Corrected QT Interval Effects of Trastuzumab Emtansine (T-DM1) in Patients With Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Locally Advanced or Metastatic Breast Cancer and the Safety and Tolerability of Combined T-DM1 and Pertuzumab in Patients With Early Disease Progression

The corrected QT interval was calculated using Bazett's correction (QTcB) from electrocardiogram (ECG) data. Each participant had triplicate QTcB intervals measured at each timepoint and the average was calculated for each patient at each timepoint. For each timepoint, a participant's corresponding baseline QTcB interval was subtracted from the average QTcB intervals to create a baseline-adjusted average QTcB interval.

The uncorrected QT interval was calculated from electrocardiogram (ECG) data. Each participant had triplicate QT intervals measured at each timepoint and the average was calculated for each patient at each timepoint. For each timepoint, a participant's corresponding baseline QT interval was subtracted from the average QT intervals to create a baseline-adjusted average QT interval.

The PR interval is the time in seconds from the beginning of the P wave to the beginning of the QRS complex, and was calculated from electrocardiogram (ECG) data. Each participant had triplicate PR intervals measured at each timepoint and the average was calculated for each patient at each timepoint. For each timepoint, a participant's corresponding baseline PR interval was subtracted from the average PR intervals to create a baseline-adjusted average PR interval.

The QRS interval represents the time it takes for depolarization of the ventricles and was calculated from electrocardiogram (ECG) data. Each participant had triplicate QRS intervals measured at each timepoint and the average was calculated for each patient at each timepoint. For each timepoint, a participant's corresponding baseline QRS interval was subtracted from the average QRS intervals to create a baseline-adjusted average QRS interval.

The corrected QT interval was calculated using Fridericia's correction (QTcF) and using Bazett's correction (QTcB) from electrocardiogram (ECG) data. Each participant had triplicate QTc intervals measured at each timepoint and the average was calculated for each patient at each timepoint. QTc interval categories are based off International Conference on Harmonisation (ICH) Tripartite Guideline E14.

The corrected QT interval was calculated using Fridericia's correction (QTcF) and using Bazett's correction (QTcB) from electrocardiogram (ECG) data. Each participant had triplicate QTc intervals measured at each timepoint and the average was calculated for each patient at each timepoint. For each timepoint, a participant's corresponding baseline QTc interval was subtracted from the average QTc intervals to create a baseline-adjusted average QTc interval.

QTc interval categories are based off International Conference on Harmonisation (ICH) Tripartite Guideline E14.

The incidence of abnormal U-wave changes from baseline was determined based on centrally read electrocardiogram (ECG) tracings comparing each of the three triplicate readings from the post baseline ECG time points to the baseline ECG reading. At each time point, if at least one of the three triplicate readings was abnormal, the participant was counted as abnormal for that ECG timepoint as follows: a large U wave, inverted U wave, or T-U fusion compared with baseline was considered an abnormal significant change from baseline.

The incidence of abnormal T-wave changes from baseline was determined based on centrally read electrocardiogram (ECG) tracings comparing each of the three triplicate readings from the post baseline ECG time points to the baseline ECG reading. At each time point, if at least one of the three triplicate readings was abnormal, the participant was counted as abnormal for that ECG timepoint as follows: an inverted T, flat T, or biphasic T compared with baseline was considered an abnormal significant change from baseline. Additionally, nonspecific T-wave changes from baseline were considered as abnormal nonsignificant changes from baseline. T-wave changes from baseline due to ventricular conduction or left ventricular hypertrophy strain were considered not evaluable. C=Cycle; D=Day.

Objective response was defined as a complete response (CR) or partial response (PR) determined on two consecutive assessments conducted by the investigator ≥ 4 weeks apart. Responses were assessed by physical examination and imaged-based evaluation using a modified version of the Response Evaluation Criteria in Solid Tumors (RECIST) version 1.0:

CR-the disappearance of all target lesions and the disappearance of all nontarget lesions and normalization of tumor marker level and no new lesions.

PR-either the disappearance of all target lesions with persistence of one or more nontarget lesion(s) and/or the maintenance of tumor marker level above the normal limits, or, at least a 30% decrease in the sum of the longest diameter of target lesions, taking as reference the baseline sum longest diameter with no new lesions or unequivocal progression of existing nontarget lesions.

In patients with an objective response during the single-agent trastuzumab emtansine treatment period, duration of response was defined as the time from the first documented objective response to the time of first documented disease progression or death, whichever occurred first. Progressive disease was defined as either at least a 20% increase in the sum of diameters of target lesions, taking as reference the smallest sum on study with an absolute increase of at least 5 mm, or the appearance of one or more new lesions, or the unequivocal progression of existing nontarget lesions.

If a patient did not die or experience disease progression before the end of the study, duration of response was censored at the day of the last tumor assessment when the patient was known to be progression free.

Progression-free survival (PFS) was defined as the time from the first day of study treatment (Day 1) to first documented disease progression or death, whichever occurred first. If a patient did not experience disease progression or die, PFS was censored at the day of the last tumor assessment that a patient was known to be progression free.

Participants were considered to have experienced clinical benefit if they had an objective response or maintained stable disease for at least 6 months from start of study treatment. Objective response was defined as a complete or partial response determined on two consecutive tumor assessments at least 4 weeks apart based on a modified version of RECIST.

Stable disease was defined as neither sufficient shrinkage to qualify for partial response nor sufficient increase to qualify for progressive disease, taking as reference the smallest sum longest diameter since the treatment started and no new lesions or unequivocal progression of existing nontarget lesions.

An serious AE is any AE that is fatal, life threatening, requires or prolongs inpatient hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect in a neonate/infant born to a mother exposed to the investigational product(s), or is considered a significant medical event by the investigator (e.g., may jeopardize the patient or may require medical/surgical intervention to prevent one of the outcomes listed above).

The severity of each AE was graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE) v3.0, or as follows: Grade 1 = Mild; Grade 2 = Moderate; Grade 3 = Severe; Grade 4 = Very severe; Grade 5 = Death related to AE.

Left ventricular ejection fraction (LVEF) was assessed on the basis of local assessments of echocardiogram or multigated acquisition scan (MUGA) data. A decrease in LVEF is defined as a decrease from Baseline of greater than or equal to 15%.

Grade 3 LVEF is an ejection fraction between 20 and 40%.

Serum samples were quantitated for T-DM1 (DM1 conjugated to trastuzumab) levels in a validated assay using an indirect sandwich enzyme-linked immunosorbent assay (ELISA). The minimum quantifiable concentration in human serum was 40 ng/mL. Serum samples were assayed for total trastuzumab (conjugated and unconjugated T-DM1) in a validated assay using an indirect sandwich ELISA method; the minimum quantifiable concentration in human serum was 40 ng/mL.

Area under the serum concentration-time curve from Time zero to time of last measurable concentration (AUClast) for T-DM1 (conjugated trastuzumab) and Total Trastuzumab (conjugated and unconjugated T-DM1) at Cycle 1 and Cycle 3.

Area under the serum concentration-time curve from Time zero extrapolated to infinity (AUCinf) for T-DM1 (conjugated trastuzumab) and total trastuzumab (conjugated and unconjugated T-DM1) at Cycle 1.

The number of participants with anti-T-DM1 antibodies was assessed using a validated bridging antibody enzyme-linked immunosorbent assay (ELISA).