Long-term adherence to GnRH agonists in men with prostate cancer. A nation-wide population-based study in prostate cancer data base Sweden.

Aim: Gonadotropin-releasing hormone (GnRH) agonists are used to treat men with prostate cancer (PCa). To date, no study has fully assessed patterns of adherence to GnRH agonists. We investigated patterns of adherence to GnRH agonists using data from Prostate Cancer data Base Sweden (PCBaSe). Methods: PCBaSe links the National Prostate Cancer Register (NPCR) Sweden to other healthcare registers and demographic databases. Men on primary or secondary GnRH agonists between 2006-2013 entered the study 45 days after GnRH agonists' initiation (run-in period) and exited at 3 years. Medication possession ratio quantified adherents (≥80%). Multivariable logistic regression models included age, injection interval, PCa risk categories, Charlson Comorbidity Index, prior PCa treatment, civil status and year of GnRH initiation. Odds ratios (OR) and 95% confidence intervals (CI) expressed odds of adherence. Results: Men on primary GnRH agonists ( n  = 8,105) were more adherent with increasing age (75-84 years compared to ≤65 years OR: 1.49; 95% CI: 1.23-1.81), longer injection intervals (365 days compared to 90 days OR: 3.29; 95% CI: 2.52-4.30) and higher PCa risk categories at diagnosis (distant metastasis compared to low risk PCa OR: 3.56; 95% CI: 2.54-5.00). Men on secondary GnRH agonists ( n  = 4,738) were more adherent with increasing age (≥85 years compared to ≤65 years OR: 1.65; 95% CI: 1.23-2.22) and prior PCa treatment (anti-androgens compared to deferred treatment OR: 1.50; 95% CI: 1.23-1.82), (radiotherapy compared to deferred treatment OR: 1.35; 95% CI: 1.11-1.64). Conclusions: Longer injection intervals could be addressed in the clinical setting to improve adherence.