Influence of erythromycin resistance, inoculum growth phase, and incubation time on assessment of the bactericidal activity of RP 59500 (quinupristin-dalfopristin) against vancomycin-resistant Enterococcus faecium

Quinupristin/Dalfopristin

Each month, subscribers to The Formulary® Monograph Service receive five to six researched monographs on drugs that are newly released or are in late Phase III trials. The monographs are targeted to your Pharmacy and Therapeutics Committee. Subscribers also receive monthly one-page summary monographs on the agents that are useful for agendas and pharmacy/nursing in-services. A comprehensive target drug utilization evaluation (DUE) is also provided each month. The monographs are published in printed form and on diskettes that allow customization. Subscribers to the The Formulary Monograph Service also receive access to a pharmacy bulletin board called The Formulary Information Exchange (The F.I.X). All topics pertinent to clinical pharmacy are discussed on The F.I.X.

Through the cooperation of The Formulary, Hospital Pharmacy publishes selected reviews in this column. If you would like information about The Formulary Monograph Service or The F.I.X., call The Formulary at 800-322-4349. The February 2000 Formulary monographs are on gatifloxacin, moxifloxacin, levetiracetam, aspirin/extended-release dipyridamole, and aminolevulinic acid HCl. The DUE is on gatifloxacin.

Characterization of Enterococcus faecium with macrolide resistance and reduced susceptibility to quinupristin/dalfopristin in a Japanese hospital: detection of extensive diversity in erm(B)-regulator regions

Cross-resistance to macrolide, lincosamide, and streptogramin B (MLSB) antibiotics is mainly mediated by the erm (erythromycin ribosome methylation) genes that encode 23S rRNA methylases in enterococi, and various mechanisms are involved in the streptogramin B resistance. Prevalence of MLSB resistance and its genetic mechanisms were analyzed for a total of 159 strains of Enterococcus faecium isolated from clinical specimens in a university hospital in Japan from 1997 to 2006. Resistance to erythromycin (EM) and clindamycin was detected in 88.1% and 89.9% of all the strains examined, respectively, and expression of resistance was totally constitutive. Although none of the strain was resistant to quinupristin/dalfopristin (Q/D), 28 strains (17.6%) showed intermediate resistance to Q/D (MIC: 2 μg/ml). The erm(B) gene was detected in 139 strains (87.4%), and msrC was found in all the strains examined, whereas no other known MLSB resistance genes were identified. The erm(B) regulator region (RR) containing a coding region of the leader peptide was classified into 13 genetic variations (L1-L3, M, S1-S7, D, and R genotypes) in 56 strains. However, no relatedness was identified between the erm(B) RR genotype and EM resistance, or reduced susceptibility to Q/D, although most of Q/D-intermediate strains were assigned to the L1, L2, and S1 genotypes. Q/D-intermediate strains were classified into five multiple-locus variable-number tandem-repeat analysis (MLVA) types, including four types of clonal complex (CC)-C1, five sequence types (STs), including four STs of CC-17, and several resistance gene/virulence factor profiles. The present study revealed the occurrence of Q/D-intermediate E. faecium, which are composed of heterogeneous strains in Japan, and more genetic diversity in the erm(B) RRs than those reported previously.

Toxic Effects of Chromium(VI) on Anaerobic and Aerobic Growth of Shewanella oneidensis MR-1

Cr(VI) was added to early- and mid-log-phase Shewanella oneidensis (S. oneidensis) MR-1 cultures to study the physiological state-dependent toxicity of Cr(VI). Cr(VI) reduction and culture growth were measured during and after Cr(VI) reduction. Inhibition of growth was observed when Cr(VI) was added to cultures of MR-1 growing aerobically or anaerobically with fumarate as the terminal electron acceptor. Under anaerobic conditions, there was immediate cessation of growth upon addition of Cr(VI) in early- and mid-log-phase cultures. However, once Cr(VI) was reduced below detection limits (0.002 mM), the cultures resumed growth with normal cell yield values observed. In contrast to anaerobic MR-1 cultures, addition of Cr(VI) to aerobically growing cultures resulted in a gradual decrease of the growth rate. In addition, under aerobic conditions, lower cell yields were also observed with Cr(VI)-treated cultures when compared to cultures that were not exposed to Cr(VI). Differences in response to Cr(VI) between aerobically and anaerobically growing cultures indicate that Cr(VI) toxicity in MR-1 is dependent on the physiological growth condition of the culture. Cr(VI) reduction has been previously studied in Shewanella spp., and it has been proposed that Shewanella spp. may be used in Cr(VI) bioremediation systems. Studies of Shewanella spp. provide valuable information on the microbial physiology of dissimilatory metal reducing bacteria; however, our study indicates that S. oneidensis MR-1 is highly susceptible to growth inhibition by Cr(VI) toxicity, even at low concentrations [0.015 mM Cr(VI)].

Daptomycin for the treatment of vancomycin resistant Enterococcus faecium bacteremia

The best therapeutic options for serious infections due to vancomyci resistant Enterococcus (VRE) remain unclear. We describe the successful treatment of vancomycin resistant Enterococcus faecium bacteremia in 2 patients using daptomycin. We also briefly review the literature on antibiotic options for VRE infection including the use of daptomycin.

Transposon mutagenesis identifies genes which control antimicrobial drug tolerance in stationary-phaseEscherichia coli

Tolerance to antimicrobial agents is a universal phenomenon in bacteria which are no longer multiplying or whose growth rate slows. Since slowly multiplying bacteria occur in clinical infections, extended periods of antimicrobial chemotherapy are needed to eradicate these organisms and to achieve cure. In this study, the molecular basis of antibiotic tolerance was investigated using transposon mutagenesis. We screened 5000 Escherichia coli Tn10Cam mutants for reduction of kanamycin tolerance in late stationary phase and found that 4935 mutants were able to grow to late stationary phase. Reduced tolerance was observed in nine mutants which became sensitive to killing by kanamycin. The mutant KS639 was the most sensitive one to kanamycin, and its genome was disrupted in an intergenic region which lies between aldB and yiaW open reading frames. This mutant showed increased sensitivity not only to kanamycin but also to gentamicin, ciprofloxacin and rifampicin. Reduced tolerance of KS639 to kanamycin was also observed in a murine thigh infection model. P1 transduction to the wild type strains confirmed that the intergenic region was responsible for the tolerance of the bacterium to antibiotics. Using PCR-directed one-step gene replacement, we inactivated the genes aldB, yiaW and yiaV. We also deleted the intergenic region. There was no difference in kanamycin tolerance between each mutant (DeltaaldB, DeltayiaW and DeltayiaV) and the parental strain. But the mutant lacking the intergenic region showed reduced tolerance to kanamycin. These data suggest that the intergenic region between aldB and yiaW genes may be involved in tolerance to antimicrobial agents in E. coli. Furthermore, they show that it is important in murine infection during antibiotic treatment and lead to a faster kill of the mutant bacteria.

Determination of fungicidal activities against yeasts and molds: lessons learned from bactericidal testing and the need for standardization

In certain unique clinical settings, the ability of the antimicrobial agent administered to kill the pathogen outright may be quite important. These situations invariably involve infection of a site not easily accessed by host defenses and/or of a structure with essential anatomic or physiologic function such as the heart (endocarditis), central nervous system (meningitis), or bone (osteomyelitis). Likewise, infections in immunosuppressed hosts, especially those who are neutropenic, are often thought to require microbicidal therapy. Proof of the cidal nature of an antimicrobial agent in vitro is tedious, complex, and fraught with error. Although several methods for assessing in vitro bactericidal activity have been standardized (NCCLS M26-A and M21-A), the clinical relevance of these determinations is questionable and the tests are performed infrequently in most laboratories. Most of the clinical data supporting the need for microbicidal therapy and testing have focused on bacterial infections. However, given the fact that most serious fungal infections occur in profoundly immunosuppressed individuals, it is generally assumed that a cidal regimen would be preferable in that setting as well. In view of this clinical concern and the perceived need to assess the fungicidal activity of a variety of agents, we considered that it would be useful to review what is known about the issues and problems in assessing bactericidal activity and the clinical utility of such measurements. Following this review, we discuss the issue of how one defines fungicidal activity in vitro and in vivo and how feasible it might be to determine the fungicidal activity of organism-drug combinations for purposes of both drug development and clinical care. Proposed methods for fungal time-kill determinations and minimal fungicidal concentration determinations are also discussed.

Role of quinupristin/dalfopristin in the treatment of Gram-positive nosocomial infections in haematological or oncological patients

Gram-positive pathogens, primarily Staphylococcus aureus, coagulase-negative staphylococci, viridans group streptococci, and enterococci, are now the predominant causes of infection in neutropenic haematology/oncology patients, but are often resistant to multiple antibiotics. Glycopeptides have been the only alternative antibiotic treatments for multidrug-resistant Gram-positive infections to date. However, glycopeptides are not always effective or well tolerated, and can produce nephrotoxic or ototoxic effects. Quinupristin/dalfopristin is a recently introduced streptogramin antibiotic that is active in vitro against most of the major Gram-positive pathogens causing infection in neutropenic patients. Recent studies of the in vitro susceptibility of clinical isolates of Gram-positive pathogens to quinupristin/dalfopristin are summarized. Pre-clinical and clinical studies of the efficacy and safety of quinupristin/dalfopristin in the treatment of Gram-positive infections are reviewed. Quinupristin/dalfopristin is active in vitro against the vast majority of recent isolates of relevant Gram-positive pathogens, including methicillin-resistant staphylococci, viridans group streptococci, and vancomycin-resistant Enterococcus faecium, but excluding Enterococcus faecalis. Pre-clinical and clinical data indicate the efficacy of quinupristin/dalfopristin in infections caused by these organisms, including bacteraemia and catheter-related infections. Quinupristin/dalfopristin is not associated with nephrotoxicity or ototoxicity. Quinupristin/dalfopristin is a potential alternative to glycopeptides in haematology or oncology patients with multidrug-resistant Gram-positive infections, especially those who are unresponsive to, or intolerant of, glycopeptides.

Vancomycin-resistant enterococci: a road map on how to prevent the emergence and transmission of antimicrobial resistance

Nosocomial acquisition of microorganisms resistant to multiple antibiotics represents a threat to patient safety. Here we review the mechanisms that have allowed highly resistant strains belonging to the Enterococcus genus to proliferate within our health-care institutions. These mechanisms indicate that decreasing the prevalence of resistant organisms requires active surveillance, adherence to vigorous isolation, hand hygiene and environmental decontamination measures, and effective antibiotic stewardship. We suggest how to tailor such a complex, multidisciplinary program to the needs of a particular health-care setting so as to maximize cost-effectiveness.

Quinupristin-dalfopristin and linezolid: evidence and opinion

Quinupristin-dalfopristin and linezolid demonstrate in vitro activity against a wide range of gram-positive bacteria, including many isolates resistant to earlier antimicrobials. Quinupristin-dalfopristin is inactive against Enterococcus faecalis but has been effective for treatment of infections due to vancomycin-resistant Enterococcus faecium associated with bacteremia. In comparative trials, linezolid proved to be equivalent to comparator agents, resulting in its approval for several clinical indications. The almost-complete bioavailability of linezolid permits oral administration. Each agent can cause adverse effects that may limit use in individual patients. Resistance to these drugs has been encountered infrequently among vancomycin-resistant E. faecium. Resistance to quinupristin-dalfopristin is rare among staphylococci in the United States, and resistance to linezolid is very rare. Whether there is any benefit to use of these agents in combination regimens, and whether there are circumstances in which they might be alternatives to cell-wall active antibiotics for treatment of bone or endovascular infections, are questions that deserve further study.

Emergence of resistance of vancomycin-resistant Enterococcus faecium in a thermal injury patient treated with quinupristin-dalfopristin and cultured epithelial autografts for wound closure

Vancomycin-resistant Enterococcus faecium and faecalis (VRE) remains a major complication among critically ill patients. A 26-year-old patient with 65% total body surface area burns (TBSA) was infected with several E. faecium strains during his admission that were resistant to vancomycin. Because chloramphenicol was the standard treatment at this time, this drug was initiated until, the organism was identified as E. faecium and reported as susceptible to quinupristin-dalfopristin. Given these data, it was then decided to discontinue the chloramphenicol therapy. Quinupristin-dalfopristin therapy resulted in initial reduction of fever and white blood cell counts that continued over the next 5 days. However, on day 7 of quinupristin-dalfopristin therapy, a return of fever and elevation of the white blood cell count was noted and a repeated E. faecium blood culture demonstrated sudden resistance to quinupristin-dalfopristin (Bauer-Kirby zone size <14 mm). Chloramphenicol was restarted and the patient improved slowly over a period of 16 days. Our indigenous VRE had limited exposure to quinupristin-dalfopristin in the recent past; however, resistance emerged with the first commercial use of this agent in our burn treatment center. High-dose chloramphenicol treatment did not appear to impair engraftment of cultured epithelial autografts (CEA) in this patient.

Quinupristin/dalfopristin

Gram-positive pathogens are associated with both community- and hospital-acquired infections. These infections may be life-threatening in hospitalised patients, especially in those with significant underlying acute or chronic diseases. Prompt and appropriate antimicrobial therapy is essential for avoiding morbidity and mortality. The concept of appropriate therapy is being redefined by increasing antimicrobial resistance, especially amongst Gram-positive pathogens. This has been most dramatic with penicillin-resistant Streptococcus pneumoniae in the community, including cross-resistance to other classes of antimicrobial agents. In the US, the incidence of methicillin-resistant Staphylococcus aureus (MRSA) with community isolates is significant. For hospital-acquired Gram-positive pathogens, MRSA, vancomycin-resistant Enterococcus species and vancomycin-intermediate resistant and -resistant S. aureus are a great concern, particularly as the frequency of recovery of these pathogens from infected patients increases. The net result of these various resistance issues is a reduction in the number of appropriate antimicrobial agents for treating infected patients. Quinupristin/dalfopristin is a parental streptogramin with a spectrum of activity that includes Gram-positive pathogens, including those resistant to other classes of antimicrobial compounds. In this review, data summarising the frequency of recovered Gram-positive pathogens from various infectious diseases, the escalating prevalence of resistance amongst Gram-positive pathogens and the factors making quinupristin/dalfopristin a suitable agent for treating patients infected with Gram-positive organisms will be discussed.

Treatment options for vancomycin-resistant enterococcal infections

Serious infection with vancomycin-resistant enterococci (VRE) usually occurs in patients with significantly compromised host defences and serious co-morbidities, and this magnifies the importance of effective antimicrobial treatment. Assessments of antibacterial efficacy against VRE have been hampered by the lack of a comparator treatment arm(s), complex treatment requirements including surgery, and advanced illness-severity associated with a high crude mortality. Treatment options include available agents which don't have a specific VRE approval (chloramphenicol, doxycycline, high-dose ampicillin or ampicillin/sulbactam), and nitrofurantoin (for lower urinary tract infection). The role of antimicrobial combinations that have shown in vitro or animal-model in vivo efficacy has yet to be established. Two novel antimicrobial agents (quinupristin/ dalfopristin and linezolid) have emerged as approved therapeutic options for vancomycin-resistant Enterococcus faecium on the basis of in vitro susceptibility and clinical efficacy from multicentre, pharmaceutical company-sponsored clinical trials. Quinupristin/dalfopristin is a streptogramin, which impairs bacterial protein synthesis at both early peptide chain elongation and late peptide chain extrusion steps. It has bacteriostatic activity against vancomycin-resistant E. faecium [minimum concentration to inhibit growth of 90% of isolates (MIC(90)) = 2 microg/ml] but is not active against Enterococcus faecalis (MIC(90 )= 16 microg/ml). In a noncomparative, nonblind, emergency-use programme in patients who were infected with Gram-positive isolates resistant or refractory to conventional therapy or who were intolerant of conventional therapy, quinupristin/dalfopristin was administered at 7.5 mg/kg every 8 hours. The clinical response rate in the bacteriologically evaluable subset was 70.5%, and a 65.8% overall response (favourable clinical and bacteriological outcome) was observed. Resistance to quinupristin/dalfopristin on therapy was observed in 6/338 (1.8%) of VRE strains. Myalgia/arthralgia was the most frequent treatment-limiting adverse effect. In vitro studies which combine quinupristin/dalfopristin with ampicillin or doxycyline have shown enhanced killing effects against VRE; however, the clinical use of combined therapy remains unestablished. Linezolid, an oxazolidinone compound that acts by inhibiting the bacterial pre-translational initiation complex formation, has bacteriostatic activity against both vancomycin resistant E. faecium (MIC(90) = 2 to 4 microg/ml) and E. faecalis (MIC(90) = 2 to 4 microg/ml). This agent was studied in a similar emergency use protocol for multi-resistant Gram-positive infections. 55 of 133 evaluable patients were infected with VRE. Cure rates for the most common sites were complicated skin and soft tissue 87.5% (7/8), primary bacteraemia 90.9% (10/11), peritonitis 91.7% (11/12), other abdominal/pelvic infections 91.7% (11/12), and catheter-related bacteraemia 100% (9/9). There was an all-site response rate of 92.6% (50/54). In a separate blinded, randomised, multicentre trial for VRE infection at a variety of sites, intravenous low dose linezolid (200mg every 12 hours) was compared to high dose therapy (600 mg every 12 hours) with optional conversion to oral administration. A positive dose response (although statistically nonsignificant) was seen with a 67% (39/58) and 52% (24/46) cure rate in the high- and low-dose groups, respectively. Adverse effects of linezolid therapy have been predominantly gastrointestinal (nausea, vomiting, diarrhoea), headache and taste alteration. Reports of thrombocytopenia appear to be limited to patients receiving somewhat longer courses of treatment (>14 to 21 days). Linezolid resistance (MIC > or = 8 microg/ml) has been reported in a small number of E. faecium strains which appears to be secondary to a base-pair mutation in the genome encoding for the bacterial 23S ribosome binding site. At present a comparative study between the two approved agents for VRE (quinupristin/dalfopristin and linezolid) has not been performed. Several investigational agents are currently in phase II or III trials for VRE infection. This category includes daptomycin (an acidic lipopeptide), oritavancin (LY-333328; a glycopeptide), and tigilcycline (GAR-936; a novel analogue of minocycline). Finally, strategies to suppress or eradicate the VRE intestinal reservoir have been reported for the combination of oral doxycyline plus bacitracin and oral ramoplanin (a novel glycolipodepsipeptide). If successful, a likely application of such an approach is the reduction of VRE infection during high risk periods in high risk patient groups such as the post-chemotherapy neutropenic nadir or early post-solid abdominal organ transplantation.

Antimicrobial activity of quinupristin-dalfopristin combined with other antibiotics against vancomycin-resistant enterococci

Interactions between quinupristin-dalfopristin and six other antimicrobials were examined by checkerboard arrays against 50 clinical isolates of vancomycin-resistant Enterococcus faecium selected to represent a range of susceptibilities to individual agents. Unequivocal synergistic or antagonistic interactions at clinically relevant concentrations were infrequently encountered when the streptogramin was combined with chloramphenicol, ampicillin, imipenem, vancomycin, or teicoplanin. Combinations with doxycycline resulted in synergistic inhibition in 36% of checkerboards. Against 10 strains of Enterococcus faecalis, synergistic interactions were found when quinupristin-dalfopristin was combined with doxycycline (four strains), either glycopeptide (three strains), or ampicillin (two strains). Combination with quinupristin-dalfopristin increased the ampicillin MIC from 1 to 4 microg/ml for one strain. For 10 strains of E. faecium, interactions were also assessed by time-kill methods using concentrations of the agents attainable in human serum. Most of these antimicrobials augmented killing by quinupristin-dalfopristin to a minor degree. Against 2 of the 12 strains in this collection that were not highly resistant to gentamicin, the combination of quinupristin-dalfopristin (2 microg/ml) plus gentamicin (5 microg/ml) resulted in killing approaching 3 log(10) CFU/ml. With the exception of doxycycline, inhibitory interactions between quinupristin-dalfopristin and other agents tested against vancomycin-resistant strains of E. faecium were uncommon at clinically relevant concentrations.

Clinical prevalence, antimicrobial susceptibility, and geographic resistance patterns of enterococci: results from the SENTRY Antimicrobial Surveillance Program, 1997-1999

As part of the SENTRY Antimicrobial Resistance Surveillance Program, a total of 4998 strains of enterococci isolated from 1997 to 1999 were processed. The occurrence of enterococcal infections by species and site of infection was analyzed, as were the occurrence of vancomycin-resistant enterococci (VRE) and their resistance phenotypes and genotypes. Trends in antimicrobial susceptibility to a variety of agents (including experimental compounds) were also reported. Enterococci accounted for >9% of isolates from all bloodstream infections (BSIs) in North America. Ampicillin was active against strains from Latin America and Europe but not against those from the United States and Canada. US isolates were considerably more resistant to vancomycin (17% resistant strains in 1999) than were those from patients in the rest of the world. The highest proportion of VRE was observed among BSI isolates (81.7%). Quinupristin-dalfopristin, chloramphenicol, and doxycycline were the most active agents tested against VRE. The results of this study confirm the worldwide trend in increasing occurrence of enterococci and the emerging pattern of antimicrobial resistance among such isolates.

Quinupristin/dalfopristin: a therapeutic review

BACKGROUND

The proliferation of multidrug-resistant gram-positive bacteria, including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecium (VREF), has created a pressing need for effective alternative antibiotics. Quinupristin/dalfopristin is a new combination streptogramin product with a selective spectrum of antibacterial activity, mainly against gram-positive aerobic bacteria. It has been assessed primarily in emergency-use protocols, in hospitalized patients with skin and skin-structure infections, and in patients with VREF bacteremia.

OBJECTIVES

The objectives of this review were to summarize important results of in vitro microbiologic studies; to provide information on relevant pharmacokinetic parameters, drug interactions, and Y-site compatibility; and to assess efficacy and safety data from clinical studies of quinupristin/dalfopristin.

METHODS

Articles included in this review were identified by a MEDLINE search of the literature published between 1966 and September 2000 using the terms Synercid, quinupristin, and dalfopristin. Additional articles were retrieved from the reference lists of articles identified in the MEDLINE search.

RESULTS

In vitro analysis of the spectrum of activity of quinupristin/dalfopristin has confirmed its relatively selective coverage of gram-positive aerobic bacteria. Both quinupristin and dalfopristin undergo hepatic metabolism and are extensively excreted in the feces. Combination quinupristin/dalfopristin inhibits the cytochrome P450 3A4 pathway, and caution is warranted with concomitant use of other medications eliminated via this pathway. In trials in patients with VREF infections, treatment success with quinupristin/dalfopristin varied depending on the site of infection, ranging from 51.9% in bacteremia of unknown origin to 88.9% in urinary tract infections. The results of comparative clinical trials suggest that quinupristin/dalfopristin has similar efficacy to that of commonly used antibiotics, including cefazolin, oxacillin, and vancomycin, in patients with skin and skin-structure infections or nosocomial pneumonia. The most frequently reported adverse effects with administration of quinupristin/dalfopristin were infusion-site inflammation, pain, and edema; other infusion-site reactions; and thrombophlebitis. Arthralgia, myalgia, nausea, diarrhea, vomiting, and rash occurred in 2.5% to 4.6% of patients and were the most frequently reported systemic adverse events.

CONCLUSIONS

Outcomes data from clinical trials indicate that quinupristin/dalfopristin has the potential to play an important role in the treatment of bacteremia, complicated skin and skin-structure infections, and nosocomial pneumonia caused by VREF. Issues of bacterial resistance to quinupristin/dalfopristin and other appropriate uses of this combination agent remain to be elucidated.

Increasing Antimicrobial Resistance: Therapeutic Implications for Enterococcal Infections

Enterococcus was designated a genus distinct from the streptococci in 1984. Enterococci cause a variety of monomicrobial and polymicrobial infections, mainly in compromised patients. These infections include bacteremia, urinary and biliary tract infections, intra-abdominal sepsis, and decubitus and diabetic foot ulcers. Enterococcal infections may be acquired from the patient's endogenous intestinal flora or exogenously from a fecally contaminated environment. Enterococci are inherently resistant to many antimicrobial agents and readily acquire additional resistances, which is likely the reason that enterococci have become prominent nosocomial pathogens. Only the combination of a cell wall-active antibiotic to which the Enterococcus is susceptible (ie, certain beta-lactams or vancomycin) plus an aminoglycoside (ie, gentamicin or streptomycin) is bactericidal, and is required for cure of endocarditis, meningitis and probably infection in neutropenic patients; bacteriostatic activity is sufficient to treat most other infections. Treatment of infections caused by strains resistant to beta-lactams, glycopeptides and aminoglycosides has become problematic due the limited number of therapeutic options. No medical therapy is reliably effective for endocarditis caused by strains resistant to all cell wall-active antibiotics and all aminoglycosides. New antimicrobial agents, such as linezolid and quinupristin/dalfopristin, have recently become available, but their activity against enterococci is mainly bacterostatic.

In vitro activities of LY333328 and comparative agents against nosocomial gram-positive pathogens collected in a 1997 global surveillance study

The in vitro activity of LY333328 was evaluated for 1,479 nosocomial gram-positive pathogens isolated in 12 countries during 1997. LY333328 MICs at which 90% of the isolates tested were inhibited for Enterococcus faecalis (n = 351), Enterococcus faecium (n = 100), Staphylococcus aureus (n = 593), coagulase-negative Staphylococcus species (n = 325), and Streptococcus pneumoniae (n = 110) were 1, 1, 2, 2, and 0.015 microg/ml, respectively. LY333328 demonstrated potent activity against isolates of vancomycin-resistant enterococci, oxacillin-resistant staphylococci, and penicillin-resistant pneumococci.

Quinupristin/dalfopristin: a review of its use in the management of serious gram-positive infections

Quinupristin/dalfopristin is the first parenteral streptogramin antibacterial agent, and is a 30:70 (w/w) ratio of 2 semisynthetic pristinamycin derivatives. The combination has inhibitory activity against a broad range of gram-positive bacteria including methicillin-resistant staphylococci, vancomycin-resistant Enterococcus faecium (VREF), drug-resistant Streptococcus pneumoniae, other streptococci, Clostridium perfringens and Peptostreptococcus spp. The combination also has good activity against selected gram-negative respiratory tract pathogens including Moraxella catarrhalis, Legioniella pneumophila and Mycoplasma pneumoniae. Quinupristin/dalfopristin has poor activity against E. faecalis. The combination is bactericidal against staphylococci and streptococci, although constitutive erythromycin resistance can affect its activity. As for many other agents, quinupristin/dalfopristin is generally bacteriostatic against E. faecium. In patients with methicillin-resistant S. aureus (MRSA) or VREF infections participating in prospective emergency-use trials, quinupristin/dalfopristin 7.5 mg/kg every 8 or 12 hours achieved clinical or bacteriological success in > or =64% of patients. Emergence of resistance to quinupristin/dalfopristin was uncommon (4% of patients) in those with VREF infections. Quinupristin/dalfopristin 7.5 mg/kg 8- or 12-hourly also achieved similar clinical success rates to comparator agents in patients with presumed gram-positive complicated skin and skin structure infections or nosocomial pneumonia (administered in combination with aztreoman) in 3 large multicentre randomised trials. Systemic adverse events associated with quinupristin/dalfopristin include gastrointestinal events (nausea, vomiting and diarrhoea), rash and pruritus. Myalgias and arthralgias also occur at an overall incidence of 1.3%, although higher rates (2.5 to 31%) have been reported in patients with multiple comorbidities. Venous events are common if the drug is administered via a peripheral line; however, several management options (e.g. use of central venous access, increased infusion volume) may help to minimise their occurrence. Hyperbilirubinaemia has been documented in 3.1% of quinupristin/dalfopristin recipients versus 1.3% of recipients of comparator agents. Quinupristin/dalfopristin inhibits cytochrome P450 3A4 and therefore has the potential to increase the plasma concentrations of substrates of this enzyme.

CONCLUSIONS

Quinupristin/dalfopristin, the first parenteral streptogramin, offers a unique spectrum of activity against multidrug-resistant gram-positive bacteria. In serious gram-positive infections for which there are other treatment options available, the spectrum of activity and efficacy of quinupristin/ dalfopristin should be weighed against its tolerability and drug interaction profile. However, in VREF or unresponsive MRSA infections, where few proven treatment options exist, quinupristin/dalfopristin should be considered as a treatment of choice for these seriously ill patients.

Vancomycin-resistant enterococci: recent advances in genetics, epidemiology and therapeutic options

Vancomycin-resistant enterococci (VRE) have gained much attention in the last decade. Currently, there are five known types of vancomycin resistance based on genes encoding ligase enzymes that the organisms use to produce their cell wall precursors, namely, VanA, VanB, VanC, VanD and VanE. An additional unclassified type was discovered in Australia. The basis of resistance among these phenotypes appears to be similar in that the resistant organisms produce peptidoglycan precursors that end in moieties other than D-alanyl-D-alanine, the usual target of vancomycin. The other dipeptide-like termini identified to date include D-alanyl-D-lactate and D-alanyl-D-serine, which have low affinity for glycopeptides. Recent evidence suggests that glycopeptide-producing organisms might be the remote origin of the vancomycin resistance genes. In European countries, avoparcin, a glycopeptide used in farm animals as a growth promoter, has been linked to the occurrence of VRE and occasional common strains have been identified in food products, farm animals, healthy subjects and hospitalized patients. There have been no such reports in the USA where heavy use of vancomycin and use of broad spectrum antibiotics such as cephalosporins have been identified as important risk factors for acquisition of VRE. Transmission within the same or between hospitals has been reported in many countries. Infection control measures and efforts to use antibiotics, particularly vancomycin, more appropriately have been implemented in a number of healthcare facilities with varying degrees of success. Many antibiotics, as a single agent or a combination of drugs, as well as various new antibiotics have been tested in vitro, in animal models, or used in anecdotal cases but clinical data from large comparative trials are not available to date. Because of the limited susceptibility of many VRE to other agents, efforts to control these organisms are particularly important. Copyright 1999 Harcourt Publishers LtdCopyright 1999 Harcourt Publishers Ltd.

The efficacy and safety of quinupristin/dalfopristin for the treatment of infections caused by vancomycin-resistant Enterococcus faecium. Synercid Emergency-Use Study Group

A progressive increase in the incidence of vancomycin resistance in strains of Enterococcus faecium (VREF) has severely constrained treatment options for patients with infection caused by this emerging pathogen. Quinupristin/dalfopristin (Synercid), the first injectable streptogramin antibiotic, is active in vitro against VREF, with an MIC90 of 1.0 mg/L. We studied the clinical efficacy and safety of quinupristin/dalfopristin in the treatment of VREF infection. Two prospective studies were conducted simultaneously. The first enrolled only patients with VREF infection; the second included patients with infection caused by other gram-positive bacterial pathogens in addition to VREF. Patients were enrolled if they had signs and symptoms of active infection and no appropriate alternative antibiotic therapy. The recommended treatment regimen of quinupristin/dalfopristin was 7.5 mg/kg i.v. every 8 h for a duration judged appropriate by the investigator. A total of 396 patients with VREF infection were enrolled. The most frequent indications for treatment included intra-abdominal infection, bacteraemia of unknown origin, urinary tract infection, catheter-related bacteraemia, and skin and skin structure infection. This patient population had a high prevalence of severe underlying illness, including a history of diabetes mellitus, transplantation, mechanical ventilation, dialysis, chronic liver disease with cirrhosis and oncological disorders. The mean (+/- S.D.) duration of treatment was 14.5 +/- 10.7 days (range: 1-108). The majority of patients (82.1%) were treated every 8 h, as assessed on day 2 of treatment, while 15.9% were treated every 12 h. The clinical success rate was 73.6% [142/193 clinically evaluable patients; 95% confidence interval (CI): 67.4%, 79.8%], the bacteriological success rate 70.5% (110/156 bacteriologically evaluable patients; 95% CI: 63.4%, 77.7%) and the overall success (both clinical and bacteriological success) rate 65.8% (102/156 bacteriologically evaluable patients; 95% CI: 57.9%, 72.9%). VREF bacteraemia at entry, mechanical ventilation and laparotomy were associated with a worse outcome. Quinupristin/dalfopristin was generally well tolerated. The most common systemic adverse events related to treatment were arthralgias (9.1%) and myalgias (6.6%). Related laboratory abnormalities were infrequent. In these severely ill patients with VREF infection and no other clinically appropriate therapeutic alternatives, quinupristin/dalfopristin demonstrated substantial efficacy and a good nervous system, cardiovascular, gastrointestinal, renal and hepatic tolerability.

Prevalence of resistance to MLS antibiotics in 20 European university hospitals participating in the European SENTRY surveillance programme. Sentry Participants Group

Macrolide, lincosamide and streptogramin (MLS) antibiotics are chemically distinct inhibitors of bacterial protein synthesis. Resistance to MLS antibiotics may be constitutive or inducible. The purpose of this study is to update our understanding of the prevalence of different forms of MLS resistance in Europe. The analysis of 3653 clinical pneumococcal, staphylococcal and enterococcal isolates exhibited an average percentage of 21.3% and 6.2% intermediate and high-level penicillin-resistant Streptococcus pneumoniae, 21.8% methicillin-resistant Staphylococcus aureus and 11% vancomycin-resistant Enterococcus faecium. Geographical differences in erythromycin and clindamycin resistance in isolates of S. pneumoniae and S. aureus strongly reflect geographical variations in susceptibility to penicillin and methicillin, respectively. A very narrow range of MICs was obtained with quinupristin/dalfopristin, with no S. pneumoniae, S. aureus and E. faecium isolate having an MIC of > 4 mg/L, indicating a possible role of quinupristin/dalfopristin in the treatment of infections by multi-resistant Gram-positive bacteria.

In vitro activity of quinupristin/dalfopristin against clinical isolates of common gram-positive bacteria in Taiwan

The MICs of quinupristin/dalfopristin against common Gram-positive bacteria isolated from various clinical specimens at a university hospital in Taiwan were determined by the agar dilution method. The tested bacteria included methicillin-resistant Staphylococcus aureus (MRSA), methicillin-susceptible S. aureus (MSSA), methicillin-resistant Staphylococcus epidermidis (MRSE), methicillin-susceptible S. epidermidis (MSSE), Streptococcus pyogenes, Streptococcus pneumoniae, and Enterococcus faecalis. With the exception of E. faecalis all bacteria were susceptible to quinupristin/dalfopristin. The MIC50 and MIC90 were, respectively, 0.25 microgram/mL and 0.5 microgram/mL for both MRSA and MSSA; 0.25 microgram/mL and 0.5 microgram/mL for MRSE; 0.25 microgram/mL and 0.25 microgram/mL for MSSE; 0.125 microgram/mL and 0.125 microgram/mL for S. pyogenes; and < or = 0.03 microgram/mL and 0.25 microgram/mL for S. pneumoniae. Eighty-two percent of the tested E. faecalis isolates were intermediately resistant or resistant to quinupristin/dalfopristin, with an MIC50 of 2 micrograms/mL and an MIC90 of 4 micrograms/mL. Quinupristin/dalfopristin seems to be a promising antimicrobial agent against common Gram-positive bacteria other than E. faecalis in Taiwan.

Epidemiologic trends in nosocomial and community-acquired infections due to antibiotic-resistant gram-positive bacteria: the role of streptogramins and other newer compounds

The Gram-positive cocci have clearly re-emerged as important pathogens world-wide in the past two decades. Staphylococci, including the coagulase-negative staphylococci and Staphylococcus aureus, and the enterococci account for approximately one-third of all blood stream infections and as much as 50% of nosocomial blood stream infections. Although Streptococcus pneumoniae is often considered a community-acquired pathogen, it is also an important cause of nosocomial infection. The hallmark of these Gram-positive pathogens is increasing resistance to available antimicrobial agents. Of particular note is resistance to glycopeptides (vancomycin and teicoplanin), aminoglycosides (high-level), and penicillins among the enterococci (especially E. faecium), resistance to penicillinase-resistant penicillins (oxacillin and methicillin) and fluoroquinolones (ciprofloxacin and ofloxacin) among staphylococci, and resistance to penicillin, other beta-lactams and macrolides among the pneumococci. The recent detection of decreased susceptibility to vancomycin among S. aureus is also quite ominous. In many instances the ability of the clinical laboratory to accurately characterize these resistant isolates is suboptimal, further compounding the problem. Increased understanding of resistance mechanisms and correlations of resistance genes with the phenotypic expression of resistance has allowed for modifications and improvements of reference susceptibility tests and interpretive breakpoints. New compounds for effective therapy of infection with multi-resistant Gram-positive species are clearly needed. To this end, the streptogramin combination, quinupristin/dalfopristin, has demonstrated significant activity against oxacillin-resistant staphylococci, penicillin-resistant streptococci, and vancomycin-resistant E. faecium. Other candidate drugs including Gram-positive active fluoroquinolones (clinafloxacin, grepafloxacin, moxifloxacin, gatifloxacin, and trovafloxacin) and novel compounds such as the everninomicin derivatives (SCH27899), ketolides, and oxazolidinones (linezolid) have been shown to be active against these organisms and are under rapid clinical development.

Streptogramins and their potential role in geriatric medicine

Despite advances in antimicrobial chemotherapy over recent decades, morbidity and mortality secondary to infection continues to rise. In addition, the incidence of infection caused by resistant organisms has also increased. Concurrently, the elderly are living longer than prior generations, often with disabling chronic diseases. The more debilitated of the geriatric population are at greater risk for infection, and more likely to acquire or develop antimicrobial resistant organisms. Gram-positive organisms are a source of resistance and commonly cause infection in older patients. Whereas resistance is a concern in all patients, in the elderly this is magnified by limitations in treatment options because of differences in pharmacokinetics and tolerance as compared with younger counterparts. Pharmacokinetic differences include changes in drug distribution and may arise as a result of diminished end organ function. Age-related decreases in renal function often impact on commonly prescribed antimicrobials. In addition, the elderly are more susceptible to drug-drug interactions because polypharmacy is common in this patient population. Streptogramins may offer a useful alternative in the treatment of infections in the elderly due to their coverage of organisms commonly causing infections in this population and because of their favourable pharmacokinetic profiles. While published experience is limited, streptogramins are not appreciably eliminated by the kidney and, therefore, they are less subject to age-related changes in renal elimination. What is required is multi-dose pharmacokinetic analysis of streptogramins in geriatric populations and subset analysis of patient use data on file. The following will provide the reader with the most recently presented data on streptogramin use and their potential. While focusing on potential use in the elderly, we have cited data and issues which we believe will be relevant in the geriatric population.

Treatment of vancomycin-resistant Enterococcus faecium with RP 59500 (quinupristin-dalfopristin) administered by intermittent or continuous infusion, alone or in combination with doxycycline, in an in vitro pharmacodynamic infection model with simulated endocardial vegetations

Quinupristin-dalfopristin is a streptogramin antibiotic combination with activity against vancomycin-resistant Enterococcus faecium (VREF), but emergence of resistance has been recently reported. We studied the activity of quinupristin-dalfopristin against two clinical strains of VREF (12311 and 12366) in an in vitro pharmacodynamic model with simulated endocardial vegetations (SEVs) to determine the potential for resistance selection and possible strategies for prevention. Baseline MICs/minimal bactericidal concentrations (microg/ml) for quinupristin-dalfopristin, quinupristin, dalfopristin, and doxycycline were 0.25/2, 64/>512, 4/512, and 0.125/8 for VREF 12311 and 0.25/32, 128/>512, 2/128, and 0.25/16 for VREF 12366, respectively. Quinupristin-dalfopristin regimens had significantly less activity against VREF 12366 than VREF 12311. An 8-microg/ml simulated continuous infusion was the only bactericidal regimen with time to 99.9% killing = 90 hours. The combination of quinupristin-dalfopristin every 8 h with doxycycline resulted in more killing compared to either drug alone. Quinupristin-dalfopristin-resistant mutants (MICs, 4 microg/ml; resistance proportion, approximately 4 x 10(-4)) emerged during the quinupristin-dalfopristin monotherapies for both VREF strains. Resistance was unstable in VREF 12311 and stable in VREF 12366. The 8-microg/ml continuous infusion or addition of doxycycline to quinupristin-dalfopristin prevented the emergence of resistance for both strains over the 96-h test period. These findings replicated the development of resistance reported in humans and emphasized bacterial factors (drug susceptibility, high inoculum, organism growth phase) and infectious conditions (penetration barriers) which could increase chances for clinical resistance. The combination of quinupristin-dalfopristin with doxycycline and the administration of quinupristin-dalfopristin as a high-dose continuous infusion warrant further study to determine their potential clinical utility.

Pharmacodynamic analysis of the activity of quinupristin-dalfopristin against vancomycin-resistant Enterococcus faecium with differing MBCs via time-kill-curve and postantibiotic effect methods

Quinupristin-dalfopristin (Q-D) is a new water-soluble, semisynthetic antibiotic that is derived from natural streptogramins and that is combined in a 30:70 ratio. A number of studies have described the pharmacodynamic properties of this drug, but most have investigated only staphylococci or streptococci. We evaluated the relationship between Q-D, quinupristin (Q), and/or dalfopristin (D) susceptibility parameters and antibacterial activities against 22 clinical isolates of vancomycin-resistant Enterococcus faecium (VREF) by using the concentration-time-kill-curve method and by measuring postantibiotic effects. Q-D, Q, and D MICs and minimum bactericidal concentrations (MBCs) ranged from 0.125 to 1 and 0.25 to 64, 8 to 512 and >512, and 2 to 8 and 8 to 512 microgram/ml, respectively. There were no significant relationships between susceptibilities to the individual components and the susceptibilities to the Q-D combination product. In the time-kill-curves studies, Q-D at a concentration of 6 microgram/ml was at least bacteriostatic against all VREF tested. There was increased activity against more susceptible isolates when the isolates were grouped either by Q-D MBCs or by Q MICs. By multivariate regression analyses, the percent change in the inoculum from that at the baseline was significantly correlated with the Q MIC (R = 0.74; P = 0.008) and the Q-D concentration-to-MBC ratio (R = 0.58; P = 0.02) and was inversely correlated with the Q-D MBC-to-MIC ratio (R = 0.68; P = 0.003). A strong correlation existed between the killing rate and the Q-D concentration-to-MBC ratio (R = 0.99; P < 0.0001). Time to 99.9% killing was best correlated with the Q-D MBC (R = 0.96; P < 0.0001). The postantibiotic effect ranged from 0.2 to 3.2 h and was highly correlated with the Q-D concentration-to-MBC ratio (R = 0.96; P < 0.0001) and was less highly correlated with the Q MIC (R = 0.42; P = 0.04). Further study of these relationships with in vitro or in vivo infection models that simulate Q-D pharmacokinetics should further define the utility of these pharmacodynamic parameters in the prediction of Q-D activity for the treatment of VREF infections in humans.

Antimicrobial activity of quinupristin-dalfopristin (RP 59500, Synercid) tested against over 28,000 recent clinical isolates from 200 medical centers in the United States and Canada

A total of 200 medical center laboratories in the USA and Canada contributed results of testing quinupristin-dalfopristin, a streptogramin combination (formerly RP 59500 or Synercid), against 28,029 Gram-positive cocci. Standardized tests [disk diffusion, broth microdilution, Etest (AB BIODISK, Solna, Sweden)] were utilized and validated by concurrent quality control tests. Remarkable agreement was obtained between test method results for characterizing the collection by the important emerging resistances: 1) oxacillin resistance among Staphylococcus aureus (41.0 to 43.7%); 2) vancomycin resistance among Enterococcus faecium (50.0 to 52.0%); and 3) the penicillin nonsusceptible rate for pneumococci (31.1% overall, with 10.6% at MICs of > or = 2 micrograms/mL). The quinupristin-dalfopristin MIC90 for oxacillin-susceptible and -resistant S. aureus was 0.5 microgram/mL and 1 microgram/mL, respectively. The quinupristin-dalfopristin MIC90 for vancomycin-resistant E. faecium was 1 microgram/mL, and only 0.2% of isolates were resistant. Other Enterococcus species were generally not susceptible to the streptogramin combination but were usually inhibited by ampicillin (86 to 97% susceptible; MIC50, 1.0 microgram/mL) or vancomycin (86 to 95%; MIC50, 1.0 microgram/mL). Among all tested enterococci, the rate of vancomycin resistance was 16.2%. The quinupristin-dalfopristin MIC90 (0.75 microgram/mL) for 4,626 tested Streptococcus pneumoniae strains was not influenced by the penicillin or macrolide susceptibility patterns. When five regions in the USA and Canada were analyzed for significant streptogramin and other antimicrobial spectrum differences, only the Farwest region had lower numbers of streptogramin-susceptible E. faecium. Canadian strains were generally more susceptible to all drugs except chloramphenicol and doxycycline when tested against E. faecalis (73% and 89% susceptible, respectively). The U.S. Southeast region had S. pneumoniae strains less susceptible to macrolides (73%) but had more susceptibility among E. faecium isolates tested against vancomycin and ampicillin. The Northeast region of the USA had the greatest rate of vancomycin resistance among enterococci. Strains retested by the monitor because of quinupristin-dalfopristin resistance (MICs, > or = 4 micrograms/mL) were generally not confirmed (2.2% validation), and only 0.2% of E. faecium isolates were identified as truly resistant. The most common errors were: 1) species misidentification (28.0%); 2) incorrect susceptibility results (65.6%); and 3) mixed cultures (4.3%) tested by participants. Overall, quinupristin-dalfopristin was consistently active (> or = 90% susceptible) against major Gram-positive pathogens in North America, regardless of resistance patterns to other drug classes and geographic location of their isolation.