A novel antimalarial lead compound: in vitro properties and mode of action studies

Malaria remains a major public health problem and the increasing number of resistant
strains underscores the need for new drugs with new modes of action (MOAs).
It was the aim of the present thesis to characterize a novel antimalarial lead compound
with respect to MOA and in vitro properties.
The lead compound, ACT-AM, inhibited in vitro proliferation of all tested P. falciparum
strains, irrespective of their drug resistance properties, with IC50 values in the low singledigit
nanomolar range. ACT-AM was further shown to equally and rapidly affect all
asexual blood stages of the parasite. The novel molecule is therefore comparable to the
most efficacious registered antimalarial drugs in terms of in vitro activity.
To investigate the MOA of ACT-AM, a chemical derivative of the compound able to
form covalent bonds upon UV activation was utilized. This advantageous UV-dependent
system was adapted and implemented for P. falciparum- notably for the use in intact cells
and proved to be appropriate for various biochemical methods including pull-down
experiments, fluorescent imaging and Far Western blotting. Pull-down experiments
revealed numerous target candidates, three of which were shown to interact with ACTAM
in vitro, namely MDR (multidrug resistance protein), ENT4 (equilibrative nucleoside
transporter 4) and CRT (chloroquine resistance transporter). These proteins could
represent actual targets or might confer resistance to the compound.
Microarray and hematin interaction studies suggested that ACT-AM has an MOA distinct
from that of several registered antimalarials, a factor that bodes well for possible
combination therapies.
The promising in vitro activity of the compound and the indication of a novel MOA
emphasize the potential of ACT-AM or analogues of the same chemical class as
therapeutic agents for the treatment of malaria.