A Dose-Limiting Toxicity (DLT) is defined as any Treatment Emergent Adverse Event not attributable to disease or disease-related processes that occurs during the DLT evaluation period (2 complete cycles during Part 1) and is Grade 3 or above according to National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE) version 5.0. Each cycle is 21 days. Low dose was set at 3.2 mg/kg and high dose was set at 5.4 mg/kg.

DLT was assessed in the DLT evaluable set.

The Objective Response Rate (ORR) was the defined as the percentage of participants who achieved a best overall response of confirmed Complete Response (CR) or Partial Response (PR), assessed by independent central review (ICR) committee based on RECIST version 1.1. CR was defined as a disappearance of all target lesions and PR was defined as at least a 30% decrease in the sum of diameters of target lesions. Confirmed ORR based on ICR is reported. As prespecified in the protocol, participants enrolled in Part 1 were pooled according to their trastuzumab deruxtecan dose and HER2 expressing levels (HER2 positive or HER2 low) for efficacy analyses. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Objective response rate was assessed in the Full Analysis Set. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Duration of Response (DoR) was defined as the time from the date of the first documentation of objective response (complete response [CR] or partial response [PR]) to the date of the first objective documentation of progressive disease (PD) or death due to any cause. CR was defined as a disappearance of all target lesions and PR was defined as at least a 30% decrease in the sum of diameters of target lesions. DoR in participants with confirmed CR/PR based on independent central review and investigator assessment is reported. As prespecified in the protocol, participants enrolled in Part 1 were pooled according to their trastuzumab deruxtecan dose and HER2 expressing levels (HER2 positive or HER2 low) for efficacy analyses. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Duration of Response (DoR) was assessed in the Full Analysis Set of participants with available data with confirmed CR/PR. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Disease Control Rate (DCR) was defined as the percentage of participants who achieved a best overall response of CR, PR, or stable disease (SD) during study treatment. CR was defined as a disappearance of all target lesions, PR as at least a 30% decrease in the sum of diameters of target lesions, and SD as neither sufficient shrinkage to qualify for PR nor sufficient increase to qualify for progressive disease. Confirmation of CR/PR was required. DCR based on independent central review and investigator assessment is reported. As prespecified in the protocol, participants enrolled in Part 1 were pooled according to their trastuzumab deruxtecan dose and HER2 expressing levels (HER2 positive or HER2 low) for efficacy analyses. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Disease control rate was assessed in the Full Analysis Set. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Progression-free survival (PFS) was defined as the time from the date of enrollment to the earlier of the dates of the first objective documentation of disease progression (as per RECIST v1.1) or death due to any cause. Progressive disease was defined as at least a 20% increase in the sum of diameters of target lesions. PFS based on independent central review and investigator assessment is reported. As prespecified in the protocol, participants enrolled in Part 1 were pooled according to their trastuzumab deruxtecan dose and HER2 expressing levels (HER2 positive or HER2 low) for efficacy analyses. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Progression-free survival (PFS) was assessed in the Full Analysis Set. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Time to response (TTR) is defined as the time from the date of first dose of study treatment to the date of the first documented objective response (CR or PR). CR was defined as a disappearance of all target lesions and PR was defined as at least a 30% decrease in the sum of diameters of target lesions. Confirmation of CR/PR was required. TTR based on independent central review and investigator assessment is reported. As prespecified in the protocol, participants enrolled in Part 1 were pooled according to their trastuzumab deruxtecan dose and HER2 expressing levels (HER2 positive or HER2 low) for efficacy analyses. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Time to response was assessed in the Full Analysis Set. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Overall survival (OS) was defined as the time from the date of first dose of study drug to the date of death due to any cause. As prespecified in the protocol, participants enrolled in Part 1 were pooled according to their trastuzumab deruxtecan dose and HER2 expressing levels (HER2 positive or HER2 low) for efficacy analyses. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Overall survival (OS) was assessed in the Full Analysis Set. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

The Objective Response Rate (ORR) was the defined as the percentage of participants who achieved a best overall response of confirmed Complete Response (CR) or Partial Response (PR), assessed by investigator assessment (IA) based on RECIST version 1.1. CR was defined as a disappearance of all target lesions and PR was defined as at least a 30% decrease in the sum of diameters of target lesions. Confirmed ORR based on IA is reported. As prespecified in the protocol, participants enrolled in Part 1 were pooled according to their trastuzumab deruxtecan dose and HER2 expressing levels (HER2 positive or HER2 low) for efficacy analyses. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

Objective response rate was assessed in the Full Analysis Set. Participants who were dosed at 5.4 mg/kg of trastuzumab deruxtecan and have HER2-positive expression Breast Cancer (IHC score 3+ or IHC score 2+/ISH+) in Part 1 were pooled with the same participants from Part 2 (in this case, participants enrolled in Cohort 1).

A treatment-emergent adverse event (TEAE) was defined as an AE that occurred, having been absent before the first dose of study drug, or had worsened in severity or seriousness after initiating the study drug until 47 days after last dose of study drug.

TEAEs were assessed in the Safety Analysis Set.