Chloroquine uptake and activity is determined by binding to ferriprotoporphyrin IX in Plasmodium falciparum.

The selective antimalarial activity of chloroquine and related compounds stems from the extensive saturable uptake of these drugs into malaria parasites. Chloroquine resistant strains of Plasmodium falciparum have evolved a mechanism to reduce the saturable uptake. The molecular mechanism of saturable chloroquine uptake is controversial and attention is currently focused on mutually exclusive models of active chloroquine uptake and intracellular chloroquine binding. We sum up recent evidence which conclusively proves that the saturable accumulation of chloroquine is due to intracellular binding to ferriprotoporphyrin IX rather than active transport into the parasite via the sodium/hydrogen exchanger. We discuss recent findings that the affinity of chloroquine binding to ferriprotoporphyrin IX is reduced in resistant parasites. The mechanism responsible for reduced binding affinity can be overcome by verapamil and various lysosomotropic agents, and is thought to be the basis of chloroquine resistance.