A Phase 1b/2 Dose Escalation and Expansion Study of Tucatinib in Combination With Trastuzumab and Oxaliplatin-based Chemotherapy or Pembrolizumab-containing Combinations for HER2+ Gastrointestinal Cancers

Number of Participants With Renal Dose-Limiting Toxicities (DLTs): Phase 1b (Cohort 1A and 1B)

Time Frame: From first dose of tucatinib until end of Cycle 3 (up to 42 days)

A renal DLT was defined as an increase in serum cystatin C \>1.5\* baseline that was not related to pre-renal or post-renal etiologies (including disease progression, dehydration and intercurrent illness) and occurred during the period of treatment with tucatinib in combination with trastuzumab and FOLFOX between the first dose of tucatinib and the end of Cycle 3. Increased serum cystatin C for which there was an alternative clinical explanation (eg, clearly related to an intercurrent illness or disease progression) was not considered renal DLTs.

Number of Participants With Treatment Emergent Adverse Events (TEAEs): Phase 1b (Cohort 1E and 1F)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.7 months)

An adverse event (AE) was defined as any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil, oxaliplatin and pembrolizumab) through 30 days after the last dose of study treatment.

Number of Participants With Treatment Related TEAEs: Phase 1b (Cohort 1E and 1F)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.7 months)

An AE was defined was any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil, oxaliplatin and pembrolizumab) through 30 days after the last dose of study treatment. Treatment related TEAEs were AEs related to any study treatment and relatedness was judged by investigator.

Number of Participants With Greater Than or Equal to (>=) Grade 3 TEAEs: Phase 1b (Cohort 1E and 1F)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.7 months)

An AE was defined was any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil, oxaliplatin and pembrolizumab) through 30 days after the last dose of study treatment. TEAEs were graded according to National Cancer Institute, Common Terminology Criteria for Adverse Events (NCI-CTCAE) version (v) 5.0 where grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening. Number of participants with \>= grade 3 TEAEs are reported in this outcome measure.

Number of Participants With >= Grade 3 Treatment Related TEAEs: Phase 1b (Cohort 1E and 1F)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.7 months)

An AE was defined was any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil, oxaliplatin and pembrolizumab) through 30 days after the last dose of study treatment. TEAEs were graded according to NCI-CTCAE v 5.0 where grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening. Treatment related TEAEs were AEs related to study treatment and relatedness was judged by investigator. Number of participants with \>= grade 3 treatment related TEAEs are reported in this outcome measure.

Number of Participants With Any Serious Adverse Event (SAE): Phase 1b (Cohort 1E and 1F)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.7 months)

An SAE was any untoward medical occurrence at any dose that was fatal; life-threatening; required hospitalization or prolongation of existing hospitalization; resulted in persistent or significant disability/incapacity; was a congenital anomaly/birth defect or was medically significant.

Number of Participants With Any Treatment Related SAE: Phase 1b (Cohort 1E and 1F)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.7 months)

An SAE was any untoward medical occurrence at any dose that was fatal; life-threatening; required hospitalization or prolongation of existing hospitalization; resulted in persistent or significant disability/incapacity; was a congenital anomaly/birth defect or was medically significant. Treatment related SAEs were SAEs related to any study treatment and relatedness was judged by investigator.

Number of Participants With DLTs: Phase 1b (Cohort 1E)

Time Frame: Cycle 1 (Up to 21 days)

DLTs were AEs/ laboratory abnormalities that were considered to be related to tucatinib/ tucatinib in combination with chemotherapy(mFOLFOX6/CAPOX) and/or trastuzumab and/or pembrolizumab. DLTs was defined any of following: a-)Hepatic- any instance of aspartate aminotransferase(AST)/ alanine aminotransferase(ALT)\>3\*upper limit of normal(ULN) and total bilirubin \>2\*ULN that is not thought to be due to progressive disease(PD)/other medical illness. PD:at least 20% increase in sum of diameters of target lesions,taking as reference smallest sum on study. In addition to relative increase of 20%,sum must also demonstrate an absolute increase of at least 5 millimeters(mm). b-) Non-hematologic- any clinically significant, non-hematologic treatment-related AE\>= grade(G)3, with following exceptions:G3 fatigue=\< 7days,G3 diarrhea,nausea/vomiting without optimal use of anti-emetics/antidiarrheals c-)hematologic:\>=G3 febrile neutropenia and absolute neutrophil count(ANC) decreased G4 for \>7days.

Number of Participants With DLTs: Phase 1b (Cohort 1F)

Time Frame: Cycle 1 (Up to 28 days)

DLTs were AEs/ laboratory abnormalities that were considered to be related to tucatinib/ tucatinib in combination with chemotherapy (mFOLFOX6/CAPOX) and/ or trastuzumab and/ or pembrolizumab. DLTs was defined any of following: a-) Hepatic- any instance of AST/ ALT \>3\*ULN and total bilirubin \>2\*ULN that is not thought to be due to PD/ other medical illness. PD: at least 20% increase in sum of diameters of target lesions, taking as reference smallest sum on study. In addition to relative increase of 20%, sum must also demonstrate an absolute increase of at least 5 mm. b-) Non-hematologic- any clinically significant, non-hematologic treatment-related AE\>= grade 3, with following exceptions: grade 3 fatigue=\< 7days, grade 3 diarrhea, nausea/vomiting without optimal use of anti-emetics/antidiarrheals c-) hematologic: \>= grade 3 febrile neutropenia and ANC decreased grade 4 for \>7days.

Number of Participants With Treatment Emergent Hematological Laboratory Abnormalities: Phase 1b (Cohort 1E and 1F)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.7 months)

The following hematological parameters were assessed: hemoglobin decreased, leukocytes decreased, lymphocytes decreased, neutrophils decreased and platelets decreased. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil, oxaliplatin and pembrolizumab) through 30 days after the last dose of study treatment. Hematological laboratory abnormalities were graded according to NCI CTCAE v 5.0 (grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening). Participants with hematological abnormalities of any grade were reported.

Number of Participants With Treatment Emergent Chemistry Laboratory Abnormalities: Phase 1b (Cohort 1E and 1F)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.7 months)

The following chemistry laboratory parameters were assessed: alanine aminotransferase (ALT) increased, albumin decreased, alkaline phosphatase increased, aspartate aminotransferase (AST) increased, calcium corrected for albumin decreased, creatinine increased, calcium corrected for albumin increased, lactate dehydrogenase increased, potassium decreased, potassium increased, sodium decreased, sodium increased and total bilirubin increased. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil, oxaliplatin and pembrolizumab) through 30 days after the last dose of study treatment Chemistry laboratory abnormalities were graded according to NCI CTCAE v 5.0 (grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening). Participants with chemistry abnormalities of any grade were reported.

Number of Participants With Clinically Significant Post-Baseline Vital Signs: Phase 1b (Cohort 1E and 1F)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.7 months)

Vital signs included temperature, blood pressure, heart rate and respiratory rate. Clinically significant vital signs were defined as: temperature greater than or equal to (\>=) 38 degrees Celsius, respiratory rate greater than (\>) 20 breaths per minute, systolic blood pressure (SBP) \>= 120 millimeter of mercury (mmHg) or diastolic blood pressure (DBP) \>= 80 mmHg, SBP \>= 140 mmHg or DBP \>= 90 mmHg and heart rate \> 100 beats per minute (bpm).

Number of Participants With TEAEs: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

An AE was defined as any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment.

Number of Participants With Treatment Related TEAEs: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

An AE was defined was any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. Treatment related TEAEs were AEs related to study treatment and relatedness was judged by investigator.

Number of Participants With >= Grade 3 TEAEs: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

An AE was defined was any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. TEAEs were graded according to NCI-CTCAE v 5.0 where grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening. Number of participants with \>= grade 3 TEAEs are reported in this outcome measure.

Number of Participants With >= Grade 3 Treatment Related TEAEs: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

An AE was defined was any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. TEAEs were graded according to NCI-CTCAE v 5.0 where grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening. Treatment related TEAEs were AEs related to study treatment and relatedness was judged by investigator. Number of participants with \>= grade 3 treatment related TEAEs are reported in this outcome measure.

Number of Participants With Any SAE: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

An SAE was any untoward medical occurrence at any dose that was fatal; life-threatening; required hospitalization or prolongation of existing hospitalization; resulted in persistent or significant disability/incapacity; was a congenital anomaly/birth defect or was medically significant.

Number of Participants With Any Treatment Related SAE: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

An SAE was any untoward medical occurrence at any dose that was fatal; life-threatening; required hospitalization or prolongation of existing hospitalization; resulted in persistent or significant disability/incapacity; was a congenital anomaly/birth defect or was medically significant. Treatment related SAEs were SAEs related to any study treatment and relatedness was judged by investigator.

Number of Participants With DLTs: Phase 1b (Cohort 1D)

Time Frame: Cycle 1 (Up to 28 days)

DLTs were AEs/ laboratory abnormalities that were considered to be related to tucatinib/ tucatinib in combination with chemotherapy (mFOLFOX6/CAPOX) and/ or trastuzumab and/ or pembrolizumab. DLTs was defined any of following: a-) Hepatic- any instance of AST/ ALT \>3\*ULN and total bilirubin \>2\*ULN that is not thought to be due to PD/ other medical illness. PD: at least 20% increase in sum of diameters of target lesions, taking as reference smallest sum on study. In addition to relative increase of 20%, sum must also demonstrate an absolute increase of at least 5 mm. b-) Non-hematologic- any clinically significant, non-hematologic treatment-related AE\>= grade 3, with following exceptions: grade 3 fatigue=\< 7days, grade 3 diarrhea, nausea/vomiting without optimal use of anti-emetics/antidiarrheals c-) hematologic: \>= grade 3 febrile neutropenia and ANC decreased grade 4 for \>7days.

Number of Participants With Treatment Emergent Hematological Laboratory Abnormalities: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

The following hematological parameters were assessed: hemoglobin decreased, leukocytes decreased, lymphocytes decreased, neutrophils decreased and platelets decreased. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. Hematological laboratory abnormalities were graded according to NCI CTCAE v 5.0 (grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening). Participants with hematological abnormalities of any grade were reported.

Number of Participants With Treatment Emergent Chemistry Laboratory Abnormalities: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

The following chemistry parameters were assessed: ALT increased, albumin decreased, alkaline phosphatase increased, AST increased, calcium corrected for albumin decreased, creatinine increased, lactate dehydrogenase increased, potassium decreased, potassium increased, sodium decreased and sodium increased. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. Chemistry laboratory parameters abnormalities were graded according to NCI CTCAE v 5.0 (grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening). Participants with chemistry abnormalities of any grade were reported.

Number of Participants With Clinically Significant Post-Baseline Vital Signs: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

Vital signs included temperature, blood pressure, heart rate and respiratory rate. Clinically significant vital signs were defined as: temperature \>= 38 degrees Celsius, respiratory rate \> 20 breaths per minute, SBP \>= 120 mmHg or DBP \>= 80 mmHg, SBP \>= 140 mmHg or DBP \>= 90 mmHg and heart rate \> 100 beats per minute (bpm).

Number of Participants With TEAEs Leading to Dose Holds: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

An AE was defined as any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. Dose hold was considered as dose delay and dose elimination. Number of participants with TEAEs leading to dose holds are reported in this outcome measure.

Number of Participants With TEAEs Leading to Dose Reductions: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

An AE was defined as any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. Number of participants with TEAEs leading to dose reductions are reported in this outcome measure.

Number of Participants With TEAEs Leading to Dose Discontinuations: Phase 1b (Cohort 1D)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 21 months)

An AE was defined as any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. Number of participants with TEAEs leading to dose discontinuations (i.e. permanent withdrawal of tucatinib, trastuzumab, oxaliplatin, fluorouracil and leucovorin) are reported in this outcome measure.

Number of Participants With TEAEs: Phase 2 (Cohort 2B)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.9 months)

An AE was defined was any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment.

Number of Participants With Treatment Related TEAEs: Phase 2 (Cohort 2B)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.9 months)

An AE was defined was any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. Treatment related TEAEs were AEs related to study treatment and relatedness was judged by investigator.

Number of Participants With >= Grade 3 TEAEs: Phase 2 (Cohort 2B)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.9 months)

An AE was defined was any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. TEAEs were graded according to NCI-CTCAE v 5.0 (grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening). Number of participants with \>= grade 3 TEAEs are reported in this outcome measure.

Number of Participants With >= Grade 3 Treatment Related TEAEs: Phase 2 (Cohort 2B)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.9 months)

An AE was defined was any untoward medical occurrence in a clinical investigational participant administered a medicinal product and which did not necessarily have a causal relationship with this treatment. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. TEAEs were graded according to NCI-CTCAE v 5.0 (grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening). Treatment related TEAEs were AEs related to study treatment and relatedness was judged by investigator. Number of participants with \>= grade 3 treatment related TEAEs are reported in this outcome measure.

Number of Participants With Any SAE: Phase 2 (Cohort 2B)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.9 months)

An SAE was any untoward medical occurrence at any dose that was fatal; life-threatening; required hospitalization or prolongation of existing hospitalization; resulted in persistent or significant disability/incapacity; was a congenital anomaly/birth defect or was medically significant.

Number of Participants With Any Treatment Related SAE: Phase 2 (Cohort 2B)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.9 months)

An SAE was any untoward medical occurrence at any dose that was fatal; life-threatening; required hospitalization or prolongation of existing hospitalization; resulted in persistent or significant disability/incapacity; was a congenital anomaly/birth defect or was medically significant. Treatment related SAEs were SAEs related to any study treatment and relatedness was judged by investigator.

Number of Participants With Treatment Emergent Hematological Laboratory Abnormalities: Phase 2 (Cohort 2B)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.9 months)

Following hematological parameters were assessed: hemoglobin decreased, leukocytes decreased, lymphocytes decreased, neutrophils decreased and platelets decreased. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. Hematological abnormalities were graded according to NCI CTCAE v 5.0; where, grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening). Participants with hematologic abnormalities of any grade were reported.

Number of Participants With Treatment Emergent Chemistry Laboratory Abnormalities: Phase 2 (Cohort 2B)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.9 months)

Following chemistry parameters were assessed: ALT increased, albumin decreased, alkaline phosphatase increased, AST increased, creatinine increased, glomerular filtration rate (GFR) estimated decreased, lactate dehydrogenase increased, potassium decreased, potassium increased, sodium decreased, sodium increased and total bilirubin increased. TEAE was defined as a newly occurring or worsening AE after the first dose of study treatment (tucatinib, trastuzumab, leucovorin, fluorouracil and oxaliplatin) through 30 days after the last dose of study treatment. Chemistry abnormalities were graded according to NCI CTCAE v 5.0; where, grade 1= mild, grade 2= moderate, grade 3= severe and grade 4= life-threatening. Participants with chemistry abnormalities of any grade were reported.

Number of Participants With Clinically Significant Post-Baseline Vital Signs: Phase 2 (Cohort 2B)

Time Frame: From first dose of the study treatment (Day 1) up to 30 days after the last dose of study treatment, end of study date, death date, or data cutoff date, whichever was earlier (maximum of 20.9 months)

Vital signs included temperature, blood pressure and heart rate. Clinically significant vital signs were defined as: temperature \>= 38 degrees Celsius, respiratory rate \> 20 breaths per minute, SBP \>= 120 mmHg or DBP \>= 80 mmHg, SBP \>= 140 mmHg or DBP \>= 90 mmHg and heart rate \> 100 bpm.