The leishmaniases

Pentavalent antimonials: new perspectives for old drugs

Pentavalent antimonials, including meglumine antimoniate and sodium stibogluconate, have been used for more than half a century in the therapy of the parasitic disease leishmaniasis. Even though antimonials are still the first-line drugs, they exhibit several limitations, including severe side effects, the need for daily parenteral administration and drug resistance. The molecular structure of antimonials, their metabolism and mechanism of action are still being investigated. Some recent studies suggest that pentavalent antimony acts as a prodrug that is converted to active and more toxic trivalent antimony. Other works support the direct involvement of pentavalent antimony. Recent data suggest that the biomolecules, thiols and ribonucleosides, may mediate the actions of these drugs. This review will summarize the progress to date on the chemistry and biochemistry of pentavalent antimony. It will also present the most recent works being done to improve antimonial chemotherapy. These works include the development of simple synthetic methods for pentavalent antimonials, liposome-based formulations for targeting the Leishmania parasites responsible for visceral leishmaniasis and cyclodextrin-based formulations to promote the oral delivery of antimony.

In vitro antileishmanial properties of tri- and pentavalent antimonial preparations

To better understand the antileishmanial effects of antimonial agents we synthesized complexes of tri- and pentavalent antimony with mannan. The 50% inhibitory concentrations (IC50s) of these agents, along with those of potassium antimony tartrate [Sb(III)] and sodium stibogluconate [Sb(V)], were determined for promastigotes and intramacrophage amastigotes. The trivalent antimonial agents were more potent than the pentavalent agents. Although the IC50s were 60- to more-than-600-fold higher for promastigotes than for amastigotes, similar intracellular antimony concentrations in both life forms were measured after incubation with all four drugs at their respective IC50s. Macrophages accumulated antimony during a 4-h exposure that was retained intracellularly for at least 3 days. Amastigotes inside macrophages had a higher antimony content 6 days after a single 4-h treatment than they did immediately after treatment, suggesting that macrophages serve as a reservoir for antimonial agents and prolong parasite exposure. Macrophages concentrated antimony from the medium with potassium antimony tartrate, trivalent antimony-mannan, and pentavalent antimony-mannan treatments. N-Acetylcysteine antagonized the antileishmanial effects of these three drugs against intracellular amastigotes; in contrast, it had minimal effects on the action of sodium stibogluconate.

Antileishmanial activity of sodium stibogluconate fractions

Sodium stibogluconate, a pentavalent antimony derivative produced by the reaction of stibonic and gluconic acids, is the drug of choice for the treatment of leishmaniasis. It has been reported to be a complex mixture rather than a single compound. We separated sodium stibogluconate into 12 fractions by anion-exchange chromatography. One fraction accounted for virtually all the leishmanicidal activity of the fractionated material against Leishmania panamensis promastigotes, with a 50% inhibitory concentration (IC50) of 12 micrograms of Sb per ml; that of unfractionated sodium stibogluconate was 154 micrograms of Sb per ml. Further analysis of this active fraction revealed that a major component was m-chlorocresol, which had been included in the sodium stibogluconate formulation as a preservative. The IC50 of pure m-chlorocresol was 1.6 micrograms/ml, a concentration equivalent to that present in unfractionated sodium stibogluconate at a concentration of 160 micrograms of Sb per ml. After ether extraction to remove m-chlorocresol, the IC50 of sodium stibogluconate was > 4,000 micrograms of Sb per ml. In contrast, when L. panamensis amastigotes were grown in macrophages, the IC50 of ether-extracted sodium stibogluconate was 10.3 micrograms of Sb per ml. The 12 fractions of ether-extracted sodium stibogluconate obtained by anion-exchange chromatography had IC50s of 10.1 to 15.4 micrograms of Sb per ml. We conclude that preservative-free sodium stibogluconate has little activity against L. panamensis promastigotes but is highly active against L. panamensis amastigotes in macrophages. This activity is associated with multiple chemical species.

Use of in vitro method to assess different brands of anti-leishmanial drugs

Reports in the literature indicate the use of animal models for testing newer anti-leishmanial drugs in vivo. However, in certain established cell lines and macrophages in vitro models have the advantage over the in vivo system of simplicity and speed with which the results can be obtained. A simple in vitro system using peritoneal exudate macrophages of BALB/c mice infected with Leishmania donovani promastigotes has been tested for its use in determining the efficacy of several new drugs. Two well-established drugs, amphotericin B and sodium stibogluconate, as expected, could kill the intracellular parasites effectively. Two relatively new drugs not routinely used against leishmania, rifampicin and metronidazole at concentrations of 20 micrograms/ml and 10 micrograms/ml, respectively, were also able to kill the intracellular leishmania parasites effectively. Critical factors for drug testing in vitro have been elucidated: the most important being the temperature of incubation after infection.

In vitro and in vivo anti-leishmanial activity of chlorpromazine alone and combined with N-meglumine antimonate

Chlorpromazine (CPZ) was effective in vitro against leishmanial promastigotes and amastigotes. CPZ at 7.5 micrograms ml-1 killed the promastigotes of Leishmania mexicana, L. aethiopica and L. major within 24 hours of exposure. A higher concentration was required to achieve the same effect against L. donovani. N-meglumine antimonate (MGA) was only partially effective against the promastigotes of the four strains studied. Even at 1000 micrograms ml-1 total destruction of the parasites did not occur within four days of treatment. Combination chemotherapy of CPZ and MGA generally showed an additive effect against promastigotes. However, a synergistic effect was observed in the case of L. donovani promastigotes and L. major amastigotes in vitro. No significant effect was obtained against the amastigotes of L. major and L. mexicana in vivo, in cutaneous lesions of BALB/c mice.

Liposomes in leishmaniasis: effects of parasite virulence on treatment of experimental leishmaniasis in hamsters

During studies on the use of liposomes as drug carriers in experimental leishmaniasis in hamsters, we noted incidentally that the apparent virulence of the infection often varied widely between different large groups of animals. When the death rates among control animals (injected only with saline) were compared with hepatic parasite counts of survivors in the same group, three distinctive types of infection were observed: type I, low death rate, low parasite count in survivors; type II, high death rate, low parasite count in survivors; type III, high death rate, high parasite count in survivors. The apparent virulence, based on death rates both at early and late stages of infection, was in the order I less than II less than III. Therapeutic efficacy of a drug (meglumine antimoniate) or liposome-encapsulated drug against each type of infection was in the order I greater than II greater than III. Liposomes reduced the drug dose required for each infection type many hundred-fold and reduced the death rate for type I to zero. However, among animals with type III (or even type II) infection certain individuals were completely refractory to treatment, even when liposome-encapsulated drug was employed, and the lowest mortality rate achieved was approximately 30%. This latter resistance to treatment may have been due to irreversible tissue damage caused by advanced disease, or it may have reflected resistance of certain virulent infections to treatment.

Protective effect of glucan against visceral leishmaniasis in hamsters

The effect of pre- or posttreatment with glucan, a reticuloendothelial stimulant, on the course of Leishmania donovani infection was assessed in highly susceptible hamsters. Intravenous administration of glucan before or after L. donovani infection significantly suppressed proliferation of amastigote-stage parasites in liver and spleen. Glucan-activated peritoneal macrophages in vitro also significantly reduced multiplication of the intracellular parasite. Ultrastructural studies revealed a well-defined hepatic granulomatous response to glucan, with hypertrophic Kupffer cells and reduced numbers of intracellular parasites compared to the control group. In additional studies, groups of hamsters were immunized by intravenous injections of glucan with Formalin-killed promastigote-stage L. donovani cells and challenged 60 days after the last immunizing injection. This treatment regimen significantly prolonged the mean survival time of those hamsters which died after infection, relative to untreated control groups. Hamsters stimulated with the glucan-killed promastigote preparation also exhibited significant reductions in splenic amastigotes on days 10 and 21 postinfection compared with all other control groups, but on day 35, splenic amastigotes did not differ significantly from those of control animals. Our composite observations provide evidence for glucan-enhanced nonspecific resistance of hamsters to visceral leishmaniasis.