Maduramicin Rapidly Eliminates Malaria Parasites and Potentiates the Gametocytocidal Activity of the Pyrazoleamide PA21A050

Abstract

c New strategies targeting Plasmodium falciparum gametocytes, the sexual-stage parasites that are responsible for malaria transmission, are needed to eradicate this disease. Most commonly used antimalarials are ineffective against P. falciparum gametocytes, allowing patients to continue to be infectious for over a week after asexual parasite clearance. A recent screen for gametocytocidal compounds demonstrated that the carboxylic polyether ionophore maduramicin is active at low nanomolar concentrations against P. falciparum sexual stages. In this study, we showed that maduramicin has an EC 50 (effective concentration that inhibits the signal by 50%) of 14.8 nM against late-stage gametocytes and significantly blocks in vivo transmission in a mouse model of malaria transmission. In contrast to other reported gametocytocidal agents, maduramicin acts rapidly in vitro, eliminating gametocytes and asexual schizonts in less than 12 h without affecting uninfected red blood cells (RBCs). Ring stage parasites are cleared by 24 h. Within an hour of drug treatment, 40% of the normally crescent-shaped gametocytes round up and become spherical. The number of round gametocytes increases to >60% by 2 h, even before a change in membrane potential as monitored by MitoProbe DiIC1 (5) is detectable. Maduramicin is not preferentially taken up by gametocyte-infected RBCs compared to uninfected RBCs, suggesting that gametocytes are more sensitive to alterations in cation concentration than RBCs. Moreover, the addition of 15.6 nM maduramicin enhanced the gametocytocidal activity of the pyrazoleamide PA21A050, which is a promising new antimalarial candidate associated with an increase in intracellular Na ؉ concentration that is proposed to be due to inhibition of PfATP4, a putative Na ؉ pump. These results underscore the importance of cation homeostasis in sexual as well as asexual intraerythrocytic-stage P. falciparum parasites and the potential of targeting this pathway for drug development. R eports of decreasing sensitivity to artemisinin in Southeast Asia provide a sense of urgency to the development of novel antimalarial compounds (1). Although both gametocytes and asexual parasites reside within the red blood cell (RBC), their physiologies differ, resulting in various sensitivities to common antimalarial drugs (2). After RBC invasion, asexual parasites undergo four or five rounds of DNA replication, resulting in 16 to 32 new parasites every 48 h. In contrast, gametocytes do not replicate in the human host. Instead, after RBC invasion, sexual differentiation of Plasmodium falciparum progresses through five stages of development over the course of 10 to 12 days, resulting in a single female or male gametocyte (3). Once taken up in a blood meal by a mosquito, gametocytes are stimulated to emerge from the RBC and develop into female and male gametes (4). After fertilization, the zygote differentiates into an ookinete that migrates out of the mosquito midgut, where it forms an oocyst that produces tens of thousands of sporozoites that...

Na+ Influx Induced by New Antimalarials Causes Rapid Alterations in the Cholesterol Content and Morphology of Plasmodium falciparum

“…Thus, the effects we describe here could potentially be due to an increase in cytosolic pH. We assessed this possibility by treating trophozoite stage parasites with maduramicin, a Na + ionophore anti-coccidial drug (structure in the inset of Fig 5A ), that has recently been shown to have potent antimalarial activity and synergy with PA21A050 [ 23 ]. As an ionophore, maduramicin is not likely to alter cytosolic pH, and thus effects of Na + influx could be assessed independent of the pH change.…”

Anti-parasitic activity of polyether ionophores

“…Medicines for Malaria Venture suggests to focus on molecules which are active against both asexual and sexual life-cycle stages of malaria parasites[90,91]. Although most commonly used anti-plasmodial agents are ineffective against P. falciparum gametocytes (the sexual stages of the parasites responsible for transmission to the Anopheles vector), the carboxylic ionophores maduramicin and narasin(Figure 1)have been found to display potent gametocytocidal activity in low nanomolar concentrations[92]. Furthermore, maduramicin was shown to block completely malaria transmission in an in vivo rodent malaria model.…”

“…Furthermore, maduramicin was shown to block completely malaria transmission in an in vivo rodent malaria model. Moreover, in combination with the pyrazolemaide PA21A050 (another promising antimalarial drug candidate), the gametocytocidal activity of maduramicin was significantly potentiated[92]. Encouragingly, the monovalent carboxylic ionophores monensin, nigericin and salinomycin(Figure 1)have been shown to kill efficiently both P. falciparum asexual parasites and gametocytes with IC50 values in the nanomolar concentration range, and have been therefore identified as potent transmission-blocking agents[90].…”

Natural Products: A Potential Source of Malaria Transmission Blocking Drugs?

“…This ionophore is fast acting, reducing late-stage gametocyte viability by >90% 12 h post treatment, with morphological changes evident even 1 h after drug exposure. This is similarly reflected in in vivo transmission-blocking activity where oocyst development was significantly blocked by maduramicin following exposure of gametocytes to drug for only 90 min prior to mosquito feed [48].…”