • Has uncontrolled or significant cardiovascular disorder
  • Has a history of clinically significant congenital cardiac syndrome
  • Has a history of human immunodeficiency virus (HIV) infection
  • Has a known additional malignancy that is progressing or has required active treatment within the past 1 year
  • Has an active infection requiring systemic therapy
  • Has concurrent active hepatitis B (HBsAg positive and/or detectable HBV deoxyribonucleic acid [DNA]) and HCV defined as anti-HCV antibody (Ab) positive and detectable HCV ribonucleic acid [RNA]) infection
  • Has not adequately recovered from major surgery or have ongoing surgical complications

Study Plan

This section provides details of the study plan, including how the study is designed and what the study is measuring.

How is the study designed?

Design Details

  • Primary Purpose: Treatment
  • Allocation: N/A
  • Interventional Model: Single Group Assignment
  • Masking: None (Open Label)

Arms and Interventions

Participant Group / Arm
Intervention / Treatment
Experimental: Patritumab Deruxtecan
Participants receive patritumab deruxtecan via IV infusion on Day 1 of each 3-week cycle until discontinuation or progression.
Biological: Patritumab Deruxtecan
IV Infusion
Other Names:
  • U3-1402
  • HER3-DXd
  • MK-1022

What is the study measuring?

Primary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Part 1: Percentage of Participants Who Experience Dose-limiting Toxicities (DLTs)
Time Frame: Cycle 1 (up to approximately 21 days); each cycle is 21 days
A DLT is any of a prespecified list of adverse events (AEs) that occur during Cycle 1 (up to 21 days) if attributed to the study treatment and not attributed to any other clearly identifiable cause. The percentage of participants who experience DLTs will be reported. Each cycle is 21 days.
Cycle 1 (up to approximately 21 days); each cycle is 21 days
Part 1: Percentage of Participants Who Experience an Adverse Event (AE)
Time Frame: Up to approximately 5 years
An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention. The percentage of participants who experience AEs will be reported.
Up to approximately 5 years
Part 1: Area Under the Curve (AUC) of total anti-HER3 antibody liquid chromatography-mass spectrometry (LC-MS) in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of AUC.
At designated timepoints (up to approximately 5 years)
Part 1: AUC of anti-HER3 antibody-conjugated DXd (anti-HER3-ac-DXd) in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of AUC.
At designated timepoints (up to approximately 5 years)
Part 1: AUC of DXd in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of AUC.
At designated timepoints (up to approximately 5 years)
Part 1: Maximum Concentration (Cmax) of anti-HER3 antibody LC-MS in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Cmax.
At designated timepoints (up to approximately 5 years)
Part 1: Cmax of anti-HER3-ac-DXd in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Cmax.
At designated timepoints (up to approximately 5 years)
Part 1: Cmax of DXd in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Cmax.
At designated timepoints (up to approximately 5 years)
Part 1: Concentration Immediately Before the Next Dose is Administered (Ctrough) of anti-HER3 antibody LC-MS in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Ctrough.
At designated timepoints (up to approximately 5 years)
Part 1: Ctrough of anti-HER3-ac-DXd
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Ctrough.
At designated timepoints (up to approximately 5 years)
Part 1: Ctrough of DXd in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Ctrough.
At designated timepoints (up to approximately 5 years)
Part 1 and Part 2: Objective Response Rate (ORR)
Time Frame: Up to approximately 5 years
ORR is defined as the percentage of participants with Complete Response (CR: disappearance of all target lesions) or Partial Response (PR: at least a 30% decrease in the sum of diameters of target lesions) per Response Evaluation Criteria In Solid Tumors Version 1.1 (RECIST 1.1). The percentage of participants who experience CR or PR as assessed by the investigator will be presented.
Up to approximately 5 years
Part 1: Percentage of Participants Who Discontinue Study Treatment Due to an AE
Time Frame: Up to approximately 5 years
An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention. The percentage of participants who discontinue study treatment due to an AE will be reported.
Up to approximately 5 years

Secondary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Part 2: Percentage of Participants Who Experience an AE
Time Frame: Up to approximately 5 years
An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention. The percentage of participants who experience AEs will be reported.
Up to approximately 5 years
Part 1 and Part 2: Disease Control Rate (DCR)
Time Frame: Up to approximately 5 years
DCR is defined, per RECIST 1.1, as the percentage of participants who have a CR (disappearance of all target lesions) or PR (at least a 30% decrease in the sum of diameters of target lesions) or Stable Disease (SD). SD is defined as neither sufficient shrinkage to qualify for PR nor sufficient increase to qualify for progressive disease (PD: at least a 20% increase in the sum of diameters of target lesions and an absolute increase of at least 5 mm). Note: The appearance of one or more new lesions is also considered PD. The time from the first dose until the date of SD must be greater than or equal to 6 weeks. The DCR as assessed by the investigator will be presented.
Up to approximately 5 years
Part 1 and Part 2: Time to Response (TTR)
Time Frame: Up to approximately 5 years
For participants who demonstrate a confirmed CR (disappearance of all target lesions) or PR (at least a 30% decrease in the sum of diameters of target lesions) per RECIST 1.1, TTR is defined as the time from the first dose to the first documented evidence of a CR or PR. The TTR as assessed by the investigator will be presented.
Up to approximately 5 years
Part 1 and Part 2: Duration of Response (DOR)
Time Frame: Up to approximately 5 years
For participants who demonstrate a confirmed CR (disappearance of all target lesions) or PR (at least a 30% decrease in the sum of diameters of target lesions) per RECIST 1.1, DOR is defined as the time from first documented evidence of CR or PR until PD or death. PD is defined as at least a 20% increase in the sum of diameters of target lesions and an absolute increase of at least 5 mm. Note: The appearance of one or more new lesions is also considered PD. DOR as assessed by the investigator will be presented.
Up to approximately 5 years
Part 1 and Part 2: Progression-free Survival (PFS)
Time Frame: Up to approximately 5 years
PFS is defined as the time from randomization to the first documented PD or death due to any cause, whichever occurs first as assessed by RECIST 1.1. PD is defined as at least a 20% increase in the sum of diameters of target lesions and an absolute increase of at least 5 mm. Note: The appearance of one or more new lesions is also considered PD. PFS as assessed by the investigator will be presented.
Up to approximately 5 years
Part 1 and Part 2: Overall Survival (OS)
Time Frame: Up to approximately 5 years
OS is defined as time from first dose of study treatment to death due to any cause.
Up to approximately 5 years
Part 2: AUC of total anti-HER3 antibody LC-MS in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of AUC.
At designated timepoints (up to approximately 5 years)
Part 2: AUC of anti-HER3-ac-DXd in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of AUC.
At designated timepoints (up to approximately 5 years)
Part 2: AUC of DXd in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of AUC.
At designated timepoints (up to approximately 5 years)
Part 2: Cmax of anti-HER3 antibody LC-MS in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Cmax.
At designated timepoints (up to approximately 5 years)
Part 2: Cmax of anti-HER3-ac-DXd in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Cmax.
At designated timepoints (up to approximately 5 years)
Part 2: Cmax of DXd in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Cmax.
At designated timepoints (up to approximately 5 years)
Part 2: Ctrough of anti-HER3 antibody LC-MS in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Ctrough.
At designated timepoints (up to approximately 5 years)
Part 2: Ctrough of anti-HER3-ac-DXd in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Ctrough.
At designated timepoints (up to approximately 5 years)
Part 2: Ctrough of DXd in plasma
Time Frame: At designated timepoints (up to approximately 5 years)
Blood samples will be collected at specified intervals for the determination of Ctrough.
At designated timepoints (up to approximately 5 years)
Part 2: Percentage of Participants Who Discontinue Study Treatment Due to an AE
Time Frame: Up to approximately 5 years
An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of a study intervention. The percentage of participants who discontinue study treatment due to an AE will be reported.
Up to approximately 5 years

Collaborators and Investigators

This is where you will find people and organizations involved with this study.

Sponsor

Merck Sharp & Dohme LLC

Collaborators

Daiichi Sankyo

Investigators

  • Study Director: Medical Director, Merck Sharp & Dohme LLC

Publications and helpful links

The person responsible for entering information about the study voluntarily provides these publications. These may be about anything related to the study.

Helpful Links

Study record dates

These dates track the progress of study record and summary results submissions to ClinicalTrials.gov. Study records and reported results are reviewed by the National Library of Medicine (NLM) to make sure they meet specific quality control standards before being posted on the public website.

Study Major Dates

Study Start (Actual)

May 26, 2025

Primary Completion (Estimated)

December 30, 2030

Study Completion (Estimated)

December 30, 2030

Study Registration Dates

First Submitted

February 27, 2025

First Submitted That Met QC Criteria

April 15, 2025

First Posted (Actual)

April 23, 2025

Study Record Updates

Last Update Posted (Actual)

May 12, 2026

Last Update Submitted That Met QC Criteria

May 8, 2026

Last Verified

May 1, 2026

More Information

Terms related to this study

Additional Relevant MeSH Terms

Other Study ID Numbers

  • 9999-01C
  • LIGHTBEAM-U01 (Other Identifier: MSD)
  • MK-9999-01C (Other Identifier: MSD)
  • U1111-1314-1866 (Registry Identifier: UTN)
  • 2024-518771-66-00 (Registry Identifier: EU CT)

Plan for Individual participant data (IPD)

Plan to Share Individual Participant Data (IPD)?

YES

IPD Plan Description

https://trialstransparency.msdclinicaltrials.com/pdf/ProcedureAccessClinicalTrialData.pdf

Drug and device information, study documents

Studies a U.S. FDA-regulated drug product

Yes

Studies a U.S. FDA-regulated device product

No

This information was retrieved directly from the website clinicaltrials.gov without any changes. If you have any requests to change, remove or update your study details, please contact register@clinicaltrials.gov. As soon as a change is implemented on clinicaltrials.gov, this will be updated automatically on our website as well.

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