A Trial to Evaluate Ovarian Suppression Following Subcutaneous ZOLADEX 10.8 mg in Premenopausal Women With Early-stage and Advanced HR+ Breast Cancer

Inclusion Criteria: 1. Age and gender: Female participants aged 18 to 55 years, inclusive. 2. Hormone receptor positive (HR+): Either estrogen receptor positive (ER+) or progesterone receptor positive (PR+) breast cancer, defined as 1% to 100% of tumor nuclei are positive for ER or PR via immunohistochemistry. 3. Participants must be of premenopausal status. 4. Recent radiotherapy: 1. Prior radiotherapy is allowed. 2. All radiotherapy-related toxicities (except alopecia) must have recovered to Common Terminology Criteria for Adverse Events (CTCAE) v6.0 Grade ≤1 before trial enrollment. 5. Concurrent medications: 1. Cyclin-dependent kinase 4 (CDK4/6) inhibitors are permitted. 2. PIK3CA/AKT inhibitors are permitted. 3. Concomitant use of bisphosphonates is permitted. 4. Concomitant use of endocrine therapy (eg, aromatase inhibitor, fulvestrant, or other FDA-approved selective estrogen receptor degrader \[SERDs\]) is permitted. 5. Concurrent standard-of-care human epidermal growth factor receptor 2 (HER2)- targeted therapy (including but not limited to trastuzumab, pertuzumab, trastuzumab deruxtecan, or T-DM1) is permitted. 6. Informed consent: Able to understand and willing to provide informed consent and able to comply with the trial procedures and restrictions. 7. Abstinence and contraception: Female participants may be enrolled if they are: 1. Practicing true abstinence for at least 28 days prior to investigational product (IP) administration until 12 weeks after the last IP administration and having a negative serum pregnancy test at Screening and a negative urine pregnancy test at Day -1, OR 2. Using 2 forms of highly effective nonhormonal contraception, including 1 physical barrier (condom or diaphragm) plus another nonhormonal method (eg, intrauterine device, spermicidals) from Screening or at least 2 weeks prior to IP administration (whichever is earlier) until 12 weeks after the last IP administration and having a negative serum pregnancy test at Screening and a negative urine pregnancy test at Day -1. 8. Performance status: Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2. Inclusion Criteria specific to the advanced breast cancer cohort: 9. Advanced breast cancer diagnosis: Histological confirmation of breast cancer with evidence of locally advanced or metastatic disease, which is not amenable to surgical resection or radiation therapy with curative intent per investigator's assessment. 10. Chemotherapy History: 1. A participant may have received chemotherapy provided the criteria for premenopausal status, as defined in Inclusion Criterion #3, are met at trial screening. 2. All chemotherapy-related toxicities must have recovered to CTCAE v6.0 ≤Grade 1 before trial participation, except for alopecia, peripheral neuropathy, or paresthesia (≤Grade 2). 11. Prior treatment: 1. Participants may have received prior radiotherapy. 2. Participants may have received or be receiving a cyclin-dependent kinase (CDK) 4/6 inhibitor, a PIK3CA-AKT inhibitor, or bisphosphonates. 3. Gonadotropin-releasing hormone (GnRH) treatment-naïve participants will have no history of GnRH agonist or other endocrine therapy in the advanced/metastatic setting. 4. GnRH treatment-exposed participants may have received a GnRH agonist or other endocrine therapy in the advanced/metastatic setting, provided they had established premenopausal status (Inclusion Criterion #3) prior to initiating GnRH agonist therapy, within 6 months prior to trial enrollment. 5. Participants may have received HER2-targeted therapy (including but not limited to trastuzumab, pertuzumab, trastuzumab deruxtecan, or T-DM1). Inclusion Criteria specific to the early-stage breast cancer cohort: 12. Early-stage breast cancer diagnosis: 1. Histologically confirmed unilateral or bilateral invasive adenocarcinoma of the breast. 2. Pathologically staged as Stage I, II, or III (specifically T1-T4, N0-N3, M0) at the time of definitive surgery, according to the current American Joint Committee on Cancer (AJCC) Staging Manual. 13. Chemotherapy history: 1. A participant may have received adjuvant or neoadjuvant chemotherapy provided the criteria for premenopausal status, as defined in Inclusion Criterion #3, are met at trial Screening. 2. All chemotherapy-related toxicities must have recovered to CTCAE v6.0 ≤Grade 1 before trial participation except for alopecia, peripheral neuropathy, or paresthesia (≤Grade 2). 14. Prior treatment: 1. Participants may have received prior radiotherapy. 2. Participants may have received or be receiving a CDK 4/6 inhibitor, a PIK3CA-AKT inhibitor, or bisphosphonates. 3. GnRH treatment-naïve participants will have no history of GnRH agonist or other endocrine therapy. 4. GnRH treatment-exposed participants may have received a GnRH agonist or other endocrine therapy provided they had established premenopausal status (Inclusion Criterion #6) prior to initiating GnRH agonist therapy, within 6 months prior to trial enrollment. 5. Participants may have received HER2-targeted therapy (including but not limited to trastuzumab, pertuzumab, trastuzumab deruxtecan, or T-DM1). Exclusion Criteria: 1. Postmenopausal: Naturally or surgically postmenopausal (regardless of age). 2. Body mass index (BMI): \<18 kg/m\^2 or \>35.0 kg/m\^2. 3. Prior surgical or radiation procedures: History of bilateral oophorectomy or prior radiotherapy to the ovaries. 4. Selective estrogen receptor modulator (SERM) use during trial: Participants may not receive tamoxifen or other SERMs during the trial and must discontinue any SERMs prior to enrollment, other endocrine therapies (eg, aromatase inhibitor, fulvestrant, or other FDA-approved SERDs) are allowed. 5. Hypersensitivity: Known hypersensitivity, idiosyncratic, or allergic reactions to goserelin, GnRH, GnRH agonists/analogs, or any trial drug components. 6. Expected survival