M. Amélia Santos

Hydroxypyridinone complexes with aluminium. In vitro/vivo studies and perspectives

Abstract Hydroxypyridinones are a class of dioxo ligands under active development, as efficient Al (and Fe) chelators for potential medical uses. A wide range of those compounds has been designed aimed at improving their physico-chemical and pharmacokinetic properties. In this paper, we make a review on the literature results related to the design, chemistry, metal binding interaction, lipo-hydrophilic character and some biological assays of bidentate and hexadentate hydroxypyridinones. Among the different types of hydroxypyridinones, the 3-hydroxy-4-pyridinones deserved special attention because they are good orally active aluminium-chelators, and they seem to be the main candidates for replacement of desferrioxamine. The interaction of the bidentate hydroxypyridinones with Al as well as the in vivo studies have been more systematically reported than those of the hexadentate derivatives. The development of the hexadentate hydroxypyridinones is quite recent but, at physiological conditions, they have higher affinity for these M3+ ions than the bidentate derivatives. Despite only studies on hexadentate hydroxypyridinone–iron interactions are known and described herein, special attention was deserved to these results because of the in vivo/vitro similarity between the physico-chemical properties of these ions. The high Al affinity and favourable lipo-hydrophilic balance of the hydroxypyridinones suggest that their use as Al scavengers should be highly considered in future prospects.

Recent progress in the drug development of coumarin derivatives as potent antituberculosis agents.

Tuberculosis (TB) is still a challenging worldwide health problem and mycobacterium tuberculosis (MTB) remains one of the most deadly human pathogens. TB is the second leading infectious cause of mortality today behind only HIV/AIDS. The impetus for developing new structural classes of antituberculosis drugs comes from the emergence of multi-drug resistant (MDR) strains. The development of MDR strains to commonly used drugs is due to, longer durations of therapy as results of resistance, and the resurgence of the disease in immune compromised patients. Therefore, there is an urgent need to explore new antitubercular (anti-TB) agents. Ironically, the low number of potentially new chemical entities which can act as anti-TB candidates is of great importance at present situation. Considering the severity of the problem, WHO has prepared a strategic plan in Berlin declaration 2007 to stop TB, globally. Among the oxygen heterocycles, coumarin derivatives are important motifs, which can be widely found in many natural products, and many of them displaying diverse biological activities. This spectacular spectrum of applications has intrigued organic and medicinal chemists for decades to explore the natural coumarins or their synthetic analogs for their applicability as anti-TB drugs. To pave the way for the future research, there is a need to collect the latest information in this promising area. In the present review, we collated published reports on coumarin derivatives to shed light on the insights on different types of methods reported for their preparations, characterizations and anti-TB applications, so that its full therapeutic potential class of compounds can be utilized for the treatment of tuberculosis. Therefore, the objective of this review is to focus on important coumarin analogs with anti-TB activities, and structure-activity relationships (SAR) for designing the better anti-TB agents. It is hoped that, this review will be helpful for new thoughts in the quest for rational designs of more active and less toxic coumarin-based anti-TB drugs.

N-Carboxyalkyl derivatives of 3-hydroxy-4-pyridinones: synthesis, complexation with Fe(III), Al(III) and Ga(III) and in vivo evaluation

A set of three N-carboxyalkyl 3-hydroxy-4-pyridinones has been studied as bidentate M(III) chelators (M=Fe, Al, Ga), with potential for oral administration. After preparation of the ligands, their protonation constants (log K(i)) and the stability constants of their metal complexes have been determined. The distribution coefficients of these compounds, between 1-octanol and Tris buffer pH 7.4, were measured. The effect of these compounds on the biodistribution of 67Ga-citrate loaded rats was investigated and compared with that of the administered 67Ga-complexes. Results indicated that, among these chelating agents, the N-carboxyethyl derivative has the highest affinity towards this set of metal ions, irrespective of the metal, and that it could even compete with transferrin, the main Fe-plasma protein. The binding affinity and the hydrophilic character decrease with the increase in the size of the alkylic chain. The biological assays indicate that the complex formation in vivo is characterized by a high kinetics and thermodynamic stability, suggesting a competition with the transferrin. All the ligands were found to enhance the excretion of the gallium. Noteworthy is the observed Ga bone fixation, mostly with the ethyl derivative, thus suggesting the potential use of the complex as a bone seeking agent.

Design, synthesis and neuroprotective evaluation of novel tacrine–benzothiazole hybrids as multi-targeted compounds against Alzheimer’s disease

Alzheimers disease (AD) is a multifactorial disorder with several target proteins contributing to its etiology. In search for multifunctional anti-AD drug candidates, taking into account that the acetylcholinesterase (AChE) and beta-amyloid (Aβ) aggregation are particularly important targets for inhibition, the tacrine and benzothiazole (BTA) moieties were conjugated with suitable linkers in a novel series of hybrids. The designed compounds (7a-7e) were synthesized and in vitro as well as in ex vivo evaluated for their capacity for the inhibition of acetylcholinesterase (AChE) and Aβ self-induced aggregation, and also for the protection of neuronal cells death (SHSY-5Y cells, AD and MCI cybrids). All the tacrine-BTA hybrids displayed high in vitro activities, namely with IC₅₀ values in the low micromolar to sub-micromolar concentration range towards the inhibition of AChE, and high percentages of inhibition of the self-induced Aβ aggregation. Among them, compound 7a, with the shortest linker, presented the best inhibitory activity of AChE (IC₅₀=0.34 μM), while the highest activity as anti-Aβ₄₂ self-aggregation, was evidenced for compound 7b (61.3%, at 50μM. The docking studies demonstrated that all compounds are able to interact with both catalytic active site (CAS) and peripheral anionic site (PAS) of AChE. Our results show that compounds 7d and 7e improved cell viability in cells treated with Aβ₄₂ peptide. Overall, these multi-targeted hybrid compounds appear as promising lead compounds for the treatment of Alzheimers disease.

Benzofuran: an emerging scaffold for antimicrobial agents

Resistance to antibiotics is a major global problem and there is an urgent need to develop new therapeutic agents. Although many classes of active compounds have been established as efficient derivatives in diverse fields of antimicrobial therapy, they have not yet found wide application against a few deadly microbes. In recent years, compounds have been developed that have solved some of the problems posed; for example improved bioavailability is one of the targets achieved with most of the more recent compounds, allowing for once-daily dosing. Benzofuran and its derivatives are found to be suitable structures, existing widely in natural products and unnatural compounds with a wide range of biological and pharmacological applications; thus, considerable attention has been focused on the discovery of new drugs in the fields of drug invention and development. Some benzofuran derivatives, such as psoralen, 8-methoxypsoralen and angelicin have been used in the treatment of skin diseases such as cancer or psoriasis. The unique structural features of benzofuran and its wide array of biological activities make it a privileged structure in the field of drug discovery, especially in the search for efficient antimicrobial candidates. Recently, this scaffold has emerged as a pharmacophore of choice for designing antimicrobial agents that are active toward different clinically approved targets. To pave the way for future research, there is a need to collect the latest information in this promising area. In the present review, we collated the published reports on this versatile core to provide a deeper insight, so that its full therapeutic potential can be utilized for the treatment of microbial diseases. This study systematically provides a comprehensive report on current developments in benzofuran-based compounds as antimicrobial agents and is also helpful for the researchers working on a substitution pattern around the nucleus, with an aim to help medicinal chemists to develop structure activity relationships (SAR) on these derivatives as antimicrobial drugs.

Dual inhibitors of matrix metalloproteinases and carbonic anhydrases: iminodiacetyl-based hydroxamate-benzenesulfonamide conjugates.

Matrix metalloproteinases (MMPs) and carbonic anhydrases (CAs) are two classes of zinc enzymes with different roles and catalytic targets, such as the degradation of most of the extracellular matrix (ECM) proteins and the regulation of the CO(2)/HCO(3)(-) equilibrium in the cells, respectively. Both families have isoforms which were proved to be involved in several stages of carcinogenic processes, and so the selective inhibition of these enzymes might be of interest in cancer therapy. We report herein the design, synthesis, and in vitro evaluation of a series of compounds possessing the iminodiacetic acid as the main backbone and two functional groups attached, namely, the hydroxamic acid and the arylsulfonamide (ArSO(2)NH(2)) moieties, to enable the inhibition of MMPs and CAs, respectively. These compounds were demonstrated to strongly inhibit both MMPs and CAs, some of them from the nanomolar to subnanomolar range. Furthermore, a docking study for MMPs was reported for the most promising compound in order to investigate its binding interactions with the different MMPs.

New tripodal hydroxypyridinone based chelating agents for Fe(III), Al(III) and Ga(III) : Synthesis, physico-chemical properties and bioevaluation

Two new tris-hydroxypyridinone based compounds (KEMPPr(3,4-HP)(3) and KEMPBu(3,4-HP)(3)) have been developed and studied as strong sequestering agents for iron and the group III of metal ions, aimed as potential pharmacological applications on metal-chelation therapy. Their structure is based on the KEMP acid scaffold to which three 3-hydroxy-4-pyridinone chelating moieties are attached via two different size spacers. After the preparation and characterization of the compounds their physico-chemical properties were studied, in relation with their metal binding affinity and lipophilicity. The KEMPPr(3,4-HP)(3) ligand was also bioassayed to evaluate its in vivo metal sequestering capacity from most organs using an animal model overload with (67)Ga. These studies showed that, for both in solution and in vivo conditions, the compounds have higher metal chelating efficacy than Deferriprone, the commercially available iron chelator in medical application, thus some perspectives are envisaged as potential pharmaceutical drug candidates for chelating therapy.

Alkylaryl-amino derivatives of 3-hydroxy-4-pyridinones as aluminium chelating agents with potential clinical application

The neurotoxicity of aluminium is well established and so strategies for suitable aluminium chelating therapies, aimed at the treatment and/or amelioration of some neurological disorders, are of current interest. The present work describes a set of new bifunctional compounds containing a 3-hydroxy-4-pyridinone (3,4-HP) unit, as the aluminium chelating moiety, which is extra-functionalised with different alkyl-arylamine molecular segments, to account for the improvement on the biodistribution specificity of the chelating agents or the corresponding complexes. Besides the synthetic scheme, studies are performed to assess the properties of these compounds, namely in terms of lipophilicity, Al-chelating ability, speciation and in vivo 67Ga biodistribution. These studies show that the extrafunctional groups fortunately have small effects on the high Al chelating affinity of the 3,4-HP units, over a wide range of pH, but they lead to favourable changes on the lipo-hydrophilic balance of the ligands and on the complex speciation. Differences found in the biodistribution, namely the decrease of the blood-clearance rate and increase of the bone retention or the hepatobiliary excretion, seem to be mostly rationalized in terms of the increasing lipophilic character of the ligands.

A gallium complex with a new tripodal tris-hydroxypyridinone for potential nuclear diagnostic imaging: solution and in vivo studies of 67Ga-labeled species

The gallium(III) complex of a new tripodal 3-hydroxy-4-pyridinone (3,4-HP) chelator has been studied in terms of its physico-chemical and in vivo properties aimed at potential application as probe for nuclear imaging. In particular, based on spectrophotometric titrations, the hexa-coordinated (1:1) gallium complex appeared as the major species in a wide physiological acid-neutral pH range and its high stability (pGa=27.5) should avoid drug-induced toxicity resulting from Ga(III) accumulation in tissues due to processes of transmetallation with endogenenous ligands or demetallation. A multinuclear ((1)H and (71)Ga) NMR study gave some insights into the structure and dynamics of the gallium(III) chelate in solution, which are consistent with the tris-(3,4-HP) coordination and an eventual pseudo-octahedral geometry. Biodistribution and scintigraphic studies of the (67)Ga(III) labelled chelate, performed in Wistar rats, confirmed the in vivo stability of the radiolabelled complex, its non interaction with blood proteins and its quick renal clearance. These results indicate good perspectives for potential application of extrafunctionalized analogues in radiodiagnostic techniques.

New Tacrine Hybrids with Natural-Based Cysteine Derivatives as Multitargeted Drugs for Potential Treatment of Alzheimer's Disease

Alzheimers disease (AD) is a devastating age‐dependent neurodegenerative disorder. The main hallmarks are impairment of cholinergic system and accumulation in brain of beta‐amyloid (Aβ) aggregates, which have been associated with oxidative damage and dyshomeostasis of redox‐active biometals. The absence of an efficient treatment that could delay or cure AD has been attributed to the complexity and multifactorial nature of this disease. With this in mind and the recent interest on natural‐based drugs, we have explored a set of natural‐based hybrid compounds by conjugation of a tacrine moiety with an S‐allylcysteine (garlic constituent) or S‐propargylcysteine moiety aimed at improving the cholinergic system and neuroprotective capacity. The docking modeling studies allowed the selection of linkers to optimize the bimodal drug interaction with acetylcholinesterase enzyme (AChE) active site. The compounds were evaluated for some representative biological properties, including AChE activity and Aβ aggregation inhibition, as well as for their neuroprotective activity to Aβ‐ and ROS‐induced cellular toxicity. The most promising results were achieved by compounds 9d for the AChE inhibition and 9l for the remarkable prevention of superoxide production and Aβ‐induced cellular toxicity.