The purpose of this study is to examine how key patient factors, including body size affect how well letrozole suppresses circulating estrogen levels.

This study has two components: (1) Part A - an observational phase to evaluate the impact of vitamin D and obesity on estrogen suppression in post-menopausal women already receiving letrozole treatment, and (2) Part B - an interventional phase for women with body mass index (BMI) > 25 kg/m2 to assess the effect of double AI dose on estrogen levels.

If BMI or other patient factors reduce the effectiveness of letrozole, modifications to treatment recommendations and studies to test higher dosing of letrozole may be needed to maximize the benefit of this treatment and minimize the risk of the breast cancer coming back.

The available evidence suggests that circulating estradiol levels play a critical role in determining effectiveness of aromatase inhibitor therapy, and that maximal suppression of aromatase activity and estrogen production is essential for optimal disease control. Host factors such as high 25-Hydroxy Vitamin D (25-OH D) levels and obesity, which might increase residual estrogen levels, could adversely impact outcome and lead to an increased risk of recurrence or death. The primary objective of Part A (which includes women regardless of BMI) is to directly evaluate the relationship between (i) 25-OH D levels and serum estrogen levels and (ii) BMI and serum estrogen levels in a cohort of postmenopausal women on standard dose letrozole therapy (2.5 mg/day). Part B (which includes only women with BMI > 25 kg/m2) will determine whether an increased dose of letrozole (5 mg/day), which had no dose limiting side effects in a Phase I study improves suppression of estrogen in overweight/obese women.

Inclusion Criteria:

• Postmenopausal female patients
• histological confirmed diagnosis of estrogen receptor and/or progesterone receptor positive breast cancer (Stage I-III) who have completed local therapy
• Currently prescribed and taking letrozole 2.5 mg daily for a minimum of 3 months
• Willing to provide written informed consent to participate
• for the experimental arm: all of the above and body mass index (BMI) > 25 kg/m2

Exclusion Criteria:

• Known abnormal liver or renal function defined by:

  1. Serum Creatinine > 1.25 times institutional upper limit of normal (ULN) or Calculated Creatinine Clearance < 40 mL/min
  2. Serum Bilirubin, Aspartate transaminase (AST) or alanine transaminase (ALT) > 1.5 times ULN

• Presence of persistent local or known metastatic cancer

The data supporting the related manuscript are kept in institutional file storage on an internal server at Lunenfeld-Tanenbaum Research Institute. There are de-identification concerns in small, regionally restricted, clinical datasets which prevent these data being openly available, but data will be made available at reasonable request from the corresponding author for up to and including 5 years from publication of the related manuscript.

For all data requests please contact:

Dr David Cescon, Princess Margaret Cancer Centre, University Health Network, Toronto, Ontario, Canada. Dave.Cescon@uhn.ca

Up to and including 5 years from publication of the related manuscript (Elliott, Mitchell J et al. "Association between BMI, vitamin D, and estrogen levels in postmenopausal women using adjuvant letrozole: a prospective study." NPJ breast cancer vol. 6 22. 12 Jun. 2020)

Reasonable request

Adverse events were collected over the duration of the study: 29 days for patients taking part in Part A only, 58 days for patients that took part in Parts A and B

National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events (CTCAE) Version 4.01