Metal complexes of anhydrotetracycline. 2. Absorption and circular dichroism study of Mg(II), Al(III), and Fe(III) complexes. Possible influence of the Mg(II) complex on the toxic side effects of tetracycline

Development and Evaluation of EDTA-Treated Rabbits for Bioavailability Study of Chelating Drugs Using Levofloxacin, Ciprofloxacin, Hemiacetal Ester Prodrugs, and Tetracycline

Laboratory rabbits are fed foods rich with cationic metals, and while fasting cannot empty gastric contents because of their coprophagic habits. This implies that, in rabbits, the oral bioavailability of chelating drugs could be modulated by the slow gastric emptying rates and the interaction (chelation, adsorption) with gastric metals. In the present study, we tried to develop a rabbit model with low amounts of cationic metals in the stomach for preclinical oral bioavailability studies of chelating drugs. The elimination of gastric metals was achieved by preventing food intake and coprophagy and administering a low concentration of EDTA 2Na solution one day before experiments. Control rabbits were fasted but coprophagy was not prevented. The efficacy of rabbits treated with EDTA 2Na was evaluated by comparing the gastric contents, gastric metal contents and gastric pH between EDTA-treated and control rabbits. The treatment with more than 10 mL of 1 mg/mL EDTA 2Na solution decreased the amounts of gastric contents, cationic metals and gastric pH, without causing mucosal damage. The absolute oral bioavailabilities (mean values) of levofloxacin (LFX), ciprofloxacin (CFX) and tetracycline hydrochloride (TC), chelating antibiotics, were significantly higher in EDTA-treated rabbits than those in control rabbits as follows: 119.0 vs. 87.2%, 9.37 vs. 13.7%, and 4.90 vs. 2.59%, respectively. The oral bioavailabilities of these drugs were significantly decreased when Al(OH)₃ was administered concomitantly in both control and EDTA-treated rabbits. In contrast, the absolute oral bioavailabilities of ethoxycarbonyl 1-ethyl hemiacetal ester (EHE) prodrugs of LFX and CFX (LFX-EHE, CFX-EHE), which are non-chelating prodrugs at least in in vitro condition, were comparable between control and EDTA-treated rabbits irrespective of the presence of Al(OH)₃, although some variation was observed among rabbits. The oral bioavailabilities of LFX and CFX from their EHE prodrugs were comparable with LFX and CFX alone, respectively, even in the presence of Al(OH)₃. In conclusion, LFX, CFX and TC exhibited higher oral bioavailabilities in EDTA-treated rabbits than in control rabbits, indicating that the oral bioavailabilities of these chelating drugs are reduced in untreated rabbits. In conclusion, EDTA-treated rabbits were found to exhibit low gastric contents including metals and low gastric pH, without causing mucosal damage. Ester prodrug of CFX was effective in preventing chelate formation with Al(OH)₃ in vitro and in vivo, as well as in the case of ester prodrugs of LFX. EDTA-treated rabbits are expected to provide great advantages in preclinical oral bioavailability studies of various drugs and dosage formulations. However, a marked interspecies difference was still observed in the oral bioavailability of CFX and TC between EDTA-treated rabbits and humans, possibly due to the contribution of adsorptive interaction in rabbits. Further study is necessary to seek out the usefulness of the EDTA-treated rabbit with less gastric contents and metals as an experimental animal.

Interaction between tetracycline and microorganisms during wastewater treatment: A review

Tetracycline (TC) is a commonly used human and veterinary antibiotic that is mostly discharged into wastewater in the form of the parent compounds. At present, wastewater treatment plants (WWTPs) use activated sludge processes that are not specifically designed to remove such pollutants. Considering the biological toxicity of TC in aquatic environment, the migration and fate of TC in the process of wastewater treatment deserve attention. This paper reviews the influence of TC on the functional bacteria in the sludge matrix and the development of tetracycline-resistant genes, and also discusses their adsorption removal rates, their adsorption kinetics and adsorption isotherm models, and infers their adsorption mechanism. In addition, the biodegradation of TC in the process of biological treatment is reviewed. Co-metabolism and the role of dominant bacteria in the degradation process are described, along with the formation of degradation byproducts and their toxicity. Furthermore, the current popular integrated coupling-system for TC degradation is also introduced. This paper systematically introduces the interaction between TC and activated sludge in WWTPs. The review concludes by providing directions to address research and knowledge gaps in TC removal from wastewater.

Toxicity of tetracycline and its transformation products to a phosphorus removing Shewanella strain

Tetracycline (TC) as an emerging contaminant has raised serious concerns about its toxicity and removal in wastewater treatment processes. The more toxic transformation products of TC, 4-epitetracycline (ETC), anhydrotetracycline (ATC) and 4-epianhydrotetracycline (EATC) are also widely detected. This study investigated the antibacterial and bactericidal activity of TC, ETC, ATC, EATC against Shewanella sp, using Escherichia coli and Pseudomonas aeruginosa strains as quality controls. Further, batch assays were conducted to investigate the inhibition of these antibiotics on the phosphorus removal of the Shewanella strain, and removal mechanisms of TC and its transformation products (TCs). The inhibition on phosphorus removal by the Shewanella strain at 20 mg L^-1 was in the order of ATC > EATC > TC > ETC. COD removal, poly-P accumulation and glycogen synthesis by the Shewanella strain were also inhibited. Biodegradation was the main removal mechanism of TC and ETC, while adsorption was the main one of ATC and EATC. This study helps to further understand the structure-activity relationship of TC.

An Artificial Yeast Genetic Circuit Enables Deep Mutational Scanning of an Antimicrobial Resistance Protein

Understanding the molecular mechanisms underlying antibiotic resistance requires concerted efforts in enzymology and medicinal chemistry. Here we describe a new synthetic biology approach to antibiotic development, where the presence of tetracycline antibiotics is linked to a life-death selection in Saccharomyces cerevisiae. This artificial genetic circuit allowed the deep mutational scanning of the tetracycline inactivating enzyme TetX, revealing key functional residues. We used both positive and negative selections to confirm the importance of different residues for TetX activity, and profiled activity hotspots for different tetracyclines to reveal substrate-specific activity determinants. We found that precise positioning of FAD and hydrophobic shielding of the tetracycline are critical for enzymatic inactivation of doxycycline. However, positioning of FAD is suboptimal in the case of anhydrotetracycline, potentially explaining its comparatively poor degradation and potential as an inhibitor for this family of enzymes. By combining artificial genetic circuits whose function can be modulated by antimicrobial resistance determinants, we establish a framework to select for the next generation of antibiotics.

Theoretical and Experimental Studies of the Controlled Release of Tetracycline Incorporated into Bioactive Glasses

Several authors have studied the release profile of drugs incorporated in different devices. However, to the best of our knowledge, although many studies have been done on the release of tetracycline, in these release devices, no study has investigated if the released compound is actually the tetracycline, or, instead, a degraded product. This approach is exploited here. In this work, we analyse the influence of two drying methods on the tetracycline delivery behaviour of synthesised glasses using the sol-gel process. We compare the drying methods results using both theoretical models and practical essays, and analyse the chemical characteristic of the released product in order to verify if it remains tetracycline. Samples were freeze-dried or dried in an oven at 37°C and characterised by several methods such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TG), differential thermogravimetric analysis (DTG), differential thermal analyses (DTA) and gas adsorption analysis (BET). The released concentration of tetracycline hydrochloride was studied as a function of time, and it was measured by ultraviolet spectrophotometry in the tetracycline wavelength. The drug delivery profiles were reasonably consistent with a diffusion model analysis. In addition, we observed higher release rates for the freeze-dried compared to those dried in an oven at 37°C. This higher release can be attributed to larger pore size for the freeze-dried sample systems with tetracycline, which promoted more water penetration, improving the drug diffusion. The analysis of the solution obtained in the release tests using high-performance liquid chromatography- mass spectrometry (HPLC-MS) confirmed that tetracycline was being released.

Adsorption of tetracycline on Fe (hydr)oxides: effects of pH and metal cation (Cu2+, Zn2+ and Al3+) addition in various molar ratios

Iron (Fe) (hydr)oxides control the mobility and bioavailability of tetracycline (TC) in waters and soils. Adsorption of TC on Fe (hydr)oxides is greatly affected by polyvalent metals; however, impacts of molar metal/TC ratios on TC adsorptive behaviours on Fe (hydr)oxides remain unclear. Results showed that maximum TC adsorption on ferrihydrite and goethite occurred at pH 5-6. Such TC adsorption was generally promoted by the addition of Cu²⁺, Zn²⁺ and Al³⁺. The greatest increase in TC adsorption was found in the system with molar Cu/TC ratio of 3 due to the formation of Fe hydr(oxide)-Cu-TC ternary complexes. Functional groups on TC that were responsible for the complexation with Cu²⁺shifted from phenolic diketone groups at Cu/TC molar ratio < 1 to amide groups at Cu/TC molar ratio ≥ 1. For the addition of Al³⁺, the complexation only took place with phenolic diketone groups, resulting in the enhanced TC adsorption at a molar Al/TC ratio of 1. However, TC adsorption decreased for Al/TC molar ratio > 1 as excess Al³⁺ led to the competitive adsorption with Al/TC complexes. For the Zn²⁺ addition, no significant correlation was found between TC adsorption capacity and molar Zn/TC ratios.

A novel fluorescent sensor based on sulfur and nitrogen co-doped carbon dots with excellent stability for selective detection of doxycycline in raw milk

Sulfur and nitrogen co-doped carbon dots obtained from casein exhibited excellent sensitivity and selectivity for the detection of doxycycline.

Transformation of Tetracycline Antibiotics and Fe(II) and Fe(III) Species Induced by Their Complexation

Tetracycline antibiotics (TCs) are frequently detected micropollutants and are known to have a strong tendency to complex with metal ions such as Fe(II) and Fe(III) in aquatic environments. Experiments with Fe(II) and TCs showed that the complexation of Fe(II) with tetracycline (TTC), oxytetracycline (OTC), or chlorotetracycline (CTC) could lead to the accelerated oxidation of Fe(II) and the promoted degradation of TCs simultaneously. The reaction started with complexation of Fe(II) with TC followed by oxidation of the Fe(II)-TC complex by dissolved oxygen to generate a Fe(III)-TC complex and reactive oxygen species (ROS). The ROS (primarily ·OH) then degraded TC. The oxidation rate constants of Fe(II) in the Fe(II)-H2L and Fe(II)-HL complexes were 0.269 and 1.511 min(-1), respectively, at ambient conditions (pH 7, 22 °C, and PO2 of 0.21 atm), which were about 60 and 350 times of the oxidation rate of uncomplexed Fe(II). Humic acids (HA) compete with TCs for Fe(II), but the effect was negligible at moderate HA concentrations (≤10 mg·L(-1)). Experiments with Fe(III) and TCs showed that the complexation of Fe(III) with TC could generate oxidized TC and Fe(II) without the need of oxygen at a relatively slower rate compared to the reaction involving Fe(II), O2, and TCs. These findings indicate the mutually influenced environmental transformation of TCs and Fe(II) and Fe(III) induced by their complexation. These newly identified reactions could play an important role in affecting the environmental fate of TCs and cycling of Fe(II) and Fe(III) in TCs-contaminated water and soil systems.

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.

Oxytetracycline sorption to organic matter by metal-bridging

The sorption of oxytetracycline to metal-loaded ion exchange resin and to natural organic matter by the formation of ternary complexes between polyvalent metal cations and sorbent- and sorbate ligand groups was investigated. Oxytetracycline (OTC) sorption to Ca- and Cu-loaded Chelex-100 resin increased with increasing metal/sorbate ratio at pH 7.6 (OTC speciation: 55% zwitterion, 45% anion). Greater sorption to Cu- than Ca-loaded resin was observed, consistent with the greater stability constants of Cu with both the resin sites and with OTC. Oxytetracycline sorption to organic matter was measured at pH 5.5 (OTC speciation: 1% cation, 98% zwitterion, 1% anion). No detectable sorption was measured for cellulose or lignin sorbents that contain few metal-complexing ligand groups. Sorption to Aldrich humic acid increased from "clean" < "dirty" (no cation exchange pretreatment) < Al-amended < Fe(III)-amended clean humic acid with K(d) values of 5500, 32000, 48000, and 250000 L kg(-1) C, respectively. Calcium amendments of clean humic acid suggested that a portion of the sorbed OTC was interacting by cation exchange. Oxytetracycline sorption coefficients for all humic acid sorbents were well-correlated with the total sorbed Al-plus-Fe(III) concentrations (r(2) = 0.87, log-log plot), suggesting that sorption by ternary complex formation with humic acid is important. Results of this research indicate that organic matter may be an important sorbent phase in soils and sediments for pharmaceutical compounds that can complex metals by the formation of ternary complexes between organic matter ligand groups and pharmaceutical ligand groups.

Sorption of oxytetracycline to iron oxides and iron oxide-rich soils

The sorption interactions of oxytetracycline with goethite, hematite, and two iron oxide-rich soils were investigated using batch sorption experiments. Oxytetracycline sorption coefficients for goethite and hematite increased with pH to maximum values at pH approximately 8. The sorption edge shape and desorption treatments were consistent with a surface complexation mechanism and could be described by the interaction of divalent anion species with the oxide surface. Oxytetracycline sorption to Georgeville and Orangeburg Ultisol soils decreased with pH. Chemical digestion treatments were used to deduce that soil sorption occurred by complexation to oxide coatings on clay and quartz grains. These results indicate that sorption models must consider the interaction of oxytetracycline, and other similar ionogenic compounds, with soil oxide components in addition to clays and organic matter when predicting sorption in whole soils.

Structure and function of "metalloantibiotics"

Although most antibiotics do not need metal ions for their biological activities, there are a number of antibiotics that require metal ions to function properly, such as bleomycin (BLM), streptonigrin (SN), and bacitracin. The coordinated metal ions in these antibiotics play an important role in maintaining proper structure and/or function of these antibiotics. Removal of the metal ions from these antibiotics can cause changes in structure and/or function of these antibiotics. Similar to the case of "metalloproteins," these antibiotics are dubbed "metalloantibiotics" which are the title subjects of this review. Metalloantibiotics can interact with several different kinds of biomolecules, including DNA, RNA, proteins, receptors, and lipids, rendering their unique and specific bioactivities. In addition to the microbial-originated metalloantibiotics, many metalloantibiotic derivatives and metal complexes of synthetic ligands also show antibacterial, antiviral, and anti-neoplastic activities which are also briefly discussed to provide a broad sense of the term "metalloantibiotics."

A physicochemical study of the tetracycline coordination to oxovanadium(IV)

The interaction of tetracycline and oxovanadium(IV) in aqueous solution was studied by potentiometric and spectrophotometric methods. Oxovanadium(IV) ions form both a positively charged 1:1 and a neutral 2:1 metal-ligand complex with tetracycline. When a 1:1 ligand-to-metal ratio mixture is used at about pH 4.5 the 1:1 species predominates, being replaced at pH 6 by the binuclear complex. The binuclear complex has been isolated and fully characterised. Infrared and EPR studies suggest the existence of two distinct vanadyl binding sites. Our results indicate that the first vanadium coordinates to the BCD-ring system and the second one to the A-ring. Biological implications of the existence of a neutral complex at physiological pH are briefly discussed.

Suppressive effects of josamycin on the development of altered liver cell foci and chronic nephropathy in a carcinogenicity study

The carcinogenicity of josamycin was examined in Fischer 344 (F344) rats. Groups of 50 males and 50 females were given the compound in their diet at concentrations of 0 (control), 1.25 or 2.5% for 104-weeks; these dose levels were selected on the basis of the results of a subchronic study, in which animals rather rejected 5% josamycin. All surviving rats were killed at wk 106. A variety of tumours developed in all groups, including the control group, but all the neoplastic lesions were histologically similar to those known to occur spontaneously in this strain of rats, and no statistically significant increase in the incidence of any tumour was found in the treated groups of either sex. Interestingly, the josamycin treatment significantly reduced the development of altered liver cell foci and chronic nephropathy in a dose-dependent manner. Thus, it was concluded that, under the present experimental conditions, josamycin is not carcinogenic in F344 rats.

Importance of tautomers in the chemical behavior of tetracyclinesdagger

We advance the concept that tautomerism is crucial for the understanding of the chemical behavior of tetracycline. Indeed, considering four deprotonations, there are 64 different possible tautomers to be considered for tetracycline. Our results indicate that tetracycline is a very adaptive molecule, capable of easily modifying itself through tautomerism in response to various chemical environments. Indeed, its situation in solution can be more accurately pictured as an equilibrium among a diversity of tautomeric species-an equilibrium that can be easily displaced depending on the various possible chemical perturbations, such as varying the pH or the dielectric constant of the solvent. Moreover, we also show that tetracycline could undergo four deprotonations and predict for it a fourth pKa of 13 and refer to our experimental determination of this parameter, which yielded the value of 12. We conclude that tautomerism is essential to the comprehension of the chemical behavior of tetracycline as determined by the semiempirical method AM1 as well as by the self-consistent reaction field method, which estimates the effects of the solvent on the tautomers. All tautomers in their different conformations have been fully optimized for each of the possible degrees of protonation of this molecule. Thus, the relative stabilities of the different tautomeric species have been computed.

A theoretical study of the interaction of anhydrotetracycline with Al(III)

In this article the complexation of anhydrotetracycline (AHTC), the major toxic decomposition product of the antibiotic tetracycline, with Al(III) has been investigated using the AM1 semiempirical and ab initio Hartree-Fock levels of theory. Different modes of complexation have been considered with the structure of tetra- and pentacoordinated complexes being fully optimized. In the gas phase, processes ii and iii, which lead to the complexes with stoichiometry MHL2+, are favored. Structure II ([AlLH2(OH)(H2O)]2+) has the metal coordinated to the O11 and O12 groups and the O3 group protonated and is the global minimum on the potential energy surface for the interaction. In water solution, the Al(III) is predicted to form predominantly a tetracoordinated complex at the Oam and O3 site (V) of the AHTC with the stoichiometry MH2L3+ (process i). The experimental proposal is the complexed form with the metal ion coordinated to the O11-O12 moiety (site II). The intramolecular proton transfer, which leads to the most stable Al(III)-AHTC MHL2+ complex, has not been considered by the experimentalists. The experimental structure was found to be unfavorable in our calculations in both gas phase and water solution. All the semiempirical results are in perfect agreement with the ab initio calculations. So, we suggest that the experimental assignments should be revised, taking into account the results obtained in the present study.

Conformational analysis of the anhydrotetracycline molecule: a toxic decomposition product of tetracycline

Anhydrotetracycline (AHTC) is a toxic decomposition product of the widely used antibiotic tetracycline (TC). The side effects of AHTC have been attributed to the conformational changes in the ring system. In the present study a systematic conformational analysis has been carried out using the semiempirical quantum mechanical AM1 model. The conformational pH dependence has been analyzed through the study of all the ionized species. The results obtained showed two distinct families of conformation, referred to as A and B, with the interconversion process involving a rotation around the C4a-C12a bond. The solvent effect has been considered using the continuum model COSMO. From the population analysis in the gas phase, we conclude that form A should be dominant for the LH3+ and LH2 +/- species and B is the preferred conformer for the L2- ionized form (97.54%). For the LH- derivative, we predict that both conformations should be present in the equilibrium mixture in the gas phase, with the relative concentration found to be 68.47% (A) and 31.53% (B). The inclusion of the solvent does not change the A/B equilibrium for the LH3+ and LH2 +/- species. However, for the LH- form, the equilibrium is shifted to conformer A in water solution. The population analysis in water solution for the L2- suggest the following relative concentrations: A (34.46%) and B (65.54%). The biological activity of the TC parent compound is attributed to the zwitterionic species, which should adopt a twisted conformation. According to the results obtained in the present study, the most abundant form of the LH2 +/- zwitterionic species for the AHTC molecule is the extended one (100% in both the gas phase and water solution). Therefore, from a pharmacodynamic point of view, this conformational difference should be taken into account in order to explain the toxic effects of the anhydrous derivative. Another point related to the structure-activity relationship was analyzed through the investigation of the tautomerization process LH2(0)-->LH2 +/-. The result obtained suggests that the LH2(0) tautomer should be dominant in the gas phase (nonpolar solvent) and adopt a conformation classified as B. In water solution, the tautomer LH2 +/- is present as conformer A (96%). This result is in agreement with the conformation changes involved in the tautomerization process for the OTC active derivative.

Molecular mechanics analysis of Tet repressor TRP-43 fluorescence

A 35% decrease in the fluorescence intensity of F75 TetR Trp-43 was observed upon binding of the tetracycline derivative 5a,6-anhydrotetracycline (AnTc) to the repressor. The fluorescence decay of Trp-43 in F75 TetR and in its complex with AnTc could be described by the sum of three exponential components, with lifetimes of about 6, 3, and 0.3 ns. The amplitudes, however, were markedly altered upon binding. The minimized energy mapping of Trp-43 chi 1 x chi 2 isomerization clearly indicated the existence of three main potential wells at positions (-160 degrees, -90 degrees) (rotamer I), (-170 degrees, 90 degrees) (rotamer II), and (-70, 150 degrees) (rotamer III). Our study of Trp-43 environment for each of the three rotamers suggests that the longest decay component may be assigned to rotamer II, the middle-lived component to rotamer I, and the subnanosecond component to rotamer III. The origin of the changes in the rotamer distribution upon AnTc binding is discussed. Anisotropy decays are also discussed within the framework of the rotamer model.