salinomycin and Malaria

Human malaria infection begins with a one-time asymptomatic liver stage followed by a cyclic symptomatic blood stage. All high-throughput malaria drug discovery efforts have focused on the cyclic blood stage, which has limited potential for the prophylaxis, transmission blocking, and eradication efforts that will be needed in the future. To address these unmet needs, a high-throughput phenotypic liver-stage Plasmodium parasite screen was developed to systematically identify molecules with liver-stage efficacy. The screen recapitulates liver-stage infection by isolating luciferase-expressing Plasmodium berghei parasites directly from the salivary glands of infected mosquitoes, adding them to confluent human liver cells in 384-well plates, and measuring luciferase activity after a suitable incubation period. Screening 5,375 known bioactive compounds identified 37 liver-stage malaria inhibitors with diverse modes of action, as shown by inhibition time course experiments. Further analysis of the hits in the Food and Drug Administration-approved drug subset revealed compounds that seem to act specifically on the liver stage of infection, suggesting that this phase of the parasite's life cycle presents a promising area for new drug discovery. Notably, many active compounds in this screen have molecular structures and putative targets distinctly different from those of known antimalarial agents.

Topics: Animals; Anopheles; Antimalarials; Drug Evaluation, Preclinical; Hep G2 Cells; Humans; Inhibitory Concentration 50; Insect Vectors; Life Cycle Stages; Liver; Malaria; Malaria, Falciparum; Male; Mice; Mice, Inbred C57BL; Plasmodium berghei; Plasmodium falciparum; Treatment Outcome

Salinomycin-Na and lasalocid-Na, two ionophorous antibiotics known for their anticoccidial activity, exhibit in vivo blood schizontocidal action on the Plasmodium berghei Keyberg 173 RC/M line that has a high level of resistance to chloroquine and mepacrine. Salinomycin was found to have a greater effect than lasalocid on asexual stages of this line. Trophozoites and schizonts were no longer found after a single dose of 20 mg/kg or five doses of 1.25 mg/kg of salinomycin whereas a single dose of 40 mg/kg or five doses of 20 mg/kg of lasalocid showed no marked effect on parasitaemia within 96 h of starting treatment in rats. Some toxicological data show that lasalocid, however, is better tolerated in domestic animals than salinomycin. Early morphological changes in asexual blood stages were membrane-coiling in the cytoplasm followed by vacuolization and disruption of the cell membrane or pellicle after treatment with both compounds. In particular mature schizonts were totally destroyed showing enormously large vacuoles. Toxicological data and blood schizontocidal activity indicate the narrow safety margin in P. berghei infected rats, and place salinomycin in the 'markedly toxic' group of antimalarial compounds.

Topics: Animals; Anti-Bacterial Agents; Cell Membrane; Chloroquine; Cytoplasm; Drug Resistance, Microbial; Lasalocid; Lethal Dose 50; Malaria; Male; Microscopy, Electron; Plasmodium berghei; Pyrans; Quinacrine; Rats; Rats, Inbred Strains; Time Factors

In vitro exposure of Plasmodium falciparum and P. berghei to salinomycin-Na showed that 10 min incubation in RPMI 1640 medium containing 100 micrograms/ml of the polyether antibiotic led to complete destruction of most parasites; in media containing 10 or 1 microgram/ml salinomycin-Na some young developmental stages seemed to survive, apparently due to the protection of the mostly intact host cell. In vitro treatment of rats infected with P. berghei revealed that a single subcutaneous (oral) dose of 20 mg/kg (80 mg/kg salinomycin-Na caused complete destruction of parasites. Incipient degeneration of the parasites could be already observed 1 h after treatment. After 22 h parasites had disappeared from blood smears. Repeated subcutaneous doses of the polyether as low as 5 mg/kg (X3) completely destroyed the asexual stages of P. berghei. During the ultrastructural investigation of the action of salinomycin-Na it was found that initially the inner lacunes (such as endoplasmic reticulum and perinuclear space) and the mitochondrion were markedly swollen. This was followed by mitochondrial disruption with rupture of the parasites' pellicle. Since the infected host cells also ruptured, mainly extracellular parasites were seen in blood smears beginning 6 h after treatment. Salinomycin-Na seems to act similarly on the malarial parasites and on the free merozoites of chicken intestinal coccidia.

Topics: Animals; Coccidiostats; Erythrocytes; Female; Malaria; Male; Mice; Mice, Inbred Strains; Microscopy, Electron; Plasmodium berghei; Plasmodium falciparum; Pyrans