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Vol. 59, Issue 5, 1298-1306, May 2001
Department of Pharmacology and Therapeutics, The University of
Liverpool, Liverpool, United Kingdom (A.M.W.S., P.G.B., I.G.E., P.A.S.,
S.A.W.); Division of Infection of Immunity, Institute of Biomedical and
Life Sciences, University of Glasgow, Glasgow, United Kingdom (H.P.D.);
and Molecular Parasitology Group, Institute of Molecular Medicine,
Oxford, United Kingdom (B.C.E.)
Extensive drug resistance in Plasmodium falciparum
emphasizes the urgent requirement for novel antimalarial agents. Here
we report potent antimalarial activity of a number of diamidine
compounds. The lead compound pentamidine is concentrated 500-fold by
erythrocytes infected with P. falciparum. Pentamidine
accumulation can be blocked by inhibitors of hemoglobin digestion,
suggesting that the drug binds to ferriprotoporphyrin IX (FPIX). All of
the compounds bound to FPIX in vitro and inhibited the formation of
hemozoin. Furthermore, inhibitors of hemoglobin digestion markedly
antagonized the antimalarial activity of the diamidines, indicating
that binding to FPIX is crucial for the activity of diamidine drugs.
Pentamidine was not accumulated into uninfected erythrocytes.
Pentamidine transport into infected cells exhibits an initial rapid
phase, nonsaturable in the micromolar range and sensitive to inhibition
by furosemide and glibenclamide. Changing the counter-ion in the order
Cl
< Br
< NO2
< I
<SCN
markedly stimulated pentamidine transport. These
data suggest that pentamidine is transported although a pore or ion
channel with properties similar to those of the recently characterized `induced permeability pathway' on the infected red cell membrane. In
summary, the diamidines exhibit two levels of selectivity against P. falciparum. The route of entry and molecular target
are both specific to malaria-infected cells and are distinct from
targets in other protozoa. Drugs that target the hemoglobin degradation pathway of malaria parasites have a proven record of accomplishment. The employment of induced permeability pathways to access this target
represents a novel approach to antiparasite chemotherapy and offers an
additional level of selectivity.
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