Background The use of oral chemotherapy (OC) is an effective and safe approach in the treatment of metastatic castration resistant prostate cancer (MCRP). Abiraterone and enzalutamide offer improved patient convenience and ease of administration. However, patients are now responsible for ensuring optimal adherence to their medication.

Purpose The aim of this study was to determine adherence to abiraterone and enzalutamide in patients with MCRP.

Material and methods A retrospective longitudinal study was carried out from September 2011 to March 2016. All patients treated with OC for MCRP were included. Patients with only one drug dispensation were excluded, because adherence could not be calculated. Patients´ medical records were reviewed and the following data were collected: demographics and pharmacotherapeutics (prior chemotherapy, abiraterona or enzalutamide treatment start and end date, dosing and dispensed data). Data were obtained from electronic clinical records, oncology prescription software and outpatient dispensing records.

Adherence to OC was evaluated indirectly using dispensation records to calculate ‘medication possession rate’ (MPR). MPR is defined as the sum of all days of drug supplied within a given period, divided by the total number of days in that period. Optimal adherence was defined as MPR >80%, following previous studies. The end points were: measure of adherence to enzalutamide and abiraterone; duration of treatment; and percentage of patients who achieved optimal adherence. Data analysis was carried out using SPSS 15.0.

Results 45 patients (mean age 74 years (57–87)) with at least two drug dispensations were selected from the pharmacy database. 30 patients (66.6%) received abiraterone, 3 patients (6.66%) received enzalutamide and 12 patients (26.66%) received both drugs sequentially.

Conclusion Most patients showed high rates of adherence to OC in MCRP. The long duration of treatment and absence of symptoms in these patients could prove a threat to adherence to treatment. Oncology pharmacists have a key role by following patients with OC in MCRP and reminding them of the importance of adherence. Study limitations include measuring adherence using only one method.

No conflict of interest