In-vitro activity of 29 antimicrobial agents against penicillin-resistant and -intermediate isolates of Streptococcus pneumoniae

A critical review of oxazolidinones: an alternative or replacement for glycopeptides and streptogramins?

OBJECTIVE

To review the available data on the oxazolidinones linezolid and eperezolid.

DATA SELECTION

Published reports were obtained by searching MEDLINE for articles published between 1992 and 2000, inclusive. References of published papers were also obtained and reviewed. Abstracts from scientific proceedings were reviewed.

DATA EXTRACTION

Due to the limited data available regarding these agents, the criteria for study inclusion were not restrictive.

DATA SYNTHESIS

The oxazolidinones (eg, linezolid) are a new antimicrobial class with a unique mechanism of action. They are active against resistant Gram-positive cocci including methicillin-susceptible and -resistant Staphylococcus aureus (MRSA), methicillin-susceptible and -resistant Staphylococccus epidermidis, vancomycin-resistant enterococci (VRE) and penicillin-resistant Streptococcus pneumoniae (PRSP). Linezolid is active against anaerobes and displays modest activity against fastidious Gram-negative pathogens such as Haemophilus influenzae, but is not active against Enterobacteriaceae. Linezolid is available both orally and parenterally, and has a bioavailability of 100%. Clinical trials comparing linezolid with standard therapy have demonstrated similar bacteriological and clinical cures rates to standard therapy in community- and hospital-acquired pneumonia, uncomplicated and complicated skin and soft tissue infections, and infections caused by MRSA and VRE. Adverse effects have been minor and infrequent; however, platelets should be monitored in patients who have received more than two weeks of linezolid therapy. It is expected that these agents will have a bright future due to their excellent spectrum of activity against antibiotic-resistant Gram-positive organisms, such as MRSA, VRE and PRSP, and their excellent bioavailability.

CONCLUSION

The oxazolidinones represent a new class of antimicrobials with a unique mechanism of action. They have excellent activity against susceptible and resistant Gram-positive organisms such as MRSA, methicillin-susceptible S epidermidis, VRE and PRSP, and a good adverse effect profile; they can be administered both intravenously and orally. Their potential use in Canada may be as an intravenous and oral alternative to glycopeptides and streptogramins.

Linezolid alone or combined with rifampin against methicillin-resistant Staphylococcus aureus in experimental foreign-body infection

We investigated the activity of linezolid, alone and in combination with rifampin (rifampicin), against a methicillin-resistant Staphylococcus aureus (MRSA) strain in vitro and in a guinea pig model of foreign-body infection. The MIC, minimal bactericidal concentration (MBC) in logarithmic phase, and MBC in stationary growth phase were 2.5, >20, and >20 microg/ml, respectively, for linezolid; 0.01, 0.08, and 2.5 microg/ml, respectively, for rifampin; and 0.16, 0.63, >20 microg/ml, respectively, for levofloxacin. In time-kill studies, bacterial regrowth and the development of rifampin resistance were observed after 24 h with rifampin alone at 1x or 4x the MIC and were prevented by the addition of linezolid. After the administration of single intraperitoneal doses of 25, 50, and 75 mg/kg of body weight, linezolid peak concentrations of 6.8, 12.7, and 18.1 microg/ml, respectively, were achieved in sterile cage fluid at approximately 3 h. The linezolid concentration remained above the MIC of the test organism for 12 h with all doses. Antimicrobial treatments of animals with cage implant infections were given twice daily for 4 days. Linezolid alone at 25, 50, and 75 mg/kg reduced the planktonic bacteria in cage fluid during treatment by 1.2 to 1.7 log(10) CFU/ml; only linezolid at 75 mg/kg prevented bacterial regrowth 5 days after the end of treatment. Linezolid used in combination with rifampin (12.5 mg/kg) was more effective than linezolid used as monotherapy, reducing the planktonic bacteria by >or=3 log(10) CFU (P < 0.05). Efficacy in the eradication of cage-associated infection was achieved only when linezolid was combined with rifampin, with cure rates being between 50% and 60%, whereas the levofloxacin-rifampin combination demonstrated the highest cure rate (91%) against the strain tested. The linezolid-rifampin combination is a treatment option for implant-associated infections caused by quinolone-resistant MRSA.

Fluoroquinolone resistance in Streptococcus pneumoniae: area under the concentration-time curve/MIC ratio and resistance development with gatifloxacin, gemifloxacin, levofloxacin, and moxifloxacin

The potential for resistance development in Streptococcus pneumoniae secondary to exposure to gatifloxacin, gemifloxacin, levofloxacin, and moxifloxacin at various levels was examined at high inoculum (10(8.5) to 10(9) log10 CFU/ml) over 96 h in an in vitro pharmacodynamic (PD) model using two fluoroquinolone-susceptible isolates. The pharmacokinetics of each drug was simulated to provide a range of free areas under the concentration-time curves (fAUC) that correlated with various fluoroquinolone doses. Potential first (parC and parE)- and second-step (gyrA and gyrB) mutations in isolates with raised MICs were identified by sequence analysis. PD models simulating fAUC/MICs of 51 and<or=60, 34 and 37, <or=82 and<or=86, and<or=24 for gatifloxacin, gemifloxacin, levofloxacin, and moxifloxacin, respectively, against each isolate were associated with first-step parC (S52G, S79Y, and N91D) and second-step gyrA (S81Y and S114G) mutations. For each fluoroquinolone a delay of first- and second-step mutations was observed with increasingly higher fAUC/MIC ratios and recovery of topoisomerase mutations in S. pneumoniae was related to the fAUC/MIC exposure. Clinical doses of gatifloxacin, gemifloxacin, and moxifloxacin exceeded the fAUC/MIC resistance breakpoint against wild-type S. pneumoniae, whereas those of levofloxacin (500 and 750 mg) were associated with first- and second-step mutations. The exposure breakpoints for levofloxacin were significantly different (P<0.001) from those of the newer fluoroquinolones gatifloxacin, gemifloxacin, and moxifloxacin. Additionally, moxifloxacin breakpoints were significantly lower (P<0.002) than those of gatifloxacin. The order of resistance development determined from fAUC/MIC breakpoints was levofloxacin>gatifloxacin>moxifloxacin=gemifloxacin, which may be related to structural differences within the class.

Ramoplanin: a lipoglycodepsipeptide antibiotic

OBJECTIVE

To review the pharmacology, antimicrobial activity, pharmacokinetics, clinical applications, and safety of ramoplanin, a lipoglycodepsipeptide antibiotic.

DATA SOURCES

Information was obtained from MEDLINE and BIOSIS databases (1984-August 2004) and Oscient Pharmaceuticals using the key words ramoplanin, A 16686, A 16686A, and MDL 62198.

STUDY SELECTION AND DATA EXTRACTION

Available English-based articles and abstracts were reviewed, along with information from Oscient Pharmaceuticals.

DATA SYNTHESIS

Ramoplanin exerts its bactericidal activity against gram-positive aerobic and anaerobic bacteria by blocking peptidoglycan synthesis via lipid II. In vitro susceptibility reports have demonstrated efficacy against antibiotic-resistant gram-positive pathogens. Cross-resistance has not been documented with vancomyin and other glycopeptides. Clinical trials are investigating ramoplanin's oral administration for treatment of Clostridium difficile-associated diarrhea. Previous clinical trials had evaluated the suppression of colonization of vancomycin-resistant Enterococcus with ramoplanin. Adverse effects are minimal, and drug-drug interactions have not been documented.

CONCLUSIONS

The completion of clinical trials will determine whether ramoplanin has a promising role as a treatment option for diarrhea due to C. difficile.

In vitro activity of the new quinolone WCK 771 against staphylococci

The activity of WCK 771, an experimental quinolone developed to overcome quinolone resistance in staphylococci and other bacteria, was determined against quinolone-susceptible and -resistant Staphylococcus aureus and S. epidermidis. WCK 771 MICs for 50 and 90% of the strains tested (MIC(50) and MIC(90), respectively) were 0.008 and 0.015 microg/ml for S. aureus (n = 43) and 0.015 and 0.03 microg/ml for S. epidermidis (n = 44) for quinolone-susceptible isolates, compared to ciprofloxacin values of 0.12 and 0.25 microg/ml and 0.25 and 0.5 microg/ml, respectively. Values for levofloxacin were 0.12 and 0.25 microg/ml and 0.12 and 0.25 microg/ml, those for clinafloxacin were 0.015 and 0.03 microg/ml and 0.015 and 0.03 microg/ml, those for moxifloxacin were 0.03 and 0.06 microg/ml and 0.06 and 0.12 microg/ml, and those for gatifloxacin were 0.06 and 0.12 microg/ml and 0.12 and 0.25 microg/ml, respectively. The WCK 771 MIC(50) and MIC(90), respectively, were 0.5 and 1 microg/ml for both species of staphylococci (n = 73 for S. aureus, n = 70 for S. epidermidis) for isolates highly resistant to ciprofloxacin (MIC(50) and MIC(90), >32 and >32 microg/ml, respectively). Values for levofloxacin were 8 and 32 microg/ml and 8 and 32 microg/ml, those for clinafloxacin were 1 and 2 microg/ml and 0.5 and 2 microg/ml, those for moxifloxacin 4 and >4 microg/ml and 4 and >4 microg/ml, and those for gatifloxacin were 4 and >4 microg/ml and 2 and >4 microg/ml, respectively. WCK 771 and clinafloxacin demonstrated MICs of 1 microg/ml against three vancomycin-intermediate strains. WCK 771 showed concentration-independent killing for up to 24 h at 2, 4, and 8 times the MICs against quinolone-resistant staphylococci and was also bactericidal after 8 h for high-density inocula (10(8) CFU/ml) of quinolone-resistant strains at 5 microg/ml, whereas moxifloxacin at 7.5 microg/ml was bacteriostatic. WCK 771 was not a substrate of the NorA efflux pump as evident from the similar MICs against both an efflux-positive and an efflux-negative strain. Overall, WCK 771 was the most potent quinolone tested against the staphylococci tested, regardless of quinolone susceptibility.

Experimental Study of Clinafloxacin Alone and in Combination in the Treatment of Ciprofloxacin-Susceptible and -Resistant Pneumococcal Meningitis

The increasing incidence of ciprofloxacin resistance in Streptococcus pneumoniae may limit the efficacy of the new quinolones in difficult-to-treat infections such as meningitis. The aim of the present study was to determine the efficacy of clinafloxacin alone and in combination with teicoplanin and rifampin in the therapy of ciprofloxacin-susceptible and ciprofloxacin-resistant pneumococcal meningitis in rabbits. When used against a penicillin-resistant ciprofloxacin-susceptible strain (Clinafloxacin MIC 0.12 microg/ml), clinafloxacin at a dose of 20 mg/kg per day b.i.d. decreased bacterial concentration by -5.10 log cfu/ml at 24 hr. Combinations did not improve activity. The same clinafloxacin schedule against a penicillin- and ciprofloxacin-resistant strain (Clinafloxacin MIC 0.5 microg/ml) was totally ineffective. Our data suggest that a moderate decrease in quinolone susceptibility, as indicated by the detection of any degree of ciprofloxacin resistance, may render these antibiotics unsuitable for the management of pneumococcal meningitis.

In vitro selection and characterization of Bacillus anthracis mutants with high-level resistance to ciprofloxacin

Mutants of attenuated Bacillus anthracis with high-level ciprofloxacin resistance were isolated using a three-step in vitro selection. Ciprofloxacin MICs were 0.5 micro g/ml for first-step mutants, which had one of two gyrA quinolone resistance-determining region (QRDR) mutations. Ciprofloxacin MICs were 8 and 16 microg/ml for second-step mutants, which had one of three parC QRDR mutations. Ciprofloxacin MICs for third-step mutants were 32 and 64 microg/ml. Mutants for which MICs were 64 microg/ml had one of two additional mutations within the gyrA QRDR or one of two mutations within the gyrB QRDR. Mutants for which MICs were 32 microg/ml had no additional target modifications but showed evidence of enhanced ciprofloxacin efflux.

In vitro activities of ramoplanin, teicoplanin, vancomycin, linezolid, bacitracin, and four other antimicrobials against intestinal anaerobic bacteria

By using an agar dilution method, the in vitro activities of ramoplanin, teicoplanin, vancomycin, linezolid, and five other agents were determined against 300 gram-positive and 54 gram-negative strains of intestinal anaerobes. Ramoplanin was active at <or=2 microg/ml against 287 of 300 (95.7%) gram-positive organisms, including 18 strains of Clostridium difficile for which MICs of ramoplanin were 0.25 to 0.5 microg/ml; for 3 of these, linezolid MICs were 8 to 16 micro g/ml. Nineteen Clostridium innocuum strains for which the vancomycin MIC at which 90% of strains were inhibited was 16 microg/ml were susceptible to ramoplanin at 0.06 to 0.25 microg/ml and to teicoplanin at 0.125 to 1.0 microg/ml. All strains of Eubacterium, Actinomyces, Propionibacterium, and Peptostreptococcus spp. were inhibited by <or=0.25 microg of ramoplanin per ml and <or=1 microg of vancomycin per ml. Ramoplanin was also active at <or=4 microg/ml against 15 of 22 of the Prevotella and Porphyromonas strains tested, but ramoplanin MICs for all 31 strains of the Bacteroides fragilis group, the Fusobacterium mortiferum-Fusobacterium varium group, and Veillonella spp. were >or=256 microg/ml. Ramoplanin displays excellent activity against C. difficile and other gram-positive enteric anaerobes, including vancomycin-resistant strains; however, it has poor activity against most gram-negative anaerobes and thus potentially has a lesser effect on the ecological balance of normal fecal flora.

Activity of and resistance to moxifloxacin in Staphylococcus aureus

Moxifloxacin has enhanced potency against Staphylococcus aureus, lower propensity to select for resistant mutants, and higher bactericidal activity against highly resistant strains than ciprofloxacin. Despite similar activity against purified S. aureus topoisomerase IV and DNA gyrase, it selects for topoisomerase IV mutants, making topoisomerase IV the preferred target in vivo.

Target site concentrations of ciprofloxacin after single intravenous and oral doses

To characterize the potential of ciprofloxacin penetration into human soft tissues following intravenous (i.v.) and oral (p.o.) administration, we measured the free ciprofloxacin concentrations in interstitial space fluid of skeletal muscle and subcutaneous adipose tissue by microdialysis. In addition, ciprofloxacin concentrations were measured in cantharis-induced skin blisters, saliva, and capillary plasma and were compared to the total concentrations in venous plasma. Furthermore, a pharmacodynamic in vitro model was used to simulate in vivo pharmacokinetics in bacterial culture. Eight healthy volunteers received ciprofloxacin in an open randomized crossover fashion either as a single i.v. infusion of 400 mg over 60 min or as a single p.o. dose of 500 mg. For both tissues the mean areas under the concentration-time curves (AUCs) for interstitial space fluid (AUC(interstitial fluid)s) were significantly lower than the corresponding AUC(plasma)s, with AUC(interstitial fluid)/AUC(plasma) ratios ranging from 0.38 to 0.68. For skeletal muscle, the AUC(interstitial fluid) was significantly higher after administration of 400 mg i.v. than after administration of 500 mg p.o., with a ratio of the AUC after p.o. administration/AUC after i.v. administration of 0.64. The ratio of the concentration in skeletal muscle/concentration in plasma increased over the entire observation period, implying that ciprofloxacin concentrations were not at steady state. The ratio of the concentration in skin blister fluid/concentration in plasma reached values above 4, indicating a preferential penetration of ciprofloxacin into inflamed lesions. The concentrations in saliva and capillary blood were similar to the corresponding total levels in plasma. In vitro both in vivo ciprofloxacin concentration-time profiles were equally effective against select bacterial strains. In conclusion, single-dose administration of two bioequivalent dosage forms of ciprofloxacin might lead to differences in target site pharmacokinetics. These differences, however, are not related to a difference in target site pharmacodynamics.

Beta-lactam resistance: clinical implications for pediatric patients

The emergence of resistance to established antibiotic agents such as beta-lactams has been reported worldwide and poses a serious challenge to the management of pediatric infections. The most common mechanism of resistance involves the production of an enzyme that inactivates the antibiotic before it can be effective. Streptococcus pneumoniae, the most common cause of pediatric respiratory tract infections, exhibits variable resistance to penicillins and aminopenicillin due to alterations in its penicillin-binding proteins (PBPs). Haemophilus influenzae and Moraxella catarrhalis show moderate and high beta-lactamase-mediated resistance to aminopenicillins, although they remain susceptible to beta-lactam/beta-lactamase inhibitor combinations. Methicillin-resistant Staphylococcus aureus, a frequent cause of skin and soft-tissue infections, has shown PBP-mediated beta-lactam resistance, prompting the wide-spread use of vancomycin to eradicate this pathogen. Finally, PBP-mediated resistance has been observed in a large proportion of isolates of coagulase-negative staphylococci, which account for a high proportion of nosocomial infections, particularly in neonatal intensive care units. The challenge is to control the emergence of beta-lactamase-mediated resistance by using beta-lactams judiciously. In this regard, the beta-lactam/beta-lactamase inhibitor combinations have an important role to play in extending the usefulness of established beta-lactam agents.

Pharmacodynamic activity and efficacy of linezolid in a rat model of pneumococcal pneumonia

Linezolid is a new oxazolidinone antibiotic with potent activity against gram-positive bacteria, including Streptococcus pneumoniae. The pharmacodynamic activity and in vivo efficacy of linezolid were compared to those of ceftriaxone in an immunocompetent rat model of pneumococcal pneumonia. Rats infected intratracheally with 8 x 10(7) CFU of a penicillin-sensitive (MIC, 0.032 microg/ml) strain of S. pneumoniae were treated for 5 days beginning 18 h postinfection. Groups of rats were sham treated with oral phosphate-buffered saline or received oral liquid linezolid at 25 or 50 mg/kg of body weight twice a day (b.i.d.) or subcutaneous ceftriaxone at 100 mg/kg once daily. Mortality was monitored for 10 days postinfection; blood culturing was performed on day 1 (pretreatment) and on days 3, 5, and 10 postinfection for the determination of bacteremia. Serum also was collected for the determination of pharmacokinetic and pharmacodynamic parameters at 30 min and at 3, 5, and 12 h (linezolid) or 3, 5, and 24 h (ceftriaxone) postdose. The cumulative mortality rates were 100% for the sham-treated group, 58.3% for the low-dose linezolid group, 8.3% for the high-dose linezolid group, and 0% for the ceftriaxone group. Rats in each of the antibiotic treatment groups had significantly fewer bacteria (P < 0.00001) in their bronchoalveolar lavage fluid (BALF) on day 3 postinfection than sham-treated rats. There also were significantly fewer organisms in the BALF of rats treated with ceftriaxone than in the BALF of rats treated with either dose of linezolid. Oral linezolid at 50 mg/kg b.i.d. therefore was as effective as ceftriaxone in experimental pneumococcal pneumonia, whereas the 25-mg/kg b.i.d. dose was significantly less effective. All pharmacodynamic parameters reflected efficacy and were significantly different for the two dosage regimens of linezolid (P < 0.01). However, the free-fraction pharmacodynamic parameters predictive of outcome were a value of >39% for the percentage of time in the experimental dosing interval during which the linezolid concentration exceeded the MIC and a value of >147 for the ratio of the area under the serum concentration-time curve to the MIC.

Streptococcus pneumoniae as an agent of nosocomial infection: treatment in the era of penicillin-resistant strains

Streptococcus pneumoniae is a well-known agent of community-acquired infections such as sinusitis, otitis media, pneumonia, bacterial meningitis, bacteremia and acute exacerbations of chronic bronchitis. However, the role of S. pneumoniae as a cause of nosocomial infections of respiratory tract, bloodstream and central nervous system is more and more recognized, primarily in high-risk patients with depression of their immune function. Therapy of pneumococcal infections is made difficult by the emergence and spread of bacterial resistance to penicillin and other beta-lactams as well as to a number of antimicrobials such as macrolides, chloramphenicol, tetracyclines and sulfonamides. This epidemiological situation is a cause for concern world-wide, but it primarily affects some European countries, North America, South Africa and the Far East. The main consequence on therapeutic grounds is that in severe infections such as bacterial meningitis, the addition of vancomycin to a third-generation cephalosporin is advisable while awaiting laboratory test results, even in areas with low prevalence of penicillin-resistant pneumococci. However, a beta-lactam agent can also be a valid choice in the presence of potentially lethal infections such as pneumonia or in the case of penicillin intermediately resistant isolates. In recent years, new alternative molecules have been introduced into clinical practice for therapy of infections caused by penicillin-resistant pneumococci. In both in vivo and in vitro studies, drugs of the classes of fluoroquinolones (levofloxacin, moxifloxacin, gatifloxacin), streptogramins (quinupristin/dalfopristin) and oxazolidinones (linezolid) have shown good microbiologic and clinical efficacy against penicillin-resistant pneumococci. In this era of world-wide spread of penicillin-resistant pneumococci, use of polysaccaride or conjugated vaccines is highly recommended.

Linezolid for the treatment of resistant gram-positive cocci

OBJECTIVE

To provide a comprehensive review of linezolid, the first of a new class of antibiotics, the oxazolidinones. Therapeutic issues regarding the emergence of multidrug-resistant bacteria and a brief history of the oxazolidinones are also discussed.

DATA SOURCES

A MEDLINE search (1966-March 2001) was conducted to identify pertinent literature, including preclinical trials, clinical trials, and reviews. Unpublished clinical data, adverse effects, and dosing information were abstracted from product labeling.

STUDY SELECTION

Clinical efficacy data were extracted from clinical trials, case reports, and abstracts that mentioned linezolid. Additional information concerning antibiotic resistance, the oxazolidinones, in vitro susceptibility and the pharmacokinetic profile of linezolid also was reviewed.

DATA SYNTHESIS

Linezolid exhibits activity against many gram-positive organisms, including vancomycin-resistant Enterococcus faecium, methicillin-resistant Staphylococcus aureus, and penicillin-resistant Streptococcus pneumoniae. Linezolid inhibits bacterial protein synthesis at an early step in translation and is rapidly and completely absorbed from the gastrointestinal tract following oral administration. Efficacy has been demonstrated in a number of unpublished clinical trials in adults with pneumonia, skin and skin structure infections, and vancomycin-resistant E. faecium infections. The adverse effect profile is similar to that of comparator agents (beta-lactams, clarithrornycin, vancomycin).

CONCLUSIONS

Linezolid is the first oral antimicrobial agent approved for the treatment of vancomycin-resistant enterococci. Since the oxazoildinones have a unique mechanism of action and expanded spectrum of activity against virulent and highly resistant gram positive pathogens, linezolid is a valuable alternative to currently available treatment options. Clinical trials evaluating linezolid and other oxazolidinones for antibiotic-resistant gram-positive infections, as well as comparator studies comparing linezolid with other candidate drugs, such as quinupristin/dalfopristin and choramphenicol, will further define the role of linezolid.

Pneumococcal resistance: the treatment challenge

OBJECTIVE

To review in vitro and in vivo information dealing with pneumococcal antibiotic resistance and provide a review of the incidence, mechanisms, and controversies surrounding this growing problem. The review is also intended to provide clinicians with relevant recommendations on treatment and prevention of this organism.

DATA SOURCES AND SELECTION

Primary and review articles were identified by MEDLINE search (1966-August 2000) and through secondary resources such as conference proceedings. All of the articles identified from the data sources were evaluated, and all information deemed relevant was included in this review.

DATA SYNTHESIS

The growing incidence and reporting of pneumococcal isolates that are resistant to one or more classes of antibiotics have become a troubling trend that has resulted in significant shifts in treatment. Although clinicians have shifted to a new generation or class of antibiotics when faced with a resistance trend, data with resistant pneumococci show that this may not be necessary. By incorporating the pharmacokinetic and pharmacodynamic data of antimicrobials into the decision-making process, many of the drugs that we have become hesitant to use due to this resistance may still be appropriate if used correctly.

CONCLUSIONS

Appropriate dosing of antimicrobials, combined with optimal use of pneumococcal vaccines, will not only prolong the longevity of some agents, but also hopefully slow resistance development.

Linezolid: an oxazolidinone antimicrobial agent

BACKGROUND

Linezolid is the first oxazolidinone anti-infective agent marketed in the United States. It is indicated for the treatment of nosocomial pneumonia, complicated skin and skin-structure infections caused by methicillin-sensitive or methicillin-resistant Staphylococcus aureus and other susceptible organisms, and vancomycin-resistant Enterococcus faecium infections. It also is indicated for the treatment of uncomplicated skin and skin-structure infections caused by methicillin-sensitive S. aureus or Streptococcus pyogenes, and community-acquired pneumonia caused by penicillin-sensitive Streptococcus pneumoniae.

OBJECTIVE

This article reviews the pharmacologic properties and clinical usefulness of linezolid.

METHODS

Using the terms linezolid, PNU-100766, and oxazolidinone, we performed a literature search of the following databases: MEDLINE (1966 to September 2000), HealthSTAR (1993 to September 2000), Iowa Drug Information Service (1966 to September 2000), International Pharmaceutical Abstracts (1970 to September 2000), PharmaProjects (January 2000 version), and meeting abstracts of the Infectious Diseases Society of America and the Interscience Conference on Antimicrobial Agents and Chemotherapy (1996 to 2000).

RESULTS

Linezolid has a unique structure and mechanism of action, which targets protein synthesis at an exceedingly early stage. Consequently, cross-resistance with other commercially available antimicrobial agents is unlikely. It is primarily effective against gram-positive bacteria. To date, resistance to linezolid has been reported in patients infected with enterococci. The pharmacokinetic parameters of linezolid in adults are not altered by hepatic or renal function, age, or sex to an extent requiring dose adjustment. Linezolid is metabolized via morpholine ring oxidation, which is independent of the cytochrome P450 (CYP450) enzyme system; as a result, linezolid is unlikely to interact with medications that stimulate or inhibit CYP450 enzymes. Compassionate-use trials and other clinical studies involving mainly adult hospitalized patients with gram-positive infections have shown that linezolid administered intravenously or orally is effective in a variety of nosocomial and community-acquired infections, including those caused by resistant gram-positive organisms. Reported adverse effects include thrombocytopenia. diarrhea, headache, nausea, vomiting, insomnia, constipation, rash, and dizziness. Preliminary pharmacoeconomic data indicate that a significantly higher percentage of patients receiving linezolid therapy versus comparator could be discharged from the hospital by day 7 (P = 0.005).

CONCLUSIONS

Linezolid appears to be effective while maintaining an acceptable tolerability profile. Due to the risk of bacterial resistance, linezolid should be reserved for the treatment of documented serious vancomycin-resistant enterococcal infections.

Single dose pharmacokinetics of linezolid in infants and children

BACKGROUND

Linezolid is an oxazolidinone antibiotic with excellent in vitro activity against a number of Gram-positive organisms including antibiotic-resistant isolates. The safety and pharmacokinetics of intravenously administered linezolid were evaluated in children and adolescents to examine the potential for developmental dependence on its disposition characteristics.

METHODS

Fifty-eight children (3 months to 16 years old) participated in this study; 44 received a single 1.5-mg/kg dose and 14 received a single 10-mg/kg dose of linezolid administered by intravenous infusion. Repeated blood samples (n = 10 in children > or = 12 months; n = 8 in children 3 to 12 months) were obtained during 24 h after drug administration, and linezolid was quantitated from plasma by high performance liquid chromatography with mass spectrometry detection. Plasma concentration vs. time data were evaluated with a model independent approach.

RESULTS

Linezolid was well-tolerated by all subjects. The disposition of linezolid appears to be age-dependent. A significant although weak correlation between age and total body clearance was observed. The mean (+/- SD) values for elimination half-life, total clearance and apparent volume of distribution were 3.0 +/- 1.1 h, 0.34 +/- 0.15 liter/h/kg and 0.73 +/- 0.18 liter/kg, respectively. Estimates of total body clearance and volume of distribution were significantly greater in children than historical values of adult data. As such maximum achievable linezolid plasma concentrations were slightly lower in children, and concentrations 12 h after a single 10-mg/kg dose were below the MIC90 for selected pathogens with in vitro susceptibility to the drug.

CONCLUSION

Based on these data a linezolid dose of 10 mg/kg given two to three times daily would appear appropriate for use in pediatric therapeutic clinical trials of this agent.

Microbiological and pharmacodynamic considerations in the treatment of infection due to antimicrobial-resistant Streptococcus pneumoniae

The incidence of antimicrobial-resistant strains of Streptococcus pneumoniae has increased alarmingly in recent years. The problem is exacerbated by the global spread of resistant organisms. Currently, the incidence of penicillin-resistant pneumococci isolated from clinical specimens in the United States is > or = 35%. For empirical oral treatment of community-acquired respiratory infections, 3 choices are available: beta-lactam agents, macrolides, and fluoroquinolones. In considering the therapeutic efficacy of these agents, it is essential to also take pharmacokinetic and pharmacodynamic (PK/PD) factors into account. Many drugs are effective against penicillin-susceptible strains. However, the higher the minimum inhibitory concentration of penicillin, the more likely that cross-resistance to beta-lactam agents and macrolides will occur. Currently, the incidence of fluoroquinolone-resistant pneumococci is low; it is proposed that adequate dosing based on the PK/PD properties of fluoroquinolones may help reduce the emergence of resistant organisms. Prudent use of all antimicrobials is essential to decrease the emergence of strains resistant to these agents.

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.