Discrepancy in plasma melarsoprol concentrations between HPLC and bioassay methods in patients with T. gambiense sleeping sickness indicates that melarsoprol is metabolized

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Abstract
Summary objective With the use of a specific high‐performance liquid chromatography (HPLC) method and a bioassay which determines trypanocidal activity concentrations of melarsoprol were assassed in plasma, urine and cerebrospinal fluid (CSF) from 8 patients with late‐stage Trypanosoma gambiense sleeping sickness. The aim was to unravel to what extent the bioassay codetermines biologically active metabolites of melarsoprol. methods Subjects were given one dose of melarsoprol i.v. per day for 4 days (1.2, 2.4, 3.0–3.6, 3.0–3.6 mg per kg b.w., respectively). Plasma samples were obtained before the first melarsoprol injection, immediately after and at 1 h, 24 h and 5 days after the 4th injection. Urine was collected before melarsoprol therapy and at 24 h after the 4th injection. CSF samples were taken once before treatment and at 24 h after the 4th injection. results HPLC analyses showed that plasma concentrations immediately after the 4th injection varied from 2200 to 15 900 nmol/l; dropping to 0–1800 nmol/l at 1 h; and to undetectable levels at 24 h. In urine small amounts of melarsoprol were recovered. Melarsoprol could not be detected in CSF by HPLC. Immediately after injection, bioassay analyses showed plasma concentrations of the same magnitude as HPLC assays but at 1 h they were 4–65‐fold higher than the levels assessed by HPLC. Even 24 h and 5 days after the 4th injection there was significant but decreasing activity. Urine levels were 40–260‐fold higher than the measured HPLC concentrations, whereas there was low but detectable activity in CSF. conclusion Results indicate that melarsoprol is rapidly eliminated from plasma. The significant trypanocidal activity determined by bioassay and simultaneuos low or undetectable levels of melarsoprol assayed by HPLC indicate that the compound is transformed into metabolites with parasiticidal activity.