trans

DRUG RESISTANCE IN KINETOPLASTID PARASITES 417
constituents, thereby increasing membrane
permeability. The exact contribution of this
membrane defect to pentamidine resistance
needs to be assessed, however.
modulating the expression of several genes of
the ergosterol biosynthetic pathway. In yeast
the expression of several genes was modulated
in response to ergosterol depletion.
Resistance to amphotericin B
The polyene antibiotic amphotericin B (AmB),
a standard antifungal drug used in systemic
fungal infections, is a second-line drug for the
treatment of visceral leishmaniasis. The toxicity
of AmB has limited its use, but liposomal formulations
of AmB are highly effective against
Leishmania, although high cost may be an
issue for treating visceral leishmaniasis in several
areas where it is endemic. However, short
course treatment of lipid AmB formulations
may help in the treatment of patients not
responding to antimony therapy. AmB interacts
with fungal membrane sterols (notably
ergosterol), and the mode of action against
Leishmania, which also have ergosterol, is
apparently the same.
Resistance to AmB in field isolates is not
widespread, although successive relapses in
HIV-infected patients could enhance the emergence
of AmB resistant Leishmania isolates.
Resistance to AmB has been induced in vitro
in Leishmania donovani promastigotes. The
mutants showed increased membrane fluidity
with changes in lipid composition including
an ergosterol precursor. Changes in membrane
fluidity may also be important in AmB resistance
in yeast. Resistance to AmB has also
been induced in L. tarentolae and two gene
amplification events have been observed and
characterized, although the link between these
amplicons and resistance is yet to be confirmed.
Some of the AmB-resistant mutants were crossresistant
to azole drugs such as ketoconazole,
which inhibits steps in ergosterol biosynthesis.
It is thus possible that Leishmania responds
to alterations in ergosterol biosynthesis by
Miltefosine
Miltefosine, an alkylphosphocholine, possesses
potent in vitro and in vivo anti-kinetoplastid
activity, and has proved highly effective in
visceral leishmaniasis clinical trials. Leishmania
have high levels of ether-lipids present
on the surface of the parasite. The glycosyl
phosphatidyl inositol-anchored glycolipids
and glycoproteins are perturbed in Leishmania
promastigotes put in contact with miltefosine.
Studies on the mode of action suggest that
miltefosine may inhibit the glycosomal enzyme
alkyl-specific-acyl-CoA acyltransferase involved
in ether-lipid remodeling. Alkyl-lysophospholipids
were initially developed as anti-cancer
agents, but resistance to these agents developed
readily against them. Possible resistance
mechanisms include reduced drug uptake,
differential plasma membrane permeability,
faster drug metabolism and efflux mediated
by the P-glycoprotein MDR1. Leishmania cells
selected for resistance to the anti-cancer agent
daunomycin overexpress their MDR1 gene.
These cells are cross-resistant to miltefosine,
and it is highly possible that this ABC transporter
efflux pump is involved in resistance to
this new promising anti-leishmanial agent.
In vitro work shows that it is relatively easy to
select miltefosine-resistant mutants, which may
suggest that miltefosine should be used in combination
with other anti-leishmanial agents to
reduce the emergence of resistant strains.
Resistance to other anti-leishmanial drugs
Leishmania cells are sensitive to a number of
aminoglycosides. It is this susceptibility which
MEDICAL APPLICATIONS