Trial Outcomes & Findings for Anti-HER2 Bispecific Antibody Zanidatamab (ZW25) Activity in Combination With Chemotherapy With/Without Tislelizumab (NCT NCT04276493)

Last Updated: 2026-09-02

Results Overview

An AE is defined as any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease (new or exacerbated) temporally associated with the use of study treatments, whether considered related to study treatments or not. A serious AE (SAE) was any untoward medical occurrence that, at any dose - resulted in death, - was life threatening, - required hospitalization or prolongation of existing hospitalization, - resulted in disability/incapacity, - was a congenital anomaly/birth defect.

Recruitment status

COMPLETED

Study phase

PHASE1/PHASE2

Target enrollment

71 participants

Primary outcome timeframe

From the first dose of study drug(s) to 30 days after the last dose; up to approximately 42 months

Results posted on

2026-09-02

Participant Flow

The study was conducted at 22 study centers in China, Korea, and Taiwan; 22 centers enrolled a total of 71 participants.

Participant milestones

Participant milestones
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC : Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.
Overall Study STARTED	11	27	19	14
Overall Study COMPLETED	4	20	5	7
Overall Study NOT COMPLETED	7	7	14	7

Reasons for withdrawal

Reasons for withdrawal
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC : Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.
Overall Study Withdrawal by Subject	4	2	3	1
Overall Study Death	2	3	11	5
Overall Study Lost to Follow-up	1	1	0	1
Overall Study Physician Decision	0	1	0	0

Baseline Characteristics

Anti-HER2 Bispecific Antibody Zanidatamab (ZW25) Activity in Combination With Chemotherapy With/Without Tislelizumab

Baseline characteristics by cohort

Baseline characteristics by cohort
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel n=11 Participants Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel n=27 Participants Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX n=19 Participants Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC : Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX n=14 Participants Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Total n=71 Participants Total of all reporting groups
Age, Continuous	59.2 years STANDARD_DEVIATION 10.78 • n=136 Participants	53.7 years STANDARD_DEVIATION 7.87 • n=136 Participants	63.3 years STANDARD_DEVIATION 11.44 • n=272 Participants	59.8 years STANDARD_DEVIATION 8.15 • n=60 Participants	58.3 years STANDARD_DEVIATION 10.05 • n=24 Participants
Sex: Female, Male Female	11 Participants n=136 Participants	27 Participants n=136 Participants	2 Participants n=272 Participants	2 Participants n=60 Participants	42 Participants n=24 Participants
Sex: Female, Male Male	0 Participants n=136 Participants	0 Participants n=136 Participants	17 Participants n=272 Participants	12 Participants n=60 Participants	29 Participants n=24 Participants
Ethnicity (NIH/OMB) Hispanic or Latino	0 Participants n=136 Participants	0 Participants n=136 Participants	0 Participants n=272 Participants	0 Participants n=60 Participants	0 Participants n=24 Participants
Ethnicity (NIH/OMB) Not Hispanic or Latino	11 Participants n=136 Participants	27 Participants n=136 Participants	19 Participants n=272 Participants	14 Participants n=60 Participants	71 Participants n=24 Participants
Ethnicity (NIH/OMB) Unknown or Not Reported	0 Participants n=136 Participants	0 Participants n=136 Participants	0 Participants n=272 Participants	0 Participants n=60 Participants	0 Participants n=24 Participants
Race/Ethnicity, Customized Asian	11 Participants n=136 Participants	27 Participants n=136 Participants	19 Participants n=272 Participants	14 Participants n=60 Participants	71 Participants n=24 Participants
Eastern Cooperative Oncology Group (ECOG) Performance Status Grade 0	5 Participants n=136 Participants	6 Participants n=136 Participants	5 Participants n=272 Participants	6 Participants n=60 Participants	22 Participants n=24 Participants
Eastern Cooperative Oncology Group (ECOG) Performance Status Grade 1	6 Participants n=136 Participants	21 Participants n=136 Participants	14 Participants n=272 Participants	8 Participants n=60 Participants	49 Participants n=24 Participants

PRIMARY outcome

Timeframe: From the first dose of study drug(s) to 30 days after the last dose; up to approximately 42 months

Population: Safety analysis population: All participants who have received ≥ 1 dose of any component of study treatment.

Outcome measures

Outcome measures
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel n=11 Participants Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel n=27 Participants Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX n=19 Participants Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC: Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX n=14 Participants Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.
Number of Participants Experiencing Adverse Events (AEs) and Serious Adverse Events (SAEs) Number of Participants experiencing AEs	11 Participants	27 Participants	19 Participants	14 Participants
Number of Participants Experiencing Adverse Events (AEs) and Serious Adverse Events (SAEs) Number of Participants experiencing SAEs	2 Participants	10 Participants	12 Participants	5 Participants

PRIMARY outcome

Timeframe: Response was assessed every 6 weeks from cycle 1 day 1 for the first 36 weeks and then every 12 weeks thereafter; maximum time on study follow-up was 42months.

Population: The efficacy evaluable analysis set included participants who received ≥1 dose of study drug, had baseline measurable disease per RECIST v1.1, and had ≥1 postbaseline tumor assessment unless clinical progression or death occurred within 10 weeks of first dose. One participant from Cohort 1 was excluded for an inclusion-criteria violation; the resulting per-protocol set was used for efficacy analyses. All the participants from Cohort 2 were included in the efficacy analysis set.

ORR is defined as the percentage of participants who had a best overall response of complete response or partial response per the Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1. Complete Response (CR): Disappearance of all target lesions. Any pathological lymph nodes (whether target or non-target) must have reduction in short axis to \< 10 mm. Partial Response (PR): At least a 30% decrease in the sum of diameters of target lesions, taking as reference the baseline sum diameters.

Outcome measures

Outcome measures
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel n=8 Participants Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel n=25 Participants Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX n=19 Participants Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC: Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX n=14 Participants Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.
Objective Response Rate (ORR)	100.0 Percentage of participants Interval 63.1 to 100.0	88.0 Percentage of participants Interval 68.8 to 97.5	78.9 Percentage of participants Interval 54.4 to 93.9	71.4 Percentage of participants Interval 41.9 to 91.6

SECONDARY outcome

Timeframe: From the start date of study treatment to the first documentation of progression or death, whichever occurs first, up to approximately 42 months

Population: The efficacy evaluable analysis set only included participants with a response who received ≥1 dose of study drug, had baseline measurable disease per RECIST v1.1, and had ≥1 postbaseline tumor assessment unless clinical progression or death occurred within 10 weeks of first dose. One participant from Cohort 1 was excluded for an inclusion-criteria violation; the resulting per-protocol set was used for efficacy analyses.

DOR is defined as the time from the first determination of an objective response until the first documentation of disease progression or death, whichever occurred first.

Outcome measures

Outcome measures
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel n=8 Participants Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel n=22 Participants Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX n=15 Participants Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC: Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX n=10 Participants Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.
Duration of Response (DOR)	12.4 months Interval 5.5 to Values could not be estimated due to an insufficient number of events	23.5 months Interval 11.3 to Values could not be estimated due to an insufficient number of events	15.4 months Interval 4.9 to Values could not be estimated due to an insufficient number of events	23.7 months Interval 7.4 to Values could not be estimated due to an insufficient number of events

SECONDARY outcome

Timeframe: From the start date of study treatment to the first documentation of response, whichever occurs first, up to approximately 42 months

Population: Cohort 1 Per Protocol Analysis set; Cohort 2: Efficacy Evaluable Analysis Set. Only participants with best overall response of complete response or partial response confirmed per RECIST v1.1 were included in the analysis.

Time to response is defined as the time from the start date of study treatment to the first determination of an objective response assessed by investigator per RECIST v 1.1.

Outcome measures

Outcome measures
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel n=8 Participants Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel n=22 Participants Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX n=15 Participants Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC: Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX n=10 Participants Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.
Time to Response (TTR)	5.71 Weeks Interval 5.36 to 6.0	6.00 Weeks Interval 5.71 to 7.0	5.71 Weeks Interval 5.43 to 11.0	6.14 Weeks Interval 5.29 to 6.14

SECONDARY outcome

Timeframe: From the start date of study treatment to the first documentation of progression or death, whichever occurs first, up to approximately 42 months

PFS is defined as the time from the start date of study drug to the date of the first objectively documented tumor progression assessed by investigator per RECIST Version 1.1 or death, whichever occurred first.

Outcome measures

Outcome measures
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel n=8 Participants Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel n=25 Participants Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX n=19 Participants Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC: Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX n=14 Participants Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.
Progression-free Survival (PFS)	13.7 months Interval 6.8 to Values could not be estimated due to an insufficient number of events	22.1 months Interval 12.7 to Values could not be estimated due to an insufficient number of events	8.3 months Interval 5.6 to 32.4	24.6 months Interval 8.8 to Values could not be estimated due to an insufficient number of events

SECONDARY outcome

Timeframe: Response was assessed every 6 weeks from cycle 1 day 1 for the first 36 weeks and then every 12 weeks thereafter; maximum time on study follow-up was 42 months.

DCR is defined as the percentage of participants with best overall response (BOR) of CR, PR, and stable disease by investigator per RECIST Version 1.1. BOR is the best response recorded from the start of the study drug treatment until the end of treatment taking into account any requirement for confirmation.

Outcome measures

Outcome measures
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel n=8 Participants Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel n=25 Participants Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX n=19 Participants Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC: Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX n=14 Participants Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.
Disease Control Rate (DCR)	100.0 Percentage of participants Interval 63.1 to 100.0	96.0 Percentage of participants Interval 79.6 to 99.9	100.0 Percentage of participants Interval 82.4 to 100.0	100.0 Percentage of participants Interval 76.8 to 100.0

SECONDARY outcome

Timeframe: From the start date of study treatment to the documented death date or the last known alive date, up to approximately 41 months

Population: Cohort 1 Per Protocol Analysis set; Cohort 2: Safety Analysis set

Time from the start date of study drug to the date of death due to any cause.

Outcome measures

Outcome measures
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel n=8 Participants Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel n=25 Participants Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX n=19 Participants Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC: Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX n=14 Participants Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.
Overall Survival (OS)	NA months Interval 36.9 to Values could not be estimated due to an insufficient number of events	NA months Values could not be estimated due to an insufficient number of events	31.4 months Interval 9.4 to Values could not be estimated due to an insufficient number of events	NA months Interval 11.8 to Values could not be estimated due to an insufficient number of events

SECONDARY outcome

Timeframe: Day 1 end of infusion of Cycle 1, 2, 5, 9, 17, 26 and 35. Each cycle is 21 days

Population: The Pharmacokinetic (PK) Analysis Set included all participants who contributed ≥ 1 quantifiable zanidatamab PK sample. The Number Analyzed indicates the number of participants with available data at each time point.

Outcome measures

Outcome measures
Measure	Cohort 1A: Breast Cancer: Zanidatamab 30 mg/kg + Docetaxel n=11 Participants Participants with HER2+ breast cancer received zanidatamab 30 mg/kg intravenously (IV) and docetaxel 75 mg/m\^2 IV once every 3 weeks (Q3W) for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 1B: Breast Cancer: Zanidatamab 1800 mg + Docetaxel n=26 Participants Participants with HER2+ breast cancer received zanidatamab 1800 mg/kg IV and docetaxel 75 mg/m\^2 IV Q3W for an initial period of up to six cycles. After cycle 6, continuation of docetaxel treatment was at the discretion of the investigator. Zanidatamab was administered until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met.	Cohort 2A: Gastric/GEJC: Zanidatamab 30 mg/kg + Tislelizumab + CAPOX n=19 Participants Participants with HER2+ gastric / gastroesophageal junction cancer (GEJC) received zanidatamab 30 mg/kg IV and tislelizumab 200 mg IV once every 3 weeks until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.	Cohort 2B: Gastric/GEJC: Zanidatamab 1800/2400 mg + Tislelizumab + CAPOX n=14 Participants Participants with HER2+ gastric / GEJC received zanidatamab 1800 mg (participants with body weight \< 70 kg) or 2400 mg (participants with body weight ≥ 70 kg) IV and tislelizumab 200 mg IV Q3W until disease progression, intolerable toxicity, or another criterion for treatment discontinuation was met. Participants also received CAPOX chemotherapy (1000 mg/m\^2 oral capecitabine twice daily for 14 days in each cycle and 130 mg/m\^2 oxaliplatin IV once every 3 weeks) for up to 6 six cycles. After cycle 6, oxaliplatin was discontinued and continuation of capecitabine as maintenance treatment was at the investigator's discretion.
Serum Concentration of Zanidatamab as a Function of Time Cycle 1 Day 1 End of Infusion (Peak)	665.73 µg/ml Standard Deviation 146.655	639.57 µg/ml Standard Deviation 205.989	550.78 µg/ml Standard Deviation 153.770	514.69 µg/ml Standard Deviation 118.776
Serum Concentration of Zanidatamab as a Function of Time Cycle 2 Day 1 End of Infusion (Peak)	759.44 µg/ml Standard Deviation 119.484	743.56 µg/ml Standard Deviation 112.107	608.75 µg/ml Standard Deviation 111.424	614.15 µg/ml Standard Deviation 89.763
Serum Concentration of Zanidatamab as a Function of Time Cycle 5 Day 1 End of Infusion (Peak)	830.20 µg/ml Standard Deviation 180.227	778.48 µg/ml Standard Deviation 95.597	648.38 µg/ml Standard Deviation 133.648	667.15 µg/ml Standard Deviation 150.823
Serum Concentration of Zanidatamab as a Function of Time Cycle 9 Day 1 End of Infusion (Peak)	962.20 µg/ml Standard Deviation 129.808	817.23 µg/ml Standard Deviation 180.142	705.64 µg/ml Standard Deviation 149.080	663.45 µg/ml Standard Deviation 121.904
Serum Concentration of Zanidatamab as a Function of Time Cycle 17 Day 1 End of Infusion (Peak)	1022.75 µg/ml Standard Deviation 134.525	840.18 µg/ml Standard Deviation 198.200	762.50 µg/ml Standard Deviation 109.983	734.88 µg/ml Standard Deviation 119.881
Serum Concentration of Zanidatamab as a Function of Time Cycle 26 Day 1 End of Infusion (Peak)	1050.67 µg/ml Standard Deviation 114.374	743.33 µg/ml Standard Deviation 148.520	683.80 µg/ml Standard Deviation 175.189	740.43 µg/ml Standard Deviation 160.651
Serum Concentration of Zanidatamab as a Function of Time Cycle 35 Day 1 End of Infusion (Peak)	950.00 µg/ml Standard Deviation 21.378	908.60 µg/ml Standard Deviation 163.032	669.40 µg/ml Standard Deviation 230.838	676.80 µg/ml Standard Deviation 179.093

SECONDARY outcome

Timeframe: Cycle 1: predose and 0, 2, 4, 24, 96, 168, and 336 hours postdose. Cycle 2: predose and 0, 168, and 336 hours postdose. Each cycle is 21 days.