Designing fluoroquinolone breakpoints for Streptococcus pneumoniae by using genetics instead of pharmacokinetics-pharmacodynamics

Prevalence of first-step mutants among levofloxacin-susceptible isolates of Streptococcus pneumoniae in north Lebanon

Resistance of Streptococcus pneumoniae to fluoroquinolones arises by stepwise accumulation of spontaneous point mutations in the quinolone-resistance determining regions (QRDRs). Fluoroquinolones treatment of infections caused by first-step mutants (pre-resistant) can lead to the selection of resistant isolates, resulting in treatment failure. First-step mutants cannot however be reliably detected by routine resistance testing. Levofloxacin has been used as a surrogate marker to predict fluoroquinolone susceptibility in clinical laboratories. By use of a PCR followed by pyrosequencing, we examined 45 levofloxacin-susceptible pneumococcal strains [minimal inhibitory concentration (MIC) < 0.5 μg/ml] for first-step parC and parE mutations in the QRDR: 51.2% of isolates were recovered from pulmonary and nasal secretions, 22.2% from blood, 24.4% from ear and eye discharge, and 2.2% from cerebrospinal fluid. The results showed that three strains (6.6%) had first-step parC (Asp83-Asn) or parE (Asp435-Asn) mutations. This test could be useful for some high-risk patients or in national surveys.

Antimicrobial breakpoints for gram-negative aerobic bacteria based on pharmacokinetic-pharmacodynamic models with Monte Carlo simulation

OBJECTIVES

This study describes a comprehensive programme designed to develop pharmacokinetic-pharmacodynamic (PK-PD) breakpoints for numerous antimicrobial classes against key gram-negative aerobic bacteria.

METHODS

A 10,000 subject Monte Carlo simulation was constructed for 13 antimicrobials (21 dosing regimens). Published pharmacokinetic data and protein binding were varied according to log-normal and uniform distributions. MICs were fixed at single values from 0.03 to 64 mg/L. The PK-PD susceptible breakpoint was defined as the MIC at which the probability of target attainment was > or = 90%. PK-PD, CLSI and European Committee on Antimicrobial Susceptibility Testing breakpoints were applied to MICs from the 2005 worldwide Meropenem Yearly Susceptibility Test Information Collection database to evaluate the impact of breakpoint discrepancies.

RESULTS

PK-PD breakpoints were within one dilution of the CLSI and European breakpoints for all antimicrobials tested--with a few exceptions. When discrepancies were noted, the PK-PD breakpoint was lower than the CLSI breakpoint [ceftriaxone (0.5 versus 8 mg/L), ertapenem (0.25 versus 2 mg/L), ciprofloxacin (0.125 versus 1 mg/L) and levofloxacin (0.25-0.5 versus 2 mg/L)] and higher than the European breakpoint [ceftazidime (4-8 versus 1 mg/L), aztreonam (4-8 versus 1 mg/L), although ciprofloxacin was an exception to this pattern (0.125 versus 0.5-1 mg/L)]. For Enterobacteriaceae, breakpoint discrepancies resulted in modest (< or = 10%) differences in the percentages susceptible. In contrast, large (> 15%) discrepancies were noted for Pseudomonas aeruginosa and Acinetobacter baumannii.

CONCLUSIONS

Breakpoint agreement exists for imipenem, meropenem and the aminoglycosides. In contrast, discrepancies exist for piperacillin/tazobactam, cephalosporins, ertapenem, aztreonam and the fluoroquinolones. These discrepancies are most pronounced for P. aeruginosa and A. baumannii.

In vitro and in vivo antibacterial activities of DC-159a, a new fluoroquinolone

DC-159a is a new 8-methoxy fluoroquinolone that possesses a broad spectrum of antibacterial activity, with extended activity against gram-positive pathogens, especially streptococci and staphylococci from patients with community-acquired infections. DC-159a showed activity against Streptococcus spp. (MIC(90), 0.12 microg/ml) and inhibited the growth of 90% of levofloxacin-intermediate and -resistant strains at 1 microg/ml. The MIC 90s of DC-159a against Staphylococcus spp. were 0.5 microg/ml or less. Against quinolone- and methicillin-resistant Staphylococcus aureus strains, however, the MIC 90 of DC-159a was 8 microg/ml. DC-159a was the most active against Enterococcus spp. (MIC 90, 4 to 8 microg/ml) and was more active than the marketed fluoroquinolones, such as levofloxacin, ciprofloxacin, and moxifloxacin. The MIC 90s of DC-159a against Haemophilus influenzae, Moraxella catarrhalis, and Klebsiella pneumoniae were 0.015, 0.06, and 0.25 microg/ml, respectively. The activity of DC-159a against Mycoplasma pneumoniae was eightfold more potent than that of levofloxacin. The MICs of DC-159a against Chlamydophila pneumoniae were comparable to those of moxifloxacin, and DC-159a was more potent than levofloxacin. The MIC 90s of DC-159a against Peptostreptococcus spp., Clostridium difficile, and Bacteroides fragilis were 0.5, 4, and 2 microg/ml, respectively; and among the quinolones tested it showed the highest level of activity against anaerobic organisms. DC-159a demonstrated rapid bactericidal activity against quinolone-resistant Streptococcus pneumoniae strains both in vitro and in vivo. In vitro, DC-159a showed faster killing than moxifloxacin and garenoxacin. The bactericidal activity of DC-159a in a murine muscle infection model was revealed to be superior to that of moxifloxacin. These activities carried over to the in vivo efficacy in the murine pneumonia model, in which treatment with DC-159a led to bactericidal activity superior to those of the other agents tested.

TARGETed surveillance: susceptibility of Streptococcus pneumoniae isolated from community-acquired respiratory tract infections in 2003 to fluoroquinolones and other agents

We assessed antibiotic resistance in Streptococcus pneumoniae collected worldwide in 2003. Resistance to clarithromycin was the highest overall (34.1%) followed by penicillin G (22.1%). Patient age and/or country of origin had the greatest effect on susceptibility. Resistance was highest in children<6 years of age and in patients from South Africa or France. Resistance to penicillin or amoxicillin/clavulanic acid decreased in adults and was low in Germany. Fluoroquinolone resistance was very low overall, but 3.0% levofloxacin resistance (2.6% gatifloxacin and 0.4% moxifloxacin) was observed in Italy. Interestingly, many isolates with minimum inhibitory concentrations (MICs) at the top of the fluoroquinolone susceptibility breakpoints possessed single quinolone resistance-determining region (QRDR) mutations. Care should be taken when treating fluoroquinolone-susceptible isolates with a higher MIC, which are likely to harbour QRDR mutations and may become fully resistant and cause treatment failure. We concur with the conclusions of other recent studies that suggest fluoroquinolone breakpoints should be lowered to ensure these isolates are categorised as resistant. Fluoroquinolones would still remain an important alternative treatment for respiratory tract infections (albeit for adults only), with moxifloxacin being the most potent fluoroquinolone tested in this study.

Detection of parC mutations in Streptococcus pneumoniae by Real-time PCR and Taqman–MGB probes

A Real-time PCR assay was developed by using Taqman-MGB probes to screen mutations at codons Ser79 and Asp83 of Streptococcus pneumoniae parC. One hundred and thirty levofloxacin-susceptible and forty-two levofloxacin-resistant clinical strains were assayed. Mutations at codon 79 were found among all the levofloxacin-resistant strains. Mutations at codon 79 or 83 were found in ten levofloxacin-susceptible strains. This procedure is a reliable method for a rapid detection of mutations in the QRDRs of parC gene of S. pneumoniae and could be carried out in a diagnostic laboratory for some high-risk patients or in epidemiological surveys.

Selection of agyrAMutation and Treatment Failure with Gatifloxacin in a Patient withStreptococcus pneumoniaewith a PreexistingparCMutation

An 81-year-old woman had pneumonia caused by Streptococcus pneumoniae (levofloxacin Etest minimum inhibitory concentration [MIC] 1.5 microg/ml) and was treated with intravenous gatifloxacin 200 mg/day. After 3 days of therapy, repeat sputum cultures were positive for S. pneumoniae, which was resistant to levofloxacin (Etest MIC > 32 microg/ml). The isolate obtained before therapy showed a preexisting parC mutation of aspartic acid-83 to asparagine (Asp83-->Asn), and the isolate obtained during therapy showed an acquired gyrA mutation from serine-81 to phenylalanine (Ser81-->Phe) and a second parC mutation from lysine-137 to Asn (Lys137-->Asn). Both isolates were the same strain, as determined with pulsed-field gel electrophoresis. This case demonstrates the potential for resistance to emerge during 8-methoxy fluoroquinolone therapy for fluoroquinolone-susceptible S. pneumoniae with a preexisting parC mutation. Additional clinical failures with a fluoroquinolone may occur unless these first-step parC mutants can be identified to assist clinicians in selecting appropriate antimicrobial therapy.

Changes in the nasal carriage of drug-resistant Streptococcus pneumoniae in urban and rural Vietnamese schoolchildren

Studying the antimicrobial drug resistance of nasopharyngeal or nasal carriage isolates of Streptococcus pneumoniae in children is likely to have predictive potential for invasive isolates. Streptococcus pneumoniae nasal carriage was studied in 1422 Vietnamese children. Forty-six percent of 536 isolates showed reduced susceptibility to penicillin and 7% showed intermediate susceptibility to ceftriaxone; and 50% of 518 isolates showed resistance to erythromycin. All isolates were sensitive to levofloxacin and gatifloxacin. Urban and suburban children were significantly more likely to carry drug-resistant isolates than rural children. Rates of non-susceptibility to penicillin and erythromycin increased significantly in the rural province Khanh Hoa in 2003/2004 compared with rates obtained in 1997. An emerging clone of penicillin non-susceptible S. pneumoniae of serogroup 15 was identified, which was widely distributed in addition to the pandemic clones Spain(23F)-1 and Taiwan(19F)-14. Although resistance to fluoroquinolones was not observed, 6 (18%) of 34 isolates had a Lys137Asn mutation in the parC gene. This study shows that drug resistance is increasing in carriage isolates of S. pneumoniae in rural areas in Vietnam owing to spread of pandemic and emerging resistant clones.

La fenêtre de mutation pour le couple « pneumocoque–fluoroquinolone ». Apport des modèles expérimentaux

Low-level resistance to fluoroquinolones (in vitro susceptible but with topoisomerase mutation, parC) is currently rare among pneumococci in France. However, this resistance is more frequently observed in previously exposed patients and therapeutic failure has been reported. These issues were investigated by using a humanized model of experimental pneumonia induced by pneumococci exhibiting this low-level resistance profile. The results are as follows: 1) when the pneumonia is due to a wild type pneumococcus, humanized ciprofloxacin treatment is not effective because of resistant mutants with parC mutation; moreover, levoflaxin treatment is less bactericidal than gatiflo- or moxifloxacin (-4 vs -6 log CFU/g); 2) when an efflux strain is used, levo-treatment is not efficient but there are no mutants, a gatiflo-treatment is combined when mutants appear and moxiflo-treatment is effective; 3) when the pneumonia is induced with susceptible parC strains, treatment with either levo, or gati, or moxifloxacin is completely ineffective because resistant mutants appear (acquisition of another gyrA mutation). Measure of the mutation prevention concentration (MPC) allows anticipating these results since the mutation window can be determined. These results stress the necessity to identify patients with such pneumococcal strains in order to avoid therapeutic failure and the emergence of fluoroquinolone resistant mutants.

Call for the international adoption of microbiological breakpoints for fluoroquinolones and Streptococcus pneumoniae

The use of current Clinical and Laboratory Standards Institute levofloxacin breakpoints for assessing fluoroquinolone resistance in Streptococcus pneumoniae is inadequate for detecting isolates possessing first-step parC mutations. Consequently, the risk for development of fluoroquinolone resistance is greatly underestimated. Adopting microbiological breakpoints for fluoroquinolones and S. pneumoniae, where parC mutations are rare in susceptible isolates, more accurately describes the emergence of resistance and may help to prevent a number of future fluoroquinolone treatment failures. Additionally, we propose that the use of a second fluoroquinolone marker, such as ciprofloxacin, offers the best prediction for detecting an isolate possessing a first-step parC mutation.

New real-time PCR assay using locked nucleic acid probes to assess prevalence of ParC mutations in fluoroquinolone-susceptible Streptococcus pneumoniae isolates from France

A real-time PCR assay with locked nucleic acid probes was developed to screen mutations at codons 79 and 83 of the Streptococcus pneumoniae parC gene. Only silent mutations were detected among 236 French invasive fluoroquinolone-susceptible strains. This test could be useful for some high-risk patients or in national surveys.

Infrequent occurrence of single mutations in topoisomerase IV and DNA gyrase genes among US levofloxacin-susceptible clinical isolates of Streptococcus pneumoniae from nine institutions (1999–2003)

OBJECTIVES

Prevalence of single quinolone-resistance determining region (QRDR) mutations in Streptococcus pneumoniae was studied from nine institutions over 5 years to track the incidence of single QRDR mutations.

METHODS

All 1106 levofloxacin-susceptible pneumococci (MICs < or = 2.0 mg/L) identified from 1112 total isolates (99.5% susceptibility) in TRUST 3 (1999), TRUST 5 (2001) and TRUST 7 (2003) surveillance studies from the same nine hospitals in nine states were screened for QRDR mutations. Using pyrosequencing, the strains were screened for mutations corresponding to hot spots Asp-78, Ser-79 and Asp-83 in ParC; Asp-80, Ser-81 and Glu-85 in GyrA; Asp-435 in ParE and Asp-435 in GyrB. DNA sequencing of QRDRs was performed to confirm mutations.

RESULTS

No QRDR mutations were found in any of the isolates with levofloxacin MICs < or = 0.5 mg/L and no gyrA or gyrB QRDR mutations were found in any of the screened isolates (MICs < or = 2 mg/L). Four single-step QRDR mutants with the following amino acid substitutions were found: ParE Asp-435 to Asn (isolated in 1999 in Colorado); ParC Asp-83 to Asn (isolated in 2001 in Kentucky); ParC Ser-79 to Phe (isolated in 2003 in Indiana) and ParC Ser-79 to Tyr (isolated in 2003 in California). These non-clonal strains had levofloxacin MICs of 1 mg/L and were non-susceptible to ciprofloxacin (MIC 2-4 mg/L).

CONCLUSIONS

Overall prevalence of single QRDR mutations in levofloxacin-susceptible S. pneumoniae with MICs of < or = 2 mg/L was 0.4% (4/1106) and has remained <1% within nine institutions over 5 years (1999-2003).

Optimal pharmacological therapy for community-acquired pneumonia: the role of dual antibacterial therapy

The optimal pharmacological therapy of community-acquired pneumonia (CAP) is one of the most ardently debated issues in medicine. Presently, most guidelines recommend either a fluoroquinolone alone or dual therapy with a third-generation cephalosporin plus a macrolide in patients hospitalised with CAP, but few provide clinicians with specific considerations for selecting from these agents. Despite a similar spectrum of activity and favourable resistance patterns (for fluoroquinolones and third-generation cephalosporins) against CAP pathogens, there is emerging evidence that dual therapy may be superior to monotherapy in certain populations.In patients with non-severe CAP, the evidence supports the use of either monotherapy or dual therapy in most patients; however, patients with severe CAP or bacteraemic pneumococcal CAP experience improved survival when treated with dual therapy. It is unclear from this evidence if any specific combination of agents is the most effective, but the combination of a third-generation cephalosporin plus a macrolide is the most extensively studied. Dual therapy was superior to monotherapy irrespective of the susceptibility of the aetiological pathogen, thus insufficient antimicrobial spectrum does not explain the disparity. The most likely explanation for improved outcomes with dual therapy is the combined effect of optimised antimicrobial spectrum (including atypicals), decreased impact of resistance to a single agent and the immunomodulatory effects of macrolides. Increasing resistance in patients with non-severe CAP warrants the consideration of dual therapy and perhaps a reappraisal of agents usually reserved for second-line therapy, including doxycycline, in these populations as well. In light of the available evidence, dual therapy should be strongly considered in all patients with severe CAP, especially when complicated by pneumococcal bacteraemia.

Therapeutic options among broad-spectrum beta-lactams for infections caused by levofloxacin-nonsusceptible Streptococcus pneumoniae

Streptococcus pneumoniae has consistently become more resistant to primary, orally administered treatment regimens used for community-acquired respiratory tract infections (CARTI; sinusitis, bronchitis, pneumonia). As resistance rates approach 40-50% in the United States and North America for penicillin and macrolides, other agents also have exhibited coresistance rates of 10-20% (tetracycline, clindamycin, trimethoprim/sulfamethoxazole). These facts led to altered clinical treatment guidelines (IDSA) supporting the use of respiratory fluoroquinolones (levofloxacin, gatifloxacin, gemifloxacin, and moxifloxacin). This report from the SENTRY Antimicrobial Surveillance Program lists possible parenterally administered treatment alternatives for the fluoroquinolone (levofloxacin)-nonsusceptible pneumococci. The SENTRY Program isolates from CARTI (1997-2003), totaling 21605 strains from Europe, Asia Pacific, and the Americas, were screened for fluoroquinolone-resistant S. pneumoniae. A total of 157 (0.7%) levofloxacin-nonsusceptible (MIC > or = 4 microg/mL) strains were identified and tested by reference broth microdilution methods against 27 antimicrobials. Quinolone resistance-determining region (QRDR) mutations were determined by PCR amplification and gene sequencing. The entire population of S. pneumoniae had the following antibiogram demographics: penicillin-nonsusceptible (32%), macrolide resistance (24%), tetracycline resistance (21%), clindamycin resistance (11%), trimethoprim/sulfamethoxazole resistance (33%), and 6% of strains were resistant to all 5 drugs. Levofloxacin-resistant strains routinely had 2 or more QRDR mutations most frequently in gyrA at Ser81Phe or Tyr and in parC at Ser79Phe or Tyr and Lys137Asn. Four agents had extremely low rates of resistance when tested against the 157 levofloxacin-nonsusceptible strains (e.g., quinupristin/dalfopristin, 0% resistance; vancomycin, 0%; cefepime, 1%; ceftriaxone, 1%). Levofloxacin-nonsusceptible pneumococcal isolates remain uncommon, but are a growing problem in CARTI (1.4% in 2003), especially in previously fluoroquinolone-treated cases. Parenteral cephalosporins (cefepime or ceftriaxone) continue to be potent and safe for use in hospitalized patients with S. pneumoniae community-acquired pneumonia, used with or without co-drugs according to published guidelines.

Are fluoroquinolone-susceptible isolates of Streptococcus pneumoniae really susceptible? A comparison of resistance mechanisms in Canadian isolates from 1997 and 2003

OBJECTIVES

To assess the prevalence of efflux and amino acid substitutions in ParC and GyrA in Canadian clinical isolates of fluoroquinolone-susceptible Streptococcus pneumoniae with levofloxacin MICs of 1 mg/L collected before the introduction of the respiratory fluoroquinolones (1995-1997) and after 7 years of use (2003).

METHODS

Quinolone resistance determining regions of parC and gyrA were sequenced for 111 clinical isolates collected from 1995 to 1997 and 665 isolates collected in 2003. Efflux was assessed using a reserpine agar dilution method.

RESULTS

No isolates exhibited efflux. No significant increase in isolates harbouring amino acid substitutions was observed over time (0.9% in 1995-1997 to 2.1% in 2003, P = 0.32). However, the proportion of isolates with a ciprofloxacin MIC = 2 mg/L and a levofloxacin MIC = 1 mg/L versus ciprofloxacin MIC = 1 mg/L and a levofloxacin MIC = 1 mg/L increased over time (3.6% to 6.5%, P = 0.0021).

CONCLUSIONS

No increase in prevalence of first-step parC mutations was observed among all fluoroquinolone-susceptible clinical isolates of S. pneumoniae with levofloxacin MICs of 1 mg/L after the introduction of the respiratory fluoroquinolones; however, fluoroquinolones appear to be selecting for isolates with elevated ciprofloxacin MICs.