Early synergistic interactions between amikacin and six beta-lactam antibiotics against multiply resistant members of the family Enterobacteriaceae

Ureidopenicillins and beta-lactam/beta-lactamase inhibitor combinations

Although research and development of new penicillins have declined, penicillins continue to be essential antibiotics for the treatment and prophylaxis of infectious diseases. The most recent additions are the ureidopenicillins and beta-lactam/beta-lactamase inhibitor combinations. This article reviews the spectrum of activity, toxicity, pharmacokinetics, and clinical uses of the ureidopenicillins, and the beta-lactam/beta-lactamase inhibitor combination agents.

Comparison of methodologies for synergism testing of drug combinations against resistant strains of Pseudomonas aeruginosa

The purpose of this study was to determine if synergism was maintained for various combinations of beta-lactams with an aminoglycoside against four clinical strains and one laboratory strain of Pseudomonas aeruginosa which were resistant, according to the MICs, to the beta-lactams and/or aminoglycoside. The results from both the checkerboard and killing curve methodologies were compared. The laboratory strain (ATCC 27853) was manipulated in vitro by serial passage onto agar containing increasing concentrations of each antibiotic to select for resistance. One clinical isolate (R61) was also serially passed to raise the MIC of piperacillin from 128 to 1,024 micrograms/ml. The fractional inhibitory concentration indices for all isolates indicated indifference for all combination therapies, with values ranging from 0.6 to 3. In contrast, killing curve results for all isolates demonstrated synergism with drug concentrations at either one-fourth or one-half the MIC for each organism. The MIC of piperacillin for the laboratory-manipulated clinical isolate R61 was 1,024 micrograms/ml, and synergism was still observed with concentrations of one-half the MIC of piperacillin and amikacin. For clinical isolate R166, which was beta-lactam and tobramycin resistant, synergism continued to be demonstrated with concentrations of tobramycin (1/16 MIC) in combination with piperacillin and cefepime at 1/2 the MIC. The results of this study indicate that against P. aeruginosa, synergism is observed in spite of resistance to beta-lactams and/or aminoglycosides. Synergism appears to be maintained even at very high MICs (piperacillin, 1,024 micrograms/ml; tobramycin, 128 micrograms/ml) with drug concentrations within achievable therapeutic ranges. With current definitions of synergism there was a complete lack of correlation between the results obtained by the checkerboard and killing curve methodologies, with the fractional inhibitory concentration indices showing indifference and killing curves resulting in synergism. The methodologies and definitions of synergism or antagonism are variable and not standardized and should be reevaluated.

Antipseudomonal penicillins

Antipseudomonal penicillins retain most of the antibacterial activity of penicillin and aminopenicillins. This group of penicillins has added activities against many gram-negative rods, including P. aeruginosa. Similar to the earlier penicillins, this group continues to be susceptible to hydrolysis by many beta-lactamases and are, therefore, not consistently active against Staphylococcus, some gram-negative rods, and certain beta-lactamase-producing gram-negative anaerobes. The ureidopenicillins, especially piperacillin, appear to have better activity against Enterococcus, Klebsiella, and P. aeruginosa than ticarcillin. The advantages over the newer cephalosporins are (1) better activity against Enterococcus, (2) more consistent activity against Clostridium, and (3) more consistent synergy with aminoglycosides. The ureidopenicillins have certain advantages over carboxypenicillins, including lower sodium load, less frequent hypokalemia, reduced platelet dysfunction, minimal dosage adjustment in patients with renal failure, and a wider spectrum of antibacterial activity, especially against Enterococcus, Pseudomonas, and Klebsiella. The utility of the antipseudomonal penicillins by themselves is limited as agents for monotherapy when the infecting organism is not known. In addition, monotherapy is not recommended in certain infections to avoid the development of resistance. When combined with a beta-lactamase inhibitor or with an aminoglycoside, however, some of the weaknesses can be overcome.

Synergistic antibacterial interaction of cefotaxime and desacetylcefotaxime

Cefotaxime (CTX) is metabolized in desacetylcefotaxime (dCTX), a less potent compound which shows, however, a higher stability against selected beta-lactamases produced by Gram-negative organisms. The aim of this study was to verify if the antimicrobial activity of CTX against 260 clinical aerobic and anaerobic pathogens isolated in our institution was enhanced by its metabolic derivative dCTX. The combination of CTX and dCTX, assessed by checkerboard titration, was completely or partially synergistic towards 61% of the 220 aerobic organisms tested and against 68% of the 40 Bacteroides strains analyzed. In addition we have investigated, by the time-kill method, the in-vitro interactions against 50 aerobic strains of CTX and dCTX alone and in combination with netilmicin, a drug often employed in severe infections in combination with beta-lactam agents in order to provide effective killing of resistant nosocomial pathogens. Time-kill studies indicated that 36% of the aerobic nosocomial strains were synergistically inhibited by the combination of CTX/dCTX with netilmicin. These results indicate that dCTX makes an important contribution to the clinical efficacy of CTX.

Activity of cefotiam in combination with beta-lactam antibiotics on enterobacterial hospital strains

By using checkerboard titrations the effect of cefotiam combined with different beta-lactam antibiotics on fifty strains of Enterobacteriaceae moderately susceptible (minimal inhibiting concentration greater than or equal to 8 mg/l) or resistant (minimal inhibiting concentration greater than or equal to 64 mg/l) to cefotiam was evaluated. The following compounds were tested: cefamandole, cefazolin, cefmenoxime, cefotaxime, cefotiam, ceftazidime, cefuroxime, mecillinam and piperacillin. The synergistic effect varied markedly. The combination cefotiam-mecillinam showed the highest rate of synergistic activity. Antagonism was found in 1% of the combinations.

Combination Antibiotic Treatment of Chemotherapy-Induced Neutropenia in Non-Leukemic Patients

The use of more aggressive chemotherapies in the treatment of patients with some tumors has caused a higher frequency of neutropenia and subsequent serious infections. To verify the role in these patients of a combination therapy of amikacin (300 mg/m2 i.v. every 12 hours) plus ceftazidime (2 g/m2 i.v. every 8 hours) adminsitered as initial empiric treatment, followed in non-responsive cases by a second-line therapy with clindamycin (300 mg/m2 i.v. every 8 hours), we conducted a prospective study in 45 febrile episodes (temperature ≥38.5 °C) in neutropenic patients (neutrophils ≤500/ml). The patients' median age was 58 (range, 19-80); 29 were women and 16 were men. The median performance status was 50 (range, 30-90), and 71 % of the patients had progressive tumoral disease. Before antibiotic therapy the median duration of fever was 12 hours (range, 4-48 hours). The median granulocyte count was 350/ml (range, 100-500 cells/ml), and the median peak temperature was 38.8 °C (range, 38.5-41 °C). The median time for neutrophils to rise towards 1000/ml was 4 days (range, 2-12), and the median duration of therapy was 8 days (range, 3-12). Documented bacterial infections were present in 28 patients whereas 17 had clinically possible infections or fever of unknown origin. The infection sites in microbiologically documented infections were: septicemia (12), multiple sites (4), tonsillitis (4), urinary tract (4), pneumonia (2) and fistula (2). Complete response to first-line therapy was obtained in 36 out of 45 episodes (80 %; 95 % confidence limits from 65 % to 90 %). Five out of 8 cases responded to second-line therapy with clindamycin for an overall recovery rate of 91 %. The amikacin-ceftazidime combination followed by clindamycin in non-responsive cases is effective, with moderate toxicity in non-leukemic febrile neutropenic patients.

Activity of cefotaxime/desacetylcefotaxime with two aminoglycosides against gram-negative pathogens: an example of interactive synergy

The susceptibility patterns of clinical Gram-negative isolates were determined to cefotaxime (CTX) and desacetylcefotaxime (dCTX) alone and in combination with gentamicin (GENT) or tobramycin (TOB) by an agar dilution technique. A constant ratio of 1:1 (CTX to dCTX) was tested throughout the study. Isolates were challenged with subinhibitory levels of TOB or GENT in combination with clinically achievable levels of CTX, dCTX and CTX/dCTX to examine the interactions of the agents. Results of this study demonstrate that CTX/dCTX interacts synergistically with aminoglycosides against many Gram-negative pathogens. Synergy (defined as a fourfold or greater decrease in minimum inhibitory concentration (MIC) when CTX/dCTX was compared to CTX/dCTX/TOB) was demonstrable for 55% of isolates tested. Similarly, 45% were synergistically inhibited by CTX/dCTX/GENT. Additivism (a 2-fold decrease in MIC with the same comparisons) was evident for an additional 18 isolates for CTX/dCTX/TOB and 19 with CTX/dCTX/GENT. When data for Pseudomonas spp. were excluded from the analysis, synergy or additivism was evident with CTX/dCTX/TOB for 88% of the organisms tested and 72% with CTX/dCTX/GENT. Synergistic synergy for CTX/dCTX/TOB (an 8- to greater than 16-fold decrease in MIC for CTX) was demonstrable for 35 and 32 of 82 nonspeudomonal isolates respectively with the TOB and GENT combinations. Ninety nine percent of the nonspeudomonal isolates were inhibited by less than 4 micrograms/ml of CTX, 4 micrograms/ml of dCTX and 0.12 micrograms/ml of TOB, or 0.25 micrograms/ml of GENT, respectively.

Antibiotic combinations: should they be tested?

When antibiotic combinations are used to provide a broader spectrum of antimicrobial activity or in an attempt to prevent the emergence of resistant organisms, it is rarely necessary or practical to perform tests of drug interactions in vitro. In vitro testing of combinations may be useful when combinations are used in an attempt to attain synergistic interactions. In some cases, screening methods can be used as substitutes for formal synergy testing. This paper examines the mechanisms of antibiotic interaction leading to synergism or antagonism, surveys attempts to correlate in vitro observations with efficacy in animal models, and reviews clinical data providing evidence for or against a useful role of synergistic antibiotic interactions in the treatment of human infections.

Early effects of beta-lactams on aminoglycoside uptake, bactericidal rates, and turbidimetrically measured growth inhibition in Pseudomonas aeruginosa

In vitro studies of tircarcillin or cefsulodin combined with [3H]tobramycin were performed with Pseudomonas aeruginosa. The rate of bacterial killing, the uptake of tobramycin, and the effects on optical density were measured. Both beta-lactams increased the uptake of subinhibitory concentrations of tobramycin. This result was quantitatively associated with a 2- to 4-h time-kill potentiation and confirmed earlier studies on the mechanism of beta-lactam-aminoglycoside synergy in Escherichia coli (P. H. Plotz and B. D. Davis, Science 135:1067-1068, 1962).

In vitro activity of piperacillin, ticarcillin, and mezlocillin alone and in combination with aminoglycosides against Pseudomonas aeruginosa

A total of 103 isolates of Pseudomonas aeruginosa were studied to compare the in vitro effectiveness of three beta-lactam antibiotics (piperacillin, ticarcillin, and mezlocillin) when used alone and in combination with four aminoglycosides (tobramycin, gentamicin, amikacin, and netilmicin). All drugs were tested as single agents against a standard inoculum (5 X 10(5) CFU/ml). The three antipseudomonal penicillins were also tested against the isolates at a higher inoculum concentration (10(7) CFU/ml). Synergy testing was performed by the two-dimensional checkerboard method and was defined by a fractional bactericidal index of less than or equal to 0.5 and bacterial killing accomplished at antibiotic concentrations no greater than those achievable in serum. All combinations were assessed for synergy. The degree of synergy was further analyzed by dividing the isolates into groups based on their susceptibility and resistance to the individual agents in the combination. The overall effectiveness of the various aminoglycoside-antipseudomonal penicillin combinations was assessed regarding their ability to kill the isolates either as single agents or through synergy. Piperacillin was the most active antipseudomonal penicillin, and tobramycin and amikacin were the most active aminoglycosides when used as single agents. When tested against isolates at a higher inoculum concentration, ticarcillin was significantly more active than the other beta-lactams. The highest degree of overall synergy was noted with gentamicin-ticarcillin (78.2% of strains) and amikacin-piperacillin (77% of strains). When assessed for overall effectiveness, all combinations containing amikacin were the most active. The combination of amikacin-piperacillin was the most effective, with activity against 96% of all isolates.

Aminoglycosides plus beta-lactams against gram-negative organisms. Evaluation of in vitro synergy and chemical interactions

Combination antibiotic therapy has been used mainly to broaden the antibacterial spectrum and prevent the development of resistance. Antibiotic combinations proven to be synergistic in vitro are associated with a significantly better in vivo response, particularly in the compromised host in whom traditional treatment combines an antipseudomonal penicillin plus an aminoglycoside. Several investigators have examined combining new agents, such as the third-generation cephalosporins (cefotaxime, ceftriaxone, ceftizoxime, ceftazidime, cefoperazone, and moxalactam), aztreonam, or the ureidopenicillins, with amikacin. When compared with combinations of an older cephalosporin, carbenicillin or ticarcillin, plus gentamicin or tobramycin, these newer combinations produce higher rates of clinically meaningful synergy and rapid enhancement of in vitro bactericidal activity against the difficult-to-treat Enterobacteriaceae (i.e., Serratia, Citrobacter, Enterobacter, Providencia, and indole-positive Proteus species). This effect, without any evidence of antagonism, has been reported even for strains moderately or completely resistant to the former antibiotics. Unsatisfactory and unpredictable synergistic interactions against both resistant and susceptible strains of Pseudomonas aeruginosa--the most difficult nosocomial pathogen to treat--have been noted with combinations of tobramycin or gentamicin plus cefotaxime, moxalactam, or cefoperazone. Conversely, the use of amikacin plus various beta-lactams against multi-resistant strains is more frequently synergistic. Agents have been observed to exhibit such synergy in the following order of activity, from most to least synergistic: ceftazidime, ceftriaxone, moxalactam, aztreonam, cefotaxime, azlocillin, cefoperazone, cefsulodin, and carbenicillin. The combination of amikacin plus imipenem or ciprofloxacin against strains of P. aeruginosa resistant to the former and moderately resistant to the latter was recently reported to have a low probability of synergy; the combination of two of the newer beta-lactams had mostly an unpredictable or even antagonistic result. In vitro studies have also demonstrated that high concentrations of the antipseudomonal penicillins can inactivate the aminoglycosides. Among the latter compounds, the inactivation order, from most to least inactivated, was as follows: tobramycin, gentamicin, netilmicin, and amikacin. To date, the reports of aminoglycoside inactivation by the newer cephalosporins have been rather contradictory; only moxalactam has been shown produce a significant decrease in activity.