Interaction of ferriprotoporphyrin IX with the antimalarials amodiaquine and halofantrine

Light-absorption spectra in the range of 350 to 650 nm of ferriprotoporphyrin IX (ferriheme), both in the presence and absence of either amodiaquine or halofantrine, have been compared in aqueous solution (pH 7.4) at room temperature. In the presence of the drugs, a distinct absorption maximum in the region of 600 to 610 nm, a shoulder near 490 to 500 nm, a maximum around 393 nm and a shoulder near 370 nm have been recorded. The spectral characteristics observed differ significantly from those of free ferriheme under similar conditions and indicate complex formation between ferriheme and either amodiaquine or halofantrine. Analogous absorption spectra were also described previously for complexes of ferriheme with either chloroquine, quinine or quinidine. The present results are considered to be consistent with a receptor action of ferriheme suggested previously and therefore remove recent objections to this hypothesis.