Deferoxamine: Stimulation of hematin polymerization and antagonism of its inhibition by chloroquine

The iron chelator deferoxamine enhances the clearance of Plasmodium falcipa rum parasitemia and may be useful in drug combinations for the treatment of cerebral malaria. However, the deferoxamine-chloroquine drug combination i s antagonistic, or at best additive, against P. falciparum in vitro. As chl oroquine is thought to exert its antimalarial activity by interacting with hematin released from the proteolytic degradation of hemoglobin in the para site food vacuole, we hypothesized that deferoxamine might interfere with t he ability of chloroquine to inhibit hematin polymerization, since it was r eported that deferoxamine interacts with hematin. Therefore, we assessed de feroxamine-hematin binding in more detail and investigated the effect of de feroxamine on hematin polymerization in the presence and absence of chloroq uine. Isothermal titration calorimetry (ITC) experiments demonstrated an en thalpy-driven deferoxamine:hematin mu-oxo dimer binding with an association constant of 2.8 x 10(4) M-1 at pH 6.5, a binding affinity 14-fold lower th an that measured for chloroquine. At least two of the three hydroxamic acid functional groups of deferoxamine must be unionized for effective binding. We also discovered that deferoxamine antagonized chloroquine-mediated inhi bition of hematin polymerization. Unexpectedly, deferoxamine increased the concentration of soluble forms of hematin and enhanced the rate of hematin polymerization. Deferoxamine also could initiate hematin polymerization. In contrast, chloroquine decreased the concentration of soluble forms of hema tin and inhibited hematin polymerization. This work supports the postulate that initiation of hematin polymerization requires a higher concentration o f soluble hematin monomer than does the elongation phase of polymerization and provides one possible explanation for the observed antagonism between d eferoxamine and chloroquine against parasites in culture. (C) 1999 Elsevier Science Inc.