4-[4, 4-Dimethyl-3-(4-hydroxybutyl)-5-oxo-2-thioxo-1-imidazolidinyl]-2-+ ++trif luoromethylbenzonitrile (RU 59063) is a prototype of a new class of high-affinity nonsteroidal androgen receptor (AR) ligands. The search for a radioiodinated AR ligand prompted us to synthesize 4-[4, 4-dimethyl-3-(4-hydroxybutyl)-5-oxo-2-thioxo-1-imidazolidinyl]-2-i odo benzonitrile (DTIB) wherein the trifluoromethyl group of RU 59063 was substituted with the similarly hydrophobic iodine atom. DTIB displayed subnanomolar binding affinity (K(i) = 0.71 +/- 0.22 nM) for the rat AR in competitive binding assays. Additionally, DTIB demonstrated potent agonist activity, comparable to that of the natural androgen 5alpha-dihydrotestosterone (DHT), in a cell-based functional assay (cotransfection assay). DTIB represents a new lead for the development of high-affinity radioiodinated AR radioligands.