Serum inhibitory and bactericidal activity against methicillin-resistant Staphylococcus aureus in volunteers receiving novobiocin and rifampin alone and in combination.

The combination of novobiocin and rifampin is effective in eliminating colonization due to methicillin-resistant Staphylococcus aureus (MRSA) and in treating experimental MRSA soft tissue infections. To evaluate novobiocin, rifampin, and the combination of the two agents for potential oral therapy in patients with MRSA infections, we measured the serum inhibitory and bactericidal activity from four volunteers against 20 MRSA strains obtained from seven different institutions. When Stratton-Reller methods employing 50% human serum were used to perform the assay, rifampin produced peak mean serum inhibitory titers of 1:40, whereas novobiocin alone produced essentially no inhibitory activity. The combination of novobiocin plus rifampin had similar inhibitory activity as rifampin alone. The bactericidal titers produced by the three regimens were significantly less than inhibitory titers. In additional studies, involving serum from five volunteers tested against seven representative strains, peak mean serum inhibitory activity of novobiocin was 1:232 when Mueller-Hinton broth was used as the diluent compared with < 1:2 when 50% human serum was used. We conclude that despite the high degree of activity of novobiocin in broth, its activity against MRSA in serum is minimal, probably related to the high degree of protein binding of that antibiotic.

Bactericidal activity of ciprofloxacin compared with that of cefotaxime in normal volunteers.

We compared ciprofloxacin (200 mg) with cefotaxime (2 g) when each was administered intravenously over a 30-min period to six volunteers in a crossover manner 1 week apart. To integrate the pharmacologic and microbiologic activity, inhibitory and bactericidal activities in serum were obtained for both antibiotics 1 and 6 h after administration against 10 strains of Escherichia coli, 10 strains of Klebsiella pneumoniae, 15 strains of Pseudomonas aeruginosa, and 10 strains each of methicillin-susceptible and -resistant Staphylococcus aureus. Geometric mean bactericidal titers for E. coli 1 h after the infusion were 1:60 for ciprofloxacin and 1:252 for cefotaxime, and for K. pneumoniae they were 1:20 and 1:256, respectively. However, geometric mean titers were poor for both antibiotics against methicillin-susceptible S. aureus (less than 1:2 for ciprofloxacin versus 1:5 for cefotaxime) and methicillin-resistant S. aureus (less than 1:2 for both antibiotics), as well as against P. aeruginosa (1:3 for ciprofloxacin versus 1:2 for cefotaxime). These data suggest that ciprofloxacin may be useful for the treatment of serious infections caused by E. coli and K. pneumoniae. However, caution is suggested when this dose of ciprofloxacin is used in situations in which septicemia is caused by P. aeruginosa or S. aureus and originates outside the urinary tract.

Imipenem coadministered with cilastatin compared with moxalactam: integration of serum pharmacokinetics and microbiologic activity following single-dose administration to normal volunteers.

We administered 1 g of imipenem along with equal amounts of cilastatin (a dehydropeptidase I inhibitor) or 2 g of moxalactam intravenously over a period of 30 min to six volunteers in a crossover manner 1 week apart. The antibiotic concentrations and pharmacokinetics for each drug were determined and integrated with the microbiologic activity by measuring the duration of time that the free drug concentrations remained above the MICs for 90% of 581 clinical isolates and by measuring serum bactericidal activities against organisms which commonly infect granulocytopenic cancer patients. Moxalactam produced serum levels at 1 h after infusion of 99.9 micrograms/ml; these levels were four times greater than the plasma levels of imipenem (22.8 micrograms/ml). The trough (5.5-h) moxalactam serum levels were 10 times greater than those of imipenem (18.5 and 1.7 micrograms/ml, respectively). Essentially all of the imipenem was unbound to protein, whereas 36 to 42% of the moxalactam was unbound. Moxalactam produced free antibiotic concentrations that were above the MIC for 90% of the strains tested for more than 6 h against all of the species tested except Staphylococcus aureus (5.3 h), Enterobacter hafnia (1.6 h), and Pseudomonas aeruginosa (0 h). The imipenem concentrations were above the MIC for 90% of the strains tested for 5.6 h or more against all of the bacteria tested except Proteus spp. and Pseudomonas aeruginosa (4.5 h). The geometric mean peak bactericidal titers from volunteers receiving imipenem were more than 1:8 against all bacteria and were significantly higher than the titers from volunteers receiving moxalactam against S. aureus (1:7.3) and Pseudomonas aeruginosa (1:4.5). These data, in addition to information obtained from animal models, indicate that imipenem is a promising new candidate for carefully controlled clinical trials as a single agent for therapy of serious infections, including empiric therapy for fever in granulocytopenic cancer patients.

Technical aspects and clinical correlations of the serum bactericidal test.

A review of the studies using 50% human serum as a diluent for the serum bactericidal test has shown correlations with patient outcome. Human serum used as diluent of the patient's serum appears to be essential because of high protein binding of some antibiotics. An inoculum of 10(5)-10(6) bacteria/ml and a bactericidal criteria of 99.9% killing are technical aspects that have gained popularity. Careful timing of serum collection for the assay is important. Neither the macrotube nor microtiter techniques are entirely satisfactory. The latter method, however, has the advantage of being more reproducible than the macrotube method, less cumbersome and requiring less serum. Preliminary guidelines for performing and interpreting the test are provided. Future research should be directed toward making the microtiter technique more sensitive for identifying antibiotic tolerance, developing effective methods to eliminate the need for human serum as a diluent and obtaining more clinical correlations.

Correlation of predicted serum bactericidal activities and values measured in volunteers.

A method was devised for predicting the serum bactericidal activity of new drugs. Six healthy volunteers received 2 g moxalactam, cefoperazone and cefotaxime, respectively, as 30-min infusions in a crossover manner at one-week intervals. The pharmacokinetics of each drug was characterized and the bactericidal activity of the serum 1 h after infusion was measured against panels of six strains of Pseudomonas aeruginosa, six strains of Escherichia coli, six strains of Staphylococcus aureus, and four strains of Klebsiella pneumoniae. The minimum bactericidal concentration of each antibiotic was determined for each organism by the standard NCCLS reference method and the method of Stratton and Reller. On the basis of these values and a serum concentration-time curve constructed from individual patient pharmacokinetic parameters, the bactericidal activity of the serum 1 h after infusion was predicted. These predictions showed a 90% agreement with measured values calculated according to the method of Stratton and Reller, whereas an agreement of 74% was obtained with the reference method. This difference was statistically significant (p less than 0.001).

Novobiocin and rifampicin in combination against methicillin-resistant Staphylococcus aureus: an in-vitro comparison with vancomycin plus rifampicin.

Novobiocin and rifampicin were evaluated in vitro as a possible new antibiotic combination against methicillin-resistant Staphylococcus aureus. An evaluation of 20 strains of methicillin-resistant Staph. aureus using microdilution checkerboard techniques at 10(5) cfu/ml showed neither synergy nor antagonism between novobiocin and rifampicin or between vancomycin and rifampicin. Agar surface inoculation of six strains of methicillin-resistant Staph. aureus showed increased synergy with increased inocula (10(6)-10(9) cfu) for novobiocin plus rifampicin compared to vancomycin plus rifampicin. Time-kill curves showed indifference at 6h for all combinations, whereas, at 24 and 48 h, they generally showed indifference, occasionally synergy, but never antagonism. The 'synergy' between novobiocin and rifampicin at higher inocula of methicillin-resistant Staph. aureus appears to be due to prevention of emergence of resistant organisms and may have clinical relevance. The combination of novobiocin-rifampicin merits further investigation.

Measurement of amikacin in serum by a latex agglutination inhibition test.

Levels of amikacin in serum were determined in 106 serum specimens by a latex agglutination inhibition card test and by radioimmunoassay (RIA). Linear regression analysis demonstrated a high degree of correlation between the two assays (latex = 0.95 (RIA) + 0.69; r = 0.97). Assay of three control sera containing 7.5, 15, and 30 micrograms of amikacin per ml on 7 separate days showed good reproducibility with a coefficient of variation of 0 to 11.7% for the latex assay compared with 7.01 to 22.2% for RIA. Recovery of amikacin in spiked sera varied between 93 and 108% for the latex assay compared with 90 and 100% for RIA. Because the procedure involves a titer, the latex agglutination inhibition card test produces results which are categorized rather than results which are continuous. However, it is a rapid and specific method for determining amikacin levels in clinical specimens and is particularly useful when processing small numbers of specimens.

Safety and tolerability of multiple doses of imipenem/cilastatin.

The safety and tolerability of 1 gm imipenem and cilastatin given together every 6 hours for 10 days was evaluated in a randomized, double-blind, placebo-controlled trial in normal subjects. Nausea was more common in the drug-treated group (five of six subjects) than in the control group (two of six subjects). No consistent changes in hepatic function indices were noted. Although beta 2-microglobulin excretion showed a significant trend of rising over time in the drug group, there were no differences between groups with regard to 24-hour urinary excretion of either N-acetyl-beta-glucosaminidase or beta 2-microglobulin. Urinalysis did not reveal any casts and serial creatinine clearance determinations showed no change in renal function in either the drug- or placebo-treated groups. Pure tone audiograms were performed before and after dosing in 11 of 12 subjects; no changes were noted. We conclude that the combination of imipenem and cilastatin was well tolerated and safe.

Comparative pharmacokinetics and serum bactericidal activity of mezlocillin, ticarcillin and piperacillin, with and without gentamicin.

We compared the serum levels, pharmacokinetics and serum bactericidal activity after intravenous infusions of 5 g of each of three anti-pseudomonal penicillins--ticarcillin, mezlocillin and piperacillin alone and in combination with 80 mg gentamicin in normal volunteers. Gentamicin levels were significantly lower when administered with ticarcillin than when administered with mezlocillin or piperacillin. Serum levels of ticarcillin and piperacillin immediately after the infusion were similar (approximately 450 mg/l) and were higher than mezlocillin at 350 mg/l, but by 6 h, levels of all three penicillins were less than 10 mg/l. Overall, the geometric mean bactericidal titres produced in the serum against organisms which commonly infect cancer patients with granulocytopenia were highest with the piperacillin-gentamicin combination. However, except for the mezlocillin-gentamicin combination against Pseudomonas aeruginosa, serum bactericidal titres of all three penicillin-gentamicin regimes were greater than 1:8 at zero and 2 h after the infusion against the organisms tested. It is unlikely that these differences in the three penicillin-gentamicin combinations will be apparent in the outcome of clinical trials for empiric therapy of fever in the cancer patient with granulocytopenia.

Activity of novobiocin against methicillin-resistant Staphylococcus aureus.

As infections due to methicillin-resistant Staphylococcus aureus become increasingly prevalent, newer alternative antibiotics, especially those which are orally administered, will be required. In order to provide an in-vitro basis for selecting alternative antibiotics, we studied the susceptibility of 103 strains of methicillin-resistant Staph. aureus from seven institutions to oral antimicrobial agents. Novobiocin, rifampicin, and trimethoprim-sulphamethoxazole had excellent in-vitro activity against virtually all strains of methicillin-resistant Staph. aureus. The MIC90 and MBC90 of novobiocin against methicillin-resistant Staph. aureus were 0.25 mg/l. Since previously reported achievable serum levels with oral novobiocin are 100 to 200 times its MIC90 against methicillin-resistant Staph. aureus, novobiocin should be evaluated further for combination therapy with rifampicin or trimethoprim-sulphamethoxazole.

Latamoxef in combination with aminoglycosides against Pseudomonas aeruginosa: similarity with ticarcillin.

The in vitro effect of latamoxef against 50 clinical strains of Pseudomonas aeruginosa was compared to that of ticarcillin, both alone and in combination with the aminoglycosides gentamicin, tobramycin and amikacin. Alone, the MIC90 of latamoxef was consistently one-half the MIC90 of ticarcillin. The two antibiotics appeared similar in regard to inoculum effect and bacterial killing. Adding of one-quarter the minimum inhibitory concentration of the aminoglycoside antibiotic to the beta lactam caused reduction in MIC90 of the latter (either ticarcillin or latamoxef) by one-half and decreased the MIC50 by almost one-quarter the concentration required by the beta lactams singly. Therefore, latamoxef singly or in combination with aminoglycosides behaved similarly but was more active than ticarcillin. Using combinations of antibiotics likely to be achieved in the serum of patients, a beneficial in vitro effect (either additive, partially synergistic or synergistic) generally occurred for the beta lactam-aminoglycoside combination if the strain was relatively sensitive to the aminoglycoside used in this combination. It occurred much less frequently for the highly aminoglycoside-resistant isolates.

The serum bactericidal activity of latamoxef (moxalactam), cefoperazone and cefotaxime.

We determined the serum bactericidal activity 1 h after the end of 2 g, 30 min infusions of latamoxef, cefoperazone and cefotaxime in six volunteers against six strains each of Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus and Pseudomonas aeruginosa. All produced excellent serum bactericidal activity against E. coli. Latamoxef and cefotaxime were superior for K. pneumoniae. Cefoperazone produced the highest titres against Staph. aureus. None of these agents produced sufficient bactericidal activity against Ps. aeruginosa to be useful in initial single agent therapy for the septic, granulocytopenic cancer patient.

Multiple-dose pharmacokinetics of imipenem-cilastatin.

We characterized the pharmacokinetic profile of imipenem-cilastatin administered intravenously to six normal volunteers in a dose of 1,000 mg of each drug every 6 h for 40 doses. The plasma concentrations of imipenem and cilastatin 1 h after the end of a 30-min infusion were 18.7 (+/- 2.1) and 19.1 (+/- 4.6), 20.0 (+/- 3.2) and 17.8 (+/- 4.8), and 23.4 (+/- 2.3) and 19.1 (+/- 3.5) micrograms/ml in the 1st, 17th, and 37th dosing intervals, respectively. The central compartment volumes of distribution for imipenem and cilastatin were 0.16 (+/- 0.05) and 0.14 (+/- 0.03) liter/kg, respectively. Elimination half-lives were short: 0.93 (+/- 0.09) h for imipenem and 0.84 (+/- 0.11) h for cilastatin. Plasma clearances were 12.1 (+/- 0.06) liters/h per 1.73 m2 for imipenem and 12.4 (+/- 1.1) liters/h per 1.73 m2 for cilastatin. Renal clearance accounted for 54% of the plasma clearance of imipenem and 69% of the plasma clearance of cilastatin. The concentrations of imipenem in plasma and urine remained above the MICs of the vast majority of pathogens throughout the dosing interval.

Postantibiotic effect of imipenem on Pseudomonas aeruginosa.

Imipenem (formerly N-formimidoyl thienamycin) and ceftazidime were investigated for their postantibiotic effect on Pseudomonas aeruginosa. Four strains of P. aeruginosa in the logarithmic phase of growth were exposed for 1 and 2 h to concentrations of antibiotics achievable in human serum. Recovery periods of test cultures were evaluated after dilution or addition of beta-lactamase. A consistent postantibiotic effect against all strains was obtained with imipenem but not with ceftazidime. Although ceftazidime did not have a postantibiotic effect, it did suppress the growth of the organisms at concentrations equivalent to one-third of the MIC. The clinical implications of these effects need further evaluation.

The plasma pharmacokinetics of high dose (1 g) imipenem coadministered with 1 g cilastatin in six normal volunteers.

The plasma pharmacokinetics of imipenem, a beta-lactam antibiotic of the new class of carbapenems, was evaluated by administering 1 g imipenem plus 1 g of the dehydropeptidase inhibitor cilastatin to six healthy male volunteers over 30 min. Subsequently intensive plasma sampling was done for 5.5 h. The volume of distribution of the central compartment was 0.16 +/- 0.04 1/kg; the volume of distribution (area) was 0.28 +/- 0.07 1/kg; the volume of distribution (steady state) was 0.23 +/- 0.03 1/kg. The mean plasma clearance was 10.1 +/- 1.0 1/h/1.73 m2. Model-independent analysis showed excellent agreement with these values. The terminal excretion half-life was short (1.3 +/- 0.4 h). For approximately 6 h after administration, the observed plasma levels were above those required to inhibit most nosocomial pathogens in vitro.

Comparison of the pharmacokinetics of ceftazidime and moxalactam and their microbiological correlates in volunteers.

We compared ceftazidime with moxalactam, a commonly utilized, currently available drug. The microbiological activities of ceftazidime and moxalactam were studied. In addition, single-dose pharmacokinetics and serum bactericidal activity 1 and 6 h after a 2.0-g, 30-min infusion of each drug were determined in a crossover study in human volunteers. In vitro, both drugs had MICs for 90% of the isolates of less than 1.0 microgram/ml against the common members of the family Enterobacteriaceae and of 8.0 micrograms/ml against Staphylococcus aureus. Against Pseudomonas aeruginosa ceftazidime was more active than moxalactam, the respective MICs for 90% of the isolates being 8 and 128 micrograms/ml. Mean half-lives were 1.75 (+/- 0.21) h for ceftazidime and 2.5 (+/- 0.38) h for moxalactam. The serum bactericidal titers for both compounds against Escherichia coli and Klebsiella pneumoniae were high. Titers against S. aureus 6 h after infusion were negative. The mean (geometric) serum bactericidal titer of ceftazidime against 31 strains of P. aeruginosa (1:44) was higher than that of moxalactam (1:3.4).

Bactericidal activity of ceftazidime in serum compared with that of ticarcillin combined with amikacin.

We compared the bactericidal activity of serum attained 1 and 6 h after the termination of infusions of either ceftazidime (2 g) or ticarcillin plus amikacin (5 g and 7.5 mg/kg, respectively) in 6 volunteers against a panel of the most common pathogens found in the blood of febrile granulocytopenic cancer patients. Ceftazidime consistently produced significantly higher serum bactericidal titers at both 1 and 6 h against all species of gram-negative bacilli. Its performance against Pseudomonas aeruginosa was especially impressive. The geometric mean titer against this organism was 1:41 at 1 h, contrasted with 1:12 for ticarcillin plus amikacin (P = 0.025). However, for Staphylococcus aureus, the geometric mean serum bactericidal titer of ceftazidime was 1:3.6 at 1 h and undetectable at 6 h. Ceftazidime shows promise as single-agent therapy for serious gram-negative bacillary infections. Whether this promise is fulfilled and whether the observed antistaphylococcal activity is adequate for empiric therapy in infected granulocytopenic patients need further investigation.

Comparative pharmacokinetics of moxalactam, cefoperazone, and cefotaxime in normal volunteers.

The pharmacokinetic parameters of moxalactam were compared with those of cefoperazone and cefotaxime in normal volunteers in a crossover manner. Following 30-min intravenous infusions of 2 g of each of the three antibiotics, serum levels at 1 hr were slightly lower for moxalactam (88 micrograms/ml) than for cefoperazone (112 micrograms/ml) but more than three times those obtained for cefotaxime (29 micrograms/ml). By 8 hr, levels of moxalactam (9.2 micrograms/ml) were slightly higher than those of cefoperazone (6.5 micrograms/ml), and levels of cefotaxime in serum were unmeasurable (less than 1 micrograms/ml). These values reflect differences in half lives of the three agents. Peak serum levels following intramuscular injection and serum levels during constant intravenous infusion were similar for moxalactam and cefoperazone because of counterbalancing differences in the apparent volume of distribution and rates of elimination of the two antibiotics. Serum levels of cefotaxime were much lower than those of the other two antibiotics primarily because of the rapid elimination of cefotaxime from the body. The kidney was the major route of excretion of moxalactam, whereas extrarenal mechanisms were more important for elimination of cefoperazone. These differences in pharmacokinetics may have significant implications for the clinical use of these new antibiotics.