Citalopram enhances the activity of chloroquine in resistant plasmodium in vitro and in vivo.

Citalopram, is an extremely potent inhibitor of neuronal serotonin reuptake. It is structurally unrelated to other antidepressants, but it contains the chemical features associated with reversal of drug resistance and exhibits minimal cardiotoxic side effects and fewer of the anticholinergic and adrenolytic side effects associated with other psychotropic agents. Sensitivity tests to citalopram alone and in combination with chloroquine were performed against chloroquine-resistant and chloroquine-sensitive strains of Plasmodium falciparum and Plasmodium chabaudi. Citalopram alone showed intrinsic activity against the chloroquine-resistant strains of P. falciparum (IC50 = 1.51 +/- .6 microM) but only limited activity against the chloroquine-sensitive strain (IC50 = 33.27 +/- 5.87 microM) and no activity in vivo. The interaction of chloroquine and citalopram in vitro resulted in a synergistic response in the chloroquine-resistant strain but there was no interaction between the drugs in the chloroquine-sensitive strain--a pattern found with other reversal agents. Citalopram enhanced chloroquine susceptibility in both strains of P. chabaudi, however, the potentiating effect was seen at lower doses in the chloroquine-resistant strain. The results of this study suggest that citalopram may have potential as a chemosensitizer in Plasmodium infections on the basis of the low toxicity of citalopram at concentrations potentiating chloroquine activity both in vitro and in vivo.