Apalutamide‐associated skin rash in patients with prostate cancer: Histological evaluation by skin biopsy

INTRODUCTION: Apalutamide‐associated skin rash is a more common adverse event in the Japanese population than in the global population. However, its mechanism remains elusive, and limited histopathological information hampers further understanding. CASE PRESENTATION: Case 1: a 71‐year‐old man with metastatic castration‐sensitive prostate cancer developed a skin rash after 70 days of apalutamide treatment. Case 2: a 71‐year‐old man with non‐metastatic castration‐resistant prostate cancer developed a skin rash after 71 days of apalutamide treatment. In both cases, the skin rash presented as a slightly exudative erythema. The histology showed spongiosis of the epidermis and perivascular and interstitial infiltration of lymphocytes and eosinophils in the upper dermis without necrotic keratinocytes. CONCLUSION: Apalutamide‐induced skin rashes can involve an eczematous reaction clinically and histologically.