Solithromycin

Synthesis, Biological Evaluation, and Computational Analysis of Biaryl Side-Chain Analogs of Solithromycin

There is an urgent need for new antibiotics to mitigate the existential threat posed by antibiotic resistance. Within the ketolide class, solithromycin has emerged as one of the most promising candidates for further development. Crystallographic studies of bacterial ribosomes and ribosomal subunits complexed with solithromycin have shed light on the nature of molecular interactions (π-stacking and H-bonding) between from the biaryl side-chain of the drug and key residues in the 50S ribosomal subunit. We have designed and synthesized a library of solithromycin analogs to study their structure-activity relationships (SAR) in tandem with new computational studies. The biological activity of each analog was evaluated in terms of ribosomal affinity (K_d determined by fluorescence polarization), as well as minimum inhibitory concentration assays (MICs). Density functional theory (DFT) studies of a simple binding site model identify key H-bonding interactions that modulate the potency of solithromycin analogs.

Differentiating the Pharmacodynamics and Toxicology of Macrolide and Ketolide Antibiotics

This is a review of the macrolide and ketolide field focusing on differentiating the pharmacodynamics and especially the toxicology of the macrolides and ketolides. We emphasize the diversity in pharmacodynamics and toxicity of the macrolides and ketolides, resulting from even small structural changes, which makes it important to consider the various different compounds separately, not necessarily as a class. The ketolide, telithromycin, was developed because of rising bacterial macrolide resistance but was withdrawn postapproval after visual disturbances, syncope, myasthenia gravis, and hepatotoxicity were noted. These diverse adverse effects could be attributed to inhibition of nicotinic acetylcholine receptors. Solithromycin, a later generation ketolide, was effective in treating bacterial pneumonia, but it was not approved by the U.S. Food and Drug Administration owing, in part, to its structural similarity to telithromycin. This Miniperspective describes that structurally similar macrolides/ketolides have clearly mechanistically distinct effects. Understanding these effects could help in developing and securing regulatory approval of a new macrolide/ketolide that is active against macrolide-resistant pathogenic bacteria.

Synthesis and biological evaluation of solithromycin analogs against multidrug resistant pathogens

Novel antibacterial drugs that treat multidrug resistant pathogens are in high demand. We have synthesized analogs of solithromycin using Cu(I)-mediated click chemistry. Evaluation of the analogs using Minimum Inhibitory Concentration (MIC) assays against resistant Staphylococcus aureus, Escherichia coli, and multidrug resistant pathogens Enterococcus faecium and Acinetobacter baumannii showed they possess potencies similar to those of solithromycin, thus demonstrating their potential as future therapeutics to combat the existential threat of multidrug resistant pathogens.

Ribosome-Templated Azide-Alkyne Cycloadditions Using Resistant Bacteria as Reaction Vessels: in Cellulo Click Chemistry

In situ click chemistry has been a powerful method for fragment-based drug design since its discovery in 2002. Recently, we demonstrated that the bacterial ribosome can template the azide-alkyne cycloaddition reaction to expedite the discovery of novel antibiotics. We now report this process can be performed in an antibiotic-resistant bacterial cell. The corresponding triazole products formed in cellulo are potent antibiotics that inhibit bacterial growth; moreover, the potency of each cycloadduct can be visualized using the traditional MIC assay in a 96-well plate format. We characterized the in cellulo clicked products by independent chemical synthesis and LC-MS analysis, which showed that mass count percent increase was directly proportional to 1/MIC. In other words, potent compounds detected by MIC were formed in greater amounts. Control experiments unambiguously showed the ribosome was responsible for templating triazole formation. Significantly, our method (1) obviates the need to isolate bacterial ribosomes; (2) could be applied to different bacterial strains, which broadens the scope and facilitates the discovery of narrow-spectrum antibiotics; and (3) does not require the knowledge of mode-of-action and thus could uncover novel antibiotic targets. We believe this method could be expanded and implemented as a novel approach for antibiotic drug discovery.

Nature nurtures the design of new semi-synthetic macrolide antibiotics

Erythromycin and its analogs are used to treat respiratory tract and other infections. The broad use of these antibiotics during the last 5 decades has led to resistance that can range from 20% to over 70% in certain parts of the world. Efforts to find macrolides that were active against macrolide-resistant strains led to the development of erythromycin analogs with alkyl-aryl side chains that mimicked the sugar side chain of 16-membered macrolides, such as tylosin. Further modifications were made to improve the potency of these molecules by removal of the cladinose sugar to obtain a smaller molecule, a modification that was learned from an older macrolide, pikromycin. A keto group was introduced after removal of the cladinose sugar to make the new ketolide subclass. Only one ketolide, telithromycin, received marketing authorization but because of severe adverse events, it is no longer widely used. Failure to identify the structure-relationship responsible for this clinical toxicity led to discontinuation of many ketolides that were in development. One that did complete clinical development, cethromycin, did not meet clinical efficacy criteria and therefore did not receive marketing approval. Work on developing new macrolides was re-initiated after showing that inhibition of nicotinic acetylcholine receptors by the imidazolyl-pyridine moiety on the side chain of telithromycin was likely responsible for the severe adverse events. Solithromycin is a fourth-generation macrolide that has a fluorine at the 2-position, and an alkyl-aryl side chain that is different from telithromycin. Solithromycin interacts at three sites on the bacterial ribosome, has activity against strains resistant to older macrolides (including telithromycin), and is mostly bactericidal. Pharmaceutical scientists involved in the development of macrolide antibiotics have learned from the teachings of Professor Satoshi Omura and progress in this field was not possible without his endeavors.

SOLITAIRE-IV: A Randomized, Double-Blind, Multicenter Study Comparing the Efficacy and Safety of Intravenous-to-Oral Solithromycin to Intravenous-to-Oral Moxifloxacin for Treatment of Community-Acquired Bacterial Pneumonia

BACKGROUND

Solithromycin, a novel macrolide antibiotic with both intravenous and oral formulations dosed once daily, has completed 2 global phase 3 trials for treatment of community-acquired bacterial pneumonia.

METHODS

A total of 863 adults with community-acquired bacterial pneumonia (Pneumonia Outcomes Research Team [PORT] class II-IV) were randomized 1:1 to receive either intravenous-to-oral solithromycin or moxifloxacin for 7 once-daily doses. All patients received 400 mg intravenously on day 1 and were permitted to switch to oral dosing when clinically indicated. The primary objective was to demonstrate noninferiority (10% margin) of solithromycin to moxifloxacin in achievement of early clinical response (ECR) assessed 3 days after first dose in the intent-to-treat (ITT) population. Secondary endpoints included demonstrating noninferiority in ECR in the microbiological ITT population (micro-ITT) and determination of investigator-assessed success rates at the short-term follow-up (SFU) visit 5-10 days posttherapy.

RESULTS

In the ITT population, 79.3% of solithromycin patients and 79.7% of moxifloxacin patients achieved ECR (treatment difference, -0.46; 95% confidence interval [CI], -6.1 to 5.2). In the micro-ITT population, 80.3% of solithromycin patients and 79.1% of moxifloxacin patients achieved ECR (treatment difference, 1.26; 95% CI, -8.1 to 10.6). In the ITT population, 84.6% of solithromycin patients and 88.6% of moxifloxacin patients achieved clinical success at SFU based on investigator assessment. Mostly mild/moderate infusion events led to higher incidence of adverse events overall in the solithromycin group. Other adverse events were comparable between treatment groups.

CONCLUSIONS

Intravenous-to-oral solithromycin was noninferior to intravenous-to-oral moxifloxacin. Solithromycin has potential to provide an intravenous and oral option for monotherapy for community-acquired bacterial pneumonia.

CLINICAL TRIALS REGISTRATION

NCT01968733.

Ribosome-Templated Azide-Alkyne Cycloadditions: Synthesis of Potent Macrolide Antibiotics by In Situ Click Chemistry

Over half of all antibiotics target the bacterial ribosome-nature's complex, 2.5 MDa nanomachine responsible for decoding mRNA and synthesizing proteins. Macrolide antibiotics, exemplified by erythromycin, bind the 50S subunit with nM affinity and inhibit protein synthesis by blocking the passage of nascent oligopeptides. Solithromycin (1), a third-generation semisynthetic macrolide discovered by combinatorial copper-catalyzed click chemistry, was synthesized in situ by incubating either E. coli 70S ribosomes or 50S subunits with macrolide-functionalized azide 2 and 3-ethynylaniline (3) precursors. The ribosome-templated in situ click method was expanded from a binary reaction (i.e., one azide and one alkyne) to a six-component reaction (i.e., azide 2 and five alkynes) and ultimately to a 16-component reaction (i.e., azide 2 and 15 alkynes). The extent of triazole formation correlated with ribosome affinity for the anti (1,4)-regioisomers as revealed by measured Kd values. Computational analysis using the site-identification by ligand competitive saturation (SILCS) approach indicated that the relative affinity of the ligands was associated with the alteration of macrolactone+desosamine-ribosome interactions caused by the different alkynes. Protein synthesis inhibition experiments confirmed the mechanism of action. Evaluation of the minimal inhibitory concentrations (MIC) quantified the potency of the in situ click products and demonstrated the efficacy of this method in the triaging and prioritization of potent antibiotics that target the bacterial ribosome. Cell viability assays in human fibroblasts confirmed 2 and four analogues with therapeutic indices for bactericidal activity over in vitro mammalian cytotoxicity as essentially identical to solithromycin (1).

Synthesis and antibacterial evaluation of novel 4″-glycyl linked quinolyl-azithromycins with potent activity against macrolide-resistant pathogens

A new azithromycin-based series of antibacterial macrolones is reported, which features the use of a 4″-ester linked glycin for tethering the quinolone side chain to the macrolide scaffold. Among the analogs prepared, compounds 9e and 22f with a quinolon-6-yl moiety were found to have potent and well-balanced activity against clinically important respiratory tract pathogens, including erythromycin-susceptible and MLSB resistant strains of Streptococcus pneumoniae, Streptococcus pyogenes, and Haemophilus influenzae. In addition, potential lead compounds 9e and 22f demonstrated outstanding levels of activity against Moraxella catarrhalis and inducibly MLSB resistant Staphylococcus aureus. The best member of this series 22f rivals or exceeds, in potency, some of the most active ketolide antibacterial agents known today, such as telithromycin and cethromycin.

The wobbly status of ketolides: where do we stand?

Ketolides are erythromycin A derivatives with a keto group replacing the cladinose sugar and an aryl-alkyl group attached to the lactone macrocycle. The aryl-alkyl extension broadens its antibacterial spectrum to include all pathogens responsible for community-acquired pneumonia (CAP): Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis as well as atypical pathogens (Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella pneumophila). Ketolides have extensive tissue distribution, favorable pharmacokinetics (oral, once-a-day) and useful anti-inflammatory/immunomodulatory properties. Hence, they were considered attractive additions to established oral antibacterials (quinolones, β-lactams, second-generation macrolides) for mild-to-moderate CAP. The first ketolide to be approved, Sanofi-Aventis' telithromycin (RU 66647, HMR 3647, Ketek®), had tainted clinical development, controversial FDA approval and subsequent restrictions due to rare, irreversible hepatotoxicity that included deaths. Three additional ketolides progressed to non-inferiority clinical trials vis-à-vis clarithromycin for CAP. Abbott's cethromycin (ABT-773), acquired by Polymedix and subsequently by Advanced Life Sciences, completed Phase III trials, but its New Drug Application was denied by the FDA in 2009. Enanta's modithromycin (EDP-420), originally codeveloped with Shionogi (S-013420) and subsequently by Shionogi alone, is currently in Phase II in Japan. Optimer's solithromycin (OP-1068), acquired by Cempra (CEM-101), is currently in Phase III. Until this hepatotoxicity issue is resolved, ketolides are unlikely to replace established antibacterials for CAP, or lipoglycopeptides and oxazolidinones for gram-positive infections.

Pipeline of Known Chemical Classes of Antibiotics

Many approaches are used to discover new antibiotic compounds, one of the most widespread being the chemical modification of known antibiotics. This type of discovery has been so important in the development of new antibiotics that most antibiotics used today belong to the same chemical classes as antibiotics discovered in the 1950s and 1960s. Even though the discovery of new classes of antibiotics is urgently needed, the chemical modification of antibiotics in known classes is still widely used to discover new antibiotics, resulting in a great number of compounds in the discovery and clinical pipeline that belong to existing classes. In this scenario, the present article presents an overview of the R&D pipeline of new antibiotics in known classes of antibiotics, from discovery to clinical trial, in order to map out the technological trends in this type of antibiotic R&D, aiming to identify the chemical classes attracting most interest, their spectrum of activity, and the new subclasses under development. The result of the study shows that the new antibiotics in the pipeline belong to the following chemical classes: quinolones, aminoglycosides, macrolides, oxazolidinones, tetracyclines, pleuromutilins, beta-lactams, lipoglycopeptides, polymyxins and cyclic lipopeptides.