A Study in Patients With Advanced Breast Cancer

Inclusion Criteria:

All 3 parts of Study:

Male or females, Age 18 years or older
Histologically or cytologically diagnosed with ER+/HER2- adenocarcinoma of the breast cancer with an evidence of metastatic/loco-regionally recurrent disease/unresectable advanced disease not amenable to treatment with curative intent
Documentation of ER-positive, HER2-negative status determined based on a biopsy performed at or after diagnosis of local or metastatic recurrence, utilizing an assay consistent with local standards
Not more than 3 prior chemotherapeutic regimens
ECOG performance status 0-1.
Resolution of all adverse events of prior therapy or surgical procedures to National Cancer Institute (NCI) CTCAE v 5.0 Grade ≤1 (except alopecia)
Adequate organ and immune system function as indicated by laboratory values
Patients of childbearing potential must practice an acceptable method of birth control as judged by the Investigator
Female subjects must be non-lactating and non-breast feeding
Male subjects should not father a child and must practice an acceptable method of birth control measures Willing and available to participate for the entire study
Willing and able to comply with protocol requirements

For Part 1& 2:

Patient must have evaluable disease (according to RECIST 1.1).
Documented disease progression or resistance to at least 1 prior endocrine therapy (with or without CDK 4/6 therapy).

For Part 3

Patient must have measurable lesions (according to RECIST 1.1)
Part 3a: HR+ve, HER2- MBC patients with ESR1 mutations, resistance to atleast one priro endocrine therapy
Part 3b: HR+ve HER2- MBC patients resistant to atleast one priro endocrine therapy
Part 3c: HR+ve HER2- MBC patients resistant to atleast one priro endocrine therapy, disease progression on Fulvestrant and CDK4/6i
Part 3d: Brain metastases secondary to ER+ve HER-ve Breast Cancer:

Measurable brain lesion (≥ 1) as per RANO-BM Criteria, Tretament naive/ Treated- Stable/ Not requiring immediate local therapy known/ Suspected leptomeningeal disease on Stable corticosteriod dose for 7 days prior screeing

Exclusion Criteria:

All 3 parts of Study

Major surgery <4 weeks of C1D1
Evidence of organ dysfunction or inadequate bone marrow reserve or any clinically significant finidngs
Patients with visceral crisis or impending visceral crisis and rapidly progressing disease
Serology tests +ve for HIV, HCV, HBsAg
Inability to swallow oral medication
H/o any relevant allergy/hypersensitivity/idiosyncrasy to drugs/ chemically related to Study drug or its excipients
Received an IMP within 30 days/5 half life to C1D1
Prior treatment with other oral SERDs
Use of concomitant medication that might reasonably influence the results or interpretation of the study
Requires concurrent systemic anticancer treatment at any time during the study treatment period
Known or suspected history of significant drug abuse/Alcohol as judged by the Investigator
Known or suspected history of excessive intake of alcohol in the 12 months prior to study entry
Malabsorption syndrome/IBD/other illness that would affect oral absorption of Study drug
Uncontrolled intercurrent illness that would limit compliance with study requirements / have impact on endpoints / safety
≤6 months H/o MI/unstable angina, ongoing > G2 cardiac dysrhythmia, prolonged QTcF/ uncontrolled AF, coronary/peripheral artery bypass graft, HF of NYHA_Class II or greater and CVA (+TIA)
H/o Endometrial intraepithelial neoplasia, other malignancy < 5 yrs prior to enrollment
Known active uncontrolled or symptomatic Central Nervous System (CNS) metastases, or leptomeningeal disease as indicated by clinical symptoms (not applicable to Part 3d), carcinomatous meningitis, cerebral edema, and/or progressive growth or pulmonary lymphangitic metastases.
Current abnormal vaginal bleeding or symptomatic endometrial disorders.
For Part 2: Use of other ET that block the estrogen receptor: atleast 8 weeks before enrollment (28 weeks for fulvestrant) For Part 2: Liver-only metastases (are not evaluable by FES-PET/CT imaging)
For Part 3: Any brain lesion requiring immediate local therapy (which includes but is not limited to WBRT, SRS, or surgical resection, for treatment of brain metastases) Requires increase in the dose of corticosteroids for control of CNS symptoms due to brain metastases Poorly controlled (> 2 per month ) generalized or complex partial seizures Who are taking concurrent enzyme-inducing antiepileptic drugs (EIAED) Who has evidence of significant (ie, symptomatic) intracranial haemorrhage Contra indications for repeated MRI assessments