trans

422 DRUG RESISTANCE
near future. The challenge of MelB-resistant
HAT has led to the empirical use of drug
combinations, and some case reports show
successful treatment of refractory parasites.
For example, a number of nitroheterocyclecontaining
drugs like nifurtimox (see below)
and metronidazole (see below) have been used
in combination with a number of classical antitrypanosome
drugs. Megazol is an experimental
5-nitroimidazole drug, which was shown to
partially cross the blood–brain barrier, and was
effective in curing chronic experimental infections
in primates. However, megazol is mutagenic
in the Ames test, so other non-mutagenic
analogs need to be identified. Megazol
appears to enter cells by simple diffusion. Once
inside the parasite, megazol and related molecules
need to be activated by reduction of the
nitro group. Change in the enzymatic reduction
of megazol leading to a loss in its activation
would potentially lead to drug resistance.
Drug resistance is also a major problem in the
treatment of veterinary trypanosomiasis. Treatment
relies mostly on isometamidium chloride
(Samorin) and diminazene aceturate (Berenil).
Isometamidium, an ethidium-containing compound,
crosses the plasma membrane, possibly
by facilitated diffusion, and is accumulated in
the mitochondria using the mitochondrial electrical
potential as the driving force. Changes
in the mitochondrial electrical potential
have been demonstrated in isometamidiumresistant
trypanosomes. It is thus possible that
isometamidium resistance is linked to a reduction
in mitochondrial accumulation, where
possibly its target is located (Figure 16.7).
Berenil is a diamidine, structurally related to
pentamidine. Its mode of action is unknown
and the P2 transporter seems to be the main
route of entry. A loss of P2 could correspond to
one mechanism by which parasites become
resistant. This has not been tested, however, in
parasites directly isolated from animals.
T. cruzi and chemotherapy
The protozoan kinetoplastid parasite Trypanosoma
cruzi is the cause of Chagas disease.
It is endemic from Northern Mexico to
Argentina with an estimated 16 to 18 million
people in Latin America infected. Considerable
progress has been made in vector control (vectorial
transmission is now eradicated in some
countries) but several million people are still
infected. The parasite invades a variety of cells
including muscle and nerve cells of the heart
and the gastrointestinal tract, leading to potentially
fatal cardiomyopathy and gastrointestinal
tract lesions. There is no prospect for a vaccine
in the near future and chemotherapy, as with
several other parasitic diseases, is unsatisfactory
and relies on nifurtimox and benznidazole.
These molecules are effective in acute cases,
but are not effective against the chronic form
of the disease. Both drugs are toxic and their
efficacy varies, probably as a consequence
of variation among parasite strains. Indeed,
the efficacy of nitrofurans and nitroimidazoles
varies greatly across Latin America, and this
appears to be due to biological differences
between the T. cruzi strains, some of which are
naturally refractory to the existing drugs. More
recently, azole antimicrobial agents were shown
to have excellent activity in infected mice with
both the acute and the chronic diseases. Work
in vitro suggests, however, that resistance
against azoles can develop rapidly in T. cruzi.
Benznidazole
Benznidazole is a nitroimidazole, and its mode
of action is unclear. It has been suggested that
it interferes with the synthesis of T. cruzi
macromolecules, including RNA and protein.
How the latter is achieved, however, has not yet
been elucidated. Benznidazole also has numerous
effects on host cells, and it is possible that
MEDICAL APPLICATIONS