Tamoxifen (TAM) is increasingly administered to new early breast cancer patients. Because it is not devoid of toxic effects, we studied factors potentially predictive of its efficacy.

From 1978 to 1983, 433 patients were enrolled in the GUN randomized trial: 206 were assigned to TAM versus 227 controls (no-TAM). Premenopausal patients with axillary lymph node involvement (60 TAM versus 65 no-TAM) also received nine CMF cycles. Eight biological markers were retrospectively assayed for most patients: estrogen; progesterone; prolactin receptors (PrlRs); microvessel count (MVC); S-phase fraction; tumor ploidy; epidermal growth factor receptor (EGFR); and HER2. We performed a multivariate test of the TAM/covariate interactions to establish whether these variables predicted for TAM efficacy. Estimates of the TAM effect were expressed as hazard ratio (HR) of death of TAM over no-TAM patients with 95% confidence intervals (95% CIs).

At a median follow-up of 15 years, PrlRs, MVC, S-phase fraction, ploidy, and EGFR did not influence TAM efficacy. Differently, HER2 had an overall significant predictive effect: HR = 0.59 (95% CI: 0.40-0.87) in HER2-negative subjects versus HR = 1.09 (95% CI: 0.63-1.87) in HER2-positive subjects (interaction test: P = 0.04). The predictive effect of HER2 was also evident in the subgroup of patients with steroid receptor-positive tumors (HER2 positive: HR = 1.33, 95% CI: 0.70-2.51; HER2 negative: HR = 0.73, 95% CI: 0.47-1.14).

With the statistical power of the present randomized trial, S-phase, ploidy, EGFR, PrlR, and MVC do not seem to predict for TAM efficacy. Conversely, our data support the hypothesis that tumors overexpressing HER2 might not benefit from adjuvant TAM.