trans

CLINICAL IMPLICATIONS AND OUTLOOK 429
TABLE 16.1 Genes implicated in drug resistance in parasites for which DNA-based
tests have been developed
Parasite Genes Drugs
Plasmodium falciparum PfCRT, Pfmdr1 Chloroquine
Plasmodium falciparum DHFR, DHPS Antifolates and sulfonamides
Trypanosoma brucei TbTA1 (P2) Melarsoprol
Haemonchus contortus TUB Benzimidazole
can help in finding strategies to increase the
efficacy or the life span of the few drugs available.
From reading the various sections of this
chapter, it is clear that considerable progress
has been made in recent years in our understanding
of drug resistance in parasites. The
availability of gene transfection for most parasites
has facilitated the testing of putative
resistance genes in sensitive parasites allowing
an unambiguous reconstruction of resistance
mechanisms. With genomes of parasites rapidly
finding their place in databanks, the identification
of genes involved in drug resistance will
be simplified, and will become a driving force
in the improvement of the use of existing drug
combinations and in the development of new
drugs. We anticipate that transcriptomic (DNA
microarrays) and proteomic approaches will
accelerate the pace by which we will understand
the mode of action of drugs and their
resistance mechanisms.
The first obvious applied outcome of understanding
resistance mechanisms is to develop
tools and assays for rapidly detecting resistance
in clinical isolates. Clinical studies will first be
required to test whether a resistance genotype
correlates with the clinical response of the
infected patient under drug treatment. In the
case of a positive correlation, a rapid DNAbased
diagnostic test can have tremendous
impact for treatment. Indeed, in the case of
a resistance genotype, useless and toxic drug
formulations would not be used, and alternative
treatments would be sought early during
infection. Where resistance is mediated by
numerous genes or mutations it could be
possible to use low-density DNA microarrays,
whereas if that resistance is mediated by a
change in gene expression, real-time PCR
assays could prove practical.
The monitoring of drug resistance by the
use of DNA-based diagnostic procedures is at
present limited by our lack of knowledge about
the molecular and biochemical mechanisms
of action and resistance of several of the antiparasitic
agents. There are nonetheless notable
exceptions for which molecular markers are
now available to detect some forms of resistance
occurring in field isolates (Table 16.1). The
list is still short, and it is clear that for chloroquine
and melarsoprol, resistance genes other
than those listed in Table 16.1 are implicated
in resistance. Thus, to be useful for therapeutic
interventions, all the possible resistance mechanisms
should be targeted in diagnostic assays.
Once these tests are available and validated,
and if they can be used in areas where the diseases
are endemic, they would have the potential
to decrease dramatically treatment failure
and human suffering.
These DNA markers and diagnostic assays
could also be used to monitor the pool of resistant
parasites in diverse geographical regions,
and to carry out large epidemiological studies.
The high prevalence of antibiotic resistance in
bacteria, and our relatively good understanding
of several of the genes implicated in antibiotic
resistance, have mobilized international
MEDICAL APPLICATIONS