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cell membrane. The difference in activity between 20
and 21 is harder to rationalize. Although both com-
pounds are more potent inhibitors of leishmanial tubu-
lin assembly than oryzalin, N1-phenyl sulfanilamide 20
is much more effective than 3 at blocking parasite
growth, while N1-alkyl sulfanilamide 21 and 3 possess
similar antiparasitic activity. Poor accumulation of 21
within the parasite may also explain these data, but
further experiments are required to adequately address
this issue.
Acknowledgements
We would like to thank the College of Pharmacy at The
Ohio State University for providing financial support,
Courtney G. Havens for assisting with the purification
of leishmanial tubulin, and Dr. Denise Cottrell for
helpful discussions.
References and Notes
The results presented here provide motivation for fur-
ther exploration of dinitroaniline sulfonamides as anti-
leishmanial agents in particular and continued study of
tubulin as an antiprotozoal drug target in general.
Synthesis of new oryzalin analogues based on the
structure–activity relationshipoutlined in this reoprt
will almost certainly yield new compounds with
improved antileishmanial and antitubulin activity, and
additional biological testing will hopefully provide new
drug candidates against leishmaniasis and perhaps other
parasitic diseases. A critical issue pertaining to the
development of these agents as antiparasitic drug can-
didates is the selectivity of their antimicrotubule activ-
ity. Oryzalin binds more avidly to plant tubulin than to
mammalian tubulin.19 Although a single filter binding
experiment suggests that the related dinitroaniline tri-
fluralin binds more strongly to leishmanial tubulin than
to mammalian tubulin,5 the determination of dissocia-
tion constants between oryzalin and both leishmanial
and mammalian tubulin would be very helpful in quan-
tifying selectivity. It will also be vital to quantitate the
selectivity for the new compounds reported here that
possess superior antiparasitic and antitubulin activity
compared to oryzalin. Another issue to be addressed is
the potential host toxicity of these compounds due to
the presence of aromatic nitro groups.20 The main point
of this work, however, is that we have significantly
improved on the activity of oryzalin against Leishmania
and the leishmanial tubulin target protein by making
simple modifications to the lead compound. It is our
hope that future compounds, perhaps those like com-
pound 20 that are substituted at the sulfonamide nitro-
gen, will retain antitubulin and antiparasitic activity in
the absence of aromatic nitro groups. By employing a
wide range of drug discovery tools in the future,
including synthetic medicinal chemistry, biochemical
assays and computer modeling of the target protein,
evaluation of new compounds against parasites and
mammalian cells, and studies in animal models of
parasitic disease, we are optimistic that the further study
of leishmanial tubulin will lead to the development of
selective antiparasitic drug candidates in the future.
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