Clinical efficacy of the aromatase inhibitor anastrozole in relation to prolactin secretion in heavily pretreated metastatic breast cancer

Aims and background: It is known that the aromatase inhibitors may act by decreasing estrogen levels. Moreover, it is known that estrogens may stimulate the release of prolactin (PRL), which is a growth factor for breast cancer. This phase II study was performed to evaluate the effects of the never aromatase inhibitor anastrozole on PRL secretion in metastatic breast cancer and the possible influence of PRL pretreatment levels on the efficacy of therapy. Methods: The study involved 14 pretreated metastatic breast cancer patients with a poor clinical status. Anastrozole was given orally once a day at 1 mg/day for at least 2 months. To evaluate PRL secretion, venous blood samples were collected before treatment and at 1-monthly intervals during treatment. Results: The clinical response consisted of partial response (PR) in 2, stable disease (SD) in 5 and progressive disease (PD) in the remaining 7 patients. Abnormally high pretreatment levers of PRL were seen in 5/14 (36%) patients. Progressing patients showed significantly higher pretreatment levels of PRL than those who achieved PR or SD. None of the patients with high PRL pretreatment levers showed a decline in PRL levers on treatment with anastrozole. Conclusions: This preliminary study suggests that anastrotole has no inhibitory effect on PRL secretion in metastatic breast cancer and that the evidence of abnormally elevated concentrations of PRL prior to therapy is generally associated with a lack of efficacy.