trans

DRUG RESISTANCE IN KINETOPLASTID PARASITES 413
are possible. For example antifolates are not
useful against this parasite, but the sequence
of the C. parvum DHFR active site contains
novel residues at several positions analogous
to those at which point mutations have been
shown to produce antifolate resistance in other
parasite DHFRs.
DRUG RESISTANCE IN
KINETOPLASTID PARASITES
Leishmania and chemotherapy
Leishmania is a significant cause of morbidity
and mortality, with a disease prevalence of 12
million cases and a worldwide annual incidence
of 1.5–2 million cases. Leishmania manifestations
range, depending on the species,
from self-healing cutaneous lesions to lifethreatening
visceral infections. Although several
experimental approaches (killed parasites
BCG; subunit vaccines; DNA vaccines; attenuated
organisms) have been used as vaccination
strategies against Leishmania, chemotherapy
is still the only effective way to control all forms
of diseases. The first agents with a favorable
therapeutic index, the pentavalent antimonials
(SbV), were introduced in the 1940s, and despite
the need for prolonged parenteral administration
are still the mainstay of therapy for all
forms of leishmaniasis. Formulations available
include sodium stibogluconate (Pentostam)
in which antimony is complexed with gluconic
acid, and meglumine antimonate (Glucantime)
where antimony is complexed with the sugar
meglumine (Figure 16.5). Although antimonials
are likely to remain the pillar of anti-leishmanial
therapy for the foreseeable future, we
remain uncertain of the mechanism of action
or even the biologically active component of
the most commonly utilized formulations. Over
the years, in face of slowly developing clinical
resistance in the 1970s and 1980s, both the
daily dose of SbV and the duration of treatment
were increased to maintain satisfactory levels
of efficacy. Resistance to SbV is now endemic
with more than 50% resistance rate in the state
of Bihar, India. Undertreatment and noncompliance
are likely to have contributed to
SbV resistance, but in India anthroponotic
transmission is possibly a determining factor
in disseminating SbV resistance. Second-line
drugs against Leishmania infections include
pentamidine and lipid forms of amphotericin
B (AmB), as well as a number of other drugs with
more specific indications.
Pentavalent antimonials
Clinical unresponsiveness to SbV has been
attributed to several factors, but resistant parasites
have now been formally documented in
the laboratory and contribute to treatment
failure in several endemic regions. A strong
correlation between clinical response and SbV
sensitivity in vitro was observed only when
strains were assessed as intracellular amastigotes,
while no correlation with clinical response
was found using extracellular promastigotes.
This complicates the testing of resistance in
the laboratory, as measuring and evaluating
intracellular Leishmania is laborious. However,
recent advances using Leishmania expressing
reporter genes, such as firefly luciferase,
may facilitate susceptibility testing within
macrophages. It is also possible to grow amastigotes
in axenic culture, and these could possibly
be used for drug testing, although several
species are refractory to axenization and it is
possible that intracellular parasites will be
required for drug testing.
The rate of appearance of resistance to SbV
in the field has been slow, which suggests that
resistance, as discussed for chloroquine resistance
in Plasmodium, is the consequence of
MEDICAL APPLICATIONS