Tetracycline and its analogues as inhibitors of amyloid fibrils: searching for a geometrical pharmacophore by theoretical investigation of their conformational behavior in aqueous solution

Targeting Amyloid Aggregation: An Overview of Strategies and Mechanisms

Amyloids result from the aggregation of a set of diverse proteins, due to either specific mutations or promoting intra- or extra-cellular conditions. Structurally, they are rich in intermolecular β-sheets and are the causative agents of several diseases, both neurodegenerative and systemic. It is believed that the most toxic species are small aggregates, referred to as oligomers, rather than the final fibrillar assemblies. Their mechanisms of toxicity are mostly mediated by aberrant interactions with the cell membranes, with resulting derangement of membrane-related functions. Much effort is being exerted in the search for natural antiamyloid agents, and/or in the development of synthetic molecules. Actually, it is well documented that the prevention of amyloid aggregation results in several cytoprotective effects. Here, we portray the state of the art in the field. Several natural compounds are effective antiamyloid agents, notably tetracyclines and polyphenols. They are generally non-specific, as documented by their partially overlapping mechanisms and the capability to interfere with the aggregation of several unrelated proteins. Among rationally designed molecules, we mention the prominent examples of β-breakers peptides, whole antibodies and fragments thereof, and the special case of drugs with contrasting transthyretin aggregation. In this framework, we stress the pivotal role of the computational approaches. When combined with biophysical methods, in several cases they have helped clarify in detail the protein/drug modes of interaction, which makes it plausible that more effective drugs will be developed in the future.

Minocycline Inhibits Candida albicans Budded-to-Hyphal-Form Transition and Biofilm Formation

Candida albicans frequently causes bloodstream infections; its budded-to-hyphalform transition (BHT) and biofilm formation are major contributors to virulence. During an analysis of antibacterial compounds that inhibit C. albicans BHT, we found that the tetracycline derivative minocycline inhibited BHT and subsequent biofilm formation. Minocycline decreased expression of hypha-specific genes HWP1 and ECE1, and adhesion factor gene ALS3 of C. albicans. In addition, minocycline decreased cell surface hydrophobicity and the extracellular β-glucan level in biofilms. Minocycline has been widely used for catheter antibiotic lock therapy to prevent bacterial infection; this compound may also be prophylactically effective against Candida infection.

A new face for old antibiotics: tetracyclines in treatment of amyloidoses

The use of tetracyclines has declined because of the appearance of resistant bacterial strains. However, the indications of nonantimicrobial activities of these drugs have considerably raised interest and triggered clinical trials for a number of different pathologies. About 10 years ago we first reported that tetracyclines inhibited the aggregation of prion protein fragments and Alzheimer's β peptides, destabilizing their aggregates and promoting their degradation by proteases. On the basis of these observations, the antiamyloidogenic effects of tetracyclines on a variety of amyloidogenic proteins were studied and confirmed by independent research groups. In this review we comment on the data available on their antiamyloidogenic activity in preclinical and clinical studies. We also put forward that the beneficial effects of these drugs are a result of a peculiar pleiotropic action, comprising their interaction with oligomers and disruption of fibrils, as well as their antioxidant, anti-inflammatory, antiapoptotic, and matrix metalloproteinase inhibitory activities.

Classification Framework and Chemical Biology of Tetracycline-Structure-Based Drugs

By studying the literature about tetracyclines (TCs), it becomes clearly evident that TCs are very dynamic molecules. In some cases, their structure-activity-relationship (SAR) are well known, especially against bacteria, while against other targets, they are virtually unknown. In other diverse fields of research-such as neurology, oncology and virology-the utility and activity of the tetracyclines are being discovered and are also emerging as new technological fronts. The first aim of this paper is to classify the compounds already used in therapy and prepare the schematic structure that includes the next generation of TCs. The second aim of this work is to introduce a new framework for the classification of old and new TCs, using a medicinal chemistry approach to the structure of those drugs. A fully documented Structure-Activity-Relationship (SAR) is presented with the analysis data of antibacterial and nonantibacterial (antifungal, antiviral and anticancer) tetracyclines. The lipophilicity and the conformational interchangeability of the functional groups are employed to develop the rules for TC biological activity.

Fast and ultrafast spectroscopic investigation of tetracycline derivatives in organic and aqueous media

The photophysical properties of seven tetracycline derivatives (tetracycline, oxytetracycline, demeclocycline, chlortetracycline, doxycycline, minocycline and meclocycline) in organic solvents and aqueous solution were studied using steady-state absorption and fluorescence techniques and transient absorption spectroscopies with nanosecond and femtosecond time resolution. The molecular structure, solvent and pH effects on the optical properties of this class of pharmaceutically interesting compounds were investigated in detail. The investigation furnished a complete description of the nature, the spectral and kinetic properties of the excited states formed upon irradiation. All the tetracycline derivatives exhibited a similar behaviour, and the photophysics of these molecules is different in organic solvents and in aqueous medium, where they exhibit a significant pH dependence. In water, compared to organic solvents, these compounds showed a blue-shifted bathochromic absorption band, a red-shifted emission spectrum, an increased Stokes shift and a decreased fluorescence quantum yield. These findings, together with the overall investigated solvent effect, suggested that in aqueous solvent additional fast and non-radiative deactivation processes, responsible for the large Stokes Shift and for the reduced fluorescence efficiency, are present. In fact, in organic media just two transients were observed during the ultrafast time-resolved investigation: the vibrationally hot S(1) state which was quickly stabilized by solvent reorganization to the relaxed S(1) state. This state showed lifetimes of tens of picoseconds and relaxed by fluorescence and internal conversion. No longer-lived transients were detected. In aqueous solution the excited-state deactivation of tetracyclines was found to be more complicated. Different protonated and tautomeric forms of the S(1) state were detected: a component which showed decay times of tens of picoseconds and a component which was longer-lived. A significant pH effect on the nature and number of these components was found. In fact, a remarkable change in the Stokes shift and in the fluorescence efficiency was also observed on going from acidic to basic aqueous solutions. The most important variations in the absorption properties were found in the pH range in which the second acid-base equilibrium takes place. The tetracycline lowest excited triplet state was observed as a 'rest absorption' during the femtosecond-resolved measurements in aqueous solution; through the nanosecond-resolved laser flash photolysis study, lower-energy radical species were detected, characterized by lifetimes of tens of microseconds. The formation of these species may be involved in the observed phototoxicity of the tetracycline drugs.

The efficacy of tetracyclines in peripheral and intracerebral prion infection

We have previously shown that tetracyclines interact with and reverse the protease resistance of pathological prion protein extracted from scrapie-infected animals and patients with all forms of Creutzfeldt-Jakob disease, lowering the prion titre and prolonging survival of cerebrally infected animals. To investigate the effectiveness of these drugs as anti-prion agents Syrian hamsters were inoculated intramuscularly or subcutaneously with 263K scrapie strain at a 10⁻⁴ dilution. Tetracyclines were injected intramuscularly or intraperitoneally at the dose of 10 mg/kg. A single intramuscular dose of doxycycline one hour after infection in the same site of inoculation prolonged median survival by 64%. Intraperitoneal doses of tetracyclines every two days for 40 or 44 days increased survival time by 25% (doxycycline), 32% (tetracycline); and 81% (minocycline) after intramuscular infection, and 35% (doxycycline) after subcutaneous infection. To extend the therapeutic potential of tetracyclines, we investigated the efficacy of direct infusion of tetracyclines in advanced infection. Since intracerebroventricular infusion of tetracycline solutions can cause overt acute toxicity in animals, we entrapped the drugs in liposomes. Animals were inoculated intracerebrally with a 10⁻⁴ dilution of the 263K scrapie strain. A single intracerebroventricular infusion of 25 µg/ 20 µl of doxycycline or minocycline entrapped in liposomes was administered 60 days after inoculation, when 50% of animals showed initial symptoms of the disease. Median survival increased of 8.1% with doxycycline and 10% with minocycline. These data suggest that tetracyclines might have therapeutic potential for humans.

Computational approaches to shed light on molecular mechanisms in biological processes

Computational approaches based on Molecular Dynamics simulations, Quantum Mechanical methods and 3D Quantitative Structure-Activity Relationships were employed by computational chemistry groups at the University of Milano-Bicocca to study biological processes at the molecular level. The paper reports the methodologies adopted and the results obtained on Aryl hydrocarbon Receptor and homologous PAS proteins mechanisms, the properties of prion protein peptides, the reaction pathway of hydrogenase and peroxidase enzymes and the defibrillogenic activity of tetracyclines.

Role of zwitterionic structures in the solid-phase extraction based method development for clean up of tetracycline and oxytetracycline from honey

Solid-phase extraction (SPE) based sample clean up of tetracycline (TC) and oxytetracycline (OTC) from honey samples was evaluated using a neutral polymeric sorbent (strata-X), a weak cation exchange polymeric sorbent (strata-X-CW) and a combination of neutral and strong cation exchange sorbents (strata-X plus Strata-Screen-C or strata-X-C). Both TC and OTC are recovered in low yields from the strong cation exchange sorbents strata-X-C and Strata-Screen-C under basic (pH >9) or acidic (pH >or=2) elution conditions. The cleanest extracts along with quantitative recoveries were obtained with strata-X-CW under either acidic or basic elution conditions, as demonstrated by analysis of eluates by both LC/UV and LC/MS-MS. On the other hand, the neutral sorbent (strata-X) was less efficient in eliminating the honey matrix constituents, since it could not retain the tetracyclines with the strong organic wash needed to remove other compounds. The differences in the elution behavior of the strong and weak cation exchange sorbents is rationalized on the basis of divergent deprotonation mechanisms of the tetracyclines, which exist in the zwitterionic structures under the SPE conditions.

The structure of 5a,6-anhydrotetracycline and its Mg2+ complexes in aqueous solution

Semiempirical molecular orbital theory has been used for a systematic scan of the binding positions for a Mg2+ ion with 5a,6-anhydrotetracycline taking both conformational flexibility and possible different tautomeric forms into account. The magnesium ion has been calculated alone and with four or five complexed water molecules in order to simulate the experimental situation more closely. The results are analyzed by comparing the behavior of the title compound with that of tetracycline itself and possible causes for the stronger induction of the Tetracycline Receptor (TetR) by 5a,6-anhydrotetracycline than by tetracycline are considered. Energetically favored 3D-structure of the zwitteranionic 5a,6-anhydrotetracycline magnesium complex in solution.

Prion 2005: Between Fundamentals and Society's Needs

Prion diseases for the most part affect individuals older than 60 years of age and share features with other diseases characterized by protein deposits in the brain, such as Alzheimer's disease and Parkinson's disease. The international conference "Prion 2005: Between Fundamentals and Society's Needs," organized by the German Transmissible Spongiform Encephalopathies Research Platform, aimed to integrate and coordinate the research efforts of participants to better achieve prevention, treatment, control, and management of prion diseases, including Creutzfeldt-Jakob disease and fatal familial insomnia in humans. Several main topics were discussed, such as the molecular characteristics of prion strains, the cell biology of cellular and pathogenic forms of the prion proteins, the pathogenesis of the diseases they cause, emerging problems, and promising approaches for therapy and new diagnostic tools. The presentations at the Prion 2005 conference provided new insights in both basic and applied research, which will have broad implications for society's needs.