Arimidex, Tamoxifen, Alone or in Combination (ATAC) Trial

  • The ATAC trial:
    • Was designed to compare the efficacy and safety of anastrozole (1 mg) with tamoxifen (20 mg):
      • As adjuvant treatment for postmenopausal women:
        • With early-stage breast cancer
    • Patients were treated every day for 5 years
    • The study was a double-blind, prospective, randomized trial:
      • With 9366 postmenopausal women
    • A proportional hazards model was used to assess:
      • The primary endpoints of:
        • DFS
      • Secondary endpoints of:
        • Time to recurrence
        • Time to distant recurrence
        • Overall survival
        • Death with or without recurrence
    • The combination arm of anastrozole and tamoxifen:
      • Was discontinued after the initial analysis:
        • As it was found to have no efficacy or tolerability benefits over tamoxifen alone
  • Long-term follow-up of 120 months:
    • Showed significant improvements in the anastrozole group versus the tamoxifen group:
      • For DFS
      • Time to recurrence
      • Time to distant recurrence
    • In hormone receptor-positive patients:
      • These benefits were seen to increase over time
    • Recurrence rates:
      • Were found to remain lower on anastrozole after treatment was completed
    • There was little difference in overall survival:
      • Hazard ratio, 0.95; 95% confidence interval, 0.84–1.06; P=0.4
  • Fractures were more frequent:
    • During the active treatment:
      • In patients receiving anastrozole:
        • But were similar between the two groups:
          • In post-treatment follow-up
  • Treatment-related serious adverse events:
    • Were less common in the anastrozole group:
      • But were also found to be similar between the two groups:
        • After treatment completion
  • Anastrozole showed a non-significant:
    • Increased incidence of:
      • Colorectal cancer
      • Lung cancers
    • Decreased incidence of:
      • Endometrial
      • Melanoma
      • Ovarian cancers
        • However, only the decrease in endometrial cancers:
          • Remained statistically significant after Bonferroni correction (P<0.001)
  • Overall:
    • Anastrozole was found to have superior long-term efficacy and safety than tamoxifen:
      • As initial adjuvant therapy for:
        • Postmenopausal women with hormone-sensitive early-stage breast cancer
    • Outcomes from the ATAC trial:
      • Made anastrozole the preferred treatment for postmenopausal women with localized hormone receptor-positive breast cancer


  1. Cuzick J, Sestak I, Baum M et al; ATAC/LATTE investigators. Effect of anastrozole and tamoxifen as adjuvant treatment for early-stage breast cancer: 10-year analysis of the ATAC trial. Lancet Oncol. 2010;11:1135-1141.
  2. Howell A, Cuzick J, Baum M, et al. Results of the ATAC (Arimidex, Tamoxifen, Alone or in Combination) trial after completion of 5 years’ adjuvant treatment for breast cancer. Lancet. 2005;365:60-62.

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