Despite considerable therapeutic success with the antimalarial
4-aminoquinolines such as chloroquine, there is serious
doubt about the future of this drug class due mainly to the development
and
spread of parasite resistance throughout
endemic areas. In this article we review the possible biochemical and
molecular basis of resistance. Based on our current
understanding we have considered the possibility of developing strategies
which may allow the aminoquinolines to once
again be used effectively against P. falciparum. Our conclusions
are that drug resistance is the result of a reduced rate of
drug uptake which in turn reduces the amount of drug available to bind
the target. The basis for this reduced accumulation
could be an altered pH gradient making the food vacuole more alkaline or
the parasite cytosol more acidic, an efflux pump
removing drug directly from the membrane or any other process which will
reduce the rate of drug uptake. Central to the
effectiveness of this resistance mechanism is the transient availability
of a high affinity, low capacity drug binding site
(possibly haem) within the parasite. Resistance reversers such as verapamil
influence the apparent Ka for this drug binding
phenomenon via an increased drug uptake rate. We demonstrate that by
chemical modification of the aminoquinolines,
producing predictable alterations in their physicochemical properties,
that it
is possible to minimise the verapamil sensitive
component of resistance and reduce significantly cross-resistance patterns
without loss in absolute activity. Based on these
views we suggest that the aminoquinoline antimalarials still have a role
to
play in the cheap, safe and effective chemotherapy of falciparum malaria.