Structural insights into drug transport by an aquaglyceroporin.

Pentamidine and melarsoprol are primary drugs used to treat the lethal human sleeping sickness caused by the parasite Trypanosoma brucei. Cross-resistance to these two drugs has recently been linked to aquaglyceroporin 2 of the trypanosome (TbAQP2). TbAQP2 is the first member of the aquaporin family described as capable of drug transport; however, the underlying mechanism remains unclear. Here, we present cryo-electron microscopy structures of TbAQP2 bound to pentamidine or melarsoprol. Our structural studies, together with the molecular dynamic simulations, reveal the mechanisms shaping substrate specificity and drug permeation. Multiple amino acids in TbAQP2, near the extracellular entrance and inside the pore, create an expanded conducting tunnel, sterically and energetically allowing the permeation of pentamidine and melarsoprol. Our study elucidates the mechanism of drug transport by TbAQP2, providing valuable insights to inform the design of drugs against trypanosomiasis.

Determination of the trypanocidal drug melarsoprol and its conversion products in biological fluids with HPLC-ICPMS/ESMS.

Although melarsoprol, an organoarsenic compound, is widely used for the treatment of trypanosomiasis (human African sleeping sickness), very little is known about its fate in the human body, its active metabolites passing the blood-brain barrier and the mode of action. Previous pharmacological studies based on the determination of melarsoprol by HPLC-UV or by a bioassay method produced different results. We report a HPLC-ICPMS method suitable for determining melarsoprol and its metabolites in biological fluids. The arsenic selective capability of the method allowed the quantitative measurement of melarsoprol and two arsenic-containing conversion products produced when melarsoprol was incubated with human serum and blood. The major product was identified as melarsen [4-[(4,6-diamino-1,3,5-triazin-2-yl)amino]phenyl]arsonic acid by HPLC/electrospray MS, and by accurate mass measurements. Investigations about the stability of melarsoprol in serum showed that within 30 h about 10% of melarsoprol is converted to melarsen. In blood, however, most of the melarsoprol was bound to proteins and only 1% was converted to melarsen after 30 hours. The limit of detection for melarsoprol and its conversion products were in the range of 1 µg AsL(-1) (13 nmol As L(-1)) based on signal to noise ratio of 3 with a 10 µL injection volume allowing direct determination of the compounds in blood and serum (after protein precipitation) at therapeutically realistic concentrations.

The antitumor effects of an arsthinol-cyclodextrin complex in a heterotopic mouse model of glioma.

In this paper, we examined arsthinol-cyclodextrin complexes, which display an anticancer activity. The association constants were 17,502±522 M(-1) for hydroxypropyl-ß-cyclodextrin and 12,038±10,168 M(-1) for randomized methylated ß-cyclodextrin. (1)H NMR experiments in solution also confirmed the formation of these complexes and demonstrated an insertion of the arsthinol (STB) with its dithiarsolane extremity into the wide rim of the hydroxypropyl-ß-cyclodextrin cavity. Complexed arsthinol was more effective than arsenic trioxide (As2O3) and melarsoprol on the U87 MG cell line. Importantly, in the in vivo study, we observed significant antitumor activity against heterotopic xenografts after i.p. administration and did not see any signs of toxicity. This remains to be verified using an orthotopic model.

Investigation of the biotransformation of melarsoprol by electrochemistry coupled to complementary LC/ESI-MS and LC/ICP-MS analysis.

Melarsoprol is the only currently available drug for treatment of the late stage of African trypanosomiasis (sleeping sickness). Unfortunately, the arsenic-containing drug causes serious side effects, for which the mechanisms have not been elucidated so far. This investigation describes the study of the melarsoprol biotransformation processes by electrochemical (EC) techniques. Based on EC, potential oxidation reactions of melarsoprol are examined. Moreover, the reactivity of melarsoprol, its metabolite melarsen oxide, and their oxidation products toward the tripeptide glutathione and the proteins hemoglobin and human serum albumin is evaluated. The combination of different analytical techniques allows the identification as well as the quantification of the biotransformation products. The hyphenation of liquid chromatography (LC) and electrospray ionization mass spectrometry (ESI-MS) is applied for identification and structure elucidation, which implies the determination of exact masses and fragmentation patterns. For the selective detection of arsenic containing metabolites, LC coupled to inductively coupled plasma mass spectrometry is utilized. Based on the obtained data, the oxidative biotransformation of melarsoprol can be predicted, revealing novel species which have been suspected, but not been identified up to now. The results of the protein studies prove that melarsen oxide, the active derivative of melarsoprol, strongly binds to human hemoglobin and forms different adducts via the free cysteinyl groups of the hemoglobin α- and ß-chain.

Melarsoprol cyclodextrin inclusion complexes as promising oral candidates for the treatment of human African trypanosomiasis.

Human African trypanosomiasis (HAT), or sleeping sickness, results from infection with the protozoan parasites Trypanosoma brucei (T. b.) gambiense or T. b. rhodesiense and is invariably fatal if untreated. There are 60 million people at risk from the disease throughout sub-Saharan Africa. The infection progresses from the haemolymphatic stage where parasites invade the blood, lymphatics and peripheral organs, to the late encephalitic stage where they enter the central nervous system (CNS) to cause serious neurological disease. The trivalent arsenical drug melarsoprol (Arsobal) is the only currently available treatment for CNS-stage T. b. rhodesiense infection. However, it must be administered intravenously due to the presence of propylene glycol solvent and is associated with numerous adverse reactions. A severe post-treatment reactive encephalopathy occurs in about 10% of treated patients, half of whom die. Thus melarsoprol kills 5% of all patients receiving it. Cyclodextrins have been used to improve the solubility and reduce the toxicity of a wide variety of drugs. We therefore investigated two melarsoprol cyclodextrin inclusion complexes; melarsoprol hydroxypropyl-ß-cyclodextrin and melarsoprol randomly-methylated-ß-cyclodextrin. We found that these compounds retain trypanocidal properties in vitro and cure CNS-stage murine infections when delivered orally, once per day for 7-days, at a dosage of 0.05 mmol/kg. No overt signs of toxicity were detected. Parasite load within the brain was rapidly reduced following treatment onset and magnetic resonance imaging showed restoration of normal blood-brain barrier integrity on completion of chemotherapy. These findings strongly suggest that complexed melarsoprol could be employed as an oral treatment for CNS-stage HAT, delivering considerable improvements over current parenteral chemotherapy.

Evaluation of nucleoside hydrolase inhibitors for treatment of African trypanosomiasis.

In this paper, we present the biochemical and biological evaluation of N-arylmethyl-substituted iminoribitol derivatives as potential chemotherapeutic agents against trypanosomiasis. Previously, a library of 52 compounds was designed and synthesized as potent and selective inhibitors of Trypanosoma vivax inosine-adenosine-guanosine nucleoside hydrolase (IAG-NH). However, when the compounds were tested against bloodstream-form Trypanosoma brucei brucei, only one inhibitor, N-(9-deaza-adenin-9-yl)methyl-1,4-dideoxy-1,4-imino-d-ribitol (UAMC-00363), displayed significant activity (mean 50% inhibitory concentration [IC(50)] +/- standard error, 0.49 +/- 0.31 microM). Validation in an in vivo model of African trypanosomiasis showed promising results for this compound. Several experiments were performed to investigate why only UAMC-00363 showed antiparasitic activity. First, the compound library was screened against T. b. brucei IAG-NH and inosine-guanosine nucleoside hydrolase (IG-NH) to confirm the previously demonstrated inhibitory effects of the compounds on T. vivax IAG-NH. Second, to verify the uptake of these compounds by T. b. brucei, their affinities for the nucleoside P1 and nucleoside/nucleobase P2 transporters of T. b. brucei were tested. Only UAMC-00363 displayed significant affinity for the P2 transporter. It was also shown that UAMC-00363 is concentrated in the cell via at least one additional transporter, since P2 knockout mutants of T. b. brucei displayed no resistance to the compound. Consequently, no cross-resistance to the diamidine or the melaminophenyl arsenical classes of trypanocides is expected. Third, three enzymes of the purine salvage pathway of procyclic T. b. brucei (IAG-NH, IG-NH, and methylthioadenosine phosphorylase [MTAP]) were investigated using RNA interference. The findings from all these studies showed that it is probably not sufficient to target only the nucleoside hydrolase activity to block the purine salvage pathway of T. b. brucei and that, therefore, it is possible that UAMC-00363 acts on an additional target.

Comparison of nanosuspensions and hydroxypropyl-beta-cyclodextrin complex of melarsoprol: pharmacokinetics and tissue distribution in mice.

The aim of this work was to develop and compare two formulations of melarsoprol (nanosuspension and hydroxypropyl-beta-cyclodextrin inclusion complex). The arsenic concentrations in the organs have been assessed on a mouse model. Since this organoarsenical drug has been proposed for the treatment of cerebral trypanosomiasis and refractory leukaemias, special emphasis has been put on the bone marrow and on the brain. The organic solution of melarsoprol (Mel B, 0.039mmol/kg), injected intravenously as control formulation, was found to concentrate significantly in the bone marrow (C(max)=1.64mmol/g), though, not surprisingly, the brain concentration was quite high (C(max)=0. 093mmol/g) and the LD(50) was 0.12mmol/kg. The hydroxypropyl-beta-cyclodextrin inclusion complex was found to concentrate much more in the brain (C(max)=0.25mmol/g) leading to a higher acute toxicity (i.e., lower LD(50); 0.056mmol/kg). Nevertheless, even if the encephalopathy risk has to be taken in to account, this could be considered as a positive point for the treatment of the cerebral trypanosomiasis, which is the main indication for this drug. On the contrary, the use of nanosuspensions allowed us to reduce the cerebral concentration (C(max)=0.02micromol/g) and the acute toxicity (LD(50)=0.25mmol/kg). Moreover, nanosuspensions, especially those prepared with polxamer 407, preserved a good in vitro antileukemic activity (IC(50)=3.34+/-0.33 after 48h on K562) with high bone marrow concentrations (C(max)=1.85micromol/g). As a consequence this formulation could be proposed for the treatment of refractory leukaemias.

The role of Trypanosoma brucei MRPA in melarsoprol susceptibility.

We previously showed that over-expression of Trypanosoma brucei MRPA, a member of the multidrug resistance protein family in T. brucei, reproducibly resulted in resistance to the anti-trypanosomal drug melarsoprol in vitro. MRPA is predicted to mediate efflux of melarsoprol as a conjugate with trypanothione, a glutathione-spermidine conjugate which is the major small thiol in trypanosomes. Here, we show that depletion of MRPA by RNA interference resulted in moderate hypersensitivity to both melarsoprol and melarsen oxide. Over-expression of MRPA alone is not sufficient to cause melarsoprol resistance in vivo, although it is sufficient in vitro. This discrepancy is not an effect of drug metabolism since over-expression of MRPA alone conferred resistance to melarsoprol and its principle metabolite, melarsen oxide, in vitro. Over-expression of MRPA was not detected in four melarsoprol-resistant trypanosome isolates from sleeping sickness patients.

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.

Current chemotherapy of human African trypanosomiasis.

Human African trypanosomiasis is a fatal disease caused by Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense that has re-emerged in recent years. However, very little progress has been made in the development of new drugs against this disease. Most drugs still in use were developed one or more decades ago, and are generally toxic and of limited effectiveness. The most recently introduced compound, eflornithine, is only useful against sleeping sickness caused by T. b. gambiense, and is prohibitively expensive for the African developing countries. We present here an overview of today's approved and clinically used drugs against this disease.