Abstract. In the early 1980s, the antischistosomal aryl hydantoin Ro 13-3978 (AH01), a close structural analogue of the androgen receptor antagonist nilutamide, was discovered. Administration of 100 mg/kg oral doses of AH01 to mice infected with adult and juvenile Schistosoma mansoni produced 95% and 64% total worm burden reductions, confirming its high activity against adult worms, and showing that AH01 is also effective against juvenile infections. AH01 had no measureable interaction with the androgen receptor in a ligand competition assay, but it did block dihydrotestosteroneinduced cell proliferation in an androgen-dependent human prostate cancer cell line. For AH01, nilutamide, and three closely related aryl hydantoin derivatives, there was no correlation between antischistosomal activity and androgen receptor interaction.Schistosomiasis is a neglected but widespread tropical disease caused by infection with parasitic blood flukes. Schistosoma mansoni, Schistosoma haematobium, and Schistosoma japonicum are the most widely distributed species and cause the highest disease burden, particularly in sub-Saharan Africa.1-3 Praziquantel (PZ) is the only drug available for treatment of this disease, and it is active against adult schistosomes, but has little activity against the juvenile schistosomula, the young developmental stages of the parasite.2,4 Should serious praziquantel drug resistance arise, there are no viable alternatives to this drug. Even so, drug discovery for schistosomiasis has languished.The introduction of PZ in 1982 likely led to decisions to abandon the development of a number of promising antischistosomal agents that were discovered in the same time period. One of these was Ro 13-3978 (AH01), the lead compound from a series of aryl hydantoins that were investigated in some detail at Hoffmann La-Roche.5-8 A number of these aryl hydantoins had high oral efficacy in schistosome animal models, 8 but they also produced antiandrogenic side effects in the host, 5 a not unexpected outcome given the close structural similarity of these aryl hydantoins to the antiandrogenic drug nilutamide (Figure 1). We recently showed 9 that nilutamide, but not the three structurally diverse androgen receptor (AR) antagonists flutamide, bicalutamide, and cyproterone acetate, has antischistosomal activity. As phylogenetic evidence indicates that schistosome species do not appear to have ARs, 10 the data led us to hypothesize that for aryl hydantoins and related heterocycles, the structural requirements for antischistosomal efficacy and AR binding interactions are divergent.Based on the limited elucidation of the structureactivity-relationship of this aryl hydantoin compound series, 8 we began to test this hypothesis by the synthesis 8,11 of AH01, its imino isostere AH02, and two derivatives (AH03 and AH04) incorporating nitrile functional groups (present in bicalutamide and other antiandrogens) known to import high AR binding affinity. 12 We now present our initial characterization of the antischistosomal versus antia...