In vitro postantibiotic effect of daptomycin (LY146032) against Enterococcus faecalis and methicillin-susceptible and methicillin-resistant Staphylococcus aureus strains

Population pharmacokinetics of intravenous daptomycin in critically ill patients: implications for selection of dosage regimens

Daptomycin is gaining prominence for the treatment of methicillin-resistant Staphylococcus aureus infections. However, the dosage selection for daptomycin in critically ill patients remains uncertain, especially in Chinese patients. This study aimed to establish the population pharmacokinetics of daptomycin in critically ill patients, optimize clinical administration plans, and recommend appropriate dosage for critically ill patients in China. The study included 64 critically ill patients. Blood samples were collected at the designated times. The blood daptomycin concentration was determined using validated liquid chromatography-tandem mass spectrometry. A nonlinear mixed-effects model was applied for the population pharmacokinetic analysis and Monte Carlo simulations of daptomycin. The results showed a two-compartment population pharmacokinetic model of daptomycin in critically ill adult Han Chinese patients. Monte Carlo simulations revealed that a daily dose of 400 mg of daptomycin was insufficient for the majority of critically ill adult patients to achieve the anti-infective target. For critically ill adult patients with normal renal function (creatinine clearance rate >90 mL/min), the probability of achieving the target only reached 90% when the daily dose was increased to 700 mg. For patients undergoing continuous renal replacement therapy (CRRT), 24 h administration of 500 mg met the pharmacodynamic goals and did not exceed the safety threshold in most patients. Therefore, considering its efficacy and safety, intravenous daptomycin doses are best scaled according to creatinine clearance, and an increased dose is recommended for critically ill patients with hyperrenalism. For patients receiving CRRT, medication is recommended at 24 h intervals.

Contemporary pharmacologic treatments of MRSA for hospitalized adults: rationale for vancomycin versus non-vancomycin therapies as first line agents

INTRODUCTION

Methicillin-resistant Staphylococcus aureus (MRSA) remains an important pathogen in the hospital setting and causes significant morbidity and mortality each year. Since the initial discovery over 60 years ago, vancomycin has remained a first-line treatment for many different types of MRSA infections. However, significant concerns related to target attainment and nephrotoxicity have spurred efforts to develop more effective agents in the last two decades.

AREAS COVERED

Newer anti-MRSA antibiotics that have been approved since 2000 include linezolid, daptomycin, and ceftaroline. As clinical evidence has accumulated, these newer agents have become more frequently used, and some are now recommended as co-first-line options (along with vancomycin) in clinical practice guidelines. For this review, a scoping review of the literature was conducted to support our findings and recommendations.

EXPERT OPINION

Vancomycin remains an important standard of care for MRSA infections but is limited with respect to nephrotoxicity and rapid target attainment. Newer agents such as linezolid, daptomycin, and ceftaroline have specific indications for treating different types of MRSA infections; however, newer agents also have unique attributes which require consideration during therapy.

Effect of Penicillin on Surface Carbohydrate, Hemolysin and Morphology ofStreptococcus pyogenesDuring and After the Post-Antibiotic Phase

The post-antibiotic effect (PAE) of penicillin was measured in vitro against a group A streptococcal strain by the kinetic growth method. The duration of the effect was 2.8 h. The bacterial morphology and some streptococcal products were analyzed during and after the PAE, after being exposed to penicillin in a concentration of 1xMIC for 2 h. Bacteria not previously exposed to penicillin were used as a control culture. Morphological changes and increases in the size of treated streptococci were observed by electronic microscope during the post-antibiotic phase. The post-penicillin effect on the production of cell-bound hemolysin and free hemolysin was examined using sheep red blood cells. Production of cell-bound hemolysin rose sharply, but was inhibited by the antimicrobial agent. The free lysin diminished significantly, and concomitantly with a higher production of free toxin by the treated cells. No effect was observed on the specific carbohydrate group when the antigen was tested with streptococcal group A antiserum.

Moderate Liver Impairment Has No Influence on Daptomycin Pharmacokinetics

Daptomycin (N-decanoyl-L-tryptophyl-L-asparaginyl-L-aspartyl-L-threonylglycyl-L-ornithyl-L-aspartyl-D-alanyl-L-aspartylglycyl-D-seryl-threo-3-methyl-L-glutamyl-3-anthraniloyl-L-alanine-lactone) is a novel cyclic lipopeptide antibiotic derived from the fermentation of Streptomyces roseosporus. Daptomycin was recently approved for the treatment of complicated skin and skin structure infections caused by aerobic gram-positive bacteria, including those caused by methicillin-resistant and methicillin-susceptible Staphylococcus aureus. This single-dose, parallel-design, matched-controlled study was designed to evaluate the pharmacokinetics of daptomycin in subjects between ages 18 and 80 years with moderately impaired hepatic function (Child-Pugh Class B, n = 10). Subjects were administered a single intravenous dose (6 mg/kg total body weight) over 30 minutes using a syringe pump. A normal volunteer control group matched by weight (+/-25 lb/11 kg), age (+/-10 years), and sex was included in this study for comparison to the hepatic-impaired group. The pharmacokinetic parameters of daptomycin were similar in both groups. Adverse events occurred only in the hepatic-impaired patients and were consistent with the subjects' disease state. In conclusion, subjects with moderate hepatic impairment receiving daptomycin do not require an adjustment in daptomycin dose or dose regimen.

Antibiotics for gram-positive bacterial infection: vancomycin, teicoplanin, quinupristin/dalfopristin, oxazolidinones, daptomycin, telavancin, and ceftaroline

An overview of the mechanism of action, dosing, clinical indications, and toxicities of the glycopeptide vancomycin is provided. The emerging gram-positive bacterial resistance to antimicrobials and its mechanisms are reviewed. Strategies to control this emergence of resistance are expected to be proposed. Newer antimicrobial agents that have activity against vancomycin-resistant organisms are now available and play a critical role in the treatment of life-threatening infections.

Daptomycin: a novel lipopeptide antibiotic against Gram-positive pathogens

The aim of this review is to summarize the historical background of drug resistance of Gram-positive pathogens as well as to describe in detail the novel lipopeptide antibiotic daptomycin. Pharmacological and pharmacokinetic aspects are reviewed and the current clinical use of daptomycin is presented. Daptomycin seems to be a reliable drug in the treatment of complicated skin and skin structure infections, infective right-sided endocarditis, and bacteremia caused by Gram-positive agents. Its unique mechanism of action and its low resistance profile, together with its rapid bactericidal action make it a favorable alternative to vancomycin in multi-drug resistant cocci. The role of daptomycin in the treatment of prosthetic material infections, osteomyelitis, and urogenital infections needs to be evaluated in randomized clinical trials.

Antibiotics for gram-positive bacterial infections: vancomycin, teicoplanin, quinupristin/dalfopristin, oxazolidinones, daptomycin, dalbavancin, and telavancin

An overview of the mechanism of action, dosing, clinical indications, and toxicities of the glycopeptide vancomycin is provided. The emerging gram-positive bacterial resistance to antimicrobials and its mechanisms are reviewed. Strategies to control this emergence of resistance are expected to be proposed. Newer antimicrobial agents that have activity against vancomycin-resistant organisms are now available and play a critical role in the treatment of life-threatening infections.

Drug dosing during continuous renal replacement therapy

Continuous renal replacement therapy (CRRT) has given clinicians an important option in the care of critically ill patients. The slow and continuous dialysate and ultrafiltrate flow rates that are employed with CRRT can yield drug clearances similar to an analogous glomerular filtration rate of the native kidneys. Advantages such as superior volume control, excellent metabolic control, and hemodynamic tolerance by critically ill patients are well documented, but an understanding of drug dosing for CRRT is still a bit of a mystery. Although some pharmaceutical companies have dedicated postmarket research in this direction, many pharmaceutical companies have chosen not to pursue this information as it is not mandated and represents a relatively small part of their market. This lack of valuable information has created many challenges in the care of the critically ill patient as intermittent hemodialysis drug dosing recommendations cannot be extrapolated to CRRT. This drug dosing review will highlight factors that clinicians should consider when determining a pharmacotherapy regimen for a patient receiving CRRT.

Daptomycin: The First Approved Lipopeptide Antimicrobial

OBJECTIVE

To review the literature concerning the first Food and Drug Administration-approved lipopeptide antimicrobial, daptomycin.

DATA SOURCES

A PUBMED search was conducted to identify pertinent English-language journal articles between 1985 and November 2003, and additional references were obtained from the bibliographies of these articles. Abstracts from the Interscience Conference on Antimicrobial Agents and Chemotherapy meetings from 1985 through 2003 also were reviewed.

STUDY SELECTION

All studies evaluating any aspect of daptomycin.

DATA SYNTHESIS

Daptomycin is a semisynthetic lipopeptide, the first such antimicrobial agent to reach the marketplace. Its mechanism of action differs from that of the related agent vancomycin in that much of its effect is not because of inhibition of peptidoglycan biosynthesis, but instead is a result of alterations in cell-membrane electrical charge and transport. It exhibits a broad spectrum of activity against gram-positive aerobes and anaerobes, including methicillin-, penicillin-, aminoglycoside-, and vancomycin-resistant strains. In subjects with normal renal function, the terminal disposition half-life is about 7 to 10 hours. It is principally eliminated as unchanged drug in the urine. Available clinical trial data demonstrate efficacy in complicated skin and skin-structure infections resulting from susceptible gram-positive pathogens, but not in pneumonia. The principal adverse event of concern, although rare, is myotoxicity, manifested by muscle pain and/or weakness and elevated serum creatine phosphokinase (CPK) concentrations. The approved dosage regimen is 4 mg/kg intravenously over 30 minutes once daily for 7 days to 14 days. Studies are underway evaluating doses of up to 8 mg/kg once daily.

CONCLUSIONS

Daptomycin, the first lipopeptide antimicrobial to be marketed, exhibits activity against multiresistant gram-positive pathogens, including linezolid- and quinupristindalfopristin-resistant strains. As such, it is a potentially valuable agent to treat infections resulting from such pathogens. To preserve its utility, it should not be used indiscriminately for infections resulting from pathogens sensitive to other antimicrobials. It is probably best used with restricted access and used only for multiresistant gram-positive pathogens where alternative agents cannot be employed. If used, careful monitoring for the signs and symptoms of myotoxicity, including obtaining weekly serum CPK levels, is mandatory. In addition, bacterial sensitivities to this agent should be prospectively monitored by national antimicrobial surveillance programs like SENTRY, TRUST, and LIBRA.

Daptomycin: rationale and role in the management of skin and soft tissue infections

The emergence of community-associated methicillin-resistant Staphylococcus aureus (MRSA) and glycopeptide tolerance in S. aureus has underlined the importance of the newer anti-MRSA agents, particularly in the management of complicated skin and soft tissue infections (cSSTIs). The novel cyclic lipopeptide antibiotic daptomycin shows marked in vitro cidality against MRSA compared with both vancomycin and linezolid. Although comparative studies in cSSTIs have demonstrated non-inferiority with vancomycin and semi-synthetic penicillins, data from both clinical trials and observational studies suggest in vivo cidality as evidenced by rapid resolution of clinical signs of local inflammation and reduced duration of therapy. Overall success in SSTI post-marketing studies is >90%, and >88% in MRSA-infected patients, with no difference in the outcome observed between those with complicated versus uncomplicated infections. When used at licensed doses (4-6 mg/kg), daptomycin is safe and effective in SSTIs with significant muscle toxicity occurring in only 0.4% to 2.5% of patients. Clinical failure in daptomycin-treated SSTIs is associated with severity of infection (creatinine clearance <30 mL/min, intensive care unit stay and sepsis syndrome). Higher dosing at 6 mg/kg (with increased dosing interval in renal failure) should be considered in such patients as well as those at risk of bacteraemia, osteomyelitis, diabetic foot infection and in situations where there is more rapid drug clearance, such as infections complicating intravenous drug use and thermal burns. Once-daily dosing allows ease of use in both hospital and outpatient settings and may facilitate early discharge or avoided admission in some patient groups with SSTIs. Clinical experience to date suggests potential economic advantages associated with earlier hospital discharge and shorter duration of therapy, although further detailed cost-effectiveness comparisons are required to validate these observations in different healthcare settings.

Daptomycin and tigecycline: a review of clinical efficacy in the antimicrobial era

There is a clinical need for new treatment options as a result of continued increase in the expression of resistance among bacterial pathogens. A number of compounds currently in development show promise. However, in some cases, there is concern that resistance may develop quickly to new compounds that are based on existing antimicrobial agents. Therefore, daptomycin, a novel lipopeptide with a unique mode of action, is of particular interest. It has rapid bactericidal activity against growing and stationary-phase bacteria, once-daily dosing regimen, and has a low potential for the development of resistance. It has been approved for the treatment of complicated skin and soft tissue infections caused by Gram-positive bacteria, and registration for treatment of infective endocarditis and bacteraemia is anticipated. Daptomycin is a welcome addition to the antimicrobial armamentarium for the treatment of bacterial infections. Tigecycline is a new glycyclcycline antimicrobial recently approved for use in the USA, Europe and elsewhere. While related to the tetracyclines, tigecycline overcomes many of the mechanisms responsible for resistance to this class. It is a novel broad spectrum glycylcycline with good activity against Gram-positive, many Gram-negative, anaerobic, and some atypical pathogens that has been developed to address this need. It is efficacious in complicated skin and soft tissue infections and in intra-abdominal infections. This review aims to summarise the key clinical data of daptomycin and tigecycline which hold promise for widespread clinical use in the next decade.

The efficacy and safety of daptomycin: first in a new class of antibiotics for Gram-positive bacteria

In the face of increasing resistance to currently available antibiotics, there is a continued interest in the development of new drugs to treat Gram-positive infections. One such agent is the cyclic lipopeptide daptomycin-licensed in the USA for treatment of Gram-positive complicated skin and skin structure infections (cSSSIs) in 2003 and currently awaiting European approval for a similar indication (complicated skin and soft tissue infections). Daptomycin exerts its rapid bactericidal effect through insertion into and subsequent depolarisation of the bacterial cell membrane, a mode of action unlike that of any other available antibiotic. This novel mechanism of action makes the development of cross-resistance between daptomycin and other antibiotic classes unlikely. Daptomycin is highly active in vitro against a range of Gram-positive pathogens, including both susceptible and multidrug-resistant staphylococci and enterococci. Bactericidal activity has also been demonstrated against both growing and stationary-phase organisms, suggesting potential utility in the treatment of deep-seated infections. Two pivotal clinical studies comparing daptomycin 4 mg/kg per day intravenously with vancomycin or oxacillin-class antibiotics demonstrated the efficacy of daptomycin for treatment of cSSSIs. Daptomycin was well tolerated, with most adverse events considered to be unrelated to study medication, of mild-to-moderate intensity, and with a frequency and distribution similar to those associated with comparator antibiotics. The favourable clinical profile and low potential for development of resistance combine to make daptomycin a promising alternative to current agents for treatment of Gram-positive bacterial infections.

Daptomycin: a novel cyclic lipopeptide antimicrobial

PURPOSE

The development, activity, pharmacokinetics, pharmacodynamics, clinical efficacy, adverse effects, and dosage and administration of daptomycin are reviewed.

SUMMARY

Daptomycin, a novel cyclic lipopeptide antimicrobial, is bactericidal against a range of gram-positive bacteria, including many multiple-drug-resistant isolates. It has only minimal activity against anaerobic bacteria and no activity against gram-negative bacteria. Daptomycin exhibits linear pharmacokinetics, and the plasma concentration-versus-time relationship is best described by a two-compartment model with first-order elimination. The initial bactericidal activity is rapid, extensive, and concentration related. In clinical trials, daptomycin has shown efficacy in treating complicated skin and skin-structure infections (CSSSIs); the drug carries FDA-approved labeling for same. The adverse effects of daptomycin appear comparable to those of vancomycin and semisynthetic penicillins. The dosage for CSSSIs is 4 mg/kg by i.v. infusion every 24 hours.

CONCLUSION

Daptomycin is bactericidal against gram-positive organisms and offers an option in the treatment of CSSSIs.

Daptomycin – a novel antibiotic against Gram-positive pathogens

Daptomycin is a novel member of a new class of antimicrobial agents used in treating resistant Gram-positive infections. These infections are becoming more commonplace and treatment options are limited. At present, daptomycin is approved for use in the US for complicated skin and skin-structure infections that are a common complication of surgery, diabetic foot ulcers, and burns. The most common causative organisms in these types of infections are Staphylococcus aureus, Streptococcus pyogenes, Streptococcus agalactiae, and Group C and G streptococci. Traditionally, these infections have been treated with penicillin and cephalosporins, but resistance to these agents is widespread and increasing. Of particular concern is the rapid increase in methicillin-resistant S. aureus (MRSA). The SENTRY Antimicrobial Surveillance Programme reported that approximately 30% of S. aureus isolates from skin and skin-structure infections were MRSA. The standard treatment for MRSA infections is vancomycin but resistance to this agent is also developing. There is a continuing need for the development of new antibiotics with Gram-positive activity, to combat multi-drug-resistant Gram-positive infections.

Daptomycin: a novel agent for Gram-positive infections

The alarming increase in the incidence of Gram-positive infections, including those caused by resistant bacteria, has sparked renewed interest in novel antibiotics. One such agent is daptomycin, a novel lipopeptide antibiotic with proven bactericidal activity in vitro against all clinically relevant Gram-positive bacteria. These include resistant pathogens, such as vancomycin-resistant enterococci (VRE), methicillin-resistant Staphylococcus aureus (MRSA), glycopeptide intermediately susceptible Staphylococcus aureus (GISA), coagulase-negative staphylococci (CNS) and penicillin-resistant Streptococcus pneumoniae (PRSP), for which there are very few therapeutic alternatives. Daptomycin provides rapid, concentration-dependent killing and a relatively prolonged concentration-dependent post-antibiotic effect in vitro. Spontaneous acquisition of resistance to daptomycin occurs rarely. Daptomycin exhibits linear pharmacokinetics, minimal accumulation with once-daily dosing, and low plasma clearance and volume of distribution. Phase II clinical trials indicate that daptomycin at doses of 2 mg/kg q24 h and 3 mg/kg q12 h is efficacious against skin and soft tissue infections and bacteremia, respectively. In addition, results in endocarditis suggested potential efficacy with higher doses. On the basis of clinical trials to date, it appears that daptomycin has an excellent safety profile, with the incidence and nature of serious adverse events comparable to those observed with conventional therapy. Adverse events associated with other classes of antimicrobials (nephrotoxicity, local irritation, ototoxicity, hypersensitivity, and gastrointestinal effects) were uncommon with daptomycin. Minimal skeletal muscle toxicity was seen at only the highest dose tested (4 mg/kg q12 h), predicted by elevations in serum creatinine phosphokinase, and readily reversible upon discontinuation of treatment. There were no signs of toxicity in cardiac or smooth muscle. Phase II and III clinical trials are underway to evaluate daptomycin for the treatment of Gram-positive bacteremia and complicated skin and soft tissue infections, respectively. Daptomycin holds promise as a rapidly acting and highly effective antibiotic for Gram-positive infections.

The need for new therapeutic agents: what is in the pipeline?

There is a clinical need for new treatment options for serious Gram-positive infections. Recently introduced agents such as the newer fluoroquinolones and the ketolide telithromycin have limited use as they do not cover methicillin-resistant Staphylococcus aureus (MRSA) or glycopeptide-resistant enterococci (GRE). The clinical use of the streptogramin combination quinupristin/dalfopristin, which has activity against MRSA and vancomycin-resistant Enterococcus faecium, is limited because administration is via a slow infusion of a large volume. The oxazolidinone linezolid is active against MRSA and GRE but resistant organisms and treatment failures have been reported. A number of compounds currently in development show promise, the new glycopeptides oritavancin, dalbavancin and the glycolipodepsipeptide ramoplanin, as well as the new tetracyclines tigecycline and BAY73-7388. However, in some cases, there is concern that resistance may develop quickly to new compounds that are based on existing antimicrobial agents. Therefore daptomycin, a novel lipopeptide with a unique mode of action, is of particular interest. Daptomycin is active against MRSA (including vancomycin-resistant strains) and GRE. Daptomycin displays rapid concentration-dependent killing and is bactericidal even in the stationary phase of growth. Daptomycin-resistant strains are very difficult to generate in vitro. A dosage of 4 mg/kg intravenous once a day has been shown to be efficacious in two evaluator-blinded trials of complicated skin and soft tissue infections with clinical success rates similar for daptomycin and comparators (vancomycin or penicillinase-resistant penicillins). With its activity against key Gram-positive pathogens, including resistant strains, daptomycin has potential as a valuable addition to the available treatment options for serious Gram-positive infections.

Population pharmacokinetics of daptomycin

Data from subjects in nine phase 1 (n = 153) and six phase 2/3 (n = 129) clinical trials were combined to identify factors contributing to interindividual variability in daptomycin pharmacokinetics (PK). Over 30 covariates were considered. A two-compartment model with first-order elimination provided the best fit for data on daptomycin concentrations in plasma over time. In the final population PK model, daptomycin plasma clearance (CL) was a function of renal function, body temperature, and sex. Of these factors, renal function contributed most significantly to interindividual variability. CL varied linearly with the estimated creatinine clearance. CL among dialysis subjects was approximately one-third that of healthy subjects (0.27 versus 0.81 liter/h). CL in females was 80% that in males; however, in clinical trials, the outcome was not affected by sex and therefore this effect is not considered clinically meaningful. The relationship with body temperature should be interpreted cautiously since the analysis included only a limited number of subjects who were hyperthermic. The volume of distribution of the peripheral compartment (V2) and intercompartmental clearance (Q) were linearly related to body weight. V2 increased approximately twofold in the presence of an acute infection. No factors were identified that significantly impacted V1. This analysis supports the dosing of daptomycin on a milligram-per-kilogram-of-body-weight basis and suggests that modified dosing regimens are indicated for patients with severe renal disease and for those undergoing dialysis.

Antibiotics for gram-positive bacterial infections: vancomycin, quinupristin-dalfopristin, linezolid, and daptomycin

An overview of the mechanism of action, dosing, clinical indications, and toxicities of the glycopeptide vancomycin is provided. Emerging gram-positive bacterial resistance to antimicrobials and its mechanisms are reviewed. Strategies to control emergence of resistance are proposed. Newer antimicrobial agents with activity against vancomycin-resistant organisms are now available and play a critical role in the treatment of life-threatening infections.

In vivo pharmacodynamic activity of daptomycin

Daptomycin is a lipopeptide antibiotic with activity against a wide range of gram-positive bacteria. We used the neutropenic murine thigh model to characterize the pharmacodynamics of daptomycin. ICR/Swiss mice were rendered neutropenic with cyclophosphamide; and the thigh muscles of the mice were infected with strains of Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecium. Animals were treated by subcutaneous injection of daptomycin at doses of 0.20 to 400 mg/kg of body weight/day divided into one, two, four, or eight doses over 24 h. Daptomycin exhibited linear pharmacokinetics, with an area under the concentration-time curve (AUC) from time zero to infinity/dose of 9.4 and a half-life of 0.9 to 1.4 h. The level of protein binding was 90%. Free daptomycin exhibited concentration-dependent killing and produced in vivo postantibiotic effects (PAEs) of 4.8 to 10.8 h. Nonlinear regression analysis was used to determine which pharmacokinetic (PK) or pharmacodynamic (PD) parameter was important for efficacy by using free drug concentrations. The peak concentration/MIC (peak/MIC) ratio and 24-h AUC/MIC ratio were the PK and PD parameters that best correlated with in vivo efficacy (R(2) = 83 to 87% for peak/MIC and R(2) = 86% for the AUC/MIC ratio, whereas R(2) = 47 to 50% for the time that the concentration was greater than the MIC) against standard strains of S. aureus and S. pneumoniae. The peak/MIC ratios required for a bacteriostatic effect ranged from 12 to 36 for S. pneumoniae, 59 to 94 for S. aureus, and 0.14 to 0.25 for E. faecium. The AUC/MIC ratios needed for a bacteriostatic effect ranged from 75 to 237 for S. pneumoniae, 388 to 537 for S. aureus, and 0.94 to 1.67 for E. faecium. The free daptomycin concentrations needed to average from one to two times the MIC over 24 h to produce a bacteriostatic effect and two to four times the MIC over 24 h to produce greater than 99% killing. The long PAE and potent bactericidal activity make daptomycin an attractive option for the treatment of infections caused by gram-positive bacteria.

Daptomycin

Daptomycin, the first in a class of agents known as lipopeptides, is a novel antimicrobial agent used for the treatment of gram-positive infections. The compound has a distinctive mechanism of action that exerts its bactericidal activity by disrupting plasma membrane function without penetrating into the cytoplasm. The agent has received much interest because of its activity against multidrug-resistant, gram-positive bacteria such as methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and glycopeptide-intermediate and -resistant S. aureus. Daptomycin demonstrates concentration-dependent killing and is eliminated primarily by glomerular filtration. It was approved in September 2003 for the treatment of complicated skin and soft tissue infections. It has a safety profile similar to other agents commonly administered to treat gram-positive infections. Daptomycin is a welcome addition to the antimicrobial armamentarium for the treatment of bacterial infections. Further clinical experience with this compound will help define its role in the treatment of resistant gram-positive organisms.

Postantibiotic effects of daptomycin against 14 staphylococcal and pneumococcal clinical isolates

Daptomycin mean staphylococcal postantibiotic effects (PAEs) were 1.1 to 6.2 h, with a mean of 2.5 h. The mean pneumococcal PAEs were 1.7 h, ranging between 1.0 and 2.5 h. The staphylococcal and pneumococcal postantibiotic sub-MIC effects at 0.4 times the MIC ranged from 3.0 to >12.0 h and 1.9 to >12.0 h, respectively.

Pharmacodynamics of antimicrobial therapy in surgery

INTRODUCTION

"Pharmacodynamics" refers to the relationship of drug concentrations in serum or tissues to effects on biologic systems. Concepts used to describe antimicrobial pharmacodynamics include the minimal inhibitory concentration (MIC), the minimal bactericidal concentration (MBC), and serum bactericidal titers (SBT), as well as post-antibiotic effect.

METHODS

Pertinent published literature was identified through a MEDLINE search.

RESULTS

Aminoglycosides have a concentration-dependent effect on bacteria killing and possess a relatively long postantibiotic effect. Given these characteristics, single-daily dosing, where the total daily dose with a traditional aminoglycoside regimen is given as one dose, may be more efficacious compared with more frequent dosing. For beta-lactam antimicrobials, bacterial killing is related to the duration of time that the free drug concentration exceeds the bacterial MIC. Beta-lactam antimicrobials have been shown to have no, or a short postantibiotic effect. Beta-lactam antimicrobials may be more effective when administered as continuous intravenous infusions.

CONCLUSIONS

Pharmacodynamic variation may result from differences in drug sensitivity among individuals and the nature of the interaction between antimicrobials and microorganisms. Proper use of pharmacokinetic and pharmacodynamic principles can result in more effective and less toxic antimicrobial regimens.

Pharmacokinetic contributions to postantibiotic effects. Focus on aminoglycosides

The postantibiotic effect (PAE) refers to a period of time after complete removal of an antimicrobial during which there is no growth of the target organism. The PAE appears to be a feature of most antimicrobial agents and has been documented with a variety of common bacterial pathogens. Various factors influence the presence or duration of the PAE including the type of organism, type of antimicrobial, concentration of antimicrobial, duration of antimicrobial exposure, antimicrobial combinations, and inoculum and medium used. beta-Lactams demonstrate a PAE against Gram-positive cocci, but produce only a short PAE with Gram-negative bacilli. Antimicrobial agents that inhibit RNA or protein synthesis have a PAE against Gram-positive cocci and Gram-negative bacilli. In vivo studies of aminoglycosides suggest that area under the plasma concentration-time curve is the pharmacokinetic parameter that best correlates with clinical efficacy. This is thought to be due to the concentration-dependent killing and PAE possessed by these antimicrobials. Animal and human studies have reported that once-daily administration of aminoglycoside is as effective as, or more effective than, and possibly less toxic than traditional multiple daily administration.

Pharmacodynamic effects of antibiotics. Studies on bacterial morphology, initial killing, postantibiotic effect and effective regrowth time

Pharmacodynamics of antibiotics deals with time course of drug activity and mechanisms of action of drugs on bacteria. In this thesis pharmacodynamic parameters have been studied after brief exposure of gram-positive bacteria to daptomycin, imipenem or vancomycin and after short exposure of gram-negative bacteria to amikacin, ampicillin, aztreonam, cefepime, cefotaxime, ceftazidime, ceftriaxone, cefuroxime, imipenem, mecillinam, or piperacillin. The studies have been focused on morphological alterations, initial killing, postantibiotic effect (PAE) and effective regrowth time (ERT) and a method, based on bioluminescence assay of intracellular ATP has been used. The basic principle behind this technique is that ATP in living cells is present in a relatively constant amount, and hence affords a measure of the number of microbial cells. The PAE describes the delayed regrowth of bacteria after brief exposure to antibiotics. The number of cells measured after this antibiotic exposure describes the initial killing and is also the start value for calculating the PAE. PAEs of 2-3 h were obtained by bioluminescence for gram-positive bacteria exposed to imipenem or vancomycin. This is in agreement with results obtained by viable count and is probably due to similar weak initial decrease in cell density when assayed by both methods. Long (greater than 3 h) concentration dependent PAEs and moderate (less than or equal to 1 log10) initial decrease in intracellular ATP were in general seen for gram-positive bacteria exposed to daptomycin and for gram-negative bacteria exposed to imipenem or amikacin when assayed by bioluminescence. These very long PAEs and rather weak initial killing have to be compared with the shorter PAEs and stronger initial killing reported by us and others using viable count. Furthermore, this study showed that there was a relatively good concordance between microscopy and bioluminescence, which are direct methods, in determining the initial killing and PAE of imipenem on Escherichia coli. The ERT, defined as the time for bacterial density to increase 1 log10 from the pre-exposure inoculum, was independent of the method used for measuring regrowth of E. coli after brief exposure to imipenem. The combination of mecillinam with ampicillin, aztreonam, ceftazidime or piperacillin and the combination of amikacin with ceftazidime, ceftriaxone or piperacillin induced longer PAEs on gram-negative bacteria than the sum of PAEs of the individual antibiotics. A strong initial killing in combination with a long PAE cause a long ERT and may allow the antibiotic concentration to stay below MIC during long periods of time without any regrowth. This may, in clinical practice, have implications for long dosing intervals.

Pharmacodynamics of daptomycin and vancomycin on Enterococcus faecalis and Staphylococcus aureus demonstrated by studies of initial killing and postantibiotic effect and influence of Ca2+ and albumin on these drugs

The pharmacodynamics of daptomycin and vancomycin on Enterococcus faecalis ATCC 29212 and Staphylococcus aureus ATCC 25923 were investigated by studying the postantibiotic effect (PAE) and initial killing. The influence of Ca2+ and albumin on these drugs was also evaluated. The PAE was studied by use of bioluminescence assay of bacterial ATP. Daptomycin at clinically achievable concentrations produced a dose-dependent PAE on E. faecalis (0.6 to 6.7 h) and S. aureus (1.0 to 6.3 h). The long PAE of daptomycin was seen simultaneously with a potent dose-dependent initial killing assayed by viable count determination. The initial change in bacterial ATP was not as extensive as the decrease in viability. Vancomycin at corresponding concentrations produced shorter PAEs on E. faecalis (0.5 to 1.0 h) and S. aureus (1.3 to 1.8 h). This coincides with a weak non-dose-dependent initial change in viability and intracellular ATP. The MICs of vancomycin were not influenced by different Ca2+ concentrations or by the addition of albumin to the broth. The MICs of daptomycin for both strains were lowered, and the PAEs were prolonged with increasing concentrations of Ca2+ in the broth. The PAE of daptomycin was Ca2+ dependent to the same extent as the MIC was. In the presence of physiological concentrations of albumin and free Ca2+, the PAEs of daptomycin on both strains were reduced and the MICs were increased in comparison with the results obtained in pure Mueller-Hinton broth with approximately the same free Ca2+ concentration. This decrease in daptomycin activity was considered to be due to the albumin binding of daptomycin. Despite the albumin binding of daptomycin, the PAE produced on E. faecalis and S. aureus in the presence of a physiological free Ca2+ concentration was still over 6 h at clinically achievable concentrations.