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

An Open-Label Multicenter Phase 1b Study of H3B-6545 in Combination With Palbociclib in Women With Advanced or Metastatic Estrogen Receptor-Positive HER2-Negative Breast Cancer

The primary objective of this study is to evaluate the safety and tolerability of H3B-6545 and palbociclib when administered in combination in order to determine the maximum tolerated dose (MTD) and/or the recommended Phase 2 dose (RP2D) of this combination in women with advanced or metastatic estrogen receptor-positive (ER+) HER2- breast cancer.

Inclusion Criteria:

1. ER+ HER2- locally advanced, recurrent, or metastatic breast cancer, as per local laboratory
2. Prior therapy in the advanced/metastatic setting
3. Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
4. Has adequate bone marrow and organ function

Exclusion Criteria:

1. Uncontrolled significant active infections
2. Major surgery or other locoregional treatment within 4 weeks before the 1st dose of study drug
3. Inability to take oral medication or presence of malabsorption
4. Active cardiac disease or a history of cardiac dysfunction
5. Evidence of ongoing Alcohol or Drug Abuse