trans

DRUG RESISTANCE IN KINETOPLASTID PARASITES 421
half-life of the parasite enzyme. The higher
specific ODC activity and faster enzyme turnover
in T. b. rhodesiense compared to T. b. gambiense
has been suggested to contribute to the
intrinsic resistance of the East African species
to DFMO. Other differences in polyamine
biosynthesis were also suggested to contribute
to the higher intrinsic resistance of T. b. rhodesiense.
Field isolates of T. b. gambiense resistant
to eflornithine have not yet been reported, but
resistance was induced in vitro. In mutants
selected for DFMO resistance, the ODC activity
was shown to be similar between wild-type and
resistant trypanosomes, in contrast to Leishmania
in which the ODC gene was overexpressed
in DFMO-resistant mutants. Instead,
an important increase in the uptake of ornithine
was found in DFMO-resistant T. brucei. The high
level of the substrate ornithine was proposed
to compete with DFMO for ODC, leading to
sufficient polyamine biosynthesis (Figure 16.7).
Pentamidine
The isethionate salt of pentamidine is the drug
of choice in early-stage Gambian HAT. The
cellular target of pentamidine in trypanosomes
is unknown, although its uptake is carriermediated.
As pentamidine accumulates in the
millimolar range in trypanosomes, it is likely
that it will interact with numerous targets.
Although MelB resistance seems to be on the
rise, very few cases of pentamidine-refractory
T. b. gambiense have been reported. Most of our
understanding of pentamidine resistance
mechanisms stems from work carried out in
vitro. Trypanosomes resistant to melaminophenyl
arsenicals are often cross-resistant to
pentamidine. Mutations in the P2 nucleoside
transporter lead to decreased uptake of both
melarsen oxide and pentamidine. The common
structural feature for recognition is likely to
reside in the melamine and benzamidine
moieties of the two classes of drugs. P2 (also
known as ASPT1, for adenosine-sensitive pentamidine
transporter) is not the only route by
which pentamidine can enter T. brucei. Indeed,
evidence for both a high-affinity and a lowaffinity
pentamidine transporter, biochemically
distinct from P2, has been provided. This
apparent multiplicity in routes of uptake may
provide an explanation for the low rate of
resistance to pentamidine. The analysis of
some pentamidine-resistant T. brucei mutants
showed that they accumulated pentamidine to
the same levels as the drug-sensitive parental
strain, and that the properties of the P2 transporter
were unaltered. Mechanisms other than
reduced uptake are therefore likely to contribute
to pentamidine resistance.
Suramin
Suramin is a dye-related drug, clinically effective
in the treatment of early stage T. b. rhodesiense
HAT but with an unknown mechanism
of action. It contains six negative charges at
physiological pH, which renders its diffusion
across membranes difficult. It appears that
suramin enters the cell while bound to low
density lipoprotein (LDL) through receptormediated
endocytosis (Figure 16.7). Resistance
to suramin is rare and usually not a problem in
clinical practice. Suramin resistance can be
induced in mice by administration of subcurative
doses of the drug, and the resistance
phenotype appeared stable. The resistance
mechanisms have not been investigated, but
suramin-resistant cells are not cross-resistant
to arsenicals and diamidines.
Resistance to other classes of drugs
Only four drugs are available against HAT, and
since there are no trypanocidal drugs in phases
I–III of clinical development, it is unlikely that
new chemotherapeutic agents will arise in the
MEDICAL APPLICATIONS