Structure‐activity relationships for several series of fragment‐based inhibitors that target Trichomonas vaginalis nucleoside ribohydrolase enzymes

Abstract

Trichomoniasis, the most common sexually transmitted infection in the world, is caused by the parasitic protozoan Trichomonas vaginalis. Upon infection, T. vaginalis has been known to cause a variety of adverse effects as well as predisposition to other illnesses. Trichomoniasis is commonly treated with 5‐nitroimidazole drugs; however, some strains have developed resistance to these drugs and novel treatments are needed. Since T. vaginalis cannot synthesize nucleobases de novo, it relies on nucleoside ribohydrolases within its pyrimidine and purine salvage pathways in order to acquire nucleobases from its host. Nucleoside ribohydrolases, such as adenosine/guanosine preferring nucleoside ribohydrolase (AGNH) and uridine nucleoside ribohydrolase (UNH), cleave the N‐glycosidic bond of purine and pyrimidine nucleosides resulting in the formation of a nucleobase and a ribose sugar. Inhibiting the AGNH or UNH enzymes would block the salvage pathway resulting in parasite cell death. Fragment screening of AGNH and UNH identified ligand‐efficient scaffolds to serve as medicinal chemistry starting points for drug design. A variety of phenyl pyridine, phenyl pyrimidine, and phenyl pyrazine compounds were then synthesized and screened for inhibitory properties. 1H and 19F NMR‐based activity assays were employed, using adenosine and 5‐fluorouridine as substrates for AGNH and UNH, respectively. It was found that compounds containing a hydroxyl group at the 2‐position resulted in lower IC50 values for AGNH, and that the UNH IC50 was lower for compounds with hydroxy or methoxy functional groups at the 2’ and 3’ positions. With the enzyme inhibition data at hand structure‐activity relationships are being derived and will be used in combination with molecular modeling to design the next generation of compounds to be synthesized. Structural data is also being pursued for both enzymes to guide compound design. Highly purified AGNH and UNH, including selenomethionine‐AGNH, have been prepared for crystallography collaborations.