Low blood levels of vitamin D may worsen the bone loss associated with exemestane(Drug information on exemestane) (Aromasin), an aromatase inhibitor commonly given to postmenopausal women with breast cancer to prevent risk of recurrence, researchers from Norway reported at this year's annual meeting of the American Society of Clinical Oncology in Atlanta.

Take-Home Message

"Our findings support the use of vitamin D as well as calcium supplementation for postmenopausal women in general, and for those receiving aromatase inhibitors in particular," said Per E. Lønning, MD, PHD, professor at Haukeland University Hospital in Bergen, Norway, and the study's lead author. "Women should have adequate vitamin D levels before starting exemestane therapy."

Aromatase inhibitors, such as exemestane, anastrozole(Drug information on anastrozole) (Arimidex), and letrozole(Drug information on letrozole) (Femara), are used in postmenopausal women whose breast cancer is fueled by estrogen to lower levels of the hormone by interfering with the aromatase enzyme. Dr. Lønning's prior research had shown that postmenopausal women with early-stage breast cancer who took exemestane for 2 years had worse bone loss in the neck of the femur and a nonsignificant increase in bone loss in the lumbar spine compared with women who took a placebo.

Study Findings

In this study, the research team analyzed baseline blood levels of various biomarkers involved in bone metabolism to determine their influence on bone loss among 59 patients who received exemestane for 2 years, and 62 who received a placebo. Compared with the desired levels of vitamin D (a minimum of 30 ng/mL), most women in both groups suffered from vitamin D deficiency at baseline (52 patients in the exemestane group, with a mean vitamin D level of 21.6 ng/mL, and 56 patients in the placebo group, with a mean level of 22.6 ng/mL). This may be partially due to the reduced exposure to sunlight in Norway, although low levels of vitamin D may occur in women in all geographic areas.

After 2 years, bone loss in the femoral neck in women who were deficient in vitamin D was greater in the exemestane group (4.7% reduction in bone mineral density) compared with the placebo group (2.9% bone mineral density decrease). In the women with adequate levels of vitamin D, however, femoral bone loss was similar between the two groups (3.6% in the exemestane group vs 3.3% in the placebo group), though Dr. Lønning noted that there were only 13 women with adequate vitamin D levels. The researchers noted that additional larger follow-up studies will be needed to confirm these findings.