Bioorganic & Medicinal Chemistry Letters
Identification of proton-pump inhibitor drugs that inhibit
Trichomonas vaginalis uridine nucleoside ribohydrolase
Tara A. Shea, Paola J. Burburan, Vivian N. Matubia, Sandy S. Ramcharan, Irving Rosario Jr.,
⇑
David W. Parkin, Brian J. Stockman
Department of Chemistry, Adelphi University, 1 South Avenue, Garden City, NY 11530, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Trichomonas vaginalis continues to be a major health problem with drug-resistant strains increasing in
prevalence. Novel antitrichomonal agents that are mechanistically distinct from current therapies are
needed. The NIH Clinical Compound Collection was screened to find inhibitors of the uridine ribohydro-
lase enzyme required by the parasite to scavenge uracil for its growth. The proton-pump inhibitors ome-
prazole, pantoprazole, and rabeprazole were identified as inhibitors of this enzyme, with IC50 values
Received 13 November 2013
Revised 3 January 2014
Accepted 6 January 2014
Available online 13 January 2014
ranging from 0.3 to 14.5 lM. This suggests a molecular mechanism for the in vitro antitrichomonal activ-
ity of these proton-pump inhibitors, and may provide important insights toward structure-based drug
design.
Keywords:
NIH Clinical Compound Collection
NMR
Proton-pump inhibitors
Trichomonas vaginalis
Uridine ribohydrolase
Ó 2014 Elsevier Ltd. All rights reserved.
Trichomoniasis is the most prevalent non-viral sexually trans-
mitted disease. The causative agent is the parasitic protozoan
Trichomonas vaginalis. It affects an estimated 170 million people
worldwide with as many as 85% of cases occurring in developing
nations.1 More than 1 million new cases are reported each year
in the United States, with infection prevalence estimated to be
almost 4 million.2 Although clinical manifestations of trichomoni-
asis infections are typically mild, the immune system can be
concomitantly compromised. Individuals with trichomonal infec-
tions have a higher susceptibility to more serious conditions such
as cervical cancer, HIV-1, and pelvic inflammatory disease.3,4 More
recently, trichomonal infection has also been associated with pros-
tate cancer and benign prostatic hyperplasia.5,6 Trichomoniasis is
typically treated with 5-nitroimidazole drugs such as metronida-
zole.3,7 The compounds themselves are inactive, but are anaerobi-
cally reduced by redox enzymes in the parasitic hydrogenosome.
Pyruvate-ferredoxin oxidoreductase reduces the nitro group form-
ing toxic nitro radical anions which target thymine and adenine
residues in the pathogen’s DNA. Cleavage of DNA results in para-
sitic death within eight hours.3,7 Resistance to metronidazole and
related drugs is increasing, with an estimated 5% of trichomoniasis
clinical cases resulting from T. vaginalis strains with some
resistance.7
T. vaginalis is an obligate parasite in that it is incapable of the de
novo synthesis of purine8 and pyrimidine rings.9,10 It must
scavenge nucleosides from host cells and then use salvage pathway
enzymes to obtain the nucleobases. Once acquired, the parasite re-
lies on the activity of these enzymes to metabolize the nucleosides.
Salvage pathway enzymes are thus potential targets for therapeu-
tic intervention. One such enzyme is uridine nucleoside ribohydro-
lase (UNH),11 a fundamental constituent in the uridine salvage
pathway. UNH is responsible for cleaving the N-glycosidic bond be-
tween uracil and ribose. The identification of UNH inhibitors may
lead to novel antitrichomonal agents that are mechanistically dis-
tinct from current therapies. An 19F NMR-based activity assay,
utilizing 5-fluorouridine in place of uridine, was thus developed
and used to screen the NIH Clinical Compound Collection for
UNH inhibitors.
An NMR-based assay was developed for monitoring the UNH
enzyme reaction in order to avoid many complications associated
with screening compounds at high concentrations by other meth-
ods.12 While 1H NMR was sufficient to monitor the hydrolysis of
the natural substrate uridine, monitoring the hydrolysis of 5-flu-
orouridine with 19F NMR was found to be superior for several rea-
sons. First, the 19F NMR spectra are much simpler since they are
not compromised by the presence of test compound resonances
or a large signal from the non-deuterated DMSO used to plate
the NIH Clinical Collection. Second, the Km values for uridine and
5-fluorouridine were determined to be 54
tively, indicating that 5-fluorouridine will provide
l
M and 15
l
M, respec-
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Corresponding author. Tel.: +1 516 877 4139.
a
lower
0960-894X/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.