Determination of melarsoprol in biological fluids by high-performance liquid chromatography and characterisation of two stereoisomers by nuclear magnetic resonance spectroscopy

Melarsoprol–cyclodextrins inclusion complexes

Melarsoprol, a water-insoluble drug, is mainly used in the treatment of trypanosomiasis and has demonstrated an in vitro activity on myeloid and lymphoid leukemia derived cell lines. It is marketed as a very poorly tolerated non-aqueous solution (Arsobal). The aim of our work was to develop melarsoprol-cyclodextrin complexes in order to improve the tolerability and the bioavailability of melarsoprol. Phase-solubility analysis showed A(L)-type diagrams with beta-cyclodextrin (betaCD), randomly methylated beta-cyclodextrin (RAMEbetaCD) and hydroxypropyl-beta-cyclodextrin (HPbetaCD), which suggested the formation of 1:1 inclusion complexes. The solubility enhancement factor of melarsoprol (solubility in 250 mM of cyclodextrin/solubility in water) was about 7.2x10(3) with both beta-cyclodextrin derivatives. The 1:1 stoichiometry was confirmed in the aqueous solutions by the UV spectrophotometer using Job's plot method. The apparent stability constants K(1:1), calculated from mole-ratio titration plots, were 57 143+/-4 425M(-1) for RAMEbetaCD and 50 761+/-5 070 M(-1) for HPbetaCD. Data from 1H-NMR and ROESY experiments provided a clear evidence of inclusion complexation of melarsoprol with its dithiaarsane extremity inserted into the wide rim of the cyclodextrin torus. Moreover, RAMEbetaCD had a pronounced effect on the drug hydrolysis and the dissolution rate of melarsoprol. However, the cytotoxic properties of melarsoprol on K562 and U937 human leukemia cell lines was not modified by complexation.

Effective measures for controlling trypanosomiasis

African trypanosomiasis, otherwise known as sleeping sickness in humans and 'Nagana' in cattle, is a disease that is resurgent in Africa. Research on the disease suggests that the development of a vaccine is still far away; even existing drugs are becoming ineffective on account of the emergence of drug-resistant trypanosomes. All this contributes to heavy economic losses and a sociopolitical crisis in the continent, thus underscoring the pressure to intensify research for inexpensive, less toxic and affordable trypanocides. This review discusses the current treatment of trypanosomiasis and the progress made towards the effective control of trypanosomiasis.

Arsenicals (melarsoprol), pentamidine and suramin in the treatment of human African trypanosomiasis

Human African trypanosomiasis (HAT), otherwise known as sleeping sickness, has remained a disease with no effective treatment. Recent progress in HAT research suggests that a vaccine against the disease is far from being successful. Also the emergence of drug-resistant trypanosomes makes further work in this area imperative. So far the treatment for the early stage of HAT involves the drugs pentamidine and suramin which have been very successful. In the second stage of the disease, during which the trypanosomes reside in the cerebrospinal fluid (CSF), treatment is dependent exclusively on the arsenical compound melarsoprol. This is largely due to the inability to find compounds that can cross the blood brain barrier and kill the CSF-residing trypanosomes. This review summarises our current understanding on the treatment of HAT.

Role of glutathione in the biliary excretion of the arsenical drugs trimelarsan and melarsoprol

After administration of the inorganic sodium arsenite or arsenate to rats, the biliary excretion of arsenic is rapid, is accompanied by the biliary output of large amounts of GSH, and is completely arrested by the GSH depletor diethyl maleate (DEM). We studied the biliary excretion of trimelarsan (TMA) and melarsoprol (MAP) in rats in order to determine whether biliary excretion is also significant in the disposition of these trivalent organic arsenicals that are used as therapeutic agents and whether GSH is also involved in their hepatobiliary transport. After injection of either drug (100 micromol/kg, i.v.), arsenic was rapidly excreted in bile (up to 1 micromol/kg. min, approximately 55% of dose/100 min). Concurrently, TMA and MAP increased the biliary output of GSH 3- and 6 fold, and lowered the hepatic GSH content by 24% and 27%, respectively. In TMA-injected rats, pretreatment with DEM or buthionine sulfoximine decreased the initial biliary excretion of arsenic by 75% and 40%, respectively, whereas in MAP-injected rats these GSH depletors diminished arsenic output by 45% and 20%. Both arsenicals reacted with GSH in vitro, giving rise to the same product, which was also shown by HPLC analysis to be a major biliary metabolite of both TMA and MAP. This metabolite was sensitive to gamma-glutamyltranspeptidase in vitro and its biliary excretion was virtually prevented by the GSH depletors, confirming that it is a GSH conjugate (purportedly melarsen-diglutathione). Some TMA was excreted in the bile unchanged, whereas a significant amount of MAP also appeared there as two glucuronides. The biliary excretion of unchanged TMA and MAP glucuronides was increased by experimental depletion of GSH. These studies indicate that the biliary excretion of TMA and MAP (1) is very significant in their disposition, (2) is partially dependent on the hepatic availability of GSH, as these arsenicals are excreted in part as a GSH conjugate, and (3) is concomitant with the increased appearance of GSH in bile, probably originating from dissociation of the unstable GSH conjugate of these arsenicals. Thus, conjugation with GSH is important in the elimination of both TMA and MAP, although glucuronidation is also involved in the fate of MAP.

An automated biological assay to determine levels of the trypanocidal drug melarsoprol in biological fluids

For the investigation of the pharmacokinetic properties of a drug, methods for sensitive and precise quantification are a prerequisite. Only few functional methods exist for the determination of the trypanocidal drug melarsoprol in biological fluids: A bioassay which requires microscopical evaluation and two HPLC methods, which require sample extraction and are difficult to automatize due to the drug's properties. We report the development of an automated biological assay, based on the fluorescent dye Alamar blue. To validate the assay for melarsoprol, 108 serum and 37 cerebrospinal fluid (CSF) samples were spiked with melarsoprol at concentrations of 17-92 ng/ml for CSF and 17 ng/ml-2.2 microg/ml for serum. The precision (repeatability) expressed as the interday average coefficient of variation was 9.9% for serum and 18.8% for CSF samples over the respective concentration range. The accuracy (measurement for the systematic error) of the test was 99.4% for serum and 96.4% for CSF. The assay's limit of quantitation with the use of the trypanosome stock STI 704 BABA was 4 ng/ml for both serum and CSF samples.

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

OBJECTIVE

With the use of a specific high-performance liquid chromatography (HPLC) method and a bioassay which determines trypanocidal activity, concentrations of melarsoprol were assessed 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 simultaneous low or undetectable levels of melarsoprol assayed by HPLC indicate that the compound is transformed into metabolites with parasiticidal activity.