A qualitative systematic review of the evidence base for non-cross-resistance between steroidal and non-steroidal aromatase inhibitors in metastatic breast cancer.

Aims: The most effective sequence of tamoxifen and both steroidal (SAIs) and non-steroidal aromatase inhibitors (NSAIs) has been extensively studied in the adjuvant setting. However, treatments for women who have failed initial aromatase inhibitor therapy in the metastatic setting have received relatively little attention. A systematic review was undertaken to assess the use of SAIs and NSAIs in metastatic breast cancer.

Materials And Methods: Medline, Embase and the Cochrane library were searched using free text and MeSH terms. Studies assessing the cross-resistance, efficacy and safety of SAIs and NSAIs for postmenopausal women with advanced metastatic breast cancer confirmed by histology/cytology were included. Patients had progressed/relapsed from previous adjuvant, first- or second-line aromatase inhibitor treatment and had undergone treatment with at least two regimens consisting of aminoglutethimide, anastrozole, letrozole and/or exemestane.

Results: Nine studies reported results for patients treated with an SAI after treatment failure with an NSAI. For SAI after NSAI, clinical benefit was the most frequently reported outcome. The clinical benefit for exemestane (SAI) after any NSAI failure or before treatment ranged from 12% (complete response not recorded, partial response 2%, stable disease 10%) to 55% (complete response 6%, partial response 13%, stable disease 35%) Survival outcomes were infrequently reported; four studies reported disease progression. The time to progression ranged from 3.7 to 5.2 months. Only one study reported a median overall survival with exemestane at 15.2 months. Only one study reported information for an NSAI after SAI and an NSAI followed by another NSAI.

Discussion: This review suggests that switching from an NSAI to an SAI is a reasonable option. This would be particularly important for patients who would probably respond to further endocrine manoeuvres; strongly oestrogen receptor-positive disease, non-visceral disease, a good prior response or a long duration of response. Further research to optimise the sequence of endocrine therapies in metastatic breast cancer is needed.