In vitro antimalarial activity of retinoids and the influence of selective retinoic acid receptor antagonists

Retinol (vitamin A alcohol) may have a beneficial role in the host response to malaria in humans and previously published data have suggested that it has a direct inhibitory effect on the growth of Plasmodium falciparum in vitro. To further investigate the role of retinoids as potential antimalarial agents, we assessed the effect of all-trans-retinoic acid (RA), 9-cis-RA and 13-cis-RA, as well as retinol itself and its ester, retinyl palmitate, on 3H-hypoxanthine uptake by the laboratory-adapted strains of P. falciparum 3D7 and K1. In addition, we examined the influence of three specific RA receptor antagonists, ER 27191, Ro 415253 and AGN 194301, on retinoid-induced growth inhibition of 3D7. All-trans-RA, 9-cis-RA and 13-cis-RA in concentrations ranging from 1×10−4 to 5×10−10 M each had antimalarial activity, but at IC50 values (5.9×10−5 to 7.9×10−5 M) that were less than those of retinol (2.5×10−5 to 3.2×10−5 M). Retinyl palmitate had minimal effect on 3H-hypoxanthine uptake. Each of the three specific antagonists inhibited growth of 3D7 (IC50 range 1.2×10−5 to 3.0×10−5 M) but, in isobolographic analysis, were antagonistic to retinol (dose factor potentiation, DFP 0.46–0.79) and, in the case of Ro 415253, to all-trans-RA (DFP=0.39). Although we did not attempt to quantify losses of retinoids from the system, these data suggest that retinol has greater antimalarial activity than its RA metabolites and especially retinyl palmitate. The specific RA receptor antagonists showed paradoxical antimalarial activity but consistently antagonised the effect of retinol and all-trans-RA in isobolographic experiments. We conclude that RA metabolites may be less suitable than retinol per se as antimalarial agents and that P. falciparum might possess or acquire a RA receptor-like moiety