Bioorganic & Medicinal Chemistry Letters
Optimization of the electrophile of chloronitrobenzamide leads
active against Trypanosoma brucei
Jong Yeon Hwang a, David C. Smithson a, Gloria Holbrook a, Fangyi Zhu a, Michele C. Connelly a,
Marcel Kaiser b, Reto Brun b, R. Kiplin Guy a,
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a St. Jude Children’s Hospital, Department of Chemical Biology and Therapeutics, 262 Danny Thomas Place, Memphis, TN 38105-3678, USA
b Department of Medical Parasitology, Swiss Tropical and Public Health Institute, Socinstr. 57, 4002 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 March 2013
Revised 9 May 2013
Accepted 13 May 2013
Available online 23 May 2013
We previously reported the phenylchloronitrobenzamides (PCNBs), a novel class of compounds active
against the species of trypanosomes that cause Human African Trypanosomiasis (HAT). Herein, we
explored the potential to adjust the reactivity of the electrophilic chloronitrobenzamide core. These stud-
ies identified compound 7d that potently inhibited the growth of trypanosomes (EC50 = 120 nM for Try-
panosoma b. brucei, 18 nM for Trypanosoma b. rhodesiense, and 38 nM for Trypanosoma b. gambiense)
without significant cytotoxicity against mammalian cell lines (EC50 > 25 lM for HepG2, HEK293, Raji,
Keywords:
and BJ cell lines) and also had good stability in microsomal models (t1/2 > 4 h in both human and mouse).
Overall these properties indicate the compound 7d and its analogs are worth further exploration as
potential leads for HAT.
Trypanosomes
Parasitology
Drug discovery
Ó 2013 Elsevier Ltd. All rights reserved.
Human African trypanosomiasis (HAT), a lethal disease wide-
spread in sub-Saharan Africa, is caused by two sub-species of the
eukaryotic protozoan parasite Trypanosoma brucei (rhodesiense
and gambiense).1 T. brucei is vectored between mammalian hosts
(cattle and humans) by the tsetse fly. Few chemotherapeutics are
available for HAT. The classic drugs are suramin and pentamidine,
for treatment of early-stage disease, and melarsoprol and eflorni-
thine, for treatment of late-stage disease.1 Suramin and melarso-
prol have serious side effects and all of the drugs have poor
clinical effect in late stage disease, and emerging drug resistance.2,3
Recently a drug combination, nifurtimox–eflornithine, has entered
late stage development for T. b. gambiense HAT.4 However, there is
still an urgent need for new drugs against HAT with better efficacy
and lower toxicity.5,6
We have recently reported a new class of lead compounds
inhibiting proliferation of T. brucei spp., the chloronitrobenzamides
(PCNBs) that were discovered by phenotypic screening (Fig. 1, 1).7
The initial examination of structure–activity relationships (SARs)
revealed that the 2-chloro-4-nitro group of the PCNBs was essen-
tial for potency with any change of the substitution pattern on this
phenyl ring causing great reduction of potency. In subsequent
intensive SAR studies it was discovered that incorporation of ben-
zothiazolyl substitution on the B-ring of PCNB 2 significantly in-
creased anti-trypanosomal potency.2 Additionally, substitution
with piperazine or methyl piperazine on A-ring of PCNBs improved
the activity and pharmacological properties.
In an effort to reduce complexity and molecular weight and im-
prove pharmacological properties of the PCNBs, we examined
whether or not the chloronitrobenzamide electrophile could be re-
placed. These studies are reported herein.
All compounds used in this study were prepared using the syn-
thetic scheme as outlined in Scheme 1. A common intermediate 4
was prepared by the published method.8–10 All variant electrophile
compounds 5 were synthesized by amide formation with either
acid chlorides or acids, using standard conditions. All compounds
were purified to >95% as verified by HPLC/MS/ELSD. The identity
of all compounds was confirmed by MS and NMR.
All compounds were subjected to a standardized testing scheme
measuring: potency for inhibition of trypanosome growth, potency
for inhibition of mammalian cell growth, solubility, and permeabil-
ity. Compounds were serially diluted in 10 threefold steps from a
10,000 lM DMSO stock. The resulting concentration series of each
PCNB were transferred to the assay wells using hydrodynamic pins
(V&P Scientific, San Diego, CA), giving a final DMSO concentration
of 0.2% and a concentration range downward from 33 lM to
0.5 nM. Trypanosome growth inhibition was measured after 48 h
as previously described in 384-well plates.11,12 The cytotoxicity
of all compounds was measured after 72 h for four human cell
lines; Raji, a Burkett’s lymphoma derived line; BJ, an immortalized
foreskin fibroblast derived line; HEK 293, an embryonic kidney
fibroblast derived line; and HepG2, a hepatocellular carcinoma de-
rived line, using the CellTiter Glo method (Promega). Each growth
⇑
Corresponding author. Tel.: +1 901 595 5714; fax: +1 901 595 5715.
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
Hwang, Jong Yeon
