Sitamaquine as a putative antileishmanial drug candidate: from the mechanism of action to the risk of drug resistance

Sitamaquine is a 8-aminoquinoline in development for the treatment of visceral leishmaniasis by oral route, no activity being observed on the experimental cutaneous leishmaniasis experimental models. Recent data explain how sitamaquine accumulate in Leishmania parasites, however its molecular targets remain to be identified. An advantage of sitamaquine is its short elimination half-life, preventing a rapid resistance emergence. The antileishmanial action of its metabolites is not known. The selection of a sitamaquine-resistant clone of L. donovani in laboratory and the phase II clinical trials pointing out some adverse effects such as methemoglobinemia and nephrotoxicity are considered for a further development decision.

Sitamaquine is a 8-aminoquinoline in development for the treatment of visceral leishmaniasis by oral route, no activity being observed on the experimental cutaneous leishmaniasis experimental models. Recent data explain how sitamaquine accumulate in Leishmania parasites, however its molecular targets remain to be identified. An advantage of sitamaquine is its short elimination half-life, preventing a rapid resistance emergence. The antileishmanial action of its metabolites is not known. The selection of a sitamaquine-resistant clone of L. donovani in laboratory and the phase II clinical trials pointing out some adverse effects such as methemoglobinemia and nephrotoxicity are considered for a further development decision.

INTRODUCTION
L eishmaniases are tropical and sub-tropical diseases caused by the parasite protist belonging to the genus Leishmania. Two basic forms of leishmaniases occurs: i) visceral leishmaniasis (VL) or "Kala-azar" is caused by Leishmania donovani and Leishmania infantum (also known L. chagasi in South America), and ii) cutaneous leishmaniasis (CL) is caused by about 15 species of Leishmania, L. tropica (recidivan leishmaniasis) in the old world, and two possible forms in Latin America, diffuse CL (L. guyanensis, L. amazonensis) and mucocutaneous form with destruction of mouth mucosa, pharynx and facial tissue (L. braziliensis) (WHO, 2010). VL, the most severe form, is fatal without treatment. The leishmaniases are prevalent in about 88 countries: 350 million (M) people living in endemic aeras. The morbidity of about 12-14 M people and roughly 1.5-2 M new cases per year from whom 0.4-0.5 M for VL mainly in India, Nepal, Bengladesh, Brazil and Sudan (WHO, 2010). The global mortality is about 60,000 people (Desjeux, 2004). Conventional treatments include antimonial drugs (Glucantime ® and Pentostam ® ), amphotericin B and its liposomal formulation (AmBisome ® ) which are used by parenteral route. A phosphorylcholine ester of hexadecanol, designated as miltefosine, originally developed as an anticancer drug (Muschiol et al., 1987) was shown to be the first oral drug against visceral (Jha et al.,1999, Sundar et al., 2002 and cutaneous (Soto et al., 2004;Sinderman & Engel, 2006, Soto & Toledo, 2007. However, it can be noticed that miltefosine (Impavido ® ) possesses a long half-life able to generate resistant Leishmania isolates and exhibits contraindication in pregnancy because of adverse effects (Jha et al., 1999, Soto & Berman, 2006. Despite these limitations, miltefosine is now success-fully proposed in combination with AmBisome ® in order to prevent drug resistance to both the drugs (Sundar et al., 2008). Anyway, new compounds active by oral route should be developed in case of failure of this AmBisome ®miltefosine bitherapy in the future. Sitamaquine (WR-6026) is a 8-aminoquinoline analog ( Fig. 1) discovered Review Parasite, 2011, 18, 115-119 by the Walter Reed Army Institute of Research (WRAIR, USA) and in development with GlaxoSmithKline (UK) for the oral treatment of VL. Sitamaquine was first synthesized as part of the collaborative antimalarial program in the US (1944)(1945)(1946)(1947)(1948)(1949)(1950)) that led to primaquine (Elderfield et al., 1955). Sitamaquine is an orally active drug and appears as promising agent for treatment of VL both in India (Jha et al., 2005) and Africa (Wasunna et al., 2005).

IN VITRO AND IN VIVO SITAMAQUINE ACTIVITIES ON LEISHMANIASIS MODELS
R ecent in vitro parasite evaluation confirmed the antileishmanial properties of sitamaquine dihydrochloride against a range of Leishmania species responsible for either cutaneous or visceral leishmaniasis, with ED50 values against amastigotes in a range from 2.9 to 19.0 microM (Garnier et al., 2006). In fact, the antileishmanial activity of 8-aminoquinoline was revealed more than fifty years ago when a series of 6-methoxy-8-alkylpiperazinoalkylaminoquinoline derivatives were shown to exhibit both a higher activity than pentavalent antimonials and oral availability against L. donovani in the hamster model (Beveridge et al., 1958). Later, a series of 4-methyl-6-methoxy-8aminoquinolines called lepidines was shown to be several hundred times more active than pentavalent antimonials in a rodent model (Kinnamon et al., 1978). Structure-activity relationships of methoxy-and hydroxy-substituted compounds were further investigated in a L. tropica-macrophage model in vitro . Among them, primaquine exhibited a noteable high activity and 8- [[6-(diethylamino) hexyl]amino]-6-methoxy-4-methylquinoline or WR6026, now called sitamaquine was 708 times more active than meglumine antimoniate (Glucantime ® ) against L. donovani in hamsters (Kinnamon et al., 1978). On L. major cutaneous lesions in BALB/c mice, different topical sitamaquine dihydrochloride formulations using topically acceptable excipients were evaluated in vivo without success since no reduction of the parasite burden and lesion progression was observed (Garnier et al., 2006).

MECHANISM OF ACTION
A lthough the sitamaquine effects on the parasite have been visualized via alterations in their morphology (Langreth et al., 1983), the molecular targets of sitamaquine are still unknown. However, the sequential steps of interactions of sitamaquine with parasites are now well documented. Sitamaquine entry into Leishmania does not involve a transporter (López-Martín et al., 2008). As a lipophilic weak base, the sitamaquine accumulation into Leishmania promastigotes occurs along an electrical gradient involving two steps: first, the positively charged sitamaquine interacts with the anionic polar head groups of membrane phospholipids, and second, the sitamaquine insertion into the parasite plasma membranes results of a subsequent hydrophobic interaction between acyl chains of phospholipids and the hydrophobic quinoline ring leading to a deeper insertion of the drug into the lipid monolayer (Dueñas-Romero et al., 2007). This process is energy-and sterol-independent (Coïmbra et al., 2010). However, this affinity of sitamaquine for membranes is transitory since the main sitamaquine location was found into the cytosol (Coïmbra et al., 2010). In contrast, an energy-dependent efflux has been evidenced but the nature of the protein involved in this efflux remains to be elucidated (Coïmbra et al., 2010). NMR study of motile lipids showed that sitamaquine does not affect lipid trafficking in Leishmania (Coïmbra et al., 2010). Once internalized, sitamaquine rapidly accumulates into acidic compartments, mainly acidocalcisomes [acid vacuoles containing most of the cellular calcium] (López-Martín et al., 2008). This accumulation in acidocalcisomes allows to their alkalization (Vercesi et al., 2000). A rapid collapse of the mitochondrial innermembrane potential was also observed (Vercesi et al., 1992). However, the antileishmanial action of sitamaquine is not related to its level of accumulation in acidocalcisomes (López-Martín et al., 2008). Proteomic analysis are running now to identify the sitamaquine targets. BIOAVAILABILITY P harmacokinetics data in humans showed that sitamaquine has a short elimination half-life (about 26 hr) in contrast to miltefosine half-life (150-200 hr) (Theoharides et al., 1987). The metabolism of sitamaquine was studied in a rat hepatic microsomal system (Theoharides et al., 1985). Two metabolites were found: 8(-6-ethylaminohexylamino)-6-methoxy-lepidine and 8(-6-diethylaminohexylamino)-6-methoxy-4-hydroxy-methyl-quinoline (Yeates, 2002). The formation of both metabolites was NADPH-dependent. The formation of both metabolites seems to be

CLINICAL TRIALS ON VISCERAL LEISHMANIASIS
F irst phase II assays performed in Kenya on 16 patients were encouraging enough to be continued further (Sherwood et al., 1994). In phase II assays in India with 120 VL patients (Jha et al., 2005), and in Kenya with 95 VL patients (Wasunna et al., 2005), sitamaquine was well tolerated with the doses ranging from 1.5 to 3 mg/mg/day, with vomiting and abdominal pains (about 10 %), headache (also about 10 %). Cyanosis (3 %) in India was reported to be due to methemoglobinemia, a recognized side effect of 8-aminoquinolines for individuals with glucose-6phosphate deshydrogenase (G6PD) deficiency (Jha et al., 2005). Methemoglobinemia was not reported in the Kenyan study (Wasunna et al., 2005). Renal adverse effects (nephritic syndrome 3 % and glomerulonephritis 2 % in India) were observed only for  (Bories et al., 2008)

RISK OF DRUG RESISTANCE
T he short elimination half-life of sitamaquine in mammals is in favour of a low probability of resistance emergence. However, in order to evaluate the risk of sitamaquine resistance in the field, a L. donovani promastigote line resistant to 160 µM sitamaquine was selected by in vitro drug pressure and some charachetristics of this resistant line were studied (Bories et al., 2008). The resistant line was infective for murine peritoneal macrophages in vitro as its parent wild-type line but less infective for Balb/C mice, suggesting that a low transmission of resistant parasites could occur in the field. The sitamaquine IC 50 on the resistant line was about five and three times higher than those of the wild-type line on promastigote and intramacrophage amastigote forms, respectively. No cross-resistance with other antileishmanial agents was observed, allowing to use another antileishmanial drug in case of sitamaquine resistance. However, this resistance was stable when parasites were subcultured in drug-free medium for a long time or after in vivo passage, suggesting that a maintenance at a constant level in the parasite populations. These considerations, apparently speculative, could be indicative from an epidemiological point of view.

CONCLUSION
F ew chemical series reach the clinical development in the field of leishmaniasis because the antileishmanial screening and toxicity bottlenecks are selective. Sitamaquine is the second orally active antileishmanial drug after miltefosine. The development of sitamaquine is slow because time is needed to ensure the safety of the drug, mainly at the level of methemoglobinemia and nephrotoxicity. Recent data show that resistance is at risk. However, the level of resistance obtained by in vitro drug pressure corresponds to a loss of susceptibility of 5-fold, that would be compatible with higher dosages if sitamaquine was not toxic. The major advantages of sitamaquine are its administration route and pharmacokinetics characteristics. Thus, its bioavailability is better than those of miltefosine. From all these data gathered in Table I, it is now probable that GSK company, the developer, will take a decision concerning the marketing of sitamaquine in the next future.