A Study of Sacituzumab Tirumotecan (MK-2870) as a Single Agent and in Combination With Pembrolizumab (MK-3475) Versus Treatment of Physician's Choice in Participants With HR+/HER2- Unresectable Locally Advanced or Metastatic Breast Cancer (MK-2870-010)

Progression-Free Survival (PFS) ( sacituzumab tirumotecan versus treatment of physician's choice [TPC]; pembrolizumab + sacituzumab tirumotecan versus TPC)

Timeframe: Up to ~38 months

PFS is defined as the time from randomization to the first documented progressive disease (PD) or death due to any cause, whichever occurs first as assessed by Response Criteria in Solid Tumors Version 1.1 (RECIST 1.1). PD is defined as ≥20% increase in the sum of diameters of target lesions. In addition to the relative increase of 20%, the sum must also demonstrate an absolute increase of ≥5 mm. The appearance of one or more new lesions is also considered PD. PFS as assessed by blinded independent central review (BICR) will be presented.

Overall Survival (OS)

Timeframe: Up to ~77 months

OS is defined as the time from randomization to death due to any cause.

Progression-Free Survival (PFS) (pembrolizumab + sacituzumab tirumotecan + versus sacituzumab tirumotecan)

Timeframe: Up to ~57 months

PFS is defined as the time from randomization to the first documented progressive disease (PD) or death due to any cause, whichever occurs first as assessed by Response Criteria in Solid Tumors Version 1.1 (RECIST 1.1). PD is defined as ≥20% increase in the sum of diameters of target lesions. In addition to the relative increase of 20%, the sum must also demonstrate an absolute increase of ≥5 mm. The appearance of one or more new lesions is also considered PD. PFS as assessed by blinded independent central review (BICR) will be presented.

Objective Response Rate (ORR)

Timeframe: Up to ~57 months

ORR is defined as the percentage of participants with Complete Response (CR: disappearance of all target lesions) or Partial Response (PR: at least a 30% decrease in the sum of diameters of target lesions) per Response Evaluation Criteria In Solid Tumors Version 1.1 (RECIST 1.1). The percentage of participants who experience CR or PR as assessed by Blinded Independent Central Review (BICR) will be presented.

Duration of Response (DOR)

Timeframe: Up to ~57 months

For participants who demonstrate a confirmed Complete Response (CR: disappearance of all target lesions) or Partial Response (PR: at least a 30% decrease in the sum of diameters of target lesions) per Response Evaluation Criteria In Solid Tumors Version 1.1 (RECIST 1.1), DOR is defined as the time from first documented evidence of CR or PR until progressive disease (PD) or death. Per RECIST 1.1, PD is defined as at least a 20% increase in the sum of diameters of target lesions. In addition to the relative increase of 20%, the sum must also demonstrate an absolute increase of at least 5 mm. The appearance of one or more new lesions is also considered PD. DOR as assessed by Blinded Independent Central Review (BICR) will be presented.

Change from baseline in global health status/quality of life scores, on the European Organization for the Research and Treatment of Cancer Quality of Life Questionnaire Core 30 (EORTC QLQ-C30)

Timeframe: Baseline and up to ~77 months

The EORTC QLQ-C30 is a 30-item questionnaire to assess the overall quality of life of cancer patients. Participant responses to the questions "How would you rate your overall health during the past week?" and "How would you rate your overall quality of life during the past week?" are scored on a 7-point scale (1= Very poor to 7=Excellent). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A higher score indicates a better overall health status.

Change from baseline in physical functioning score, on the EORTC QLQ-C30

Timeframe: Baseline and up to ~77 months

The EORTC QLQ-C30 is a 30-item questionnaire to assess the overall quality of life of cancer patients. Participant responses to questions about their physical functioning are scored on a 4-point scale (1=Not at All to 4=Very Much). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A higher score indicates a better level of physical functioning.

Change from baseline in emotional functioning score, on the EORTC QLQ-C30

Timeframe: Baseline and up to ~77 months

The EORTC QLQ-C30 is a 30-item questionnaire to assess the overall quality of life of cancer patients. Participant responses to questions about their emotional functioning are scored on a 4-point scale (1=Not at All to 4=Very Much). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A higher score indicates a better level of emotional functioning.

Change from baseline in fatigue score, on the EORTC QLQ-C30

Timeframe: Baseline and up to ~77 months

The EORTC QLQ-C30 is a 30-item questionnaire to assess the overall quality of life of cancer patients. Participant responses to questions about their fatigue are scored on a 4-point scale (1=Not at All to 4=Very Much). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A lower score indicates a better level of function.

Change from baseline in diarrhea score, on the EORTC QLQ-C30

Timeframe: Baseline and up to ~77 months

The EORTC QLQ-C30 is a 30-item questionnaire to assess the overall quality of life of cancer patients. Participant responses to the question "Have you had diarrhea?" are scored on a 4-point scale (1=Not at All to 4=Very Much). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A lower score indicates a better level of function.

Time to first Deterioration (TTD) in global health status/quality of life scores, on the EORTC QLQ-C30

Timeframe: Up to ~77 months

The EORTC QLQ-C30 is a 30-item questionnaire to assess the overall quality of life of cancer patients. Participant responses to the questions "How would you rate your overall health during the past week?" and "How would you rate your overall quality of life during the past week?" are scored on a 7-point scale (1= Very poor to 7=Excellent). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A higher score indicates a better overall health status. TTD is defined as the time from baseline to the first onset of a 10-point deterioration from baseline in global health status/quality of life combined score.

TTD in physical functioning score, on the EORTC QLQ-C30

Timeframe: Up to ~77 months

The EORTC QLQ-C30 is a 30-item questionnaire to assess the overall quality of life of cancer patients. Participant responses to questions about their physical functioning are scored on a 4-point scale (1=Not at All to 4=Very Much). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A higher score indicates a better level of physical functioning. TTD is defined as the time from baseline to the first onset of a 10-point deterioration from baseline in the physical functioning score.

TTD in emotional functioning score, on the EORTC QLQ-C30

Timeframe: Up to ~77 months

The EORTC QLQ-C30 is a 30-item questionnaire to assess the overall quality of life of cancer patients. Participant responses to questions about their emotional functioning are scored on a 4-point scale (1=Not at All to 4=Very Much). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A higher score indicates a better level of emotional functioning. TD is defined as the time from baseline to the first onset of a 10-point deterioration from baseline in the emotional functioning score.

TTD in fatigue score, on the EORTC QLQ-C30

Timeframe: Up to ~77 months

The EORTC QLQ-C30 is a 30-item questionnaire to assess the overall quality of life of cancer patients. Participant responses to questions about their fatigue are scored on a 4-point scale (1=Not at All to 4=Very Much). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A lower score indicates a better level of function. TTD is defined as the time from baseline to the first onset of a 10-point deterioration from baseline in the fatigue score.

TTD in diarrhea score, on the EORTC QLQ-C30

Timeframe: Up to ~77 months

The EORTC QLQ-C30 is a 30-item questionnaire to assess the overall quality of life of cancer patients. Participant responses to the question about their diarrhea are scored on a 4-point scale (1=Not at All to 4=Very Much). Using linear transformation, raw scores are standardized, so that scores range from 0 to 100. A lower score indicates a better level of function. TTD is defined as the time from baseline to the first onset of a 10-point deterioration from baseline in the diarrhea score.

Number of participants who experience one or more adverse events (AEs)

Timeframe: Up to ~77 months

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.

Number of participants who discontinue study treatment due to an AE

Timeframe: Up to ~77 months

An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.