Skin Rash Management Based on Early Elevation in the Eosinophil Proportion Within 3 Months of Initiating Apalutamide for Prostate Cancer.

BACKGROUND: Skin rash (SR) is a key issue in the long-term use of apalutamide (APA) in patients with prostate cancer (PCa); however, there is currently no established strategy for the safe management of APA in preventing SR. AIMS: To investigate the predictive value of early elevation in the proportion of eosinophils (EE) for SR in patients treated with APA for PCa. METHODS AND RESULTS: We retrospectively reviewed the clinical data of 78 patients who received APA for PCa between April 2021 and November 2024. Early EE was defined as beyond the upper normal limit of 5.0% within 3 months of APA treatment initiation. The initial SR (I-SR) and recurrent (R-) SR incidences were compared between patients with and without early EE. During a median follow-up period of 14.7 months, 35 patients (44.9%) developed SR, and a significantly higher proportion of patients with early EE had I-SR than those without early EE (87.5% [7/8] vs. 40.0% [28/70]; p = 0.020). Six of the seven patients with early EE and I-SR had EE before the onset of I-SR (85.7%). In terms of R-SR, among patients who reduced the APA dose after I-SR, R-SR remained significantly more prevalent in patients with early EE compared with that in patients without early EE (75.0% [3/4] vs. 11.1% [2/18]; p = 0.016). CONCLUSION: The present study suggests that early EE has predictive value for I- and R-SR in patients who receive APA for PCa. Further large-scale studies are warranted to verify the clinical utility of early EE in patients with APA.