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

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

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)

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 interstitial lung disease (ILD)/pneumonitis irrespective of steroid use, or current ILD/pneumonitis, or suspected ILD/pneumonitis, or ILD that cannot be ruled out by imaging
• 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