Over-adherence to capecitabine: a potential safety issue in breast and colorectal cancer patients.

Purpose: The aim of the OCTO clinical study was to measure patients' adherence to capecitabine-based treatment.
Methods: A cohort of ambulatory patients treated with capecitabine monotherapy for either locally advanced or metastatic, breast or colorectal cancer was monitored for 6 cycles. Adherence was assessed in all patients by self-completed questionnaires on disease, pill-count and pharmacological dosage of FBAL (metabolite of capecitabine); and in half of the cohort by electronic medication event monitoring systems (MEMS™) recording the opening times of the device.
Results: Forty patients were enrolled between November 2008 and September 2011 and treated by capecitabine for an average of 4.75 cycles (range 1-6). Hand-foot syndrome (HFS) was the most frequently reported toxicity (35% patients), and to a lesser extent fatigue and/or asthenia (21%), nausea and/or vomiting (13%) and diarrhea (11%). In the MEMS™ cohort, 20 patients were included. Patients' adherence was excellent with very few missing occasions (23/2272 records). Close analysis of MEMS™ data revealed unexpected medication patterns, such as patients taking extra days of medication beyond planned cycle, patients taking extra doses per day and patients missing a day of dosing and "compensating" by taking extra the following day (N = 7, 18%). A trend was found between over-adherence and high-grade toxicity (grades 3 and/or 4): OR 4.74 [0.65-45.2], p = 0.13 and higher AUC (p = 0.16). There was a trend towards increased AUC of FBAL in over-adherent patients (p = 0.16).
Conclusion: Adherence to oral anticancer chemotherapy was found excellent in this population suggesting over-adherence to capecitabine and potential safety implications for outpatients' drugs.