Kinetic Driver of Antibacterial Drugs against Plasmodium falciparum and Implications for Clinical Dosing

Modified dosing schedule efficacy of fosmidomycin and clindamycin against murine malaria Plasmodium berghei

Fosmidomycin and clindamycin target the Plasmodium apicoplast. Combination clinical trials have produced mixed results with the primary problem being the recrudescent infection frequency by day 28. Given that antibiotic efficacy against bacterial infections often depends on the constant drug presence over several days, we hypothesized that the antimalarial blood or liver stage efficacy of fosmidomycin and clindamycin could be improved by implementing a more frequent dosing schedule. A blood stage murine malaria P. berghei GFP-luciferase low and high parasitemia model was implemented to follow pharmacodynamics and cure for modified dose, schedule and duration of individual and combination fosmidomycin and clindamycin. P. berghei sporozoites were used to investigate fosmidomycin during the 48 h murine liver stage. Here we observed that the same total dose of fosmidomycin and clindamycin, alone and in combination, are more efficacious when scheduled in smaller, more frequent doses. Fosmidomycin added measurably small additional killing in combination with clindamycin. Despite dosing every 6 h during liver stages, fosmidomycin was inhibitory, but noncurative even with addition of atorvastatin to decrease hepatocyte production of mevalonate. We have also demonstrated in vitro efficacy of fosmidomycin and clindamycin against P. falciparum C580Y with IC50s similar to those for drug sensitive P. falciparum. The dosing schedule of quinoline and artemisinin partner drugs fosmidomycin or clindamycin targeting the apicoplast should maximize time above minimum inhibitory concentration.

Clinically relevant atovaquone-resistant human malaria parasites fail to transmit by mosquito

Long-acting injectable medications, such as atovaquone, offer the prospect of a "chemical vaccine" for malaria, combining drug efficacy with vaccine durability. However, selection and transmission of drug-resistant parasites is of concern. Laboratory studies have indicated that atovaquone resistance disadvantages parasites in mosquitoes, but lack of data on clinically relevant Plasmodium falciparum has hampered integration of these variable findings into drug development decisions. Here we generate atovaquone-resistant parasites that differ from wild type parent by only a Y268S mutation in cytochrome b, a modification associated with atovaquone treatment failure in humans. Relative to wild type, Y268S parasites evidence multiple defects, most marked in their development in mosquitoes, whether from Southeast Asia (Anopheles stephensi) or Africa (An. gambiae). Growth of asexual Y268S P. falciparum in human red cells is impaired, but parasite loss in the mosquito is progressive, from reduced gametocyte exflagellation, to smaller number and size of oocysts, and finally to absence of sporozoites. The Y268S mutant fails to transmit from mosquitoes to mice engrafted with human liver cells and erythrocytes. The severe-to-lethal fitness cost of clinically relevant atovaquone resistance to P. falciparum in the mosquito substantially lessens the likelihood of its transmission in the field.

Inhibitory compounds targeting Plasmodium falciparum Gyrase B

Malaria persists as a major health problem due to the spread of drug resistance and the lack of effective vaccines. DNA gyrase is a well-validated and extremely effective therapeutic target in bacteria, and it is also known to be present in the apicoplast of malarial species, including Plasmodium falciparum. This raises the possibility that it could be a useful target for novel antimalarials. To date, characterization and screening of this gyrase have been hampered by difficulties in cloning and purification of the GyrA subunit, which is necessary together with GyrB for reconstitution of the holoenzyme. To overcome this, we employed a library of compounds with specificity for P. falciparum GyrB and assessed them in activity tests utilizing P. falciparum GyrB together with Escherichia coli GyrA to reconstitute a functional hybrid enzyme. Two inhibitory compounds were identified that preferentially inhibited the supercoiling activity of the hybrid enzyme over the E. coli enzyme. Of these, purpurogallin (PPG) was found to disrupt DNA binding to the hybrid gyrase complex and thus reduce the DNA-induced ATP hydrolysis of the enzyme. Binding studies indicated that PPG showed higher-affinity binding to P. falciparum GyrB than to the E. coli protein. We suggest that PPG achieves its inhibitory effect on gyrase through interaction with P. falciparum GyrB leading to disruption of DNA binding and, consequently, reduction of DNA-induced ATPase activity. The compound also showed an inhibitory effect against the malaria parasite in vitro and may be of interest for further development as an antimalarial agent.