Serum bactericidal activity and postantibiotic effect in serum of patients with urinary tract infection receiving high-dose amikacin

Systematic review of high-dose amikacin regimens for the treatment of Gram-negative infections based on EUCAST dosing recommendations

BACKGROUND

Updated European Committee on Antimicrobial Susceptibility Testing (EUCAST) amikacin breakpoints for Enterobacterales and Pseudomonas aeruginosa included revised dosing recommendations of 25-30 mg/kg to achieve key pharmacokinetic/pharmacodynamic parameters, higher than recommended in the British National Formulary. The objectives of this review were to identify clinical evidence for high-dose amikacin regimens and to determine drug exposures that are related to adverse events and toxicity.

METHODS

The literature search was conducted in October 2021 and updated in May 2022 using electronic databases for any study reporting adult participants treated with amikacin at doses ≥20 mg/kg/day. Reference lists of included papers were also screened for potential papers. Data were extracted for pharmacokinetic parameters and clinical outcomes, presented in a summary table and consolidated narratively. Meta-analysis was not possible. Each study was assessed for bias before, during and after the intervention using the ROBINS-I tool.

RESULTS

Nine studies (total 501 participants in 10 reports) were identified and included, eight of which were observational studies. Assessment of bias showed substantial flaws. Dosing regimens ranged from 25 to 30 mg/kg/day. Six studies adjusted the dose in obesity when participants had a body mass index of ≥30 kg/m2. Target peak serum concentrations ranged from 60 mg/L to 80 mg/L and 59.6-81.8% of patients achieved these targets, but there was no information on clinical outcomes. Two studies reported the impact of high-dose amikacin on renal function. No studies reporting auditory or vestibular toxicity were identified.

CONCLUSION

All included papers were limited by a significant risk of bias, while methodological and reporting heterogeneity made drawing conclusions challenging. Lack of information on the impact on renal function or ototoxicity means high-dose regimens should be used cautiously in older people. There is a need for a consensus guideline for high-dose amikacin to be written.

TRIAL REGISTRATION NUMBER

PROSPERO (CRD42021250022).

Optimized antimicrobial dosing strategies: a survey of pediatric hospitals

BACKGROUND

Extended-interval aminoglycoside (EIAG) and extended- and continuous-infusion β-lactam (EIBL and CIBL) dosing strategies are increasingly used in adults, but pediatric literature is limited.

OBJECTIVE

The objective of this study was to describe the use of EIAG, EIBL, and CIBL dosing in pediatric hospitals in the USA.

STUDY DESIGN, SETTING, AND PARTICIPANTS

A national survey of children's hospitals was conducted. A single practitioner from each target hospital was identified through the Children's Hospital Association. Practice-based survey questions identified whether hospitals utilize EIAG, EIBL, and CIBL dosing.

MAIN OUTCOME MEASURE

The main outcome measure was the percentage utilization of the dosing strategies, with secondary outcomes being the reasons for not using these dosing strategies.

RESULTS

Seventy-seven of 215 identified practitioners (36 %) participated in the survey. EIAG, EIBL, and CIBL dosing were utilized in 63 %, 24 %, and 13 % of responding hospitals, respectively. The most common reasons for not using EIAG were concern regarding lack of efficacy data (56 %) and concern regarding the duration of the drug-free period (41 %). Respondents who did not utilize EIBL cited concern due to lack of pediatric EIBL efficacy data (54 %), the need for more intravenous access (54 %), intravenous medication compatibility issues (39 %), and the time during which the patient is attached to an intravenous infusion (31 %).

CONCLUSION

This survey of children's hospitals indicates that EIAG is used in over 50 % of hospitals, but there is some lag in adoption of EIBL and CIBL dosing, both of which are used in fewer than 25 % of hospitals. Additional studies may provide much-needed evidence to increase the utilization of these strategies.

Therapeutic drug monitoring of amikacin in septic patients

INTRODUCTION

Use of higher than standard doses of amikacin (AMK) has been proposed during sepsis, especially to treat less susceptible bacterial strains. However, few data are available on drug concentrations during prolonged therapy and on potential adverse events related to this strategy.

METHODS

Sixty-three critically ill patients who required AMK administration for the treatment of severe infection were included in this study. After a loading dose (LD, 18 to 30 mg/kg), the daily regimen was adapted using therapeutic drug monitoring (TDM) of both peak (Cpeak) and trough (Cmin) concentrations. Target concentrations had to give a ratio of at least 8 between Cpeak and the minimal inhibitory concentration (MIC) of the isolated pathogen. A Cmin >5 mg/L was considered as potentially nephrotoxic. We recorded clinical and microbiological responses, the development of acute kidney injury (AKI) during therapy and ICU mortality.

RESULTS

The median AMK LD was 1500 (750 to 2400) mg, which resulted in a Cpeak/MIC ≥8 in 40 (63%) patients. Increasing the dose in the 23 patients with a Cpeak/MIC <8 resulted in optimal Cpeak/MIC in 15 of these patients (79%). In 23 patients (37%), Cmin was >5 mg/L after the LD, notably in the presence of altered renal function at the onset of therapy, needing prolongation of drug administration. Overall, only 11 patients (17%) required no dose or interval adjustment during AMK therapy. Clinical cure (32/37 (86%) vs. 16/23 (70%), P = 0.18)) and microbiological eradication (29/35 (83%) vs. 14/23 (61%), P = 0.07) were higher in patients with an initial optimal Cpeak/MIC than in the other patients. The proportion of patients with clinical cure significantly improved as the Cpeak/MIC increased (P = 0.006). Also, increased time to optimal Cpeak was associated with worse microbiological and clinical results. AKI was identified in 15 patients (24%) during AMK therapy; 12 of these patients already had altered renal function before drug administration. Survivors (n = 47) had similar initial Cpeak/MIC ratios but lower Cmin values compared to nonsurvivors.

CONCLUSIONS

TDM resulted in adjustment of AMK therapy in most of our septic patients. Early achievement of an optimal Cpeak/MIC ratio may have an impact on clinical and microbiological responses, but not on outcome. In patients with impaired renal function prior to treatment, AMK therapy may be associated with a further decline in renal function.

Adsorption of amikacin, a significant mechanism of elimination by hemofiltration

We used an in vitro model of continuous venovenous hemofiltration (CVVH) to characterize amikacin adsorption by polyacrylonitrile (PAN) and polyamide filters. A blood-crystalloid mixture dosed with amikacin was pumped from a reservoir through a hemofiltration circuit and back to the reservoir. All ultrafiltrate was also returned to the reservoir. The level of adsorption was calculated from the fall in the amikacin concentration. The dose and the initial concentration of amikacin were varied, as were the pH, the type of hemofilter, and the hemofilter surface area. The reversibility of adsorption and the effect of repeated dosing were also studied. The level of adsorption by 0.6-m2 PAN filters was significantly greater than that by 0.6-m2 polyamide filters. Adsorption was increased by increasing the dose of amikacin even when the initial concentration was unchanged. It was unaffected by the pH (pH 6.8 or 7.4) or the hemofilter surface area (0.6 m2 or 0.9 m2). Repeated doses of amikacin resulted in further adsorption. In a saturation experiment, the maximum adsorptive capacity of 0.6-m2 PAN hemofilters was at least 546.9 mg (range, 427.6 to 577.5 mg). The adsorption of amikacin by hemofilters is irreversible and was associated with the dose and the hemofilter material but not the hemofilter surface area. Close monitoring of peak amikacin levels should be considered for patients receiving CVVH with PAN hemofilters.

Early hospital discharge versus continued hospitalization in febrile pediatric cancer patients with prolonged neutropenia: A randomized, prospective study

BACKGROUND

Hospitalization with single or multi-agent antibiotic therapy has been the standard of care for treatment of febrile neutropenia in cancer patients. We hypothesized that an empiric antibiotic regimen that is effective and that can be administered once-daily will allow for improved hospital utilization by early transition to outpatient care.

PROCEDURE

Febrile pediatric cancer patients with anticipated prolonged neutropenia were randomized between a regimen of once-daily ceftriaxone plus amikacin (C + A) and imipenem monotherapy (control). Afebrile patients on C + A satisfying "Early Discharge Criteria" at 72 hr continued treatment as outpatients. We compared the outcome, adverse events, duration of hospitalization, and cost between both groups.

RESULTS

A prospective randomized controlled clinical trial was conducted on 129 febrile episodes in pediatric cancer patients with prolonged neutropenia. No adverse events were seen in 32 children (84% of study arm) treated on an outpatient basis. We found a statistically significant difference between the duration of hospitalization of the C + A group [median 5 days] and control [median 9 days](P < 0.001), per episode antibiotic cost (P < 0.001) and total episode cost (P < 0.001). There was no statistically significant difference in the response to treatment at 72 hr or after necessary antimicrobial modifications.

CONCLUSIONS

We conclude that pediatric febrile cancer patients initially considered at risk for sepsis due to prolonged neutropenia can be re-evaluated at 72 hr for outpatient therapy. The convenience, low incidence of adverse effects, and cost benefit of the once-daily regimen of C + A may be particularly useful to reduce the overall treatment costs and duration of hospitalization.

Once-daily gentamicin dosing for the preterm and term newborn: proposal for a simple regimen that achieves target levels

OBJECTIVE

Based on recent safety and efficacy data, combined with the known pharmacokinetic parameters of aminoglycosides in the newborn, once-daily gentamicin should be preferable to the many other dosing regimens currently in use. Although there are growing data to support its use in term newborns, experience with preterm infants is more limited. In our Neonatal Intensive Care Unit, we experienced difficulties regarding complicated dosing regimens, actual dosing errors, and the tendency to check trough and peak levels around the third dose for infants receiving only a 48 hour course. Therefore, we conducted a quality improvement initiative in which we developed and tested a clinical practice guideline for the use of once-daily gentamicin for preterm and term infants that we hoped would yield trough and peak levels in our target range.

METHODS

We combined a review of the published English language literature with pharmacokinetic analysis of our own data prior to initiation of this new regimen to design the following dosing regimen: <35 weeks gestation: 3 mg/kg q 24 hours, > or =35 weeks gestation: 4 mg/kg q 24 hours. Our goal serum levels were a trough < or =2 microg/ml and a peak between 6 and 12 microg/ml. We collected and analyzed trough and peak levels from all infants receiving this dosing regimen in the first week of life for at least 72 hours between 3/1/99 and 12/31/00.

RESULTS

In total, 214 babies met our inclusion criteria, 75 of whom were <35 weeks gestation. 100% of babies of all gestational ages had a nontoxic trough level. For infants <35 weeks gestation, 79% had a therapeutic peak level, with a mean value of 6.8 microg/ml. For infants of at least 35 weeks gestation, 93% had a therapeutic peak level, with a mean value of 8.4 microg/ml. 92% of nontherapeutic peaks were too low.

CONCLUSION

This study of once-daily gentamicin represents the largest sample size of pre-term infants published to date. The proposed regimen is simple and yields a high proportion of desirable levels. We recommend it for use in preterm and term newborns.

Clinical issues surrounding once-daily aminoglycoside dosing in children

Aminoglycoside antibiotics are first-line treatment for many infectious diseases in the pediatric population and are effective in adults. The traditional dosing interval in children is every 8-12 hours. Studies in adults reported equivalent efficacy and equal or less toxicity with once-daily regimens. Despite many studies in the adult population, this approach has yet to become standard practice in most pediatric hospitals. Reasons for lack of acceptance of this strategy in children include rapid aminoglycoside clearance, unknown duration of postantibiotic effect, safety concerns, and limited clinical and efficacy data.

Once daily, high dose versus divided, low dose gentamicin for open fractures

A prospective, randomized study was performed on 75 Gustilo Grades II and III open fractures to determine the efficacy of once daily, high dose aminoglycoside therapy, compared with more conventional dosing, in reducing the infection rate when used in conjunction with an aggressive operative treatment protocol. All patients enrolled in the study were treated with immediate irrigation, debridement, operative stabilization of the fracture, and 1 g of cefazolin every 8 hours. At the time of admission patients were randomized to two groups. Patients in Group I received gentamicin 5 mg/kg divided into twice daily doses, and patients in Group II received gentamicin 6 mg/kg given once daily. All patients were monitored for renal toxicity and observed for radiographic and clinical signs of infection until fracture union. The results of the study revealed no statistically significant difference between once daily, high dose versus divided, low dose gentamicin in infection rates. Thus, daily dosing of gentamicin was found to be safe, effective, and cost efficient in the treatment of open fractures when combined with a cephalosporin and aggressive operative debridement and stabilization.

Population pharmacokinetic study of amikacin administered once or twice daily to febrile, severely neutropenic adults

Once-daily (o.d.) administration of 20 mg of amikacin per kg of body weight to neutropenic patients has been validated by clinical studies, but amikacin pharmacokinetics have been documented only for the 7.5-mg/kg twice-daily (b.i.d.) regimen in this population. In order to determine in neutropenic patients (i) the influence of the dosing regimen on the kinetics of amikacin, (ii) the linearity of kinetics of amikacin in the range of 7.5 to 20 mg/kg, and (iii) the influence of patient characteristics on the disposition of amikacin and (iv) to provide a rationale for dosing recommendations, we evaluated the population pharmacokinetics of amikacin administered to 57 febrile neutropenic adults (neutrophil count, <500/mm3) being treated for a hematological disorder and receiving amikacin at 7.5 mg/kg b.i.d. (n = 29) or 20 mg/kg o.d. (n = 28) and administered intravenously over 0.5 h. A total of 278 blood samples were obtained (1 to 14 samples per patient) during one or several administration intervals (1 to 47). Serum amikacin levels were measured by the enzyme-multiplied immunoassay technique. A mixed-effect modeling approach was used to fit a bicompartmental model to the data (NONMEM software). The influences of the dosing regimen and the demographic and biological indices on the pharmacokinetic parameters of amikacin were evaluated by the maximum-likelihood ratio test on the population model. The dosing regimen had no influence on amikacin pharmacokinetic parameters, i.e., the kinetics of amikacin were linear over the range of 7.5 to 20 mg/kg. Amikacin elimination clearance (CL) was only correlated with creatinine clearance or its covariates, namely, sex, age, body weight, and serum creatinine level. The interindividual variability of CL was 21%, while those of the central volume of distribution, the distribution clearance, and the tissue volume of distribution were 15, 30, and 25%, respectively. On the basis of the expected distribution of amikacin concentrations in this population, dosing recommendations as a function of creatinine clearance (CL[CR]) are proposed: for patients with normal renal function (CL[CR] of 80 to 130 ml/min), 20 mg/kg o.d. is recommended, whereas for patients with severe renal impairment (CL[CR], 10 to 20 ml/min), a dosage of 17 mg/kg every 48 h is recommended.

Postexposure factors influencing the duration of postantibiotic effect: significance of temperature, pH, cations, and oxygen tension

The purpose of the present study was to assess and compare the impacts of various postexposure conditions on postantibiotic effect (PAE). PAEs were induced in Staphylococcus aureus and Escherichia coli by exposing the organisms to different antibiotics (penicillin G, ampicillin, erythromycin, ciprofloxacin, and gentamicin) at 5 or 10 times the MIC in plain Mueller-Hinton broth for 1 h at 35 degrees C. Regrowth was determined by measuring the viable counts after drug removal by a 10(-3) or 10(-4) dilution procedure under various postexposure conditions (incubation temperatures at 20, 25, 30, or 35 degrees C; growth under shaken, unshaken, anaerobic conditions; pH 6.0, 7.4, or 9.0; and with sodium chloride concentrations at 0, 1, 3, or 6%). PAE increased in response to a decrease in incubation temperature from 35 to 20 degrees C, and a significant correlation between bacterial generation times and duration of PAEs (r2, 0.82 to 0.97) was demonstrated. The duration of PAE was also modified by the pH in the regrowth medium. PAE increased considerably for S. aureus at pH 6.0 and 9.0 compared to that at pH 7.4 after induction with penicillin G, and with gentamicin the PAE against S. aureus recovering at pH 6.0 also increased considerably. A high concentration of sodium chloride in the regrowth medium produced the most extensive changes in PAE except for that against E. coli induced by ampicillin. PAE increased significantly in response to increased salinity. No recovery even after overnight incubation was detected for S. aureus after preexposure to penicillin, ciprofloxacin, or gentamicin. Only minor changes in the duration of PAE were observed in relation to recovery oxygen tension. It is concluded that many postexposure factors have a profound effect on the duration of PAE.

Antibiotic treatment of febrile episodes in neutropenic cancer patients. Clinical and economic considerations

The increased frequency of infections caused by Gram-positive microorganisms, and the expansion of resistant pathogens resulting from institutional therapeutic practices, represent some of the emerging issues of empirical drug treatment of cancer patients with febrile neutropenia. However, the therapeutic strategies for the treatment of these patients have progressed remarkably over the last decade. Individual therapy in the light of the principal clinical features (in particular, the degree and estimated duration of neutropenia, as well the presence of other potential factors favouring infection such as long-standing intravascular catheters) and local microbial ecology have emerged as the leading concepts. Empirical drug monotherapy has been recognised as a feasible alternative to combination therapy, at least in selected low-risk patients. The indiscriminate use of empirical glycopeptides should be discouraged to prevent the emergence of resistant bacteria, especially in centres where methicillin-resistant staphylococci have not yet become a major issue. Empirical antifungal therapy with amphotericin B is still essential for a successful outcome in case of fever persistence or recurrence. Finally, selected febrile neutropenic patients who exhibit a better prognosis can be handled on an outpatient basis. The prophylactic use of haemopoietic growth factors has been shown to augment cost savings substantially in the management of neutropenic patients via a reduction in the duration and severity of the neutropenia, as well as infectious complications. Although data from economic analyses are not yet available, some cost-containment strategies such as outpatient treatment, monotherapy, and use of more convenient antibiotic combinations may lead to a reduction of therapy expenditures for febrile episodes in these patients.

Once versus thrice daily gentamicin in patients with serious infections

Aminoglycosides are usually given in two or three divided doses. A once-daily regimen might be more effective and less toxic. We have conducted a randomised trial in consecutive patients with serious infections for whom an aminoglycoside seemed warranted. Exclusion criteria were neutropenia or severely impaired renal function. 123 patients were enrolled. For efficacy analysis only those patients were considered in whom treatment with the aminoglycoside was not stopped within 72 h (n = 67); toxicity was analysed on patients receiving aminoglycosides for more than 48 h and not using other nephrotoxic medication (n = 85). Gentamicin 4 mg/kg every day (OD) or gentamicin 1.33 mg/kg three times daily (MD) (with dose-reduction in case of renal dysfunction) were given intravenously. In almost all patients intravenous amoxycillin 1 g every 6 h was also started. Baseline characteristics were comparable in both arms. A good clinical response was observed in 32/35 (91%) of the OD and in 25/32 (78%) in the MD group (difference 13%, 95% confidence interval -6.4% to +26.9%). 2 patients in each group died with uncontrolled infection. An insufficient bacteriological response (persistent positive cultures, resistance, or superinfection) was observed in 2 patients with OD and 3 patients with MD. In patients treated for more than 48 h duration of therapy and mean doses were 7.0 days (1590 mg) and 7.4 days (1672 mg) in OD and MD respectively. Mean first serum trough/peak levels were 0.6/10.2 mg/L and 1.4/5.2 mg/L. Nephrotoxicity (a rise in serum creatinine of 45 mumol/L or more) developed in 2/40 (5%) in OD and 11/45 (24%) in MD (p = 0.016). Risk factors for nephrotoxicity were duration of therapy and baseline creatinine clearance rate. High-tone audiometry was performed when possible; no significant differences were found in hearing loss (3/12 and 3/11) or prodromal signs of ototoxicity (5/12 and 4/11). A once-daily dosing regimen of gentamicin is at least as effective as and is less nephrotoxic than more frequent dosing.

Pharmacodynamics of once-daily amikacin in various combinations with cefepime, aztreonam, and ceftazidime against Pseudomonas aeruginosa in an in vitro infection model

The pharmacodynamics of once-daily amikacin administered as monotherapy and in combination with aztreonam, ceftazidime, and cefepime against Pseudomonas aeruginosa ATCC 27853 and clinical isolate 16690 (moderately susceptible to ceftazidime) were investigated with an in vitro model of infection over a 24-h period. Monotherapy with aztreonam, ceftazidime, and cefepime and combinations of aztreonam with cefepime or ceftazidime were also studied. MICs and MBCs were determined for viable organisms at 24 h to test for the development of resistance. Once-daily amikacin demonstrated killing activity over the initial 8 h superior to those of all other drugs administered as monotherapy against both strains tested (P < 0.01). Regrowth by 24 h was greatest for the amikacin regimen (P < 0.01) but was apparent for all monotherapy regimens against both strains. No changes in susceptibilities were observed. All combination therapies containing once-daily amikacin achieved 99.9% reductions in log10 CFU/ml by 2.0 h against both strains, with no regrowth of organisms at 24 h. Aztreonam-cefepime and -ceftazidime combinations required approximately 5 to 6 h to achieve a 99.9% reduction in log10 CFU/ml. Although there was no difference in time to the 99.9% kill between the aztreonam-cefepime and -ceftazidime regimens against either strain, the killing activity of these combinations was significantly less than those of regimens containing once-daily amikacin (P < 0.01). Minor differences in the initial susceptibilities of beta-lactams and the monobactam aztreonam against P. aeruginosa may not be important for therapeutic outcomes when used in combination with single-daily aminoglycoside therapy.

Platelet microbicidal protein enhances antibiotic-induced killing of and postantibiotic effect in Staphylococcus aureus

The interaction of bacteria with platelets at the cardiac valve surface represents a critical event in the induction of infective endocarditis. Platelets are thought to modulate induction or propagation of endocarditis via secretion of alpha-granule-derived platelet microbicidal protein (PMP) (a low-molecular-mass, cationic, heat-stable protein distinct from lysozyme). We studied representative PMP-susceptible and PMP-resistant Staphylococcus aureus isolates to determine their in vitro bacteriostatic and bactericidal susceptibilities to combinations of PMP plus antistaphylococcal antibiotics. PMP plus oxacillin exerted a synergistic bactericidal effect, in contrast to either agent alone, regardless of the intrinsic PMP susceptibility of the isolate tested. Exposure of S. aureus to PMP alone resulted in residual postexposure growth-inhibitory effects lasting from 0.9 to 1.8 h. Sequential exposure of S. aureus isolates to PMP for 30 min followed by exposure to either oxacillin or vancomycin (each at 10x the MIC for 120 min) resulted in a significant extension of the postantibiotic-effect duration compared with antibiotic exposure alone (P less than or equal to 0.05). Collectively, these findings indicate that PMP both enhances antibiotic-induced killing of S. aureus and increases the postantibiotic-effect duration in S. aureus.

Bactericidal titers of loracarbef (LY 163892) in serum and killing rates in volunteers receiving 400 versus 200 milligrams

In a randomized crossover trial, six volunteers received 200- and 400-mg doses of loracarbef (LY 163892), a new oral cephalosporin. Mean +/- standard error of the mean concentrations in serum obtained after 1.5 and 3 h were 13.2 +/- 2.8 and 4.3 +/- 0.7 mg/liter, respectively, after the 400-mg dose and 6.9 +/- 1.0 and 1.7 +/- 0.2 mg/liter, respectively, after the 200-mg dose. Bactericidal reciprocal titers measured against respiratory pathogens in serum suggested that loracarbef would be highly effective against Streptococcus pneumoniae and Streptococcus pyogenes (median titers, 8 to 128 at 1.5 h and less than 2 to 32 at 3 h) and beta-lactamase-negative Haemophilus influenzae (median titers, 4 at 1.5 h and 2 to 4 at 3 h). Other species (Branhamella catarrhalis, Streptococcus anginosus, Staphylococcus aureus) were associated with lower bactericidal titers. Killing curves performed against 12 strains demonstrated that the bioactivity of loracarbef (measured by the reduction in the area under the control growth curve) was significantly correlated with the concentration/MIC ratio, whereas the initial rate of killing was not, once the concentration was greater than the MIC. Our results suggest that administration of 400 mg of loracarbef every 8 h might be associated with more favorable pharmacodynamic parameters against target bacteria.

Human serum enhances the postantibiotic effect of fluoroquinolones against Staphylococcus aureus

The postantibiotic effect (PAE) of fluoroquinolones against Staphylococcus aureus was determined in Mueller-Hinton broth and normal human serum. At both 4X and 10X the MIC, serum significantly increased the duration of the PAE in all strains tested (P less than 0.05). Reducing the pH of the serum from 7.9 to 7.2 had no effect on the PAE. Heat treating the serum (56 degrees C, 30 min) reduced the PAE of ciprofloxacin at 10X the MIC approximately 25% (P less than 0.05). The PAE of cloxacillin was reduced approximately 80% in serum, and PAE experiments with gentamicin and cephalexin produced findings similar to those obtained with the fluoroquinolones. Serum increased the MICs of ciprofloxacin and norfloxacin less than twofold and increased the MIC of pefloxacin approximately fourfold. We conclude that normal human serum considerably increases the PAE of fluoroquinolones against S. aureus.

Pharmacodynamic parameters and toxicity of netilmicin (6 milligrams/kilogram/day) given once daily or in three divided doses to cancer patients with urinary tract infection

The pharmacologic parameters and toxicity of netilmicin (6 mg/kg/day) given once daily (qd) or thrice daily (tid) for the treatment of urinary tract infections were studied in a randomized prospective study of 60 cancer patients. The overall efficacy was 96%. Nephrotoxicity, assessed by the measure of urinary excretion of phospholipids, was lower for the patients receiving the qd regimen than for those receiving the tid regimen. Elevation of serum creatinine (20% over baseline) occurred in one patient receiving the qd regimen and in three receiving the tid regimen. Cochleotoxicity, assessed by pure-tone audiometry (250 to 18,000 Hz) occurred in one patient receiving the qd regimen and none receiving the tid regimen. Concentrations in sera were measured on days 1 and 5. No significant accumulation was observed in either group. Median serum bactericidal titers, expressed as reciprocal values (percentage of the sera with a titer greater than or equal to 8), were measured against 25 test organisms in samples collected 6 h after the administration of netilmicin and were, for the qd group, 16 (82%) against members of the family Enterobacteriaceae and less than 2 (8%) against Pseudomonas aeruginosa, and for the tid group, 4 (57%) against members of the Enterobacteriaceae and less than 2 (0%) against P. aeruginosa. The rate of killing in serum was rapid (2 to 3 log in 2 h against P. aeruginosa; 3 to 5 log in 2 h against members of the Enterobacteriaceae) and correlated with the sampling time and hence the concentration in serum of netilmicin. The duration of the postantibiotic effect in serum depended also on the strain and the sampling time of the serum.

The postantibiotic effect: a review of in vitro and in vivo data

The term postantibiotic effect (PAE) refers to a period of time after complete removal of an antibiotic 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. Several factors influence the presence or duration of the PAE including the type of organism, type of antimicrobial, concentration of antimicrobial, duration of antimicrobial exposure, and antimicrobial combinations. In vitro, beta-lactam antimicrobials demonstrate a PAE against gram-positive cocci but fail to produce a PAE with gram-negative bacilli. Antimicrobials that inhibit RNA or protein synthesis produce an in vitro PAE against gram positive cocci and also produce a PAE against gram-negative bacilli. In vitro methods used to determine the PAE include colony counts, optical density, and measurement of adenosine triphosphate in bacteria. The exact mechanisms by which antimicrobials induce the PAE have not been clearly delineated. Animal studies reveal in vivo PAEs in accordance with PAEs obtained in vitro for most organism/antimicrobial combinations. The clinical relevance of the PAE is probably most important when designing dosage regimens. The presence of a long PAE allows aminoglycosides to be dosed infrequently; the lack of an in vivo PAE suggests that beta-lactam antimicrobials require frequent or continuous dosing. Important questions remain to be answered concerning the PAE.

Ex vivo study of serum bactericidal titers and killing rates of daptomycin (LY146032) combined or not combined with amikacin compared with those of vancomycin

Twelve volunteers, in two groups of six, received daptomycin at a dose of 1 or 2 mg/kg. In addition, they received in a randomly allocated order amikacin (500 mg), daptomycin-amikacin, and vancomycin (500 mg). Thirty-five clinical isolates, including Staphylococcus aureus, S. epidermidis, Corynebacterium sp. group JK, and Enterococcus faecalis, were tested in vitro for the measure of the serum bactericidal titers and killing rates. The mean peak concentrations of daptomycin in serum 1 h after the administration of 1 and 2 mg/kg were 11 and 20 micrograms/ml, respectively. At 24 h after the administration of 2 mg/kg, the mean level in serum was 1.9 micrograms/ml, which is higher than the MICs for susceptible pathogens. Daptomycin and amikacin provided identical concentrations in serum whether given alone or in combination. Among the six regimens tested, those including daptomycin provided the highest and the most prolonged serum bactericidal titers against S. aureus, S. epidermidis, and E. faecalis. The killing rates measured by the killing curves were correlated with the concentration/MIC and concentration/MBC ratios of daptomycin for all strains tested. Significant killing occurred once the concentration of daptomycin in the serum 4- to 6-fold the MIC or 1- to 1.2-fold the MBC. The combination of daptomycin and amikacin had no effect on either the serum bactericidal titers or the rates of killing. Only vancomycin provided significant killing of the strains of Corynebacterium sp. group JK.

Once-daily aminoglycoside administration: new strategies for an old drug

Considerable in vitro and clinical research has been conducted concerning once-daily administration of aminoglycosides. Inherent to such regimens are elevated peak concentrations and prolonged periods during which concentrations are below the MIC for the pathogen. High peak concentration to MIC ratios may maximize bactericidal rates but the problem of toxicity remains. Although sustained exposure to sublethal concentrations could undermine efficacy, antimicrobial activity may be prolonged by the post-antibiotic effect. A wide variety of animal toxicity and efficacy models has been investigated. Findings suggest that less frequent administration is associated with less toxicity while efficacy remains unaltered. Elevated doses appeared to be well tolerated in studies in volunteers and patients, however relatively few critically ill patients have been assessed. Efficacy in the treatment of urinary tract, respiratory and other infections appears unaffected by administration frequency. In an attempt to standardize dosage regimens to allow comparisons among diverse patient groups in clinical trials, guidelines for dosage adjustment based on renal function are outlined.

Serum bactericidal test

The serum bactericidal test represents one of the few in vitro tests performed in the clinical microbiology laboratory that combines the interaction of the pathogen, the antimicrobial agent, and the patient. Although the use of such a test antedates the antimicrobial era, its performance, results, and interpretation have been subject to question and controversy. Much of the confusion concerning the serum bactericidal test can be avoided by an understanding of the various factors which influence bactericidal testing. In addition, the methodologic aspects of the serum bactericidal test have recently been addressed and should place this test on firmer ground. New information on the clinical utility of this test is becoming available; additional data are needed to establish more clearly the usefulness of the serum bactericidal test in specific infections. Such clinical trials from multiple centers will enable firmer recommendations for the future use of the serum bactericidal test.