Efficacy of Exemestane After Nonsteroidal Aromatase inhibitor Use in Metastatic Breast Cancer Patients

Background : Previous studies have suggested a lack of complete cross-resistance between steroidal (exemestane) and non-steroidal aromatase inhibitors (nSAI). Methods : Eighty-eight metastatic breast cancer (MBC) patients who received 25 mg of exemestane orally once a day at the National Cancer Center, Korea, between 2003 and 2009, were reviewed retrospectively. All patients had received nSAI for metastatic disease prior to exemestane therapy. Results : The median age was 52 years (range, 33-79), and 13 (14.8%) patients were premenopausal who concomitantly received GnRH agonist. Exemestane was given as a second- (80.7%) or third-line (19.3%) hormone therapy. The clinical benefit (CB) rate (complete response + partial response + stable disease ${\geq}$ 24 weeks) was 30.7%, with a median CB duration of 10.0 months (range, 6.3-78.7). The median progression-free survival (PFS) was 3.0 months (95% confidence interval [CI], 1.99-4.01) and the overall survival (OS) 21.5 months (95% CI, 17.96-25.04), with a median followup of 50.3 months. Patients who achieved CB had longer OS than those patients who did not (29.6 vs 17.9 months; P=0.002). On univariate analysis of predictive factors, patients who had achieved CB from previous nSAI tended to show lower CB rate (24.6% vs 44.4%, respectively; P=0.063) and shorter PFS (2.8 vs 4.8 months, respectively; p=0.233) than patients who had not. Achieving CB from previous nSAI became independent predictive factor for CBR to exemestane on multivariable analysis (Odds ratio = 2.852, P = 0.040). Conclusions : Exemestane after nSAI failure was effective in prolonging CB duration. The drug's efficacy seemed to be inferior in patients who had benefit from previous nSAI use.