As an aromatase inhibitor, exemestane works by blocking the synthesis of estrogen, slowing down the growth of cancers that have estrogen receptors. Whilst the drug is effective at preventing the development of cancer, there has been an increasing concern about its effects on bone density and fracture risk.
Lead author of the study, Angela Cheung from the University Health Network, Toronto, Canada, says:
"Exemestane worsens age-related decreases in bone mineral density by about three times, even in the setting of adequate calcium and vitamin D intake".
Cheung and her colleagues carried out a report of a sub-study of patients in the Mammary Prevention 3 (MAP.3) trial. The MAP.3 trial was carried out to see how effective exemestane is at preventing breast cancer; it included over 4500 healthy postmenopausal women who were considered at "high-risk" of developing the disease.
The bone sub study observed 351 of patients in the trial - without osteoporosis - of which half were given exemestane and the other half the placebo. They compared the bone-mineral density (BMD) of the two groups at the end of the trial through the use of conventional dual-energy x-ray absorptiometry and high-resolution peripheral quantitative CT.
Although exemestane proved to be very effective at reducing the risk of breast cancer - by 65% compared to the placebo - women receiving the medication experienced a substantial loss of BMD at the distal radius and tibia.
Those in the exemestane group had a reduction in the cortical bone thickness and area of nearly 8%, compared to a reduction of only 1% in the placebo group, this finding is important, as cortical bone loss accounts for around 80% of fractures in old age.
The authors say:
"Women considering exemestane for the primary prevention of breast cancer should weigh their individual risks and benefits. For women taking exemestane, regular bone monitoring plus adequate calcium and vitamin D supplementation are important. Long-term studies are needed to assess the effect of our findings on fracture risk."
Jane Cauley from the University of Pittsburgh, Pennsylvania, USA says:
"Most bone loss occurs after 65 years of age, within the cortical compartment. Thus, if aromatase inhibitors increase cortical porosity, this effect could be a key cause of bone loss strength and non-vertebral fractures associated with their use. Thus, one might not be too reassured about the use of exemestane in the prevention setting."