Looking to possible predictive factors of primary resistance to abiraterone acetate (AA) and enzalutamide (ENZ) in pretreated patients (pts) with castration-resistant prostate cancer (CRPC).
248 Background: Abiraterone acetate (AA) and enzalutamide (ENZ) are new generation hormonal agents (NHA) which demonstrated a survival gain in patients (pts) with castration-resistant prostate cancer (CRPC) pre-treated with docetaxel. Although all patients eventually became resistant to these NHAs, some of them show primary resistance, defined as an early progression within the first 3 months, which leads to an early treatment interruption. In the present analysis we have tried to identify which factor, if any, may predict primary resistance to AA and ENZ. Methods: We evaluated a consecutive series of 57 pts, treated in our hospital in two successive named patient programs conducted in our hospital to allow pre-treated CRPC patients to receive NHAs before their approval in Italy: 26 received AA (1,000 mg po + prednisone 10 mg po daily) and 31 ENZ (160 mg po daily). For each pt we have recorded the pre- and post-NHA clinical history, the treatment details and outcomes. We have also assessed the ability of a series of 24 selected clinical factors to predict NHAs resistance, through a logistic regression analysis. Continuous variables were categorized by quartiles and chosen for the initial model after a univariate chi-square analysis. Results: Among the 24 factors, the presence of pain at baseline, high baseline lactate dehydrogenase levels and prostate-specific antigen (PSA) levels after one month of treatment were predictive of primary NHA resistance at the univariate analysis. However, only PSA levels were confirmed at the multivariate analysis [exp(beta) 0.115; p = 0.007], as patients failing to achieve a 50% or more reduction in baseline PSA levels, were more likely to show primary NHA resistance (48% vs. 15%). Conclusions: Our results suggest that PSA trend may represent a simple and rapid method of identifying patients with primary resistance to NHAs, so patients failing to achieve a 50% or more reduction within the first month of treatment should undergo intensive investigations, to verify whether they have primary resistance to NHAs. These data should be confirmed in a larger patient population.