A Phase 1/2, Open-label, Multicenter Study to Evaluate the Safety, Tolerability, Preliminary Antitumor Activity, Pharmacokinetics, and Pharmacodynamics of ZB716 as Monotherapy and in Combination With Palbociclib in Patients With Estrogen Receptor-Positive, HER2-Negative Locally Advanced or Metastatic Breast Cancer

Cohorts will follow a 3+3 study design. Approx. 3 to 6 subjects will be enrolled in each dose cohort (6 subjects in Cohort A6; food-effect evaluation). (Dose levels: 50, 100, 200, 300, 400 mg, orally QD in a 28 day cycle) The overall DLT observation period of ZB716 monotherapy will be 4 weeks following the initial dose of study drug on Cycle 1 Day 1. There will be 2 ~ 6 days between dose escalations to allow sufficient time for an adequate safety review. The max. dose may be lower than 400 mg. In the first dosing group of Part A (Cohort A1), subject dosing will be staggered such that administration of the first dose is separated by at least 7 days between the first 2 subjects. In each of Cohorts A1 to A5, 3 to 6 subjects will receive ZB716 doses according to the assigned dose level in the fasted state. For Cohort A6, Period 1, doses will be administered in the fasted state in Treatment Period 1 and 2, doses will be given 30 min. after starting a standard high fat breakfast.

Potential subjects will be screened to assess their eligibility to enter the study within 28 days prior to the first dose administration. Subjects will come to the clinical site before the first dose of study drug to confirm eligibility. Cohorts will be enrolled sequentially based on an optional Simon 2-stage study design.

Approx. 29 subjects may be enrolled in the dose expansion cohorts of Part B. For Part B, doses will be administered (self-administered by subjects at home or administered by subjects under observation of clinical staff during clinic visits) at the determined monotherapy RD of ZB716 (based on Part A) QD in a 28-day cycle. Doses will be administered in the dietary status for dosing as determined from Part A fed/fasted comparison.

Cohorts will follow a 3+3 study design. Approx. 6 to 12 subjects will be enrolled in the dose escalation phase of ZB716 in combination with Palbociclib. For Part C, doses will be administered at escalating doses starting with 1 dose level below the monotherapy RD (determined in Part A) and Palbociclib will be dosed at the fixed standard dose of 125 mg QD. ZB716 will be administered on a 28 day cycle and Palbociclib will be administered for 21 days in the cycle with 7 days off treatment. Administration of the higher dose level (at monotherapy RD) of ZB716 (with the standard dose of Palbociclib) to subsequent subjects will be based on the occurrence of DLTs during the DLT observation period (Cycle 1), until MAD of ZB716 with combination of Palbociclib is reached. Doses will be administered in the dietary status for dosing as determined from Part A fed/fasted comparison.

Cohorts will follow a 3+3 study design. Approx. 6 to 12 subjects will be enrolled in the dose escalation phase of ZB716 in combination with Palbociclib. For Part C, doses will be administered at escalating doses starting with 1 dose level below the monotherapy RD (determined in Part A) and Palbociclib will be dosed at the fixed standard dose of 125 mg QD. ZB716 will be administered on a 28 day cycle and Palbociclib will be administered for 21 days in the cycle with 7 days off treatment. Administration of the higher dose level (at monotherapy RD) of ZB716 (with the standard dose of Palbociclib) to subsequent subjects will be based on the occurrence of DLTs during the DLT observation period (Cycle 1), until MAD of ZB716 with combination of Palbociclib is reached. Doses will be administered in the dietary status for dosing as determined from Part A fed/fasted comparison.

Potential subjects will be screened to assess their eligibility to enter the study within 28 days prior to the first dose administration. Subjects will come to the clinical site before the first dose of study drug to confirm eligibility. Cohorts will be enrolled sequentially based on an optional Simon 2-stage study design.

Approx. 29 subjects may be enrolled in the dose expansion cohorts of Part D. For Part D, doses will be administered (self-administered by subjects at home or administered by subjects under observation of clinical staff during clinic visits) at the determined RD of ZB716 QD for 28 days from Part C in combination with the standard dose of Palbociclib (125 mg QD for 21 days with 7 days off treatment). Doses will be administered in the dietary status for dosing as determined from Part A fed/fasted comparison.

Potential subjects will be screened to assess their eligibility to enter the study within 28 days prior to the first dose administration. Subjects will come to the clinical site before the first dose of study drug to confirm eligibility. Cohorts will be enrolled sequentially based on an optional Simon 2-stage study design.

Approx. 29 subjects may be enrolled in the dose expansion cohorts of Part D. For Part D, doses will be administered (self-administered by subjects at home or administered by subjects under observation of clinical staff during clinic visits) at the determined RD of ZB716 QD for 28 days from Part C in combination with the standard dose of Palbociclib (125 mg QD for 21 days with 7 days off treatment). Doses will be administered in the dietary status for dosing as determined from Part A fed/fasted comparison.