Participants receive patritumab deruxtecan intravenous (IV) infusion on Day 1 of each 21-day cycle (every 3 weeks) until disease progression, intolerable toxicity, or investigator decision.

Administered via intravenous (IV) infusion

DLT will be defined as any drug-related adverse event observed during the DLT evaluation period that results in a change to a given dose or a delay in initiating the next 21-day cycle. The number of participants in the dose-escalation phase who experience a DLT will be presented.

An AE is defined as any unfavorable and unintended sign, symptom, disease, or worsening of preexisting condition temporally associated with study treatment and irrespective of causality to study treatment. The number of participants who experience an AE will be presented.

An AE is defined as any unfavorable and unintended sign, symptom, disease, or worsening of preexisting condition temporally associated with study treatment and irrespective of causality to study treatment. The number of participants who discontinue study treatment due to an AE will be presented.

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 (RECIST) 1.1. The percentage of participants who experience CR or PR as assessed by Blinded Independent Central Review (BICR) will be presented.

For participants who demonstrate a confirmed 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 RECIST 1.1, DOR is defined as the time from first documented evidence of CR or PR until progressive disease (PD) or death. Per RECIST 1.1, PD is defined as at least a 20% increase in the sum of diameters of target lesions. In addition to the relative increase of 20%, the sum must also demonstrate an absolute increase of at least 5 mm. The appearance of one or more new lesions is also considered PD. DOR as assessed by BICR will be presented.