Once-daily aminoglycoside therapy

Infection caused by Mycobacterium tuberculosis

OBJECTIVE

To update readers on the clinical management of infections caused by Mycobacterium tuberculosis, to provide a general description of the organism, culture and susceptibility testing, and clinical manifestations of the disease, and to provide several aspects of the treatment of the disease, including historical perspective, current approaches, and research opportunities for the future.

DATA SOURCES

The current medical literature, including abstracts presented at recent international meetings, is reviewed. References were identified through MEDLINE, MEDLARS II, Current Contents, and published meeting abstracts.

STUDY SELECTION

Data regarding the epidemiology, clinical manifestations, culture and susceptibility testing, and treatment of tuberculosis are cited. Specific attention has been focused on the clinical management of patients with noncontagious infection and potentially contagious active disease (TB) caused by M. tuberculosis.

DATA EXTRACTION

Information contributing to the discussion of the topics selected by the authors is reviewed. Data supporting and disputing specific conclusions are presented.

DATA SYNTHESIS

The incidence of TB is increasing in the US, despite the fact that available technologies are capable of controlling the vast majority of existing cases. Fueling the fire is the problem of coinfection with HIV and M. tuberculosis. Very few drugs are available for the treatment of TB, and few of these approach the potency of isoniazid and rifampin. Preventive therapy of patients exposed to multiple-drug-resistant M. tuberculosis (MDR-TB) is controversial and of unknown efficacy. Treatment of active disease caused by MDR-TB requires up to four times longer, is associated with increased toxicity, and is far less successful than the treatment of drug-susceptible TB. Strategies for the management of such cases are presented. The rising incidence of TB in the US reflects a breakdown in the healthcare systems responsible for controlling the disease, which reflects the past budgetary reductions. Although TB control is one of the most cost-effective public health strategies, funding has been cut repeatedly despite the fact that TB was never eliminated. This has helped to produce the current crisis, including the spread of MDR-TB in many urban areas. The elimination of TB will now take decades longer, cost hundreds of millions of dollars more, and result in vastly higher morbidity and mortality rates than would have occurred with timely, adequate measures.

CONCLUSIONS

Tremendous effort and far more funding will be required to eliminate TB in the US. The selection of drug therapy must be based on the susceptibility data for each isolate. Multiple-drug therapy must be continued for 6 to > or = 24 months, and patient adherence to prescribed regimens must be verified in all cases of TB. Significant antimycobacterial drug malabsorption has been documented in AIDS patients with TB, and may result in treatment failure. New agents are needed to improve the clinical outcome in patients with MDR-TB.

A prospective study of septic complications of endoscopic retrograde cholangiopancreatography

Prophylactic antibiotics are used in an attempt to avoid the septic complications of endoscopic retrograde cholangiopancreatography (ERCP). We prospectively performed blood cultures and surveyed patients for complications. The aims were first, to determine the incidence of bacteraemia associated with ERCP, second, to assess the incidence of clinical sepsis following the procedure and third, to evaluate the effectiveness of our antibiotic prophylaxis. One hundred and fifty successive patients underwent 179 ERCP. Bacteraemia related to the procedure or the underlying pathology was found in nine procedures (5.2%). Bacteraemias were more likely to complicate therapeutic procedures (P = 0.015), biliary obstruction (P = 0.045) or underlying pathology (P = 0.022). Although 61% of ERCP received antibiotics, 22 septic events occurred. Five bacteraemic patients were septic despite antibiotics. Septic complications were associated with the same factors as bacteraemia. It was concluded that patients with biliary obstruction and undergoing therapeutic endoscopic procedures are at greatest risk of bacteraemia. Single dose prophylactic antibiotics may not prevent sepsis in these patients and longer-acting drugs or repeated dosing may be necessary.

Choosing between the new cephalosporin antibiotics: a pharmacodynamic approach

The use of pharmacodynamic properties when formulating antibacterial administration guidelines can maximise the potential for efficacy while minimising the risk of toxicity. Aminoglycosides and quinolones demonstrate concentration-dependent bactericidal killing, which is maximised when their concentrations appreciably exceed their minimum inhibitory concentration (MIC) for an organism. beta-Lactams demonstrate time-dependent or concentration-independent bactericidal killing, which is maximised when the time that concentrations exceed the MIC is prolonged, regardless of the absolute levels attained. Methods of prolonging the time beta-lactam concentrations exceed the MIC include the following: interfering with excretion (e.g. probenecid); decreasing the dosage interval; increasing the dose; infusing continuously rather than by bolus; and choosing an agent with a prolonged elimination half-life. The optimal duration for exceeding the MIC varies with the infecting organism, site of infection, inoculum effect, and the immunocompetence of the host. Integration of the microbiological activity and pharmacokinetic properties enables estimation of the time that serum concentrations of various cephalosporins will exceed the MIC of a given organism, consequently allowing estimation of the relative potential for clinical success. Cefixime, a third generation oral cephalosporin with a long plasma elimination half-life, allowing once-daily administration, achieves serum concentrations that exceed the MIC of Haemophilus influenzae, Moraxella catarrhalis, Escherichia coli, and Group A streptococci for greater than 90% of the dosage interval, and the MIC of Streptococcus pneumoniae for 50 to 90% of the dosage interval.

Cost considerations in therapeutic drug monitoring of aminoglycosides

Aminoglycoside antibiotics are very important in the treatment of Gram-negative infections and as synergistic agents for the treatment of staphylococcal and streptococcal (group B streptococci and enterococci) infections. However, these agents have a narrow therapeutic index. Thus, a number of new antibiotics have been introduced in an attempt to reduce the number of patients treated with aminoglycosides. Unfortunately, these new antibiotics tend to be costly, and are often associated with development of resistance and treatment failure. Data suggest that a pharmacokinetic/pharmacodynamic relationship exists for some aspects of efficacy and toxicity of aminoglycosides. Serum drug concentrations and/or tissue accumulation are related to the development of nephrotoxicity, and individualised pharmacokinetic monitoring may decrease rates of nephrotoxicity. Peak serum drug concentrations and the ratio of peak serum drug concentration to minimum inhibitory concentration appear to correlate with clinical efficacy in the treatment of patients with bacteraemia or pneumonia. Therapeutic drug monitoring (TDM) has been used to optimise aminoglycoside therapy and reduce toxicity. Cost-effective approaches to drug selection and TDM are important considerations in the proper use of aminoglycosides.

Pharmacology of the antimycobacterial drugs

The management of MDR-TB requires that the clinician become familiar with the "second-line" antimycobacterial agents. These drugs are generally less potent and frequently more toxic than isoniazid and rifampin. Because they are less active, innovative dosing schedules may allow us to take advantage of the few strengths that they possess. This approach will require further research into the dose-response relationships for each agent. Based on our current knowledge of these drugs, practical guidelines for their use have been described. These guidelines include the gradual escalation of the oral doses of PAS, cycloserine, and ethionamide over several days, and the intravenous administration of streptomycin and capreomycin. Both ciprofloxacin and ofloxacin may be used for the treatment of MDR-TB, but data from clinical trials are currently lacking. Finally, because patients with AIDS appear to develop antimycobacterial drug malabsorption over the course of their HIV infection, therapeutic drug monitoring can be used to verify drug absorption in the individual patient. This approach may improve therapy for that patient and prevent the selection of additional drug resistance.

Simulated human serum profiles of one daily dose of ceftriaxone plus netilmicin in treatment of experimental streptococcal endocarditis

We performed experiments in rats aimed at determining whether a combination of ceftriaxone (CRO) and netilmicin (NET), by using once-daily administration in rats, which simulated profiles of drug in human serum, was more effective than either agent alone in the treatment of endocarditis caused by viridans group streptococci. A programmable infusion pump system enabled the production of profiles of CRO in serum that simulate those found in humans after the intravenous administration of 2 g. The subcutaneous administration of 18 mg of NET per kg of body weight produced levels in the sera of rats comparable to those after the intravenous administration of a dose of 5 mg of NET per kg in humans. Rats with catheter-induced aortic vegetations were infected intravenously with two test strains, a CRO-susceptible Streptococcus sanguis strain (MICs of CRO and NET, 0.064 and 8 mg/liter, respectively) and a relatively CRO-resistant Streptococcus mitis strain (MICs of CRO and NET, 2 and 8 mg/liter, respectively). Against both strains, the combination of CRO and NET was synergistic in vitro as determined by time-kill curves. Treatment of rats was started 48 h postinfection and lasted for 3 days. CRO alone was effective against the susceptible strain (P < 0.001 compared with control animals) but was not effective against the resistant organism. A significantly enhanced antibacterial activity of the CRO-NET combination in reducing the valvular bacterial counts was observed with both test strains (P < 0.001). The synergistic effect was obtained with a single daily injection of NET which provided detectable levels in serum for only 8 h, suggesting that in vivo synergism in the treatment of infections caused by viridans group streptococci can be obtained without 24 h of aminoglycoside coverage. These experimental data might provide a rationale for clinical trials of a once-a-day dosing regimen in the treatment of streptococcal but nonenterococcal endocarditis.

Aminoglycoside monitoring in the once- or twice-daily era. The Dutch situation considered

The results of an inquiry among Dutch hospital pharmacists on the monitoring of aminoglycosides are presented and the relevance of monitoring is discussed. The vast majority of Dutch hospitals (47 out of 65) use aminoglycosides in a twice-daily dosage regimen, whereas 12 hospitals use a once-daily dose. The timing of peak level sampling is usually 30 min after the end of an intravenous infusion of 20-30 min. Mean 'therapeutic' peak levels of gentamicin were 7-13 mg/l in the once-daily group, 6.4-9.6 mg/l in the twice-daily group and 5-9 mg/l in the small thrice-daily group. Little or no evidence has been published to substantiate a real therapeutic range for aminoglycosides, concerning a relationship between peak or trough levels of aminoglycosides and clinical efficacy, ototoxicity and nephrotoxicity. All studies have been performed with the conventional thrice-daily regimen. No therapeutic range can be defined yet for once-daily or twice-daily aminoglycosides. The monitoring of aminoglycosides may be helpful to reduce the variability in serum levels after a standard dose.

Controversies in the management of Mycobacterium avium complex infection in AIDS patients

OBJECTIVE

To update readers on the clinical management of infections secondary to Mycobacterium avium complex (MAC) in patients with AIDS. A general description of the organism, culture and susceptibility testing, and clinical manifestations of the disease is provided. Several aspects of the treatment of the disease, including an historical perspective, current approaches, and future research opportunities, are described.

DATA SOURCES

Current medical literature, including abstracts presented at international meetings, is reviewed. References were identified through MEDLINE, Current Contents, and published meeting abstracts.

STUDY SELECTION

Data regarding the epidemiology, clinical manifestations, culture and susceptibility testing, and treatment of MAC are cited. Specific attention is given to the management of patients with MAC infection.

DATA EXTRACTION

Information contributing to the discussion of the topics selected by the author is reviewed. Data supporting and disputing specific conclusions are presented.

DATA SYNTHESIS

Disseminated MAC infection is diagnosed antemortem in approximately 30 percent of patients with AIDS; postmortem rates of isolation exceed 50 percent. The incidence of MAC may increase as attempts at isolating the organism become more aggressive. The traditional approach to the isolation, susceptibility testing, and treatment of MAC has been derived from the management of Mycobacterium tuberculosis, with disappointing results. Newer radiometric in vitro methods of susceptibility testing appear to show more promise. Current mouse models of MAC are not true AIDS models; new CD4-deficient mouse models are being developed. Clinical mycobacteriologic and pharmacokinetic laboratory support have been underused, with treatment generally proceeding empirically. New agents that may contribute to the management of disseminated MAC infection include the macrolide derivatives clarithromycin and azithromycin. Research also continues with new rifamycins (including rifabutin) and fluoroquinolones (ciprofloxacin, sparfloxacin). Preliminary results suggest a central role for macrolides in the treatment of disseminated MAC; effective companion drugs are needed to prevent the rapid emergence of macrolide-resistant MAC.

CONCLUSIONS

Treatment results for disseminated MAC infection remain poor. Therapy may be improved by selecting drugs on the basis of susceptibility data for each isolate, rather than by using empiric regimens based on susceptibility trends. Significant antimycobacterial drug malabsorption has been documented, and may contribute to poor outcomes. More-potent agents are needed to improve the clinical outcome in AIDS patients with MAC.

Optimisation of antibiotic therapy in cystic fibrosis patients. Pharmacokinetic considerations

Antibiotic therapy plays a central role in the medical management of patients with cystic fibrosis. While totally convincing efficacy data are lacking, antibiotics probably have a pronounced beneficial effect on both morbidity and mortality. Much has been learned in the past 20 years about antibiotic use in this population. At the same time, new antimicrobial agents with the potential to treat this condition have become available for use. The pharmacokinetics of a number of antibiotic classes, including beta-lactams, aminoglycosides and quinolones, are altered in this patient population. Increased total body clearance is a common occurrence but is not always changed enough to warrant altered dosages. Nonetheless, in light of altered pharmacokinetics in the cystic fibrosis population, appropriate dosage and monitoring parameters for a number of antibiotics have been determined.

Outpatient parenteral antibiotic therapy. Management of serious infections. Part I: Medical, socioeconomic, and legal issues. Selecting the antibiotic

Variations in antibiotic pharmacokinetics and pharmacodynamics allow therapy to be readily adapted to the outpatient setting. Factors to be taken into account when designing an outpatient parenteral regimen include minimal inhibitory and bactericidal concentrations, post-antibiotic effect, half-life, protein binding, drug stability, IM versus i.v. administration, and continuous versus intermittent infusion.

Empiric antibiotic therapy of abdominal sepsis and serious perioperative infections

This article discusses empiric therapy for several serious infections in surgical patients. The accepted antibiotic treatment for purulent peritonitis, the empiric treatment of postsurgical wound infection, and the empiric treatment of postsurgical pneumonia are discussed. The cost of the various regimens is listed. Recommendation of the various regimens is based on the seriousness of the infection, peculiarities of the hospital flora, effectiveness of the regimens, and cost.

Kill kinetics and regrowth patterns of Escherichia coli exposed to gentamicin concentration-time profiles simulating in vivo bolus and infusion dosing

The relative influence of peak concentration (Cmax) versus the area under the antibiotic concentration-time curve (AUC) on the bactericidal effect of gentamicin against Escherichia coli NCTC 10418 was studied. Bacteria in the lag phase were exposed to an in vitro gentamicin concentration series which mirrored the concentrations determined in patients after 80-mg intravenous bolus (1 min) and 80-mg intravenous infusion (30 min) doses. Bacterial viable cell counts and gentamicin concentrations were measured before and during antibiotic exposure. Both the Cmax and AUC were shown to be factors determining antibacterial activity; however, the Cmax was an independent determinant of effect. These findings indicate that bolus intravenous dosing with gentamicin could maximize bactericidal activity. Increased efficacy could result at any given daily antibiotic dose if delivered via bolus with long intervals (12 to 24 h) between doses if appropriate precautions to avoid toxicity are taken.

Methods of minimising the cost of aminoglycoside therapy to hospitals

Aminoglycoside agents are used for a wide variety of systemic infections and can profoundly affect hospital expenditures depending upon the amounts used, acquisition costs, and costs incurred during therapy. Significant cost advantages can be gained through selection and proper use of the appropriate aminoglycoside. Institutions can improve the selections by using inservice training and other educational methods, drug-use audits, and therapeutic substitution. Institutions may also restrict or delete certain aminoglycosides from the drug formularies to help minimise costs. Therapeutic drug monitoring can reduce the incidence of aminoglycoside adverse effects. However, some methods of monitoring, such as certain clinical pharmacokinetic services, can be time consuming and require additional manpower. These measures can be shown to be cost effective if they can decrease the length of therapy, minimise toxicity, decrease the length of hospitalisation, or affect mortality. Experimental data and preliminary clinical evidence suggest that once-daily regimens of aminoglycoside agents have some possible advantages over the standard regimens. If this regimen can be widely used, cost savings secondary to decreased use of aminoglycosides can be realised in the future. Potent, broad spectrum beta-lactam monotherapy has threatened to replace conventional combination therapies that include aminoglycosides in some infectious processes and a few studies have found certain single-drug therapies to be cost effective.

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.

Synergistic activity of ceftriaxone combined with netilmicin administered once daily for treatment of experimental streptococcal endocarditis

We have conducted experiments to determine if one daily injection of netilmicin (NET) would be synergistic with the broad-spectrum cephalosporin ceftriaxone (CRO) in the treatment of experimentally induced endocarditis. Rats with catheter-induced aortic vegetations were infected intravenously with 3 x 10(7) CFU of a beta-lactam-sensitive strain of Streptococcus sanguis or a beta-lactam-resistant strain of Streptococcus mitis. Treatment with the antibiotics alone (CRO, 10 mg/kg of body weight every 8 h; NET, 18 mg/kg every 24 h) or in combinations which had proved synergistic in in vitro time-kill curves was commenced 48 h postinfection and continued for 72 h. The results show that the combination was markedly effective against S. sanguis and moderately effective against S. mitis, while, with the protocol used here, the agents alone were not. The results suggest that CRO-NET should be an effective combination for treating streptococcal endocarditis in humans and may permit a shorter duration of treatment and once-a-day dosing to be used.

Inhibition of gentamicin uptake into cultured mouse proximal tubule epithelial cells by L-lysine

Gentamicin uptake and toxicity was studied in a nontransformed cell line obtained from the S1 segment of the proximal tubule epithelium of a transgenic mouse. Cytotoxicity was assayed using the dye 3-(4,-5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Gentamicin uptake was assayed by a fluorescence polarization assay. No differences in toxicity were found among cells incubated for 4 hours in complete culture medium, enriched Kreb's buffer alone, or enriched Krebs' buffer with added 300 micrograms/mL gentamicin, 0.5 mmol/L L-lysine, or gentamicin plus L-lysine. Uptake of 300 micrograms/mL gentamicin was minimal at zero time and increased as a function of time. Uptake of gentamicin at 4 hours was positively correlated with medium gentamicin concentration. Addition of 0.5 mmol/L L-lysine inhibited uptake of 300 micrograms/mL gentamicin 38.9 +/- 10.2%. No other amino acid, including D-lysine or arginine, significantly changed gentamicin uptake. The authors conclude that gentamicin and L-lysine share a specific uptake mechanism located in the apical membrane of renal proximal tubule cells.

Optimal aminoglycoside dosing regimen for penicillin-tobramycin synergism in experimental Streptococcus adjacens endocarditis

The combination of penicillin and aminoglycoside is the recommended therapy for endocarditis caused by nutritionally variant streptococci (NVS). However, the optimal aminoglycoside dosing regimen remains controversial. We compared the efficacies of four regimens of tobramycin alone or combined with procaine penicillin in the therapy of rabbits with endocarditis caused by Streptococcus adjacens, a new species of NVS. Animals were injected intramuscularly for 4 days with procaine penicillin (150,000 U/kg of body weight twice daily) or tobramycin at a low dose (3 mg/kg every 24 h) or a high dose (12 mg/kg every 24 h) either once or three times daily (t.i.d.) alone or in combination with procaine penicillin. Additional groups of animals were treated with the combination regimens for a shorter period of time (2 days) in order to demonstrate a possible difference in the rapidity of efficacy between the regimens. The MICs and MBCs were 0.015 and 1 micrograms/ml and 8 and 16 micrograms/ml for penicillin and tobramycin, respectively. The mean peak tobramycin levels in plasma were 2.4 +/- 1.3 (1 mg/kg t.i.d.), 5.4 +/- 3.7 (4 mg/kg t.i.d.), and 25 +/- 9.3 (12 mg/kg once daily). The mean penicillin levels in serum were always above the MIC. In vitro kill curves plotted at the time that peak concentrations were reached in plasma showed a concentration-dependent killing effect of tobramycin alone but not in combination with penicillin. In vivo, low-dose tobramycin was significantly less effective than the high dose. Results for the combinations of the different dosing regimens of tobramycin with procaine penicillin were not significantly different. Our results suggest that (i) against susceptible strains of streptococci, aminoglycoside alone exhibits a concentration-dependent killing effect both in vitro and in vivo; (ii) against NVS strains, combinations of penicillin and high- or low-dose tobramycin are equally effective; and (iii) aminoglycoside given once daily or at a low dose t.i.d. with penicillin could be a cost-effective alternative with reduced toxic risk for patients with NVS endocarditis when the bacteria are susceptible to the killing activities of both compounds.

Efficacy of temafloxacin in experimental Streptococcus adjacens endocarditis and autoradiographic diffusion pattern of [14C]temafloxacin in cardiac vegetations

Temafloxacin, a new fluoroquinolone, alone or in combination with tobramycin, was compared with penicillin, tobramycin, and their combination in the therapy of rabbits with endocarditis caused by Streptococcus adjacens GaDT, a new species of nutritionally variant streptococci. Animals were injected intramuscularly for 4 days with temafloxacin (50 mg/kg of body weight twice daily [b.i.d.]) alone or combined with tobramycin (12 mg/kg once daily), with procaine penicillin (150,000 U/kg b.i.d.) alone or combined with tobramycin (12 mg/kg once daily), or with tobramycin (12 mg/kg once daily) alone. Another group of animals was treated with a higher dose of temafloxacin (100 mg/kg b.i.d.). Temafloxacin, penicillin, and tobramycin MICs and MBCs were 1 and 2, 0.015 and 1, and 8 and 16 micrograms/ml, respectively. Time-kill curves showed that the addition of tobramycin to penicillin or temafloxacin increased the killing rate. In vivo, treatment with temafloxacin (50 and 100 mg/kg b.i.d.) alone reduced the bacterial counts in vegetations (3.9 +/- 0.9 and 3.1 +/- 0.8 log10 CFU/g of vegetation) compared with those in the vegetations of control animals (7.5 +/- 0.9 log10 CFU/g of vegetation). This result was similar to that obtained with penicillin alone (4.5 +/- 0.8 log10 CFU/g of vegetation). The combination of temafloxacin (50 mg/kg) and tobramycin was as effective as penicillin plus tobramycin (2.5 +/- 0.3 versus 2.3 +/- 0.4 log10 CFU/g of vegetation, respectively). The autoradiographic pattern of [14C]temafloxacin diffusion into infected cardiac vegetations was studied. Thirty minutes after the end of infusion of 250 microCi of [14C]temafloxacin, the [14C]temafloxacin was homogeneously distributed throughout the vegetations. These data support further evaluation of quinolones in experimental endocarditis.

Adaptive resistance following single doses of gentamicin in a dynamic in vitro model

Adaptive resistance is a phenomenon recently described for Pseudomonas aeruginosa and other gram-negative bacilli following exposure to aminoglycoside antibiotics. It is a reversible form of resistance which develops within 1 to 2 h of initial exposure to an aminoglycoside and disappears several hours after removal of the antibiotic. We investigated adaptive resistance in P. aeruginosa ATCC 27853 following single doses of gentamicin by using a dynamic in vitro model which mimics in vivo pharmacokinetics. The initial peak gentamicin concentrations were 2.5, 8, and 25 mg/liter, and these were followed by an exponential decay in the concentration, with a half-life of 2.5 h. The degree of adaptive resistance was greater and the duration was longer with higher initial gentamicin concentrations. Maximal adaptive resistance occurred between 2 and 10 h following 8 mg/liter and between 2 and 16 h following 25 mg/liter. Full recovery of susceptibility occurred at approximately 36, 39, and 43 h following 2.5, 8, and 25 mg/liter, respectively, at which times the gentamicin concentrations were extremely low. Longer dosing intervals for aminoglycosides may improve efficacy by allowing time for adaptive resistance to resolve.

Single-dose antibiotic therapy: what has the past taught us?

The proper dosage schedule of antibiotics has generally been determined empirically, due to the difficulty of clinical trials. Initially, the dosage was chosen to allow high sustained levels greater than MIC in the blood. Antibiotics (beta lactams, tetracyclins, macrolides) were given at high doses three to six times daily, whatever their kinetic properties. The data obtained by Eagle3 with beta lactams in animal models of streptococcal and treponemal infections outlined the importance of interval between doses on the in vivo efficacy. They also showed that increasing the dose of penicillin had a positive effect on the bactericidal activity only through the persistence of effective levels (greater than MIC) at the site of infection. Further illustrations were given through experimental and clinical studies with beta lactams or other compounds on different types of infections: LRTIs, UTIs, meningitis, and endocarditis. The importance of both dynamic (i.e., pattern of bactericidal effect) and kinetic (elimination half-life) parameters was thus further identified. Information on toxicity with some compounds with a narrow therapeutic index, such as aminoglycosides, indicated that increasing the dose to enhance efficacy had some limitations. This led to numerous studies on the relations between concentration and toxicity, stating that nephro- or ototoxicity were not directly related to peak level in serum. Experimental studies showed that OD administration of aminoglycosides was both more efficient and less toxic than the multiple-dose regimen of the same daily amount. Economic considerations progressively justified attempts to both reduce the dose and the work load related to multiple administrations.(ABSTRACT TRUNCATED AT 250 WORDS)

What preclinical data are needed to justify once-daily therapy?

Before applying in clinical practice once-daily dosing of antimicrobials, one must take into consideration several factors that may influence the pharmacodynamic interaction between antimicrobials and microbes at the site of infection. The ideal agent should demonstrate rapid concentration-dependent killing activity and a post-antibiotic effect that could allow for a clinically significant delay with levels below the minimal inhibitory concentration before regrowth of the microorganism. The pharmacokinetic properties of the antibiotic should allow for a good therapeutic ratio (concentration/MIC) at the site of infection. To evaluate the importance of dosage schedule on outcome, investigators have to use animal models where peak levels, half-life, area under the curve, time above MIC in interstitial fluid or infected tissues, and other pharmacodynamic properties can be evaluated simultaneously. The pharmacodynamics of several antibiotics administered at different dosing interval is compared using an animal model of infected fibrin clots. In this model, once-daily therapy resulted in better killing than other modes of administration. Aminoglycosides and quinolones may be better suited for once-daily therapy than beta-lactams unless these latter agents have a long half-life.

Comparison of 5 milligrams of netilmicin per kilogram of body weight once daily versus 2 milligrams per kilogram thrice daily for treatment of gram-negative pyelonephritis in children

The efficacy and safety of netilmicin, 5 mg/kg of body weight once daily or 2 mg/kg thrice daily for 10 days, for the treatment of gram-negative pyelonephritis in children were compared in a prospective, randomized trial. Explicit criteria were used to define the site of infection, treatment outcome, and adverse effects. Netilmicin was given to 74 children once daily and to 70 children three times daily. At 1 week posttreatment, 73 (99%) of 74 children treated with netilmicin once daily and 70 (100%) of 70 children treated with netilmicin three times daily were cured. At 4 weeks posttreatment, no relapse was detected and the rate of reinfection was essentially identical in the two treatment groups. Peak serum netilmicin concentrations were higher in patients given the once-daily regimen, whereas a higher trough level was detected in patients given the three-times-daily regimen. Nephrotoxicity, which was defined as an increase in the serum creatinine level of greater than or equal to 0.3 mg/dl over the baseline, was rare (3%) and reversible and occurred regardless of the treatment regimen. Ototoxicity, which was assessed by pure-tone audiometry (250 to 8,000 Hz) and brain stem-evoked response (6,000 Hz), occurred in 2 of 32 children who were evaluated. In these two children, who were given the once-daily regimen, wave V was not evokable monolaterally below 25 and 40 dB normal hearing level, respectively. Thus, it may be possible to treat childhood pyelonephritis with netilmicin once daily. However, this new approach needs to be confirmed in other studies.

Effect of pooled human cerebrospinal fluid on the postantibiotic effects of cefotaxime, ciprofloxacin, and gentamicin against Escherichia coli

The killing and postantibiotic effects (PAE) of cefotaxime, ciprofloxacin, and gentamicin against Escherichia coli were determined in Mueller-Hinton broth (MHB) and pooled human cerebrospinal fluid (CSF). MICs performed in MHB and CSF were within one dilution for all antimicrobial agent-organism combinations. At two times the MIC, CSF significantly (P less than 0.05) increased the duration of the PAE compared with MHB when cefotaxime, ciprofloxacin, and gentamicin were used against all strains tested. This effect occurred despite similar reductions in bacterial growth in both fluids after the 2-h antimicrobial agent exposure. We conclude that pooled human CSF markedly increases the PAE of cefotaxime, ciprofloxacin, and gentamicin against E. coli compared with MHB, without affecting bacterial killing.

Identification of factors affecting in vivo aminoglycoside activity in an experimental model of gram-negative endocarditis

Aminoglycoside bactericidal activity during the first 24 h of treatment probably is a determining parameter in the prognosis of severe gram-negative infections in immunocompromised patients. To identify the predictive factors involved in the definition of the best therapeutic regimen for Enterobacter cloacae and Serratia marcescens infections, we studied different gentamicin, tobramycin, and amikacin regimens by using an experimental model of rabbit endocarditis. Two factors appear to play an important role in predicting in vivo efficacy: (i) the level of in vivo bactericidal activity, which can differ widely from one aminoglycoside to another for the same bacterial strain and from one strain to another of the same species, and (ii) the critical serum drug concentration (CSC, in milligrams per liter), defined as the lowest serum antibiotic concentration capable of producing a significant CFU reduction (P less than 0.05) in endocarditis vegetations 24 h after the beginning of a continuous infusion. Stepwise regression analysis showed that for gentamicin and S. marcescens, the area under the concentration-time curve above the CSC and then the time above the CSC are the determining parameters for efficacy (R = 0.69; F = 13.5; P = 0.001), whereas for amikacin and S. marcescens, the time above the CSC and then the area under the concentration-time curve above the CSC predict efficacy (R = 0.74; F = 24.0; P = 0.0001). The lowest CSC is that of amikacin (about 8 mg/liter); those of gentamicin and tobramycin are about 15 mg/liter. In severe S. marcescens infections, intermittent amikacin dosing offers excellent bactericidal activity within the first 24 h.