Villejif, France—Adherence to tamoxifen therapy for early-stage breast cancer in younger women might be worse than expected. That was the finding of a French prospective study presented at the recent European Society for Medical Oncology (ESMO) 2018 Congress in Berlin.

Researchers report that nearly one in six premenopausal women being treated for early-stage breast cancer did not adequately follow their drug regimen, raising their risk for recurrence and potentially reducing survival rates.

Hormonal therapy such as tamoxifen is recommended for 5 to 10 years in all patients with hormone receptor-positive breast cancer, but questions have been raised about how often long-term therapy is followed.

“This issue is important because non-adherence with hormonal therapy—meaning taking less than 80% of prescribed treatment—can be associated with higher risk of mortality and shorter time to recurrence of breast cancer,” said lead author Barbara Pistilli, MD, a medical oncologist at Institut Gustave Roussy.

The study was especially significant because it didn’t use only patient-reported data for measuring adherence. Instead, the study team actually measured serum levels of tamoxifen in the participants.

Included in the study were patients recently diagnosed with early (stage I-III) breast cancer in the CANTO cohort, which is a French prospective study investigating the long-term impact of side-effects with breast cancer treatments in around 12,000 participants.

Within the larger trial is a subgroup of 1,799 premenopausal women prescribed adjuvant hormonal therapy, researchers assessed their adherence to tamoxifen by measuring serum levels at 1, 3, and 5 years and then comparing the data with patients’ self-reports of adherence.

The following definitions were used:
• Non-adherent if tamoxifen (TAM) is ≤15 ng/mL (≤40 nM)
• Poorly-adherent if TAM 15-60 ng/mL (40-150 nM)
• Adherent if >60 ng/mL (>150 nM).

Results of the plasma assessment on all 1,228 patients reaching 1 year of follow-up found that, overall, 224 (18.2%) patients appeared not adequately adherent to TAM. Of those, 162 (13.2%) were nonadherent and 62 (5.0%) poorly adherent.

“At one year from initiation of TAM, plasma measurements show that a substantial proportion of premenopausal patients are not adequately adherent to this treatment,” study authors write. “Poorly-adherent patients could benefit from metabolic and pharmacogenetic investigations. Identification of patients at risk of non-adherence allows early targeted interventions to promote adherence in this unique population.”

“I was surprised at the high rate of non-adherence, which was considerably higher than reported previously,” Pistilli added. “Women with breast cancer should be encouraged to discuss their treatment and any side-effects they experience with their doctor to obtain help to take their therapy.”

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