ER Reactivation Therapy for Breast Cancer

Metastatic breast cancer is rarely cured by current therapies. ER+ breast cancers ultimately
become resistant to all available anti-estrogens. Response rates to estrogens are similar to
those of anti-estrogens in the metastatic setting. Given that ER+ breast cancers are often
responsive to anti-estrogens and estrogens, alternating anti-estrogen/estrogen therapies may
be more effective than continuous treatment with either type of agent. Anecdotal evidence
indicates that such a strategy of alternating therapies is effective in some patients.
Preclinical evidence suggests that anti-estrogen-resistant ER+ breast cancers are sensitized
to the anti-tumor effects of estrogens. Such cells harbor subpopulations that can ultimately
regain the ability to grow in the presence of estrogens, and revert to their
anti-estrogen-sensitive state. The investigators will formally test whether alternating
17B-estradiol/anti-estrogen therapies is effective for the management of
anti-estrogen-resistant metastatic ER+/HER2- breast cancer, and to identify molecular
biomarkers that predict tumor response to 1) 17B-estradiol and 2) alternating
17B-estradiol/anti-estrogen therapies. If successful, this study would present a novel
strategy to manage metastatic ER+/HER2- breast cancer by pre-emptively switching therapies
prior to disease progression.

Inclusion Criteria:

1. Women ≥18 years of age with clinical stage IV ER+/HER2- breast cancer, or with locally
recurrent ER+/HER2- disease not amenable to therapy for curative intent.

2. Patient must have been treated with an anti-estrogen at any time in their disease
history. Combination regimens that include an anti-estrogen and any biologic, or
targeted therapy, are permitted (e.g., any CDK inhibitor, everolimus, or any other
novel biologics), and are considered to be a single hormonal therapy based regimen.

- Any number of prior lines of anti-estrogen (i.e., hormonal) therapy is
permissible.

- One line of prior chemotherapy for advanced/metastatic disease is permissible.

3. Histologic documentation of ER strongly+/HER2- breast cancer by core needle biopsy,
fine needle aspiration, incisional biopsy, or surgical biopsy of ≥1 site(s) of
metastatic or locally advanced disease performed as standard of care within the past 4
months for assessment of eligibility for study participation (except as noted below in
c/d/e).

1. ER strongly+ status defined as ER staining by immunohistochemistry in ≥50% of
malignant cell nuclei with an intensity ≥2+ on a scale of 0-3+. These criteria
are equivalent to an Allred score ≥6.

2. HER2-negative status is defined as immunohistochemistry score of 0-1+, or with a
FISH ratio of <2 if IHC is 2+ or if IHC has not been done (as per ASCO/CAP
definitions). In cases of borderline or equivocal HER2 status, eligibility will
be determined by the PI.

3. Archived tumor specimens: Excess tumor tissue must be available for research
purposes. This will include tumor tissue sufficient to make ≥10 five-micron
sections; more tumor tissue is preferred.

Freshly acquired tumor specimens: As part of a clinically indicated biopsy
procedure, an additional 1-3 cores or tissue fragments will be obtained by core
needle or surgical biopsy for research purposes and FFPE.

4. Patients with bone-only metastatic disease with a history of ER+/HER2- breast
cancer are eligible, and bone biopsy is not required, providing their primary
cancer is consistent with the above-described ER and HER2 criteria.

5. Patients with non-bone metastatic disease in whom a safe and accurate biopsy of
recurrent/metastatic disease cannot be readily obtained are also eligible,
providing their primary cancer is consistent with the above-described ER and HER2
criteria.

4. Patient must be a candidate for treatment with 17B-estradiol and an aromatase
inhibitor.

5. If the most recent therapy was in the adjuvant setting, the recurrence-free interval
(time from initiation of adjuvant anti-estrogen therapy to clinical evidence of
disease recurrence) must have been ≥2 years.

If the most recent therapy was in the advanced/metastatic setting, the
progression-free interval must have been ≥4 months (except in the case of
investigational hormonal therapies).

6. Patient must be post-menopausal based on either a history of an oophorectomy, or ≥1
year of amenorrhea. An elevated serum gonadotropin level and estradiol level in the
postmenopausal range (as locally defined) can be used to confirm menopausal status in
a subject with <1 year of amenorrhea.

7. Baseline radiographic staging, including specifically either PET/CT, or CT (CAP) and
bone scan.

8. Patient must be capable and willing to provide informed written consent for study
participation.

9. The following laboratory values must be confirmed for eligibility within 28 days prior
to initiation of study therapy:

Hematology panel

- hemoglobin > 9 g/dL

- white blood cell (WBC) count (≥ 2,000/uL)

- platelet count ≥ 75,000/uL Serum biochemistry/metabolic panel

- creatinine ≤ 1.5 x upper limits of normal (ULN)

- total bilirubin ≤ 1.5 x upper limits of normal (ULN)

- ALT and AST ≤ 3.0 x upper limits of normal (ULN) For patients with liver metastasis: <
5 x upper limits of normal (ULN)

Exclusion Criteria:

1. Treatment with fulvestrant within 4 months prior to study enrollment.

2. Any other concurrent systemic anti-cancer treatments, including conventional
chemotherapeutic agents and biological agents, during the study period.

Anti-resorptive bone therapies (e.g., bisphosphonates, denosumab) are permitted.

3. Any investigational cancer therapy in the last 28 days.

4. Known CNS disease, unless clinically stable for ≥ 3 months.

5. History of any of the following:

- deep venous thrombosis

- pulmonary embolism

- stroke

- acute myocardial infarction

- congestive heart failure

- previous malignancy not treated with curative intent, or with an estimated
recurrence risk ≥30%