A First-in-Human Study of D-0502 Alone and in Combination With Palbociclib in Women With Advanced or Metastatic ER-Positive and HER2-Negative Breast Cancer

Inclusion Criteria:

1. Histologically-proven, ER-positive, HER2-negative locally advanced, inoperable, and/or
metastatic breast cancer.

2. Female patients with menopausal status:

i. Postmenopausal status defined as meeting at least one of the following criteria:

1. Have undergone a bilateral oophorectomy any time in life;

2. Age ≥60 years, or

3. Age <60 years but have cessation of regular menses ≥12 months with follicle
stimulating hormone (FSH) value >40 milli-international units per milliliter
(mIU/mL) and an estradiol value <40 picograms per milliliter (pg/mL) (140
picomoles per liter [pmol/L]).

ii. Premenopausal or perimenopausal concurrently given a luteinizing hormone-releasing
hormone (LHRH) agonist starting at least 4 weeks before the start of trial therapy and
is planned to continue LHRH agonist during the study.

3. Patients meeting all the following criteria:

1. Has had at least 6 months of endocrine therapy for metastatic ER+ breast cancer
prior to disease progression;

2. Receipt of ≤ 2 lines of prior chemotherapy for metastatic disease;

3. Radiological or objective evidence of disease progression on or after the last
systemic therapy prior to starting current study treatment.

4. For phase Ia: Not eligible for standard therapy that would confer clinical
benefit to the patient.

Exclusion Criteria:

1. Patients with prior anticancer or investigational drug treatment within the following
windows are excluded:

1. Tamoxifen therapy less than 6 weeks before first dose of study treatment.

2. For phase Ib: fulvestrant or other selective estrogen receptor degraders (SERD)
such as RAD1901, AZD9496, LSZ102, GDC0927 and H3B-6545 therapy less than 6 months
before first dose of study treatment.

3. Any other anti-cancer endocrine therapy less than 30 days before first dose of
study treatment.

4. Any other investigational drug therapy less than 28 days or 4 half-lives
(whichever is longer) or non-resolved toxicity such hematology issue prior to
first dose of study treatment.

2. Patients with untreated or symptomatic or progressive central nervous system (CNS)
metastases.

3. Patients with endometrial disorders, including evidence of endometrial hyperplasia,
dysfunctional uterine bleeding or cysts.

- Other protocol defined inclusion/exclusion criteria could apply