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 novel 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 levels 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 levels showed a decline in PRL levels on treatment with anastrozole. Conclusions This preliminary study suggests that anastrozole 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.