Duration of letrozole treatment and outcomes in the placebo-controlled NCIC CTG MA.17 extended adjuvant therapy trial.

PURPOSE: MA.17 was a double-blind placebo-controlled trial involving 5187 postmenopausal women that established letrozole to be of value in reducing recurrence of breast cancer when given in the extended adjuvant therapy setting after about 5 years of tamoxifen. Analyses were conducted to examine the relationships between duration of treatment on MA.17 and outcomes.

METHODS: The final MA.17 database that included all events up to the date of unblinding of the study was interrogated. A non-parametric kernel smoothing method was used to estimate the hazard rates for disease-free survival (DFS), distant DFS (DDFS) and overall survival (OS) at 6, 12, 24, 36 and 48 months of follow-up and the hazard ratios (HRs) of letrozole to placebo were determined. The trend in HRs over time was tested based on a Cox model with a time-dependent covariate.

RESULTS: Considering all patients, HRs for events in DFS and DDFS progressively decreased over time, favoring letrozole, with the trend being significant (p < 0.0001 and p = 0.0013, respectively) whereas the trend for OS was not significant. Considering the 2360 patients with node-positive status, the HRs for DFS, DDFS and OS all decreased over time with tests for trend all showing significance (p = 0.0004, 0.0005 and 0.038, respectively). Considering the 2568 patients with node-negative status, the HRs for DFS decreased over time with the test for trend being significant (p = 0.027) whereas the HRs for DDFS and OS showed no significant change over time.

CONCLUSION: These analyses suggest that, at least out to about 48 months, longer duration of letrozole treatment is associated with greater benefit in the extended adjuvant therapy setting.