New Approaches for the Identification of Drug Targets in Protozoan Parasites - International Review of Cell and Molecular Biology - page 376

completely different target (see section 4.2.) inhibits one of those signal-
ing proteins, namely calmodulin-dependent myosin light chain kinase with
high affinity ( Wang et al., 1994 ).
Moreover, adenylyl cyclases, the enzymes that synthesize cAMP, may
be valuable targets. In P. falciparum , one of the two cyclases, PfACβ, plays
an important role during the erythrocytic stage of the life cycle. PfACβ is
susceptible to a number of small molecule inhibitors, one of them being
selective for PfACβ relative to its human ortholog, soluble adenylyl cyclase
( Salazar et al., 2012 ).
PDEs have more recently been identified as novel drug targets that
exhibit a great potential as antiparasitic agents. All protozoan parasites for
which genome data are available code for at least one class1 PDE. The
amino acid sequences of these enzymes are reasonably well conserved
among the parasites, as well as between them and the human PDE families.
Considering the current wealth of inhibitors for and information about the
human PDEs, the parasite PDEs are being exploited as drug targets, with
medicinal chemistry developing corresponding inhibitors, which specifi-
cally target the parasite PDEs but not the human PDEs. This approach has
been followed up in more depth for two related parasites: Trypanosoma brucei
( Oberholzer et al., 2007 ), the causative agent of African human sleeping
sickness, and Leishmania major , the causative agent of cutaneous leishmaniasis
( Wang et al., 2007 ). Both organisms, as well as all their other relatives for
which genome information is available, namely Trypanosoma cruzi (Chagas
disease), Leishmania infantum (visceral leishmaniasis), Leishmania braziliensis
(mucocutaneous leishmaniasis) and Leishmania tarentolae (a model organ-
isms derived from lizards) code for the same set of four PDEs ( Alonso
et al., 2007 ; Kunz et al., 2005 ). A typical approach for studies on PDEs is to
express the gene of interest in a PDE-deficient yeast strain and to character-
ize it by complementation of vital functions of the deficient yeast strain and
by functional assays after purification of the recombinant protein as shown
for three Leishmania PDEs ( Johner et al., 2006 ) and for a Trypanosoma PDE
( Alonso et al., 2007 ). Leishmania PDE1 has been crystallized and structural
data for the design of novel inhibitors are available ( Wang et al., 2007 ). Thus,
PDEs from Leishmania and Trypanosoma spp. are candidates for a target-
based development of drugs against these parasites ( Seebeck et al., 2011 ).
3.3.2. Isoflavones and Receptor Kinases
Another group of compounds with antiparasite effects allegedly acting
on cell signaling are isoflavones. Targets of most isoflavones are unknown,