an adenine-adenosine transporter. Resistance can occur with point mutations within this transporter. Resistance has been present since the 1970s. Mechanism of action Melarsoprol is a prodrug, a complex of melarsen oxide (a melamine derivative of phenylarsonous acid) with dimercaprol (also known as British anti-Lewisite, or BAL). It is metabolized to melarsen oxide in the body, which then acts by irreversibly binding to sulfhydryl groups on the enzyme pyruvate kinase, thus disrupting energy production in the parasite. The inability to distinguish between the host's and the parasite's pyruvate kinase renders this drug highly toxic, with many side effects. Melarsen oxide also reacts with trypanothione (a spermidine-glutathione adduct that replaces glutathione in trypanosomes). It forms a melarsen oxide-trypanothione adduct (Mel T) that competitively inhibits trypanothione reductase, effectively killing the protist. Pharmacokinetics The half-life of melarsoprol is less than one hour, but bioassays indicate a 35-hour half-life. This is commonly associated with pharmacologic agents that have active metabolites. One such metabolite, melarsen oxide, reaches maximum plasma levels about 15 minutes after melarsoprol injection. Melarsoprol clearance is 21.5 ml/min/kg and the half-life of melarsen oxide is approximately 3.9 hours. Dosage Two arsenic-containing stereoisomers exist in a 3:1 molar ratio. Since melarsoprol is insoluble in water, dosage occurs via a 3.6% propylene glycol intravenous injection. To avoid the risk of injection site reactions, melarsoprol must be given slowly. Melarsoprol used for the treatment of African trypanosomiasis with CNS involvement is given under a complicated dosing schedule. The dosing schedule for children and adults is 2–3.6 mg/kg/day intravenously for three days, then repeated every seven days for a total of three series. To monitor for relapse, follow-up is recommended every six months for at least two years. Side effects Although melarsoprol cures about 96% of people with late stage disease, its toxicity limits its use. About 1-5% of people die during treatment. As a toxic organic compound of arsenic, melarsoprol is a dangerous treatment that is typically only administered by injection under the supervision of a licensed physician. Notable side effects are similar to arsenic poisoning. Among clinicians, it is colloquially referred to as "arsenic in antifreeze". Severe and life-threatening adverse reactions are associated with melarsoprol. It is known to cause a range of side effects including convulsions, fever, loss of consciousness, rashes, bloody stools, nausea and vomiting. In approximately 5-10% of cases, it causes encephalopathy. Of those, about 50% die due to encephalopathy-related adverse reactions.