Background and importance Abiraterone is used in combination with prednisone, is metabolised by the liver (CYP3A4) and is an enzyme inhibitor (CYP2D6/CYP2C8). Enzalutamide is metabolised by the liver (CYP2C8/CYP3A4) and is a potent enzyme inducer (CYP3A4/CYP2B6/CYP2C9/CYP2C19). Both are used to treat castrate metastatic prostate cancer (CMPC).

Aim and objectives To analyse switching between two antiandrogenic drugs, abiraterone and enzalutamide, in patients with CMPC.

Material and methods This was an observational, retrospective, descriptive, unicentre study. The study included 127 patients with CMPC who began treatment with abiraterone or enzalutamide from January 2015 to March 2019. Clinical data from an outpatient pharmacy database and from the medical history were analysed. Reasons to switch were classified as safety, pharmacological interactions and galenic advantages.

Results A total of 127 patients were analysed: 50 began treatment with abiraterone and 77 with enzalutamide. Four of the 50 patients who started with abiraterone switched to enzalutamide (8%) for safety reasons (100%, n=4) because of side effects: digestive intolerance and diarrhoea (50%, n=2), oedema (25%, n=1) and uncontrolled diabetes (25%, n=1). The last case was probably due to prednisone.

Ten of the 77 patients who started treatment with enzalutamide switched to abiraterone (13%) for safety reasons in six patients (60%) because of side effects: memory loss and disorientation (20%, n=2), asthenia (10%, n=1), depression and anxiety (10%, n=1), hypertension (10%, n=1) and parkinsonism (10%, n=1). In three patients (30%) switching was due to drug interactions, which modified the efficacy and safety of enzalutamide and the other drug involved. Four drugs were involved, 2 (50%) were antihypertensives (manidipine and verapamil) and 2 (50%) were anticoagulants (rivaroxaban and acenocoumarol). In one patient (10%), switching was due to the galenic advantage of the smaller number and size of abiraterone tablets compared with enzalutamide capsules because of difficulty in swallowing in a case of oesophageal neoplasm.

Conclusion and relevance Switching between abiraterone and enzalutamide in our patients was mostly for safety reasons. Some side effects of the treatment with abiraterone and prednisone may have a steroidal origin. Enzalutamide is involved in pharmacokinetic and pharmacodynamic interactions with clinical relevance, so this is an important reason to switch. The smaller number and size of tablets could be a galenic advantage.

References and/or acknowledgements No conflict of interest.