Chemoresistance is not a cause of the apparent failure of adjuvant chemotherapy in postmenopausal women.

Chemotherapeutic regimens used for the adjuvant treatment of breast carcinoma are less effective when applied to postmenopausal women than when applied to premenopausal women. Differences in growth fraction or altered chemosensitivity of tumors are potential causes of the differential effects of chemotherapy in younger and older patients. We have attempted to identify the presence of these putative causes by measuring size of clonogenic cell fraction and drug sensitivity of progenitor cells on the tumors from pre- and postmenopausal women. We found that the chemosensitivity of tumors was similar for patients of all ages. We further observed that the clonogenic cell fraction of tumors from women older than 65 years tended to be smaller compared to those of all other patients, while the hormone-sensitivity of tumors from these patients was higher. Our observations thus suggest that drug resistance related to inherent metabolic characteristics of tumor cells may not be a major contributing cause of failure of adjuvant chemotherapy in the treatment of postmenopausal women.