Current Pharmaceutical Design - Volume 10, Issue 25, 2004

Ever since the isolation of erythromycin from Streptomyces erythreus more than 50 years ago, macrolide antibiotics have assumed a prominent position in our pharmacological armamentarium against a variety of microbial pathogens. The broad spectrum activity and intracellular accumulation of this class of antibiotics account for the substantial clinical efficacy of macrolides against both extracellular and intracellular pathogens, particularly in the setting of infection of the upper and lower respiratory tract. More recently, 14 and 15 membered ring macrolides have been found to modulate inflammatory responses to bacterial challenge in a manner that is independent of effects on bacterial viability. For instance, macrolide antibiotics are generally unable to kill certain gram-negative bacteria such as Pseudomonas aeruginosa at physiologically achievable concentrations in-vivo. However, these agents can substantially alter the virulence of P. aeruginosa by attenuating the expression of key virulence factors that are required for bacterial persistence and invasiveness. Moreover, specific members of the macrolide family have been discovered that exert important immunomodulatory effects on host inflammatory responses. These immunomodulatory effects, which appear to be unique to the macrolide family of antibiotics, were first recognized by Professor Kudoh in Tokyo, Japan, who made the astute observation that the long term administration of erythromycin to a patient with diffuse panbronchiolitis (DPB) resulted in a striking reversal of the usually progressive nature of this disease. In fact, the use of erythromycin and other 14 and 15 membered ring macrolides for the treatment of advanced DPB has resulted in a dramatic increase in the 10-year survival of this disease from less than 15% to nearly 90%. Clinical successes in the treatment of DPB have lead to trials assessing the efficacy of macrolides in other inflammatory airways diseases, including asthma and cystic fibrosis. Importantly, several macrolides have been proven in single center and multicenter controlled trials to reduce symptoms and improve pulmonary function in patients with cystic fibrosis. These encouraging results have ignited the performance of a large body of in-vitro and in-vivo studies to determine the mechanism(s) by which this family of antibiotics modulates the immune response. This issue will focus on these novel immunomodulatory properties of macrolides, including effects on both bacterial responses and host immune / inflammatory responses. In Chapter 1, Drs. Jain and Danziger [1] have nicely outlined the unique pharmacokinetic and pharmacodynamic aspects of macrolide antibiotics, with a particular emphasis on features that contribute to antibacterial and immunomodulatory effects in the lung. Dr. Tateda and colleagues [2] provide an insightful overview of non-bactericidal effects of macrolides on P. aeruginosa, including the influence of these antibiotics on bacterial quorum-sensing systems and other virulence factors. Dr. Labros [3] has provided a comprehensive review of macrolide effects in pulmonary and non-pulmonary inflammatory disease states, with a focus on immunomodulation of leukocyte effector cell function in these diseases. In Chapter 4, Drs. Tsai and Standiford [4] discuss the influence of various macrolides on the immune / inflammatory responses of lung cells both in-vitro and in animal models of lung disease. Finally, Drs. Martinez and Simon [5] present an exciting review of macrolide effects in inflammatory sinopulmonary disease, highlighting the clinical successes realized in DPB and cystic fibrosis. Given the pace of discovery in this field, a comprehensive review of immunomodulatory properties of macrolides is indeed timely and of unquestioned clinical relevance. While work performed to date has largely focused on lung-specific effects, it is probable that macrolide immunomodulatory properties are applicable to disease states involving organ systems outside the lung. It is our hope that this review will generate interest in research aimed at further defining mechanisms of macrolide immunomodulatory effects, as well as identify new therapeutic applications of macrolides in the treatment of acute and chronic disease.

The macrolide antimicrobial family is a comprised of 14, 15 and 16 member-ringed compounds that are characterized by similar chemical structures, mechanisms of action and resistance, but vary in the different pharmacokinetic parameters, and spectrum of activity. The macrolides accumulate in many tissues such as the epithelial lining fluid and easily enter the host defense cells, predominantly macrophages and polymorphonuclear leukocytes (PMNs). Concentrations of the macrolides in respiratory tract tissues and extracellular fluids are in almost all cases higher than simultaneously measured serum concentrations, making them useful for respiratory tract infections. This review will focus on pharmacokinetic and pharmacodynamic aspects of the clinical relevant macrolides including azithromycin, clarithromycin, dirithromycin, erythromycin and roxithromycin.

Pseudomonas aeruginosa is an opportunistic pathogen, and this organism is a major cause of pulmonary damage and mortality in patients with cystic fibrosis (CF), diffuse panbronchiolitis (DPB) and other forms of bronchiectasis. A break-through in the treatment of DPB and associated chronic P. aeruginosa pulmonary infection was realized when a patient with DPB improved dramatically after treatment with erythromycin for years. Now, long-term macrolide therapy has become a first line of treatment in DPB patients, and the immunomodulatory properties have now been extended to other clinical settings, including CF. An important factor in the pathogenesis of chronic P. aeruginosa infection is a bacterial cell-to-cell signaling mechanism, referred to as “quorum sensing”, which enables bacteria to coordinately turn on and off specific virulence genes through the production of autoinducer molecules. Interference or blocking of quorum-sensing systems has been considered an attractive therapeutic strategy. Clinical and basic science data suggests the potential of macrolides as relevant inhibitors of the Pseudomonas quorum-sensing system. In fact, certain macrolides strongly suppressed quorum-sensing associated genes and autoinducer production, in addition to inhibition of a variety of virulence factors. In this review, clinical efficacy of macrolides on DPB and CF patients will be briefly summarized. Additionally, the mechanisms of action of macrolides will be discussed from the standpoint of sub-MIC macrolide effects on P. aeruginosa, particularly the ability of this antibiotic to suppress quorum-sensing systems, which may be crucial in the pathogenesis of chronic P. aeruginosa infection.

Macrolide antimicrobials have stimulated worldwide interest owing to their therapeutic effects in various inflammatory, apparently non infectious, diseases. Abundant data are now available on their interactions with host cell (specially phagocyte) functions. Modulation of oxidant production by neutrophils and of pro-inflammatory cytokine synthesis and release by leukocytes are the two main effects observed in vitro. However, despite an extensive literature, many questions remain, such as the cellular / microbial target(s) of macrolide action, the critical chemical structure(s), and the usefulness of combining antimicrobial and anti-inflammatory properties, which during long-term treatment, could lead to increased microbial resistance. Also, because of the multiplicity of macrolide effects on different cell subsets, a unifying hypothesis for macrolide interactions with host cells is lacking. Novel analytical methods will certainly lead to new macrolide-based therapeutic strategies in cancer and inflammation.

Macrolide antibiotics appear to play a role in the management of diseases of chronic airway inflammation, distinctly separate from their antibactericidal activity. In the last fifteen years, their success in human clinical trials has prompted both in-vitro and in-vivo investigations to determine the mechanisms by which this family of antibiotics modulate the immune response. A large body of evidence suggests that macrolides directly target multiple components of the inflammatory cascade that occur independent of bactericidal / bacteriostatic effects. We will review the existing data in support of immunomodulatory effects of macrolides on activated leukocytes at the site of lung inflammation, on pulmonary host cells, and in animal models of lung disease.

Macrolides have broad antibacterial spectrum and proven efficacy in the management of respiratory tract infections. Over the past decade there has been progressive interest in these agents for their potential role as tissue modifying, anti-inflammatory agents. Increasingly, the effect of macrolides on numerous cell types has been documented. Preliminary data have suggested a beneficial clinical role of chronic macrolide therapy in selected patients with chronic rhinosinusitis, chronic obstructive pulmonary disease, cystic fibrosis, non-cystic fibrosis bronchiectasis and asthma. This review presents the biological rationale and the available clinical data of chronic macrolide therapy in chronic respiratory tract diseases. When available, the data addressing the presumed mechanisms underlying clinical benefits are presented.

Alzheimer's disease (AD) is characterized by progressive dementia caused by the loss of the presynaptic markers of the cholinergic system in the brain areas related to memory and learning and brain deposits of amyloid beta peptide (Aβ) and neurofibrillary tangles (NFT). A small fraction of early onset familial AD (FAD) is caused by mutations in genes, such as the b-amyloid precursor protein (APP) and presenilins that increase the load of Aβ in the brain. These studies together with findings that Aβ is neurotoxic in vitro, provide evidence that some aggregates of this peptide are the key to the pathogenesis of AD. The yield of Aβ and the processing and turnover of APP are regulated by a number of pathways including apolipoprotein E, cholesterol and cholinergic agonists. Early studies showed that muscarinic agonists increased APP processing within the Aβ sequence (sAPPα). More recently, we have presented evidence showing that some, but not all, anticholinesterases reduce secretion of sAPPa as well as Aβ into the media suggesting that cholinergic agonists modulate Aβ levels by multiple mechanisms. Herein we review the recent advances in understanding the function of cholinesterase (ChE) in the brain and the use of ChE-inhibitors in AD. We propose and support the position that the influence of cholinergic stimulation on amyloid formation is critical in light of the early targeting of the cholinergic basal forebrain in AD and the possibility that maintenance of this cholinergic tone might slow amyloid deposition. In this context, the dual action of certain cholinesterase inhibitors on their ability to increase acetylcholine levels and decrease amyloid burden assumes significance as it may identify a single drug to both arrest the progression of the disease as well as treat its symptoms. A new generation of acetyl- and butyryl cholinesterase inhibitors is being studied and tested in human clinical trials for AD. We critically discuss recent trends in AD research, from molecular and genetic to clinical areas, as it relates to the effects of cholinergic agents and their secondary effects on Aβ. Finally, we examine different neurobiological mechanisms that provide the basis of new targets for AD drug development.

Several cellular processes could be targeted if the complex nature of Alzheimer's disease (AD) was already understood. Most of AD treatments have been focused on the inhibition of acetylcholinesterase (AChE) in order to raise the levels of its substrate, i.e. the neurotransmitter acetylcholine (ACh), to augment cognitive functions of affected patients. Effectiveness in AChE inhibition and side-effect issues of clinical (tacrine, donepezil, galanthamine and rivastigmine) as well as of novel inhibitors is reviewed here. Novel design methods for the inhibition of AChE include the use of in silico tools to predict the interactions between AChE and the desired compound, both at the active site of the enzyme, responsible of hydrolysing ACh and with the peripheral anionic site (PAS), which has been described as a promoting agent of the amyloid β-peptide (Aβ) aggregation present in the senile plaques of the brain of AD individuals.

The interest for acetylcholinesterase as a target for the palliative treatment of Alzheimer's disease has been renewed in the last years owing to the evidences that support the role of this enzyme in accelerating the aggregation and deposition of the β-amyloid peptide. A large amount of structural information on the acetylcholinesterase enzyme and of its complexes with inhibitors acting at the catalytic site, the peripheral binding site, or both is now available. Based on that, molecular modelling studies can be intensively used to decipher the molecular determinants that mediate the relationship between chemical structure and inhibitory potency. In turn, this knowledge can be exploited to design new compounds leading to more effective cholinergic strategies. At this point, inhibitors able to interact at the peripheral binding site are of particular relevance, as they might disrupt the interactions between the enzyme acetylcholinesterase and the β-amyloid peptide. Therefore, these compounds might not only ameliorate the cholinergic deficit, but also be capable of slowing down the progression of the disease.

At present the only FDA-approved therapy for Alzheimer's disease involves the administration of acetylcholinesterase inhibitors, to alleviate the cholinergic deficit associated with this disease. However, none of the approved drugs is ideal in efficacy or tolerability. One possible strategy to improve selectivity and potency is to design drugs that can simultaneously bind to the catalytic and peripheral anionic sites of AChE. In this review we will describe the development of dimeric AChE inhibitors, from the early observations of high inhibition potency by bis-quaternary inhibitors, to the structure-based design of dimers based on tacrine, huperzine A, galanthamine, and polyamines.

The knowledge about the pathogenesis and the development of the neurodegeneration associated with Alzheimer's disease (AD) has been organised throughout the years into two theories, namely the cholinergic and the amyloid hypotheses. The loss of cholinergic neurotransmission and the abnormal aggregation and deposition of the amyloid-β peptide (Aβ) in the brain are retained as the central events by the two theories, respectively. These phenomena and their pathological consequences are the main targets of the drug discovery strategies based on each hypothesis. However, the two paradigms share some common aspects as shown by several experimental evidences, such that they might even fit into a unifying scenario of neuropathology and neurodegeneration. In this context, in a perspective of drug discovery, the enzyme acetylcholinesterase (AChE) holds a key position, as it is a main target for cholinomimetic AD drugs being responsible for the breakdown of the neurotransmitter, and it is also involved in the aggregation of Aβ and the formation of the neurotoxic fibrils. Following this view, in recent years, a drug design strategy has emerged, directed to finding molecules able to inhibit both of these actions exerted by AChE. In this review, we will briefly introduce the biological basis of this strategy, and then will account for the early results obtained in this field in our and in other laboratories. The main focus will be on potential lead compounds for which some experimental evidence exists supporting the hypothesis of their dual action, as AChE inhibitors and blockers of the AChE-induced Aβ aggregation.

Insufficient cholinergic neurotransmission in AD is responsible for a progressive loss of cognition and motor capacities. The cholinergic hypothesis has provided the rationale for the current treatment approaches based on acetylcholinesterase inhibitors. However, recent data focus on the complex nature of AD and disclose the involvement of other neurotransmitters such as serotonin, noradrenalin, dopamine, histamine, excitatory amino acids and neuropeptides among others. Interestingly, recent research has revealed that in severe AD brains the levels of AChE are considerably reduced whereas BuChE activity increases, thus aggravating the toxicity of βA. In such instances, it is possible that BuChE may be a more suitable target than AChE. Oxidative stress has been implicated in CNS degenerative disorders such as AD and PD. Therefore, owing to their capacity to inhibit oxidative damage, MAOIs are potential candidates as anti-Alzheimer drugs. More recently, a novel drug - TV3326 - was designed, based upon two pharmacophores: the carbamate moiety from rivastigmine, an AChE inhibitor; and the propargyl group from rasagiline, a MAO inhibitor. This drug exhibits cholinesterase and selective brain MAO inhibitory activities, reduces apoptosis and stimulates the processing of APPα, hence reducing the possibility of generation of the toxic βA. Thus, TV3326 may be expected to contribute positively to the cognitive benefits of Alzheimer's patients. Anyhow, the development of drugs with several targets and diverse pharmacological properties may conclusively demonstrate the most beneficial therapy.

Alzheimer's disease (AD) is associated to a gradual loss of attention and memory that have been associated to impairment of brain cholinergic neurotransmission, particularly a deficit of cholinergic neurons in the nucleus basalis of Meynert. Thus, it is not surprising that the first therapeutic target that has demonstrated therapeutic efficacy on cognition, behaviour and functional daily activities has been the inhibitors of acetylcholinesterase (AChE), i.e. tacrine, donepezil, rivastigmine and galanthamine. But not all inhibitors of AChE have the same potency to block the enzyme and have a different pharmacological profile. For instance, rivastigmine is a dual inhibitor of AChE and butyrylcholinesterase (BuChE), and galanthamine is a mild inhibitor of AChE and an allosteric potentiating ligand of neuronal nicotinic receptors for acetylcholine (nAChRs). In addition, we have recently found that galanthamine has neuroprotective effects by inducing calcium signals and the induction of the antiapoptotic protein Bcl-2. In this frame, we have been synthesizing new tacrine derivatives that keep their ability to inhibit AChE but that interfere with neuronal calcium overloading and prevent apoptosis. Some of these compounds exhibit neuroprotecting properties and thus, could be useful in the treatment of neurodegenerative and ischaemic brain diseases.