Recent developments with macrolide antibiotics

Dinuclear vs. Mononuclear Copper(II) Coordination Species of Tylosin and Tilmicosin in Non-Aqueous Solutions

The veterinary 16-membered macrolide antibiotics tylosin (HTyl, 1a) and tilmicosin (HTilm, 1b) react with copper(II) ions in acetone at metal-to-ligand molar ratio of 1:2 to form blue (2) or green (3) metal(II) coordination species, containing nitrate or chloride anions, respectively. The complexation processes and the properties of 2–3 were studied by an assortment of physicochemical techniques (UV-Vis, EPR, NMR, FTIR, elemental analysis). The experimental data revealed that the main portion of copper(II) ions are bound as neutral EPR-silent dinuclear complexes of composition [Cu₂(µ-NO₃)₂L₂] (2a–b) and [Cu₂(µ-Cl)₂Cl₂(HL)₂] (3a–b), containing impurities of EPR-active mono-species [Cu(NO₃)L] (2a’–b’) and [CuCl₂(HL)] (3a’–b’). The possible structural variants of the dinuclear- and mono-complexes were modeled by the DFT method, and the computed spectroscopic parameters of the optimized constructs were compared to those measured experimentally. Using such a combined approach, the main coordination unit of the macrolides, involved in the complex formation, was defined to be their mycaminosyl substituent, which acts as a terminal ligand in a bidentate mode through the tertiary nitrogen atom and the oxygen from a deprotonated (2) or non-dissociated (3) hydroxyl group, respectively.

The isolation of water-soluble natural products - challenges, strategies and perspectives

Covering period: up to 2019Water-soluble natural products constitute a relevant group of secondary metabolites notably known for presenting potent biological activities. Examples are aminoglycosides, β-lactam antibiotics, saponins of both terrestrial and marine origin, and marine toxins. Although extensively investigated in the past, particularly during the golden age of antibiotics, hydrophilic fractions have been less scrutinized during the last few decades. This review addresses the possible reasons on why water-soluble metabolites are now under investigated and describes approaches and strategies for the isolation of these natural compounds. It presents examples of several classes of hydrosoluble natural products and how they have been isolated. Novel stationary phases and chromatography techniques are also reviewed, providing a perspective towards a renaissance in the investigation of water-soluble natural products.

Identification of impurities in macrolides by liquid chromatography-mass spectrometric detection and prediction of retention times of impurities by constructing quantitative structure-retention relationship (QSRR)

Macrolides are multicomponent drugs whose impurity control is always a challenge demanding analysis method with good sensitivity and selectivity. Three separate, sensitive, accurate liquid chromatography tandem mass spectrometry methods (LC-MS) were developed for the measurement of three 16-membered ring macrolides (josamycin, josamycin propionate and midecamycin acetate) and related substances in commercial samples. The characteristics of impurities in macrolides were summarized as useful guidance for the impurity analysis of this class of drugs. For each drug, a large number of unknown components have been detected with the high-sensitive MS detector and possible structures of the majority of them were postulated based on the summarized fragmentation rules of 16-membered ring macrolides. A QSRR model was constructed by multilinear regression to predict the retention times of identified impurities which were not detected by the LC-MS methods, without obtaining their reference standards. Satisfactory performance was obtained during leave-one-out cross-validation with a predictive ability (Q²) of 0.95. The generalisation ability of the model was further confirmed by an average error of 2.3% in external prediction. The best QSRR model, based on eight molecular descriptors, exhibited a promising predictive performance and robustness.

Application of combinatorial chemistry to antimicrobial drug discovery

The emergence of pathogens resistant to currently available treatments is seen as a public health crisis. Since few new classes of antimicrobial drugs have been developed in the last two decades, it is becoming increasingly probable that healthcare providers will be faced with infections for which no chemotherapeutic agent is available. A renewed emphasis is being placed on employing the most advanced drug discovery technologies in the development of new antimicrobials. The recently introduced technologies of combinatorial chemistry offer new sources of chemical diversity, as well as methods with which to produce and rapidly test them. In the last few years, many groups have adopted a number of approaches in order to apply combinatorial chemistry to antimicrobial drug discovery. These combinatorial strategies, and the manner in which they are used to develop new screening formats or to identify new chemical leads are, reviewed.

Synthesis and Antibacterial Activity of a Novel Class of 4‘-Substituted 16-Membered Ring Macrolides Derived from Tylosin

Novel 4'-substituted 16-membered ring macrolides were synthesized by the cleavage of the mycarose sugar of tylosin and subsequent modification of 4'-hydroxyl group. This new class of macrolide antibiotics exhibited potent activity against some key erythromycin-resistant pathogens.

Erythromycin, roxithromycin, and clarithromycin: use of slow-binding kinetics to compare their in vitro interaction with a bacterial ribosomal complex active in peptide bond formation

In a cell-free system derived from Escherichia coli, it is shown that clarithromycin and roxithromycin, like their parent compound erythromycin, do not inhibit the puromycin reaction (i.e., the peptide bond formation between puromycin and AcPhe-tRNA bound at the P-site of 70S ribosomes programmed with heteropolymeric mRNA). Nevertheless, all three antibiotics compete for binding on the ribosome with tylosin, a 16-membered ring macrolide that behaves as a slow-binding, slowly reversible inhibitor of peptidyltransferase. The mutually exclusive binding of these macrolides to ribosomes is also corroborated by the fact that they protect overlapping sites in domain V of 23S rRNA from chemical modification by dimethyl sulfate. From this competition effect, detailed kinetic analysis revealed that roxithromycin or clarithromycin (A), like erythromycin, reacts rapidly with AcPhe-tRNA.MF-mRNA x 70S ribosomal complex (C) to form the encounter complex CA which is then slowly isomerized to a more tight complex, termed C*A. The value of the overall dissociation constant, K, encompassing both steps of macrolide interaction with complex C, is 36 nM for erythromycin, 20 nM for roxithromycin, and 8 nM for clarithromycin. Because the off-rate constant of C*A complex does not significantly differ among the three macrolides, the superiority of clarithromycin as an inhibitor of translation in E. coli cells and many Gram-positive bacteria may be correlated with its greater rate of association with ribosomes.

Synthesis of new 14-membered macrolide antibiotics via a novel ring contraction metathesis

[reaction: see text] A novel ring opening ring closing metathesis (ROM-RCM) was demonstrated for cyclic conjugated dienes, effecting the excision of a C(2)H(2) unit and a net ring contraction. Applying the ring contraction metathesis, new 14-membered ring macrolide antibiotics were synthesized in a single step from existing 16-membered ring macrolides. This new class of macrolide antibiotics will provide access to new therapeutics for the treatment of macrolide-resistant bacterial infections.

Synthesis and antibacterial activity of ketolides (6-O-methyl-3-oxoerythromycin derivatives): a new class of antibacterials highly potent against macrolide-resistant and -susceptible respiratory pathogens

In the search for new antibiotics active against macrolide-resistant pneumococci and Haemophilus influenzae, we synthesized a new class of 3-oxo-6-O-methylerythromycin derivatives, so-called "ketolides". A keto function was introduced in position 3 after removal of L-cladinose, a sugar which has long been thought essential. Further modifications of the macrolactone backbone allowed us to obtain three different series of 9-oxime, 11,12-carbamate, and 11, 12-hydrazonocarbamate ketolides. These compounds were found to be very active against penicillin/erythromycin-resistant pneumococci and noninducers of MLSB resistance. The 11,12-substituted ketolide 61 (HMR 3004) demonstrated a potent activity against multiresistant pneumococci associated with a well-balanced activity against all bacteria involved in respiratory infections including H. influenzae, Mycoplasma catarrhalis, group A streptococci, and atypical bacteria. In addition HMR 3004 displayed high therapeutic activity in animals infected by all major strains, irrespective of their resistance phenotype.