In a French study reported in the Journal of Clinical Oncology, Pistilli et al found that serum detection of nonadherence to adjuvant tamoxifen therapy was common and was associated with poorer distant disease-free survival among premenopausal women with breast cancer.

Biochemical nonadherence was not concordant with patient self-reporting of adherence or nonadherence.

Study Details

The study included 1,177 women (median age = 45 years) enrolled in the prospective CANTO study. Biochemical nonadherence was defined as a tamoxifen serum level ≤ 60 ng/mL. Patients were assessed for biochemical adherence at approximately 1 year after initial tamoxifen prescription, with data on self-reported adherence/nonadherence being collected at the same time through patient interview.

Distant disease-free survival was assessed using an inverse probability weighted Cox proportional hazards model, with propensity scoring based on age, disease stage, surgery, chemotherapy, and center size.

Rate of Nonadherence and Outcomes

Overall, 188 patients (16.0%) were found to be biochemically nonadherent to tamoxifen therapy at 1 year. The overall patient-reported rate of nonadherence was 12.3%. Among the 188 patients with biochemical nonadherence, 55% self-reported adherence to tamoxifen. Median tamoxifen serum concentrations were 119 ng/mL vs 6 ng/mL among biochemically adherent vs nonadherent patients.

After median follow-up of 24.2 months from tamoxifen serum assessment, patients who were biochemically nonadherent had significantly reduced distant disease-free survival (adjusted hazard ratio = 2.31, 95% confidence interval [CI] = 1.05–5.06, P = .036), with 3-year rates of 89.5% vs 95.4%.

Biochemical nonadherence was significantly associated with patients’ not living with a partner as a couple vs living with a partner (adjusted odds ratio [OR] = 1.72, 95% CI = 1.02–2.89), increased comorbidities (adjusted OR = 1.85, 95% CI = 1.09–3.15, for Charlson comorbidity score ≥ 1 v 0), and no receipt of (neo)adjuvant chemotherapy vs receipt of chemotherapy (adjusted OR = 1.74, 95% CI = 1.04–2.91). In addition, presence of symptoms assessed at a median of 3.9 months from tamoxifen prescription was associated with biochemical nonadherence, with significant associations observed for musculoskeletal symptoms (adjusted OR = 1.58, 95% CI = 1.06–2.37) and severe fatigue (adjusted OR = 1.65, 95% CI = 1.07–2.5).

The investigators concluded, “Therapeutic drug monitoring may be a useful method to promptly identify patients who do not take adjuvant tamoxifen as prescribed and are at risk for poorer outcomes. Targeted interventions facilitating patient adherence are needed and have the potential to improve short-term breast cancer outcomes.”

Ines Vaz-Luis, MD, of Institut Gustave Roussy, Villejuif, is the corresponding author for the Journal of Clinical Oncology article.

Disclosure: The study was supported by the Institut National Cancer-France and others. For full disclosures of the study authors, visit ascopubs.org.