A Study of H3B-6545 in Combination With Palbociclib in Women With Advanced or Metastatic Estrogen Receptor-Positive Human Epidermal Growth Factor Receptor-2 (HER2)-Negative Breast Cancer

The MTD was defined as the highest dose at which no more than 1 of 6 participants experienced a Dose-Limiting Toxicity (DLT) in the dose cohort. DLT was graded as per National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) version 5.0. DLTs were defined as the following events that occurred in Cycle 1, for which a causal relationship with the study drug could not be ruled out: febrile neutropenia; Grade 4 neutropenia that was not resolved within 7 days; Grade 4 thrombocytopenia; Grade 3 thrombocytopenia lasting greater than (>) 7 days or associated with clinically significant bleeding; Grade 4 vomiting and diarrhea; Grade 3 vomiting and diarrhea lasting > 72 hours despite treatment; Grade 4 electrolyte abnormality or Grade 3 abnormality lasting > 24 hours; Grade 3 or 4 serum creatinine or bilirubin increase; Grade 4 biochemistry or Grade 3 lasting > 7 days; Grade 4 or Grade 3 or intolerable grade 2 toxicities of any non-hematologic adverse event.

TEAE was defined as an adverse event (AE) with an onset that had occurred after receiving study drug. An AE was defined as any untoward medical occurrence in a participants or clinical investigation participant administered an investigational product. An AE does not necessarily have a causal relationship with medicinal product. A serious adverse event (SAE) was defined as any AE if it resulted in death or life-threatening AE or required inpatient hospitalization or prolongation of existing hospitalization or resulted in persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions or was a congenital anomaly/birth defect. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

Ratio of palbociclib Cmax Day 21/Day 8, is the ratio of palbociclib exposure on Day 21 and palbociclib exposure on Day 8. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

Ratio of palbociclib AUC24 Day 21/Day 8, is the ratio of palbociclib exposure on Day 21 and palbociclib exposure on Day 8. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

Ratio of palbociclib C24 Day 21/Day 8, is the ratio of palbociclib exposure on Day 21 and palbociclib exposure on Day 8. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

Ratio of palbociclib Cmax Day 21/Day 28, is the ratio of H3B-6545 exposure on Day 21 and H3B-6545 exposure on Day 28. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

Ratio of H3B-6545 AUC24 Day 21/Day 28, is the ratio of H3B-6545 exposure on Day 21 and H3B-6545 exposure on Day 28. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

Ratio of H3B-6545 C24 Day 21/Day 28, is the ratio of H3B-6545 exposure on Day 21 and H3B-6545 exposure on Day 28. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

ORR is defined as the percentage of participants achieving a best overall response (BOR) of confirmed partial response (PR) or complete response (CR). The ORR will be assessed according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

DoR is defined as the time from the date of the first documented CR/PR until the first documentation of disease progression (PD) or death, whichever comes first. The DoR will be assessed according to RECIST version 1.1. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

CBR is defined as the percentage of participants with BOR of PR, CR, or durable stable disease (SD) (duration of SD greater than or equal to 23 weeks). Duration of SD is defined as the time from the date of first dose to the date of the first documentation of disease progression or death, whichever occurs first. It will be calculated for participants whose BOR is SD. The CBR will be assessed according to RECIST version 1.1. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

PFS is defined as the time from the first dose date to the date of the first documentation of PD or death (whichever occurs first). Analysis is not final for this outcome measure, and complete data will be posted at study completion date.

OS is defined as the time from first dose date to the date of death from any cause. Analysis is not final for this outcome measure, and complete data will be posted at study completion date.