Activities of a new oral streptogramin, XRP 2868, compared to those of other agents against Streptococcus pneumoniae and haemophilus species

Streptogramins for the treatment of infections caused by Gram-positive pathogens

INTRODUCTION

Streptogramins (pristinamycin and quinupristin-dalfopristin) can be interesting options for the treatment of infections due to Gram-positive cocci, especially multidrug-resistant isolates.

AREAS COVERED

This review provides an updated overview of structural and activity characteristics, mechanisms of action and resistance, pharmacokinetic/pharmacodynamic, and clinical use of streptogramins.

EXPERT OPINION

The streptogramin antibiotics act by inhibition of the bacterial protein synthesis. They are composed of two chemically distinct compounds, namely type A and type B streptogramins, which exert a rapid bactericidal activity against a wide range of Gram-positive bacteria (including methicillin-resistant staphylococci and vancomycin-resistant enterococci). Several mechanisms of resistance have been identified in staphylococci and enterococci but the prevalence of streptogramin resistance among clinical isolates remains very low. Even if only a few randomized clinical trials have been conducted, the efficacy of pristinamycin has been largely demonstrated with an extensive use for 50 years in France and some African countries. Despite its effectiveness in the treatment of severe Gram-positive bacterial infections demonstrated in several studies and the low rate of reported resistance, the clinical use of quinupristin-dalfopristin has remained limited, mainly due to its poor tolerance. Altogether, streptogramins (especially pristinamycin) can be considered as potential alternatives for the treatment of Gram-positive infections.

Synergy of streptogramin antibiotics occurs independently of their effects on translation

Streptogramin antibiotics are divided into types A and B, which in combination can act synergistically. We compared the molecular interactions of the streptogramin combinations Synercid (type A, dalfopristin; type B, quinupristin) and NXL 103 (type A, flopristin; type B, linopristin) with the Escherichia coli 70S ribosome by X-ray crystallography. We further analyzed the activity of the streptogramin components individually and in combination. The streptogramin A and B components in Synercid and NXL 103 exhibit synergistic antimicrobial activity against certain pathogenic bacteria. However, in transcription-coupled translation assays, only combinations that include dalfopristin, the streptogramin A component of Synercid, show synergy. Notably, the diethylaminoethylsulfonyl group in dalfopristin reduces its activity but is the basis for synergy in transcription-coupled translation assays before its rapid hydrolysis from the depsipeptide core. Replacement of the diethylaminoethylsulfonyl group in dalfopristin by a nonhydrolyzable group may therefore be beneficial for synergy. The absence of general streptogramin synergy in transcription-coupled translation assays suggests that the synergistic antimicrobial activity of streptogramins can occur independently of the effects of streptogramin on translation.

Advances in MRSA drug discovery: where are we and where do we need to be?

INTRODUCTION

Methicillin-resistant Staphylococcus aureus (MRSA) have been on the increase during the past decade, due to the steady growth of the elderly and immunocompromised patients, and the emergence of multidrug-resistant (MDR) bacterial strains. Although there are a limited number of anti-MRSA drugs available, a number of different combination antimicrobial drug regimens have been used to treat serious MRSA infections. Thus, the addition of several new antistaphylococcal drugs into clinical practice should broaden clinician's therapeutic options. As MRSA is one of the most common and problematic bacteria associated with increasing antimicrobial resistance, continuous efforts for the discovery of lead compounds as well as development of alternative therapies and faster diagnostics are required.

AREAS COVERED

This article summarizes the FDA-approved drugs to treat MRSA infections, the drugs in clinical trials, and the drug leads for MRSA and related Gram-positive bacterial infections. In addition, the article discusses the mode of action of antistaphylococcal molecules and the resistant mechanisms of some molecules.

EXPERT OPINION

The number of pipeline drugs presently undergoing clinical trials is not particularly encouraging. There are limited and rather expensive therapeutic options for MRSA infections in the critically ill. Further research efforts are required for effective phage therapy on MRSA infections in clinical use, which seem to be attractive therapeutic options for the future.

In vitro pharmacokinetic/pharmacodynamic activity of NXL103 versus clindamycin and linezolid against clinical Staphylococcus aureus and Streptococcus pyogenes isolates

NXL103 (linopristin/flopristin, 30/70) is a novel oral streptogramin combination with activity against a large variety of multidrug-resistant Gram-positive pathogens. The objective of this study was to evaluate the in vitro activity of NXL103 in comparison with oral comparators (clindamycin and linezolid). Six clinical isolates [four meticillin-resistant Staphylococcus aureus (MRSA) and two Streptococcus pyogenes] were exposed for 48 h in an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model at a starting inoculum of ca. 10(6) colony-forming units (CFU)/mL. Antimicrobial simulations included NXL103 500 mg every 12 h, linezolid 600 mg every 12 h and clindamycin 450 mg every 6 h. Bactericidal and static effects were defined as ≥3log(10) and <3log(10) CFU/mL kill from the starting inoculum, respectively. Experiments were performed in duplicate to ensure reproducibility, and differences between regimens were evaluated by analysis of variance (ANOVA) with Tukey's post-hoc test. NXL103 exhibited lower minimum inhibitory concentrations than comparators, with values ≤0.06 mg/L for S. pyogenes and 0.125-0.25 mg/L for MRSA isolates. In the PK/PD model, NXL103 demonstrated significantly better activity than linezolid and clindamycin (P<0.05), achieving sustained bactericidal activity within <2 h against S. pyogenes strains and between 7.3-32 h against MRSA isolates. In contrast, linezolid only exhibited a static effect, whereas clindamycin achieved 3log(10) kill at 6h against the unique clindamycin-susceptible S. pyogenes strain evaluated. In conclusion, at therapeutic concentrations NXL103 exhibits promising activity against both MRSA and S. pyogenes strains, including clindamycin-resistant organisms. Further in vitro and in vivo experiments are warranted to explore the therapeutic benefit of NXL103 for the treatment of Gram-positive skin and soft-tissue infections.

Time-kill activity of the streptogramin NXL 103 against Gram-positive and -negative bacteria

Against 33 Gram-positive and -negative bacteria, NXL 103 MICs were 0.03 to 1 μg/ml. NXL 103 was bactericidal by 12 h at 2 × MIC against all 5 pneumococci and at 2 × MIC after 24 h against all 5 group A and B β-hemolytic streptococci. NXL 103 was bactericidal against all 8 Haemophilus influenzae strains at 2 × MIC and all 5 Moraxella catarrhalis strains at 4 × MIC after 24 h but was mainly bacteriostatic against 5 methicillin-resistant Staphylococcus aureus strains. After the exposure of one strain of each species to NXL 103 for 10 daily subcultures, the MICs remained within ± 1 dilution.

NXL-103, a combination of flopristin and linopristin, for the potential treatment of bacterial infections including community-acquired pneumonia and MRSA

Novexel is developing the novel, orally active, semisynthetic streptogramin NXL-103, which has potential therapeutic application in the treatment of community-acquired pneumonia, community- or hospital-acquired MRSA, vancomycin-resistant enterococcus, and acute bacterial skin and soft tissue infections. NXL-103 is a combination of streptogramin A:streptogramin B components, initially developed in a 70:30 dose ratio. In multiple in vitro studies, NXL-103 demonstrated potent activity against different types of bacteria, such as Staphylococcus aureus (including MRSA), Streptococcus pneumoniae, Streptococcus pyogenes, Enterococcus faecium, Enterococcus faecalis, Haemophilus influenzae and Haemophilus parainfluenzae. NXL-103 was not affected by the resistance profiles of bacteria against other commonly used antibiotics. In phase I clinical trials, NXL-103 achieved bactericidal levels in plasma and was generally well tolerated, with side effects primarily on the gastrointestinal system. The first phase II trial conducted to evaluate the efficacy of NXL-103 against community-acquired pneumonia revealed that the compound was comparable with amoxicillin. NXL-103 has promise to become an important agent in the treatment of community-acquired pneumonia and complex skin and soft tissue infections, pending further development.

Natural products to drugs: natural product-derived compounds in clinical trials

Natural product and natural product-derived compounds that are being evaluated in clinical trials or are in registration (as at 31st December 2007) have been reviewed, as well as natural product-derived compounds for which clinical trials have been halted or discontinued since 2005. Also discussed are natural product-derived drugs launched since 2005, new natural product templates and late-stage development candidates.

Structural basis for streptogramin B resistance in Staphylococcus aureus by virginiamycin B lyase

The streptogramin combination therapy of quinupristin-dalfopristin (Synercid) is used to treat infections caused by bacterial pathogens, such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecium. However, the effectiveness of this therapy is being compromised because of an increased incidence of streptogramin resistance. One of the clinically observed mechanisms of resistance is enzymatic inactivation of the type B streptogramins, such as quinupristin, by a streptogramin B lyase, i.e., virginiamycin B lyase (Vgb). The enzyme catalyzes the linearization of the cyclic antibiotic via a cleavage that requires a divalent metal ion. Here, we present crystal structures of Vgb from S. aureus in its apoenzyme form and in complex with quinupristin and Mg2+ at 1.65- and 2.8-A resolution, respectively. The fold of the enzyme is that of a seven-bladed beta-propeller, although the sequence reveals no similarity to other known members of this structural family. Quinupristin binds to a large depression on the surface of the enzyme, where it predominantly forms van der Waals interactions. Validated by site-directed mutagenesis studies, a reaction mechanism is proposed in which the initial abstraction of a proton is facilitated by a Mg2+ -linked conjugated system. Analysis of the Vgb-quinupristin structure and comparison with the complex between quinupristin and its natural target, the 50S ribosomal subunit, reveals features that can be exploited for developing streptogramins that are impervious to Vgb-mediated resistance.

Effects of various media on the activity of NXL103 (formerly XRP 2868), a new oral streptogramin, against Haemophilus influenzae

The activity of NXL103 against 108 strains of Haemophilus influenzae was tested using Haemophilus test media (HTM) obtained from various sources. With the exception of those obtained with stored HTM, MICs did not differ significantly, with MIC(50) and MIC(90) values of 0.5 and 0.5 to 1 microg/ml, respectively, in each medium.

Activity of a new oral streptogramin, XRP2868, against gram-positive cocci harboring various mechanisms of resistance to streptogramins

The antibacterial activity of XRP2868, a new oral streptogramin composed of a combination of RPR132552 (streptogramin A) and RPR202868 (streptogramin B), was evaluated against a collection of clinical gram-positive isolates with characterized phenotypes and genotypes of streptogramin resistance. The effects of genes for resistance to streptogramin A or B on the activity of XRP2868 and its components were also tested by cloning these genes individually or in various combinations in gram-positive recipient strains susceptible to quinupristin-dalfopristin. The species tested included Staphylococcus aureus, coagulase-negative staphylococci, Enterococcus faecalis, Enterococcus faecium, Streptococcus pneumoniae, and other species of streptococci. XRP2868 was generally fourfold more potent than quinupristin-dalfopristin against S. aureus, E. faecium, and streptococci and had activity against E. faecalis (MICs = 0.25 to 1 microg/ml). XRP2868 appeared to be affected by the same mechanisms of resistance as those to quinupristin-dalfopristin. Nevertheless, the strong activity of factor A of the oral streptogramin enabled the combination to be very potent against streptogramin-susceptible staphylococci, streptococci, and E. faecium (MICs = 0.03 to 0.25 microg/ml) and to retain low MICs against the strains harboring a mechanism of resistance to factor A or factor B of the streptogramin. However, the combination of mechanisms of resistance to factors A and B caused an increase in the MICs of XRP2868, which reached 1 to 4 mug/ml. As with the other streptogramins, there was a reduction in the bactericidal effect of XRPR2868 when the staphylococcal strains acquired a constitutively expressed erm gene.

Pharmacodynamics of a new streptogramin, XRP 2868, in murine thigh and lung infection models

XRP 2868 is a new streptogramin antibiotic with broad-spectrum activity against gram-positive cocci. We used the neutropenic murine thigh and lung infection models to characterize the time course of antimicrobial activity of XRP 2868 and determine which pharmacokinetic/pharmacodynamic (PK/PD) parameter and magnitude best correlated with efficacy. Serum levels following four two- to fourfold-escalating single-dose levels of XRP 2868 were measured by liquid chromatography mass spectrometry assay. In vivo postantibiotic effects (PAEs) were determined after doses of 2.5, 10, and 40 mg/kg. Mice had 10(6.8) to 10(8.4) CFU/thigh of strains of Streptococcus pneumoniae ATCC 10813 or Staphylococcus aureus ATCC 29213 at the start of therapy when treated for 24 h with 2.5 to 640 mg/kg/day of XRP 2868 fractionated for 3-, 6-, 12-, and 24-h dosing regimens. Nonlinear regression analysis was used to determine which PK/PD parameter best correlated with CFU/thigh at 24 h. Pharmacokinetic studies exhibited peak dose values of 0.03 to 0.07, area under the concentration-time curve (AUC) dose values of 0.02 to 0.07, and half-lives of 0.35 to 1.27 h. XRP 2868 produced in vivo PAEs of 0.5 to 3.4 h with S. pneumoniae strain ATCC 10813 and -1.5 to 10.7 h with S. aureus strain ATCC 29213. The 24-h AUC/MIC was the PK/PD parameter that best correlated with efficacy. In subsequent studies, we used the neutropenic murine thigh infection model to determine if the magnitude of the AUC/MIC needed for the efficacy of XRP 2868 varied among pathogens (including resistant strains). Mice had 10(6.1) to 10(7.8) CFU/thigh of four isolates of S. aureus (three methicillin-susceptible and one methicillin-resistant strain) and nine isolates of S. pneumoniae (one penicillin-susceptible, four penicillin-intermediate, and four penicillin-resistant strains) when treated for 24 h with 0.16 to 640 mg/kg of XRP 2868 every 6 h. A sigmoid dose-response model was used to estimate the doses (mg/kg/24 h) required to achieve a net bacteriostatic affect over 24 h. MICs ranged from 0.06 to 0.25 microg/ml. The 24-h AUC/MICs for each static dose (20.7 to 252 mg/kg/day) varied from 3 to 70. Mean 24-h AUC/MICs +/- standard deviations (SDs) for S. pneumoniae and S. aureus isolates were 14 +/- 10 and 31 +/- 16, respectively. Beta-lactam and macrolide resistance did not alter the magnitude of AUC/MIC required for efficacy.

A comparison of a new oral streptogramin XRP 2868 with quinupristin-dalfopristin against antibiotic-resistant strains of haemophilus influenzae, Staphylococcus aureus, and Streptococcus pneumoniae

A new streptogramin antibiotic XRP 2868 was compared with quinupristin-dalfopristin for inhibitory activities against antibiotic-resistant Haemophilus influenzae, Staphylococcus aureus, and Streptococcus pneumoniae. In each organism examined, XRP 2868 had an IC(50) that was twofold to fivefold lower than quinupristin-dalfopristin, for inhibition of cell viability, protein synthesis, and ribosomal subunit formation.

Recently approved and investigational antibiotics for treatment of severe infections caused by Gram-positive bacteria

The development of resistance in the major pathogenic Gram-positive genera Staphylococcus and Streptococccus has led to the need for new agents that are able to overcome existing resistance mechanisms or that have novel mechanisms of action. There is currently a dearth of new agents that are active against resistant bacterial species. Agents that have recently been approved for clinical use include linezolid, the first oxazolidinone in clinical use, daptomycin, the first lipopeptide in clinical use, and telithromycin, a ketolide that is derived from clarithromycin. Agents currently in clinical development include tigecycline, a broad-spectrum intravenous tetracycline, ceftobiprole, a broad-spectrum cephalosporin that has activity against methicillin-resistant staphylococci, DX-619 and WCK-771, which are potent quinolones that have activity against quinolone-resistant staphylococci, oritavancin and dalbavancin, both of which are new glycopeptides, and iclaprim, which is a diaminopyrimidine. Additional agents that are in preclinical development against Gram-positive pathogens include quinoline-naphthyridine agents, which target novel DNA gyrase sites, other novel quinolones that have high potency, peptide deformylase inhibitors, and new lincosamide, oxazolidinone, lipopeptide and cephalosporin derivatives. Misuse of potent new agents will, however, result in the inevitable development of resistance to these agents; responsible use of potent new agents is required to prevent continuation of this vicious cycle.

In vitro activity of an oral streptogramin antimicrobial, XRP2868, against gram-positive bacteria

The comparative in vitro potency of XRP2868, a new oral semisynthetic streptogramin antibiotic, was evaluated against gram-positive bacteria. XRP2868 inhibited all staphylococci at < or = 1 microg/ml and all non-pneumococcal streptococci at < or = 0.25 microg/ml and was fourfold more potent than quinupristin-dalfopristin against Staphylococcus aureus and Enterococcus faecium.

Comparative in vitro activities of XRP 2868, pristinamycin, quinupristin-dalfopristin, vancomycin, daptomycin, linezolid, clarithromycin, telithromycin, clindamycin, and ampicillin against anaerobic gram-positive species, actinomycetes, and lactobacilli

A comparative study of the in vitro activities of XRP 2868, a new oral streptogramin, against 266 anaerobic gram-positive clinical isolates using the agar dilution method showed that the XRP 2868 MICs for 95% (254 of 266) of isolates were < or =0.5 microg/ml. XRP 2868 MICs for only two strains, one being Clostridium clostridioforme (MIC, 16 microg/ml) and the other being Clostridium difficile (MIC, 32 microg/ml), were >2 microg/ml. Depending on its pharmacokinetics and pharmacodynamics, XRP 2868 has potential for use against infections with gram-positive anaerobes and deserves further clinical evaluation.

Clarithromycin in 2003: sustained efficacy and safety in an era of rising antibiotic resistance

Data from surveillance studies show increasing prevalence of respiratory pathogens resistant to commonly used antibiotics. Thus, a Medline search was conducted to identify studies of clarithromycin, especially those addressing macrolide resistance. Changing trends of in vitro susceptibility have not affected clinical efficacy with clarithromycin. Over the last 12 years, clarithromycin study results have shown consistent rates of clinical cure and bacteriological eradication, which are similar to those observed with comparator agents. The incidence of clarithromycin treatment failure in patients infected with Streptococcus pneumoniae is substantially less than that predicted by macrolide resistance rates from surveillance programmes. In summary, despite widespread use since its introduction, clarithromycin remains active both in vitro and in vivo against clinically relevant respiratory tract pathogens.

Newer antibiotics for the treatment of respiratory tract infections

PURPOSE OF REVIEW

In this review, we highlight some of the developments achieved over the past 2 years in the field of novel antimicrobial compounds.

RECENT FINDINGS

Modification of existing compound classes to create more powerful compounds capable of overcoming pathogen resistance and the introduction of completely new classes of antibiotics and inhibitors of new bacterial targets or inhibitors of genes relating to virulence or pathogenesis are the strategies more commonly employed in pharmacologic research. Ketolides, oxazolidinones, streptogramins, glycylcyclines, and peptide deformylase inhibitors are among the most promising classes of antibiotics. Recently, several lines of research have documented that it is effective to target the infection process rather than killing bacteria. This is important because it is likely that such a therapeutic strategy could ablate infection without inducing resistance.

SUMMARY

Emergence of resistance to the antibiotics currently employed in clinical practice is a continual stimulus for further research aimed at identifying novel antimicrobial compounds. These drugs will perhaps effectively fight against bacteria that now are scarcely controlled by the traditional antimicrobial agents. Health care personnel must appreciate that only judicious use of antimicrobial drugs will prevent the further uncontrolled spread of bacterial resistance. Implementation of reference guidelines would probably be an effective way to limit antibiotic misuse.