Study of Trastuzumab Deruxtecan (T-DXd) vs Investigator's Choice Chemotherapy in HER2-low, Hormone Receptor Positive, Metastatic Breast Cancer

Eligible patients will be those patients who have had disease progression on at least 2 previous lines of endocrine therapies given for the treatment of metastatic disease or disease progression within 6 months of starting first line treatment for metastatic disease with an endocrine therapy combined with a CDK4/6 inhibitor. All patients must have historically confirmed HR positive (either estrogen receptor and/or progesterone receptor positive), HER2-low (defined as IHC2+/ISH- and IHC 1+) or HER2 IHC \>0 \<1+ expression, as determined by central laboratory testing results, advanced or metastatic breast cancer.

The study aims to evaluate the efficacy, safety and tolerability of trastuzumab deruxtecan compared with investigator's choice chemotherapy. This study aims to see if trastuzumab deruxtecan allows patients to live longer without the cancer getting worse, or simply to live longer, compared to patients receiving standard of care chemotherapy. This study is also looking to see how the treatment and the cancer affects patients' quality of life.

Primary Outcome Measures

1. Progression-Free Survival (PFS) Assessed by Blinded Independent Central Review (BICR) in the Hormone Receptor-Positive (HR+), Human Epidermal Growth Factor Receptor 2 (HER2)-Low Population
   PFS per Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST 1.1) assessed by BICR was defined as the time from the date of randomization until the date of PD, as defined or death (by any cause in the absence of progression) regardless of whether the participant withdrew from randomized therapy or received another anticancer therapy prior to progression. PD was defined as at least a 20% increase in the sum of diameters of target lesions (TLs), taking as reference the smallest previous sum of diameters (nadir), this included the baseline sum if that was the smallest on study. In addition to the relative increase of 20%, the sum must demonstrate an absolute increase of at least 5 millimeter (mm) from nadir. Median PFS was calculated using Kaplan-Meier method and its confidence interval (CI) using Brookmeyer-Crowley method.

Secondary Outcome Measures

1. Overall Survival (OS) in the Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-low Population
   OS was defined as the time from the date of randomization until death due to any cause regardless of whether the participant withdrew from randomized therapy or received another anticancer therapy. Median OS was calculated using Kaplan-Meier method and its CI using Brookmeyer-Crowley method.
2. Progression-Free Survival Assessed by Blinded Independent Central Review in the Intent-to-Treat, Human Epidermal Growth Factor Receptor 2 Immunohistochemistry (IHC) >0 <1+ and Human Epidermal Growth Factor Receptor 2-low Population
   PFS per RECIST 1.1 assessed by BICR was defined as the time from the date of randomization until the date of PD, as defined or death (by any cause in the absence of progression) regardless of whether the participant withdrew from randomized therapy or received another anticancer therapy prior to progression. PD was defined as at least a 20% increase in the sum of diameters of TLs, taking as reference the smallest previous sum of diameters (nadir), this included the baseline sum if that was the smallest on study. In addition to the relative increase of 20%, the sum must demonstrate an absolute increase of at least 5 mm from nadir. Median PFS was calculated using Kaplan-Meier method and its CI using Brookmeyer-Crowley method.
3. Overall Survival (OS) in the Intent-to-Treat Population
   OS was defined as the time from the date of randomization until death due to any cause regardless of whether the participant withdrew from randomized therapy or received another anticancer therapy. Median OS was calculated using Kaplan-Meier method and its CI using Brookmeyer-Crowley method.
4. Progression-Free Survival Assessed by Investigator in the Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-low Population
   PFS per RECIST 1.1 assessed by Investigator was defined as the time from the date of randomization until the date of PD, as defined or death (by any cause in the absence of progression) regardless of whether the participant withdrew from randomized therapy or received another anticancer therapy prior to progression. PD was defined as at least a 20% increase in the sum of diameters of TLs, taking as reference the smallest previous sum of diameters (nadir), this included the baseline sum if that was the smallest on study. In addition to the relative increase of 20%, the sum must demonstrate an absolute increase of at least 5 mm from nadir. Median PFS was calculated using Kaplan-Meier method and its CI using Brookmeyer-Crowley method.
5. Objective Response Rate (ORR) Assessed by Blinded Independent Central Review and Investigator in the Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-low Population
   ORR per RECIST 1.1 assessed by BICR and Investigator was defined as the percentage of participants with at least 1 visit response of complete response (CR) or partial response (PR). CR was defined as disappearance of all TLs since baseline. Any pathological lymph nodes selected as TLs must have a reduction in short axis diameter to \&lt;10 mm. PR was defined as at least a 30% decrease in the sum of the diameters of TL, taking as reference the baseline sum of diameters.
6. Duration of Response (DOR) Assessed by Blinded Independent Central Review and Investigator in the Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-low Population
   DOR per RECIST v1.1 was defined as the time from the date of first documented response until date of documented progression or death in the absence of PD. PD was defined as at least a 20% increase in the sum of diameters of TLs, taking as reference the smallest previous sum of diameters (nadir), this included the baseline sum if that was the smallest on study. In addition to the relative increase of 20%, the sum must demonstrate an absolute increase of at least 5 mm from nadir. Median DOR was calculated using Kaplan-Meier method.
7. Objective Response Rate Assessed by Blinded Independent Central Review and Investigator in the Intent-to-Treat and Human Epidermal Growth Factor Receptor 2 Immunohistochemistry >0 <1+ Population
   ORR per RECIST 1.1 assessed by BICR and Investigator was defined as the percentage of participants with at least 1 visit response of CR or PR. CR was defined as disappearance of all TLs since baseline. Any pathological lymph nodes selected as TLs must have a reduction in short axis diameter to \&lt;10 mm. PR was defined as at least a 30% decrease in the sum of the diameters of TL, taking as reference the baseline sum of diameters.
8. Duration of Response Assessed by Blinded Independent Central Review and Investigator in the Intent-to-Treat Population
   DOR per RECIST v1.1 was defined as the time from the date of first documented response until date of documented progression or death in the absence of PD. PD was defined as at least a 20% increase in the sum of diameters of TLs, taking as reference the smallest previous sum of diameters (nadir), this included the baseline sum if that was the smallest on study. In addition to the relative increase of 20%, the sum must demonstrate an absolute increase of at least 5 mm from nadir. Median DOR was calculated using Kaplan-Meier method.
9. Time From Randomization to Second Progression or Death (PFS2) in the Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-low and Intent-to-Treat Population
   PFS2 was defined as time from randomization to second progression (the earliest of the progression event subsequent to first subsequent therapy) or death; second progression was defined according to local standard clinical practice and might involve any of the following: objective radiological imaging, symptomatic progression, or death. Median PFS2 was calculated using Kaplan-Meier method and its CI using Brookmeyer-Crowley method.
10. Number of Participants With Treatment-Emergent Adverse Events (TEAEs) and Treatment-Emergent Serious Adverse Events (TESAEs)
    An adverse event (AE) was any untoward medical occurrence in a participant or clinical study participant administered a medicinal product and which did not necessarily had a causal relationship with this treatment. A serious adverse event (SAE) was an AE occurring during any study phase, that fulfilled 1 or more of following criteria: resulted in death, was immediately life-threatening, required in-participant hospitalization or prolongation of existing hospitalization, resulted in persistent or significant disability or incapacity, was congenital abnormality or birth defect, and was an important medical event that jeopardized participant or required medical treatment to prevent 1 of outcomes listed above. TEAE was any AE that occurred, having been absent before the first dose of study drug, or had worsened in severity or seriousness after initiation of the study drug until the 40-day (+7 days) safety follow-up visit.
11. Serum Concentration of Trastuzumab Deruxtecan (T-DXd), Total Anti-Human Epidermal Growth Factor Receptor 2 Antibody and MAAA-1181a
    Blood samples were collected at indicated time points to determine the concentration of T-DXd, total anti-HER2 antibody and MAAA-1181a in serum.
12. Change From Baseline in European Organization for the Research and Treatment of Cancer Quality of Life Questionnaire-Core 30 (EORTC QLQ-C30) Scale Score at Week 91
    EORTC QLQ-C30 is a 30-item self-administered questionnaire grouped into 5 multi-item functional scales(physical,role,emotional,cognitive and social),3 multiitem symptom scales(fatigue,pain and nausea/vomiting),2-item global QoL scale,5 single items assessing additional symptoms reported by cancer participants(dyspnea,loss of appetite,insomnia,constipation and diarrhea). All but 2 questions have 4-point scales:1:not at all,2:a little,3:quite a bit,4:very much.2 questions concerning global health status and QoL have 7-point scales with responses ranging from 1:very poor to 7:excellent;higher score:better level of functioning/greater degree of symptoms.For each of 15 scales,final scores were averaged from scores of component items,transformed,range: 0 to 100;higher scores:better functioning,better health related (HR)QoL,or greater level of symptoms(higher scores:opposite interpretations for functioning/QoL and symptoms/problems).Baseline:last observed measurement prior to randomization.
13. Change From Baseline in European Organization for the Research and Treatment of Cancer Quality of Life Questionnaire-Breast Module 45 (BR45) Scale Score at Week 58
    EORTC QLQ-BR45 is an updated version of the BR23. Self-administered instrument includes original 23-items yielding 5 multi-item scores (body image, sexual functioning, arm symptoms, breast symptoms, and systemic therapy side effects). Additional 22 items yield 4 additional multi-item scales (breast satisfaction, endocrine therapy symptoms, skin mucosis symptoms, and endocrine sexual symptoms). Single items assess sexual enjoyment, future perspective and being upset by hair loss. Items are scored on a 4-point verbal rating scale: 1: not at all, 2: a little, 3: quite a bit, and 4: very much; higher score: better level of functioning or greater degree of symptoms. Scores of these scales were averaged from scores of component items, transformed, and ranged from 0 to 100; higher scores for functioning scales or items indicate better functioning, whereas higher scores for symptom scales or items represent a higher level of symptoms. Baseline:last observed measurement prior to randomization.
14. Time to Deterioration in European Organization for the Research and Treatment of Cancer Quality of Life Questionnaire-Core 30 Scale Score
    Time to deterioration was defined as the time from randomization until the date of the first clinically meaningful deterioration that was confirmed at next available assessment, at least 14 days apart, regardless of whether the participant withdrew from study treatment or receives another anti-cancer therapy prior to deterioration.
15. Number of Participants With Anti-Drug Antibodies (ADAs) Against Trastuzumab Deruxtecan
    Blood samples were collected at indicated timepoints to determine ADA. Treatment-induced ADA was defined as ADA positive post-baseline and not detected at baseline (negative or missing). Treatment-boosted ADA was defined as a baseline positive ADA titer that was boosted to a 4-fold or higher-level following drug administration.
16. Time to First Subsequent Treatment or Death (TFST) in the Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-low and Intent-to-Treat Population
    TFST was defined as time from randomization to the start date of the first subsequent anti-cancer therapy after discontinuation of randomized treatment or death due to any cause.
17. Time to Second Subsequent Treatment or Death (TSST) in the Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-low and Intent-to-Treat Population
    TSST was defined as time from randomization to the start date of the second subsequent anti-cancer therapy after discontinuation of randomized treatment or death due to any cause.