1,5-Bis(4-amidinophenoxy)pentane (pentamidine) is a potent inhibitor of [3H]idazoxan binding to imidazoline I2 binding sites

Drug reformulation for a neglected disease. The NANOHAT project to develop a safer more effective sleeping sickness drug

Background

Human African trypanosomiasis (HAT or sleeping sickness) is caused by the parasite Trypanosoma brucei sspp. The disease has two stages, a haemolymphatic stage after the bite of an infected tsetse fly, followed by a central nervous system stage where the parasite penetrates the brain, causing death if untreated. Treatment is stage-specific, due to the blood-brain barrier, with less toxic drugs such as pentamidine used to treat stage 1. The objective of our research programme was to develop an intravenous formulation of pentamidine which increases CNS exposure by some 10–100 fold, leading to efficacy against a model of stage 2 HAT. This target candidate profile is in line with drugs for neglected diseases inititative recommendations.

Methodology

To do this, we evaluated the physicochemical and structural characteristics of formulations of pentamidine with Pluronic micelles (triblock-copolymers of polyethylene-oxide and polypropylene oxide), selected candidates for efficacy and toxicity evaluation in vitro, quantified pentamidine CNS delivery of a sub-set of formulations in vitro and in vivo, and progressed one pentamidine-Pluronic formulation for further evaluation using an in vivo single dose brain penetration study.

Principal Findings

Screening pentamidine against 40 CNS targets did not reveal any major neurotoxicity concerns, however, pentamidine had a high affinity for the imidazoline₂ receptor. The reduction in insulin secretion in MIN6 β-cells by pentamidine may be secondary to pentamidine-mediated activation of β-cell imidazoline receptors and impairment of cell viability. Pluronic F68 (0.01%w/v)-pentamidine formulation had a similar inhibitory effect on insulin secretion as pentamidine alone and an additive trypanocidal effect in vitro. However, all Pluronics tested (P85, P105 and F68) did not significantly enhance brain exposure of pentamidine.

Significance

These results are relevant to further developing block-copolymers as nanocarriers, improving BBB drug penetration and understanding the side effects of pentamidine.

Sleeping sickness or human African Trypanosomiasis (HAT) is a disease caused by a parasite, which is transferred to humans by the bite of an infected tsetse fly. There are two disease stages: the first stage is the blood-based stage of the disease and the second stage affects the brain. It is fatal if left untreated. The blood-brain barrier (BBB) makes the brain stage difficult to treat because it prevents 99% of all drugs from entering the brain from the blood. Those anti-HAT drugs that do enter the brain are toxic and have serious side effects. Pentamidine is a less toxic blood stage drug, which our research has shown has a limited ability to cross the BBB due to its removal by proteins called transporters. The objective of this study was to use Pluronic to improve pentamidine delivery to target sites, whilst reducing its side effects. Pluronic is a polymer, which can assemble into micelles and encapsulate the drug. Thus, prolonging its circulation time and protecting it. Our study indicated that the selected Pluronics did not increase the brain delivery of pentamidine. However. Pluronic-pentamidine formulations were identified that harboured trypanocidal activity and did not increase safety concerns compared to unformulated pentamidine.

Pentamidine analogs as inhibitors of [(3)H]MK-801 and [(3)H]ifenprodil binding to rat brain NMDA receptors

The anti-protozoal drug pentamidine is active against opportunistic Pneumocystis pneumonia, but in addition has several other biological targets, including the NMDA receptor (NR). Here we describe the inhibitory potencies of 76 pentamidine analogs at 2 binding sites of the NR, the channel binding site labeled with [(3)H]MK-801 and the [(3)H]ifenprodil binding site. Most analogs acted weaker at the ifenprodil than at the channel site. The spermine-sensitivity of NR inhibition by the majority of the compounds was reminiscent of other long-chain dicationic NR blockers. The potency of the parent compound as NR blocker was increased by modifying the heteroatoms in the bridge connecting the 2 benzamidine moieties and also by integrating the bridge into a seven-membered ring. Docking of the 45 most spermine-sensitive bisbenzamidines to a recently described acidic interface between the N-terminal domains of GluN1 and GluN2B mediating polyamine stimulation of the NR revealed the domain contributed by GluN1 as the most relevant target.

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.

Imidazoline binding sites and their ligands: an overview of the different chemical structures

Since Bousquet et al. discovered the imidazoline binding sites (IBS) two decades ago, when they realized that the antihypertensive drug clonidine interacts not only with the alpha2-adrenenoceptors (alpha2-AR) but also with a distinct imidazoline preferring binding site, these receptors have been paid a great deal of attention. At least two subtypes, I1 and I2, have been characterised based on their binding affinity for different radioligands, but their structures still remain unknown. The pharmacological profile of these IBSs has been the objective of several and very thorough reviews. However, a medicinal chemistry overview of the different IBS ligands prepared to date has never been attempted. In this study, we attempt to compile all the different chemical structures reported to date as IBS ligands and classify them in function of their chemical structure and binding affinity for the different IBS subtypes. Thus, we comment on the different endogenous IBS ligands known as well as the drugs described to interact with the I1-IBS which have found application as antihypertensive drugs. Then, we review those compounds described in the literature to interact with the I2-IBS, classifying them by their chemical families (imidazolines, guanidines, 2-aminoimidazolines, beta-carbolines). Finally, some conclusions are drawn.

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.

Polyhydroxylated azepanes as new motifs for DNA minor groove binding agents

The synthesis of 1,3-bis-[3,4,5,6-tetrahydroxyazepane-N-p-phenoxy] and 1,3-bis-[3,4,5,6-tetrahydroxyazepane-N-p-benzyloxy] propanes is reported. These compounds have been prepared to investigate the potential of incorporating iminosugars as useful recognition elements in DNA minor groove binding agents. The compounds were shown to have very moderate binding affinities for DNA in thermal denaturation and ethidium bromide displacement assays when compared with propamidine. They were also found to possess some in vitro anticancer activity that did not correlate with their DNA binding affinity.

Problems for the chemotherapy of human African trypanosomiasis

Problems associated with the current therapies of sleeping sickness include toxicity, resistance and a lack of a guaranteed supply. However, no new formulations are close to gaining a licence for clinical use and relatively few compounds have been shown to be effective in experimental systems. Many potentially good biochemical targets for drugs have been identified. Some of these have been validated and lead compounds have been developed. However, the biology of trypanosomes means that various pharmacological demands must be met in developing new trypanocides for clinical use. Foremost among these problems is the blood-brain barrier, across which trypanocides must cross to reach parasites in the cerebrospinal fluid.The principal problem, however, relates not to biological difficulties, which are technically surmountable, but to economics. Put simply, most representatives of the pharmaceutical industry believe that selling drugs to the victims of sleeping sickness will not yield sufficient income to justify expenses needed for the development of novel reagents. Only when this economic barrier can be lowered will new drugs emerge for use against sleeping sickness.

Immunoassays for pentamidine and related compounds: development of a facile inhibitory ELISA suitable for clinical use

Aromatic dicationic drugs have a broad spectrum of activity against protozoal and fungal pathogens including Pneumocystis carinii, Leishmania mexicana amazonensis, Cryptosporidium parvum and Cryptococcus neoformans. Pentamidine serves as the exemplar for an extensive collection of newly synthesized related compounds, which have reduced toxicity and a wider range of target organisms. Assays of pentamidine and related compounds have depended on HPLC-tandem mass spectrometry (HPLC-TMS) for the quantitation and identification of drug and metabolites. Immunoassays for pentamidine would have many advantages over the HPLC methods including relative simplicity of assay format and required equipment, convenience in sample preparation and reduction in time and cost of assays. In this report we describe a simple ELISA based immunoassay for pentamidine and pentamidine-like drugs with requisite sensitivity and specificity for use as a clinical assay (EC50 value of about 50 nanomolar). Immunogen was synthesized by coupling the hapten aminopentamidine to ovalbumin (chemically modified to provide an optimal number of -SH groups) using sulfo-MBS. Maleic-anhydride activated ELISA plates were covalently sensitized using the aminopentamidine hapten and used in an inhibitory ELISA assay format whereby the ability of analyte to suppress antibody binding to sensitized plate was measured. The assay detects primarily the phenolic amidine of pentamidine when in a para position and hence can also detect structurally related derivatives of pentamidine of potential interest as new therapeutic agents.