RU58841 (also written RU 58841, and catalogued as PSK-3841 or HMR-3841) is a non-steroidal androgen receptor antagonist of the N-substituted aryl hydantoin class, synthesised at the Roussel Uclaf research centre and first described in 1994 as a locally acting antiandrogen. Chemically it is 4-[3-(4-hydroxybutyl)-4,4-dimethyl-2,5-dioxoimidazolidin-1-yl]-2-(trifluoromethyl)benzonitrile. Published work is predominantly preclinical and dates mostly from the 1990s. Studies have examined receptor binding and androgen-dependent organs in hamsters and rats, sebaceous gland models, an androgen receptor reporter assay in prostate cells, a stumptailed macaque model of androgenetic alopecia and human balding scalp grafts maintained on nude mice. Later work addressed prodrug and nanoparticle formulation for follicular targeting, and metabolite identification for doping control.