Antiprotozoal Agents

Sodium stibogluconate (SSG) contains antimony and enters macrophages where Leishmania parasites live. Inside the parasite, thiol systems convert SSG's pentavalent antimony into the active trivalent form. This trivalent antimony blocks trypanothione reductase, disrupting the parasite's redox system and ultimately causing its death.

Trypanothione reductase (TR) is an enzyme that maintains redox balance in Leishmania by reducing oxidized trypanothione disulfide using NADPH. TR is absent in mammalian hosts, making it a selective target for chemotherapy. This selectivity allows antiprotozoal drugs to inhibit the parasite while minimizing harm to human cells.

Trivalent antimony blocks trypanothione reductase and binds to free thiols, interfering with iron-sulfur cluster enzymes and zinc-binding proteases. This causes reactive oxygen species to accumulate within the parasite cell, damaging proteins and membrane lipids. The drug also promotes efflux of intracellular thiols, further weakening the parasite's antioxidant defense.

Mammalian cells depend on the glutathione/glutathione reductase system for redox balance, while Leishmania uses a distinct thiol pathway based on trypanothione and trypanothione reductase. This trypanothione system acts as the main antioxidant defense in trypanosomatids, making it essential for parasite survival under oxidative stress.

After administration, sodium stibogluconate enters macrophages through normal cellular uptake. The drug then enters the Leishmania parasite via aquaglyceroporin channels. Once inside, thiol systems in both the macrophage and parasite convert the pentavalent antimony into its active trivalent form, which can then inhibit trypanothione reductase.

Crystal structures reveal that trivalent antimony coordinates with specific amino acids: cysteine 52, cysteine 57, threonine 335, and histidine 461 from the opposing subunit of the trypanothione reductase dimer. This binding geometry disrupts the enzyme's ability to reduce oxidized trypanothione disulfide, blocking the parasite's redox cycle and causing cell death.

Heavy metal drugs are effective because they target parasite-specific biochemical pathways absent in mammalian hosts. Sodium stibogluconate's trivalent antimony form selectively inhibits trypanothione reductase, disrupting the parasite's unique redox system. This selectivity allows the drug to eliminate Leishmania while minimizing toxicity to human cells.