Transport and pharmacodynamics of albitiazolium, an antimalarial drug candidate

Abstract

BACKGROUND AND PURPOSE Choline analogues, a new type of antimalarials, exert potent in vitro and in vivo antimalarial activity. This has given rise to albitiazolium, which is currently in phase II clinical trials to cure severe malaria. Here we dissected its mechanism of action step by step from choline entry into the infected erythrocyte to its effect on phosphatidylcholine (PC) biosynthesis. EXPERIMENTAL APPROACH We biochemically unravelled the transport and enzymatic steps that mediate de novo synthesis of PC and elucidated how albitiazolium enters the intracellular parasites and affects the PC biosynthesis. KEY RESULTS Choline entry into Plasmodium falciparum‐infected erythrocytes is achieved both by the remnant erythrocyte choline carrier and by parasite‐induced new permeability pathways (NPP), while parasite entry involves a poly‐specific cation transporter. Albitiazolium specifically prevented choline incorporation into its end‐product PC, and its antimalarial activity was strongly antagonized by choline. Albitiazolium entered the infected erythrocyte mainly via a furosemide‐sensitive NPP and was transported into the parasite by a poly‐specific cation carrier. Albitiazolium competitively inhibited choline entry via the parasite‐derived cation transporter and also, at a much higher concentration, affected each of the three enzymes conducting de novo synthesis of PC. CONCLUSIONS AND IMPLICATIONS Inhibition of choline entry into the parasite appears to be the primary mechanism by which albitiazolium exerts its potent antimalarial effect. However, the pharmacological response to albitiazolium involves molecular interactions with different steps of the de novo PC biosynthesis pathway, which would help to delay the development of resistance to this drug.

New Insight into the Mechanism of Accumulation and Intraerythrocytic Compartmentation of Albitiazolium, a New Type of Antimalarial

“…At physiological pH, most of the ammonium is in the NH 4 ϩ form and thus requires transport proteins for membrane passage. As for other inorganic or organic cations (choline, tetramethylammonium, or tetrapropylammonium) (18,36,37) and for albitiazolium, (18), NH 4 ϩ ion transport should be mediated by the new permeability pathways (NPP) and thus at least partly inhibits albitiazolium entry. Alternatively, presence of NH 4 ϩ ions in the host cytosol could change pump activities and thus ionic gradients, altering the transports dependent on these gradients and leading to inhibition of albitiazolium entry into IRBC.…”

“…2C) within the first 15 min of incubation in drug-free medium likely corresponded to the release of the minor portion of drug contained in the erythrocyte cytosol of IRBC, which might have been released or reexported into the external medium via parasite-induced NPP in the erythrocyte membrane (37). We recently showed that albitiazolium enters IRBC mainly through these NPP (18), which can transport molecules bidirectionally (42,43).…”

“…This massive, irreversible, and energy-dependent accumulation allowed albitiazolium to exert an antiplasmodial and antibabesial activity at a low nanomolar concentration, resulting in a high micromolar intraparasitic concentration, while being weakly toxic to mammalian cells (17). Considering that applying drug pressure of a closely related choline analogue for 9 months to a P. falciparum blood culture (47) did not lead to the appearance of drug-resistant parasites (unpublished data), it is therefore likely that this intraparasitic compartmentation underlies a dual mechanism of action (18,20,21) that thus makes this new type of antimalarial agent resistant to parasite resistance.…”

“…In this setting, the inhibition of phospholipid biosynthesis has been proposed as a novel therapeutic strategy (11)(12)(13). The most advanced approach is based on the use of choline analogues (12,(14)(15)(16)(17), whose primary interference has been associated with blocking the parasite choline carrier, thus preventing phosphatidylcholine biosynthesis (13,18). Bis-thiazolium salts exert a rapid cytotoxic effect against all P. falciparum blood stages in vitro (17,19) and cure in vivo malaria infections without recrudescence at very low doses for rodent malaria (Ͻ1 mg/kg of body weight delivered intraperitoneally) and primate malaria (17).…”

High Accumulation and In Vivo Recycling of the New Antimalarial Albitiazolium Lead to Rapid Parasite Death

“…The bisthiazolium salts exert their antimalarial activity through inhibition of de novo PC biosynthesis by blocking the parasite choline carrier (10)(11)(12)). An additional interaction with heme inside the food vacuole contributes to the antimalarial activity (13).…”