Inclusion Criteria:  Females  Able to understand the investigational nature of this study and provide written informed consent prior to the participation in the trial Age 18 to 55, inclusive Diagnosis of Stage I, II, or III HR+, HER2-negative breast cancer (ER>1% and/or, PR>1%, HER2-negative per ASCO CAP guidelines) Is a candidate for endocrine therapy + ovarian suppression  Is premenopausal at the time of study entry.  E2 > 30 pg/mL Follicle stimulating hormone (FSH) < 40 IU/L Last menstrual period within the 3 months prior to first dose of TOL2506  Exclusion Criteria:  Females  Body mass index (BMI) < 18.00 kg/m2 or > 35.00 kg/m2 Breastfeeding Life expectancy < 12 months Eastern Cooperative Oncology Group (ECOG) performance status ≥ 3  Unacceptable hepatic function as determined by any of the following:  Alanine aminotransferase (ALT) ≥ 2X upper limit of normal (ULN) Aspartate aminotransferase (AST) ≥ 2X ULN Bilirubin ≥ 2X ULN Alkaline phosphatase ≥ 2X ULN Severe hepatic impairment (Child-Pugh Class C)  Unacceptable renal function as determined by any of the following:  Creatinine ≥ 3X ULN Creatinine clearance ≤ 30 mL/minute Creatinine clearance ≤ 60 mL/minute in subjects with bone density 1.5 standard deviations below the young adult normal mean  History of significantly abnormal ECG or screening 12-lead ECG demonstrating any of the following:  HR > 100 BPM QRS > 120 msec QTc > 450 msec PR > 220 msec Prior (within 28 days prior to Day 0) and/or concomitant use of medications known to prolong the QT/QTc interval Prior use of tamoxifen, other SERMs (eg, raloxifene) or antagonists (eg, fulvestrant), aromatase inhibitor, mammalian target of rapamycin (mTOR) inhibitors, or hormone replacement therapy within 3 months before breast cancer diagnosis Prior neoadjuvant or adjuvant endocrine therapy since diagnosis of breast cancer History of treatment for osteopenia/osteoporosis Prior or current use of drugs or supplements known to increase bone mineral density (ie, bisphosphonates, denosumab, teriparatide, abaloparatide, romosozumab, calcitonin, fluoride, strontium) Low trauma fracture(s) occurring within 12 months prior to Screening (defined as a fracture that results from a fall from a standing height or less, excluding fingers, toes, face and skull) Conditions that preclude bone mineral density measurement (lumbar spine/bilateral hip surgery with hardware in place, abdominal clips, umbilical ring [not willing to remove] or weight that exceeds the DEXA machine limitation) Any other medical condition or serious illness, presence of a second malignancy under current treatment or follow-up, or the presence of clinically significant findings on the physical exam, laboratory testing, medical history (including conditions that may be associated with low bone mass), that in the opinion of the Investigator may interfere with trial conduct, subject safety, or interpretation of study results Already receiving and/or previously received GnRH analogs within 1 year before breast cancer diagnosis Psychiatric, addictive, or other disorders that would preclude study compliance  Use of medications that may impact subject safety and/or affect the PK of the drug and hormonal assessments including but not limited to:  Oral or transdermal hormonal therapy within 30 days prior to Screening Estrogen, progesterone, or androgens within 30 days prior to Screening Hormonal contraceptives within 30 days prior to Screening Medications known to result in clinically important decreases in bone mass taken within 6 months prior to Screening Known hypersensitivity, idiosyncratic, or allergic reactions to GnRH, GnRH agonist/analogs or to any of the components of the IP Sexually active with a male partner and not willing to use non-hormonal contraceptive methods throughout the study Is of childbearing potential with a positive serum pregnancy test at Screening or urine pregnancy test at Day 0 Exposure to any investigational agent within 30 days prior to the first dose of TOL2506  See contact information to obtain inclusion/exclusion criteria for males