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