Phase I Study of the Combination of MLN9708 and Fulvestrant

Subjects with metastatic ER+/HER2- breast cancer with disease progression on Fulvestrant, for
whom continuation of endocrine therapy would be an appropriate treatment, will be treated
with the combination of MLN9708 (proteasome inhibitor) and Fulvestrant (anti-estrogen).
Subjects with visceral or soft tissue disease will have a tumor biopsy while on treatment
with Fulvestrant but prior to initiation of MLN9708 to confirm ER/PR/HER2 status, and to
provide a baseline specimen for molecular analysis. A biopsy of the same tumor will be
obtained after 2 days of treatment with MLN9708 plus Fulvestrant (i.e. on day 3). Patients
with bone-only disease will be eligible for the study, but will not be biopsied.

The Investigators propose a 3x3 dose-escalation trial to assess the safety and efficacy of
Fulvestrant and three dose levels of MLN9708 (2.3, 3, and 4mg) Subjects will be treated with
Fulvestrant (500mg) once every four weeks on day 1. The dose of MLN9708 will start at 2.3mg,
which is about 50% of the phase 3 dose in another ongoing study. If 2.3mg MLN9708 does not
induce any grade 3 non-hematologic toxicity, or any grade 4 hematologic toxicity by CTCAE
v4.0 (Common Terminology Criteria for Adverse Events, version 4.0) in any of the 3 subjects
treated for 1 cycle, the Investigators will treat another 3 subjects with the combination of
Fulvestrant and MLN9708 (3mg); and if no grade 3 non-hematologic toxicity, or any grade 4
hematologic toxicity by CTCAE v4.0 is seen over the first cycle, the Investigators will treat
another 3 subjects with Fulvestrant and MLN9708 (4mg, which is the phase 3 dose). If
dose-limiting toxicity is observed in 1/3 subjects, the Investigators will treat an
additional 3 subjects at that same dose. If dose-limiting toxicity is seen in 2 of 6 subjects
at any dose level, that dose level will be considered the maximum tolerated dose, and a total
of 6 subjects will be treated at the prior dose. Plasma pharmacokinetic profiles of MLN9708
will be determined over 21 days after the first dose of the combination.

Pre- and post- treatment tumor biopsies will be formalin-fixed and paraffin-embedded. Tissue
sections will be analyzed by H&E (Hematoxylin and eosin) staining and immunohistochemistry
using antibodies against markers of proliferation, apoptosis, estrogen receptor alpha
activation, endoplasmic reticulum stress, and polyubiquitin. Proteasome activity of whole
blood will be determined using samples acquired prior to treatment initiation, and on day 3.
Tumor-specific plasma DNA will also be measured prior to treatment initiation and after the
first two doses of MLN9708.

Inclusion Criteria:

1. Female post-menopausal patients 18 years or older. Voluntary written consent must be
given before performance of any study related procedure not part of standard medical
care, with the understanding that consent may be withdrawn by the patient at any time
without prejudice to future medical care.

2. Patients must have either A) histologic documentation of metastatic or locally
advanced breast cancer by needle or incisional biopsy, or B) history of breast cancer
with radiologic evidence of bone-only metastatic disease.

3. Patients must be post-menopausal based on either a history of an oophorectomy, or at
lease one year of amenorrhea. An elevated serum gonadotropin level can be used to
confirm menopausal status in a subject with one year or more of amenorrhea.

4. The invasive cancer must be HER2-negative, defined as IHC0-1+, or with a FISH ratio of
<1.8 if IHC is 2+ or if IHC has not been performed.

5. Metastatic or locally advanced breast cancer for which endocrine therapy is an
appropriate treatment option.

6. Patients must have been treated with Fulvestrant for at least 56 days as their most
recent anti-cancer treatment, and they must be tolerating Fulvestrant with at most
grade I toxicity by CTCAE v4.0.

7. Disease progression based on RECIST criteria while the subject has been taking
Fulvestrant, and for which continuation of endocrine therapy would be appropriate.

8. The subject must agree to undergo pre- and post- treatment research biopsies if a
non-osseous metastatic site is available for biopsy.

9. Eastern Cooperative Oncology Group (ECOG) performance status 0-1

10. Life expectancy 6 months or longer.

11. Patients must meet the following clinical laboratory data:

- Absolute Neutrophil Count (ANC) ≥ 1,000/mm(3) and platelet count ≥75,000/mm(3)

- Total bilirubin ≤ 1.5 x the upper limit of normal range (ULN).

- Alanine aminotranserase (ALT) and aspartate aminotransferase (AST) ≤ 3 x ULN.

- Calculated creatinine clearance ≥ 30 mL/min

12. Ability to give informed consent.

Exclusion Criteria:

1. Failure to have fully recovered (i.e., ≤ Grade 1 toxicity) from the reversible effects
of prior chemotherapy or endocrine therapy, except for Grade 2 or greater anemia.

2. Major surgery within 14 days before enrollment.

3. Radiotherapy within 14 days before enrollment. If the involved field is small, 7 days
will be considered a sufficient interval between treatment and administration of the
MLN9708.

4. Central nervous system involvement.

5. Infection requiring systemic antibiotic therapy or other serious infection within 14
days before study enrollment.

6. Evidence of current uncontrolled cardiovascular conditions.

7. Systemic treatment, within 14 days before the first dose of MLN9708, with strong
inhibitors of CYP1A2 or CYP3A, or strong inducers of CYP3A.

8. Ongoing or active systemic infection, active hepatitis B or C virus infection, or
known human immunodeficiency virus (HIV) positive.

9. Any serious medical or psychiatric illness that could, in the investigator's opinion,
potentially interfere with the completion of treatment according to this protocol.

10. Known allergy to any of the study medication, their analogues, or excipients in the
various formulations of any agent.

11. Known gastrointestinal (GI) disease or GI procedure that could interfere with oral
absorption or tolerance of MLN9708 including difficulty swallowing.

12. Diagnosed or treated for another malignancy within 2 years before study enrollment or
previously diagnosed with another malignancy and have evidence of residual disease.

13. Patient has ≥ grade 3 peripheral neuropathy, or grade 2 with pain on clinical
examination during the screening period.

14. Participation in other clinical trials within 21 days of the start of this trial or
throughout the duration of this trial.

15. Visceral crisis or rapidly progressive disease for which chemotherapy would be
indicated.