In Vitro Activity Analysis of a New Polymyxin, SPR741, Tested in Combination with Antimicrobial Agents against a Challenge Set of Enterobacteriaceae, Including Molecularly Characterized Strains.

The activities of azithromycin, fusidic acid, vancomycin, doxycycline, and minocycline were evaluated alone and in combination with SPR741. A total of 202 Escherichia coli and 221 Klebsiella pneumoniae isolates were selected, and they included a genome-sequenced subset (n = 267), which was screened in silico for ß-lactamase, macrolide-lincosamide-streptogramin (MLS), and tetracycline (tet) genes. Azithromycin (>16 mg/liter), fusidic acid (>64 mg/liter), vancomycin (>16 mg/liter), and SPR741 (>8 mg/liter) showed off-scale MICs when each was tested alone against all isolates. MIC50/90 results of 0.5/8 mg/liter, 4/>32 mg/liter, 16/>16 mg/liter, 2/32 mg/liter, and 0.25/4 mg/liter were obtained for azithromycin-SPR741, fusidic acid-SPR741, vancomycin-SPR741, doxycycline-SPR741 and minocycline-SPR741, respectively, against all isolates. Overall, azithromycin-SPR741 (MIC90, 2 to 4 mg/liter) and minocycline-SPR741 (MIC90, 0.5 to 2 mg/liter) showed the lowest MIC90 values against different subsets of E. coli isolates, except for azithromycin-SPR741 (MIC90, 16 mg/liter) against the AmpC and metallo-ß-lactamase subsets. In general, minocycline-SPR741 (MIC90, 2 to 8 mg/liter) had the lowest MIC90 against K. pneumoniae isolates producing different groups of ß-lactamases. The azithromycin-SPR741 MIC (MIC50/90, 2/32 mg/liter) was affected by MLS genes (MIC50/90 of 0.25/2 mg/liter against isolates without MLS genes), whereas doxycycline-SPR741 (MIC50/90, 0.5/2 versus 8/32 mg/liter) and minocycline-SPR741 (MIC50/90, 0.25/1 versus 1/8 mg/liter) MIC results were affected when tested against isolates carrying tet genes in general. However, minocycline-SPR741 inhibited 88.2 to 92.9% of tet-positive isolates regardless of the tet gene. The azithromycin-SPR741 MIC results (MIC50/90, 1/16 mg/liter) against isolates with enzymatic MLS mechanisms were lower than against those with ribosomal protection (MIC50/90, 16/>32 mg/liter). SPR741 increased the in vitro activity of tested codrugs at different levels and seemed to be dependent on the species and resistance mechanisms of the respective codrug.

In Vitro Activity of Tebipenem (SPR859) against Penicillin-Binding Proteins of Gram-Negative and Gram-Positive Bacteria.

Tebipenem (SPR859) is the microbiologically active form of SPR994 (tebipenem-pivoxil), an orally available carbapenem with activity against extended-spectrum ß-lactamase (ESBL)-producing Enterobacteriaceae Measurement of the relative binding of SPR859 to the bacterial cell targets revealed that it is a potent inhibitor of multiple penicillin-binding proteins (PBPs) but primarily a Gram-negative PBP 2 inhibitor, similar to other compounds in this class. These data support further clinical development of SPR994.

Tebipenem, the first oral carbapenem antibiotic.

INTRODUCTION: Infections caused by antibiotic-resistant pathogens, particularly Gram-negative bacteria, have become increasingly challenging to successfully treat. The beta-lactam antibiotic subclass, the carbapenems, have proven valuable for the treatment of such Gram-negative bacterial infections due to their spectrum and ß-lactamase stability properties. However, all marketed carbapenems to date are parenterally administered to adult patients. Areas covered: One carbapenem, tebipenem-pivoxil (TBPM-PI), is an oral prodrug that was approved in Japan for pediatric use only in 2009. This review summarizes preclinical and clinical data for TBPM-PI, which is now in clinical development again this time for use as the first oral carbapenem available for treatment of bacterial infections in adult patients. Expert commentary: There is an urgent unmet need with an increasing prevalence of fluoroquinolone-resistant and ESBL-producing Gram-negative pathogens in the hospital and community setting. Carbapenems have traditionally been considered the drugs of choice for infections caused by enterobacteria producing ESBL and AmpC enzymes because they are not affected by these resistance mechanisms. The carbapenem, TBPM-PI, offers an oral option, particularly as step-down therapy, for use of this class in the treatment of serious Gram-negative infections.

Treatment of Gram-negative bacterial infections by potentiation of antibiotics.

Infections caused by antibiotic-resistant pathogens, particularly Gram-negative bacteria, represent significant treatment challenges for physicians resulting in high rates of morbidity and mortality. The outer membrane of Gram-negative bacteria acts as a permeability barrier to many compounds that would otherwise be effective antibacterial agents, including those effective against Gram-positive pathogens. Potentiator molecules disrupt this barrier allowing entry of otherwise impermeant molecules, thus providing a strategy to render multi-drug resistant pathogens susceptible to a broader range of antibiotics. Potentiator molecules are cationic and the mechanism of disruption involves interaction with the negatively charged outer membrane. This physical attribute, along with an often high degree of lipophilicity typically endears these molecules with unacceptable toxicity. Presented herein are examples of advanced potentiator molecules being evaluated for use in combination therapy for the treatment of resistant Gram-negative infections.

Structural and Kinetic Characterization of Diazabicyclooctanes as Dual Inhibitors of Both Serine-ß-Lactamases and Penicillin-Binding Proteins.

Avibactam is a diazabicyclooctane ß-lactamase inhibitor possessing outstanding but incomplete efficacy against multidrug-resistant Gram-negative pathogens in combination with ß-lactam antibiotics. Significant pharmaceutical investment in generating derivatives of avibactam warrants a thorough characterization of their activity. We show here through structural and kinetic analysis that select diazabicyclooctane derivatives display effective but varied inhibition of two clinically important ß-lactamases (CTX-M-15 and OXA-48). Furthermore, these derivatives exhibit considerable antimicrobial activity (MIC ≤ 2 µg/mL) against clinical isolates of Pseudomonas aeruginosa, Escherichia coli, and Enterobacter spp. Imaging of cell phenotype along with structural and biochemical experiments unambiguously demonstrate that this activity, in E. coli, is a result of targeting penicillin-binding protein 2. Our results suggest that structure-activity relationship studies for the purpose of drug discovery must consider both ß-lactamases and penicillin-binding proteins as targets. We believe that this approach will yield next-generation combination or monotherapies with an expanded spectrum of activity against currently untreatable Gram-negative pathogens.

Oritavancin Pharmacokinetics and Bone Penetration in Rabbits.

The pharmacokinetics and bone concentrations of oritavancin were investigated after a single intravenous dose was administered to rabbits. The pharmacokinetic profile of oritavancin in rabbits showed that it is rapidly distributed to bone tissues, with concentrations remaining stable for up to 168 h, the last measured time point. Based on these findings, further evaluation of oritavancin for the treatment of infections in bone tissues is warranted.

Activity of oritavancin and comparators in vitro against standard and high inocula of Staphylococcus aureus.

In this study, the impact of inoculum density on the growth inhibitory and killing activities of oritavancin and comparators (vancomycin, daptomycin and linezolid) in vitro against four Staphylococcus aureus strains at clinically relevant drug concentrations was studied. Broth microdilution and time-kill assays were performed using a standard inoculum [ca. 10(5)colony-forming units (CFU)/mL as per Clinical and Laboratory Standards Institute (CLSI) guidelines] and a high inoculum (ca. 10(7)CFU/mL). Whereas minimal inhibitory concentrations (MICs) of comparators were 2-8-fold higher when tested at high inoculum, oritavancin MICs were 16-fold higher for all strains at the high inoculum relative to the standard inoculum. However, in time-kill assays, when tested at its fC(min) [trough concentration of free (non-protein-bound) drug] and fC(max) (peak concentration of non-protein-bound drug), oritavancin retained its bactericidal activity against a vancomycin-susceptible, meticillin-susceptible S. aureus (VS-MSSA) strain and a vancomycin-susceptible, meticillin-resistant S. aureus (VS-MRSA) strain both at standard and high inocula. At its fC(max), oritavancin was bactericidal at standard inoculum but not at high inoculum against two vancomycin-intermediate S. aureus (VISA) strains. Against both VISA strains at standard inoculum, oritavancin at its fC(min) reduced cell density by between 2 and 3 log (bacteriostatic), predicting that it will retain activity against certain VISA infections. However, oritavancin had no substantial growth inhibitory effect against either VISA strain at high inoculum, suggesting that in rare VISA infections with an anticipated high bacterial burden such as endocarditis, alternative oritavancin dosing strategies, including combinations with other agents, may be explored.

In vitro characterization of oritavancin clearance from human blood by low-flux, high-flux, and continuous renal replacement therapy dialyzers.

BACKGROUND: Oritavancin is an investigational lipoglycopeptide antibiotic under clinical development for the treatment of gram-positive bacterial infections. The impact of hemodialysis on plasma concentrations of oritavancin is unknown and may be important in making dosage adjustments in such patients. The present study sought to determine the clearance of oritavancin from human blood by various commercially available dialyzers in an in vitro hemodialysis model. METHODS: Three types each of low-flux (Dicea 130, F6, and Polyflux 14L) and high-flux (Revaclear, Exeltra 150, and Optiflux F160NR) dialyzers and one type of continuous renal replacement therapy (CRRT) dialyzer (Prismaflex M100) were studied. Heparinized human blood containing oritavancin (200 mg/L) was circulated from a reservoir to the dialyzers and back to the reservoir. Fresh dialysate was pumped through the dialyzers in a countercurrent manner. Blood samples from each side of the dialyzers and contaminated dialysate samples were collected at periodic intervals. Oritavancin levels were analyzed by a liquid chromatography/mass spectrometry method with a limit of quantification of 12.5 ng/mL and plasma clearances of oritavancin were calculated. RESULTS: The mean dialytic clearance of oritavancin was insignificant for each of the low-flux, high-flux and CRRT dialyzers. Clinically significant amounts of oritavancin were not detected in dialysate during any of the experimental dialysis sessions. CONCLUSIONS: The clearance of oritavancin from human blood by the dialyzers used in this study is insignificant. Further clinical studies would be required before making changes in dosage of oritavancin in hemodialysis patients.

Oritavancin disrupts membrane integrity of Staphylococcus aureus and vancomycin-resistant enterococci to effect rapid bacterial killing.

Oritavancin is an investigational lipoglycopeptide in clinical development for the treatment of acute bacterial skin and skin structure infections. In this study, we demonstrate that oritavancin causes bacterial membrane depolarization and permeabilization leading to cell death of Gram-positive pathogens and that these effects are attributable to the 4'-chlorobiphenylmethyl group of the molecule.

Assessment of oritavancin serum protein binding across species.

Biophysical methods to study the binding of oritavancin, a lipoglycopeptide, to serum protein are confounded by nonspecific drug adsorption to labware surfaces. We assessed oritavancin binding to serum from mouse, rat, dog, and human by a microbiological growth-based method under conditions that allow near-quantitative drug recovery. Protein binding was similar across species, ranging from 81.9% in human serum to 87.1% in dog serum. These estimates support the translation of oritavancin exposure from nonclinical studies to humans.

Synthesis and in vitro evaluation of bisphosphonated glycopeptide prodrugs for the treatment of osteomyelitis.

As therapeutic agents of choice in the treatment of complicated infections, glycopeptide antibiotics are often preferentially used in cases of osteomyelitis, an infection located in bone and notoriously difficult to successfully manage. Yet frequent and heavy doses of these systemically administered antibiotics are conventionally prescribed to obtain higher antibiotic levels in the bone and reduce the high recurrence rates. Targeting antibiotics to the bone after systemic administration would present at least three potential advantages: (i) greater efficacy, by concentrating the therapeutic agent in bone; (ii) greater convenience, through a reduction in the frequency of administration; and (iii) greater safety, by reducing the levels of systemic drug exposure. We present here the design, synthesis and in vitro evaluation of eight prodrugs of the glycopeptide antibacterial agents vancomycin and oritavancin taking advantage of the affinity of the bisphosphonate group for bone for delivery to osseous tissues.

Time-kill kinetics of oritavancin and comparator agents against Streptococcus pyogenes.

The activity of oritavancin in vitro against recent clinical isolates of Streptococcus pyogenes, including antibiotic-resistant strains, was characterised by determination of broth microdilution minimal inhibitory concentrations as well as time-kill assays. Ten clinical isolates of S. pyogenes, three of which were resistant to erythromycin, as well as one reference S. pyogenes strain were tested. In the time-kill assays, oritavancin and the comparators vancomycin, teicoplanin, linezolid, penicillin, erythromycin and daptomycin were tested at static concentrations approximating their free peak (fC(max)) and free trough (fC(min)) concentrations in plasma when administered at approved doses for skin and skin-structure infections. At its fC(max) predicted from a 200 mg dose in humans, oritavancin exerted bactericidal activity (> or = 3 log kill relative to the starting inoculum) within 15 min to 3 h against all tested strains. Daptomycin exhibited bactericidal activity at its fC(max) for all but one strain; time to cidality was between 15 min and 6 h. At fC(min), only oritavancin was bactericidal against all the tested strains. Oritavancin displayed concentration-dependent killing of all isolates in vitro. Oritavancin was more rapidly bactericidal than the comparators at physiologically relevant concentrations against all strains tested. These data support the potential utility of oritavancin in infections with contemporary isolates of S. pyogenes, including drug-resistant strains.

Impact of human serum albumin on oritavancin in vitro activity against Staphylococcus aureus.

Human serum albumin (HSA) did not affect oritavancin MICs against non-vancomycin-intermediate Staphylococcus aureus (non-VISA) strains. In time-kill assays, oritavancin bactericidal activity in the presence of HSA was significantly more rapid than comparators against non-VISA strains. HSA increased oritavancin MICs by 4-fold for VISA strains, reflective of reduced oritavancin activity in time-kill assays with HSA.

Comparative in vitro activity profile of oritavancin against recent gram-positive clinical isolates.

Oritavancin activity was tested against 15,764 gram-positive isolates collected from 246 hospital centers in 25 countries between 2005 and 2008. Organisms were Staphylococcus aureus (n = 9,075), coagulase-negative staphylococci (n = 1,664), Enterococcus faecalis (n = 1,738), Enterococcus faecium (n = 819), Streptococcus pyogenes (n = 959), Streptococcus agalactiae (n = 415), group C, G, and F streptococci (n = 84), and Streptococcus pneumoniae (n = 1,010). Among the evaluated staphylococci, 56.7% were resistant to oxacillin. The vancomycin resistance rate among enterococci was 21.2%. Penicillin-resistant and -intermediate rates were 14.7% and 21.4%, respectively, among S. pneumoniae isolates. Among nonpneumococcal streptococci, 18.5% were nonsusceptible to erythromycin. Oritavancin showed substantial in vitro activity against all organisms tested, regardless of resistance profile. The maximum oritavancin MIC against all staphylococci tested (n = 10,739) was 4 microg/ml; the MIC(90) against S. aureus was 0.12 microg/ml. Against E. faecalis and E. faecium, oritavancin MIC(90)s were 0.06 and 0.12, respectively. Oritavancin was active against glycopeptide-resistant enterococci, including VanA strains (n = 486), with MIC(90)s of 0.25 and 1 microg/ml against VanA E. faecium and E. faecalis, respectively. Oritavancin showed potent activity against streptococci (n = 2,468); MIC(90)s for the different streptococcal species were between 0.008 and 1 microg/ml. These data are consistent with previous studies with respect to resistance rates of gram-positive isolates and demonstrate the spectrum and in vitro activity of oritavancin against a wide variety of contemporary gram-positive pathogens, regardless of resistance to currently used drugs. The data provide a foundation for interpreting oritavancin activity and potential changes in susceptibility over time once oritavancin enters into clinical use.

Vancomycin and oritavancin have different modes of action in Enterococcus faecium.

The increasing frequency of Enterococcus faecium isolates with multidrug resistance is a serious clinical problem given the severely limited number of therapeutic options available to treat these infections. Oritavancin is a promising new alternative in clinical development that has potent antimicrobial activity against both staphylococcal and enterococcal vancomycin-resistant pathogens. Using solid-state NMR to detect changes in the cell-wall structure and peptidoglycan precursors of whole cells after antibiotic-induced stress, we report that vancomycin and oritavancin have different modes of action in E. faecium. Our results show the accumulation of peptidoglycan precursors after vancomycin treatment, consistent with transglycosylase inhibition, but no measurable difference in cross-linking. In contrast, after oritavancin exposure, we did not observe the accumulation of peptidoglycan precursors. Instead, the number of cross-links is significantly reduced, showing that oritavancin primarily inhibits transpeptidation. We propose that the activity of oritavancin is the result of a secondary binding interaction with the E. faecium peptidoglycan. The hypothesis is supported by results from (13)C{(19)F} rotational-echo double-resonance (REDOR) experiments on whole cells enriched with l-[1-(13)C]lysine and complexed with desleucyl [(19)F]oritavancin. These experiments establish that an oritavancin derivative with a damaged d-Ala-d-Ala binding pocket still binds to E. faecium peptidoglycan. The (13)C{(19)F} REDOR dephasing maximum indicates that the secondary binding site of oritavancin is specific to nascent and template peptidoglycan. We conclude that the inhibition of transpeptidation by oritavancin in E. faecium is the result of the large number of secondary binding sites relative to the number of primary binding sites.

Impact of human serum albumin on oritavancin in vitro activity against enterococci.

Oritavancin is a lipoglycopeptide with activity against gram-positive pathogens including vancomycin-resistant enterococci. The impact of human serum albumin (HSA) on oritavancin activity against enterococci was compared to those of vancomycin, daptomycin, teicoplanin, and linezolid in vitro using MIC and time-kill methods. Oritavancin MICs increased between 0- and 8-fold in the presence of HSA. In time-kill assays with HSA, oritavancin retained activity, killing or inhibiting enterococci more rapidly than did comparators when peak concentrations were simulated.

Oritavancin kills stationary-phase and biofilm Staphylococcus aureus cells in vitro.

Slow-growing bacteria and biofilms are notoriously tolerant to antibiotics. Oritavancin is a lipoglycopeptide with multiple mechanisms of action that contribute to its bactericidal action against exponentially growing gram-positive pathogens, including the inhibition of cell wall synthesis and perturbation of membrane barrier function. We sought to determine whether oritavancin could eradicate cells known to be tolerant to many antimicrobial agents, that is, stationary-phase and biofilm cultures of Staphylococcus aureus in vitro. Oritavancin exhibited concentration-dependent bactericidal activity against stationary-phase inocula of methicillin-susceptible S. aureus (MSSA) ATCC 29213, methicillin-resistant S. aureus (MRSA) ATCC 33591, and vancomycin-resistant S. aureus (VRSA) VRS5 inoculated into nutrient-depleted cation-adjusted Mueller-Hinton broth. As has been described for exponential-phase cells, oritavancin induced membrane depolarization, increased membrane permeability, and caused ultrastructural defects including a loss of nascent septal cross walls in stationary-phase MSSA. Furthermore, oritavancin sterilized biofilms of MSSA, MRSA, and VRSA at minimal biofilm eradication concentrations (MBECs) of between 0.5 and 8 mug/ml. Importantly, MBECs for oritavancin were within 1 doubling dilution of their respective planktonic broth MICs, highlighting the potency of oritavancin against biofilms. These results demonstrate a significant activity of oritavancin against S. aureus in phases of growth that exhibit tolerance to other antimicrobial agents.