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Antiprotozoal Agents

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Heavy metal drugs are effective antiprotozoal agents.

For example, sodium stibogluconate, or SSG, contains the heavy metal antimony and is widely used to treat Leishmania infections.

In the host, Leishmania lives inside macrophages.

Once administered, the drug enters the macrophages and then enters the parasite through aquaglyceroporin channels.

Thiol systems inside the parasite and the macrophages change SSG’s pentavalent antimony into the active trivalent form.

This active form blocks the enzyme trypanothione reductase, disrupting the parasite’s redox system.

Trivalent antimony also binds to free thiols and interferes with iron-sulfur cluster enzymes and zinc-binding proteases.

As a result, reactive oxygen species accumulate within the cell, damaging its proteins and membrane lipids.

The active drug then promotes the efflux of intracellular thiols, further weakening the parasite’s defense.

These effects ultimately result in the death of Leishmania and their removal from the host’s body.

Overview

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Leishmaniasis is a widespread parasitic disease caused by several Leishmania species. It affects millions of people each year and remains a major public health problem in endemic regions. First-line treatment relies on pentavalent antimonials, including meglumine antimoniate and sodium stibogluconate. Even so, how these drugs work has not been fully clear, especially their interaction with parasite-specific biochemical pathways. One key target is trypanothione reductase (TR), an enzyme that maintains redox balance in Leishmania. TR is absent in mammalian hosts, which makes it a selective target for chemotherapy.

Redox metabolism in Leishmania

Mammalian cells depend on the glutathione/glutathione reductase system, but Leishmania uses a distinct thiol pathway based on trypanothione [N¹,N⁸-bis(glutathionyl)spermidine] and TR. This system acts as the main antioxidant defense and redox regulator in trypanosomatids. TR reduces oxidized trypanothione disulfide (TS₂) to its dithiol form using NADPH. This pathway is essential for detoxifying reactive oxygen species and supporting parasite survival under oxidative stress.

TR inhibition by antimonials

Crystal structures of Leishmania infantum TR in oxidized and reduced states show that Sb(III), generated by intracellular reduction of pentavalent antimonials, binds the TR active site. Sb(III) coordinates with Cys52, Cys57, Thr335, and His461′ from the opposing subunit of the TR dimer. This interaction disrupts TS₂ reduction and blocks the redox cycle.

Biochemical and structural implications

Enzyme assays show that Sb(III) strongly inhibits TR (Ki 1.5 ± 0.4 μM), more effectively than As(III). Conserved binding regions across species point to a shared mechanism. The crystal structures (PDB 2JK6 and 2W0H) reveal Sb(III) coordination geometry and highlight His461′ and TR dimerization as key features for drug binding. These insights support the design of improved antileishmanial agents that target the TR–trypanothione system with better specificity and lower toxicity.

Transcript

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