A 2-Week Ketogenic Diet in Combination With Letrozole to Modulate PI3K Signaling in ER+ Breast Cancer

Inclusion Criteria:

* All participants must provide written informed consent.
* Patients must have histologically confirmed primary invasive mammary carcinoma

  * The tumor must be estrogen receptor positive
  * The tumor must be HER2 negative (negative IHC or FISH)
  * The primary tumor size must be at least 2 mm in size.
* Patients must be post-menopausal defined by any of the following:

  * Subjects at least 55 years of age.
  * Subjects younger than 55 years of age and amenorrheic for at least 12 months or serum follicle-stimulating hormone (FSH) levels and estradiol levels in the post-menopausal range by local lab criteria
  * Subjects with history of bilateral oophorectomy or prior radiation castration with amenorrhea for at least 6 months.
* Patients must have clinical stage I, II, or III invasive mammary carcinoma planning to undergo surgical treatment with either segmental resection or total mastectomy.
* Patients must have BMI \>= 30.
* A core biopsy from the time of diagnosis must be available.
* Mammogram or ultrasound required prior to screening
* Patients must have adequate organ function based on the following laboratory parameters:

  * Serum creatinine \<= 1.5x ULN
  * SGOT, SGPT \<= 4x ULN (unless known steatohepatitis)
  * Serum albumin \>= 2.0 g/dL
  * Total serum bilirubin \<= 1.5x ULN (or \<= 3x ULN if known Gilbert's syndrome)

Exclusion Criteria:

Patients with locally advanced disease who are candidates for other preoperative (chemo)therapy at the time of initial evaluation. This includes patients with inflammatory breast cancer.

* Evidence of distant metastatic disease (stage IV).
* Serious medical illness that in the judgment of the treating physician places the patient at high risk of operative mortality.
* Serious medical illness that in the judgment of the treating physician would preclude the use of a ketogenic diet.
* Severe uncontrolled malabsorption condition or disease (e.g. grade II/III diarrhea, severe malnutrition, short gut syndrome).
* Diabetes mellitus requiring insulin therapy.
* Dementia, altered mental status, or any psychiatric condition that would prohibit the understanding or rendering of informed consent.
* Participation in any other neoadjuvant therapeutic clinical trial.
* Concurrent anti-cancer therapy other than endocrine therapy (e.g. chemotherapy, radiotherapy, immunotherapy, or any other biologic therapy).
* Concurrent treatment with an investigational agent.
* Use of an investigational drug within 30 days or 5 half-lives, whichever is longer, preceding the first day of dietary intervention.