Therapy of experimental Pseudomonas endocarditis with high-dose amikacin and ticarcillin

Treatment of experimental staphylococcal endocarditis due to a strain with reduced susceptibility in vitro to vancomycin: efficacy of ampicillin-sulbactam

We evaluated several 3-day antimicrobial regimens in the treatment of experimental endocarditis caused by an oxacillin-resistant Staphylococcus aureus strain exhibiting intermediate susceptibility in vitro to vancomycin (VISA). Neither vancomycin alone nor trovafloxacin exhibited in vivo efficacy; addition of amikacin to vancomycin yielded a modest in vivo effect. In contrast, the combination of ampicillin and sulbactam was highly effective in vivo, causing a mean decrease in VISA vegetation densities of >5 log(10) CFU/g versus those of untreated controls.

Intravegetation antimicrobial distribution in aortic endocarditis analyzed by computer-generated model. Implications for treatment

The distribution of antibiotics into cardiac valvular tissues is incompletely understood. By integrative computer modeling, we have used previously obtained pharmacokinetic data in experimental endocarditis to characterize aminoglycoside distribution within various geographic sectors of aortic vegetations of rabbits and humans in the current study. In rabbits with pseudomonal aortic endocarditis receiving a standard regimen of amikacin (15 mg/kg every eight hours), sub-MBC levels of the drug for the infecting organism were calculated in the center of 0.38-cm vegetations; this occurred despite supra-MBC levels calculated in plasma and more peripheral loci of the vegetation. In contrast, with a high-dose regimen of amikacin (40 mg/kg every eight hours), supra-MBC drug levels were calculated throughout the entire vegetation for at least 50 percent of the dosing interval. Using similar computer-generated approaches, these data in the rabbit were approximately in simulated aminoglycoside penetration of 10-mm human aortic vegetations. Aminoglycoside regimens designed to yield supra-MBC serum levels in both normal and rapid drug eliminators consistently achieved sub-MBC levels in the center of the vegetation. Computer simulations also confirmed that daily doses of aminoglycoside at least two to four times higher than those ordinarily recommended are necessary to consistently achieve uniform supra-MBC intravegetation levels for an entire dosing interval. Such computer-generated data support the concept of maldistribution of aminoglycosides in aortic endocarditis and provide a rationale for investigating the use of high-dose regimens of aminoglycoside in treating experimental endocarditis.

Development of beta-lactam resistance and increased quinolone MICs during therapy of experimental Pseudomonas aeruginosa endocarditis

The in vivo efficacies of pefloxacin, a new fluoroquinolone, and amikacin-ceftazidime were compared in 50 rabbits with experimental aortic endocarditis caused by Pseudomonas aeruginosa. Animals were randomly chosen to receive 4 or 10 days of no therapy (controls), pefloxacin (40 mg/kg [body weight] per day, intramuscularly [i.m.]), or amikacin (30 or 80 mg/kg per day, i.m.)-ceftazidime (150 mg/kg per day, i.m.). Pefloxacin and both amikacin regimens significantly reduced vegetation bacterial densities compared with controls at days 4 and 10 of treatment (P less than 0.0005). By day 10 of therapy, between 33 and 40% of vegetations from amikacin-ceftazidime recipients contained ceftazidime-resistant bacteria (MICs, greater than 25 micrograms/ml); nitrocefin agar overlay confirmed that these ceftazidime-resistant variants were constitutive overproducers of beta-lactamase. At therapy days 4 and 10, approximately 30% of vegetations sampled from pefloxacin recipients contained bacteria for which pefloxacin MICs were four- to eightfold higher than the MIC for the parental strain used to initially induce endocarditis (MIC, 0.19 microgram/ml). These variants also exhibited increases in ciprofloxacin and ticarcillin MICs, as well as pleotropic resistance to chloramphenicol (but not to amikacin, ceftazidime, or tetracycline). Amikacin-ceftazidime, as well as pefloxacin, was effective in this model of aortic pseudomonal endocarditis. However, in vivo development of ceftazidime resistance and step-ups in pefloxacin MICs among intravegetation isolates were associated with inability to completely eradicate P. aeruginosa from aortic vegetations.

LY146032 compared with penicillin G in experimental aortic valve endocarditis caused by group G streptococci

Rabbits with group G streptococcal aortic endocarditis received no therapy (controls); repeated doses of procaine penicillin G, 300 mg/kg (body weight) per day, administered intramuscularly; or LY146032, a new peptolide antibiotic, 20 mg/kg per day, administered intravenously. Penicillin G and LY146032 reduced mean intravegetation group G streptococcal densities significantly below those observed in controls at both day 3 and day 6 of therapy. Penicillin G effected a more rapid clearance of intravegetation streptococci than LY146032 by day 3, but not by day 6, of therapy.

Bactericidal interactions of a beta-lactam and beta-lactamase inhibitors in experimental Pseudomonas aeruginosa endocarditis caused by a constitutive overproducer of type Id beta-lactamase

We investigated the in vitro and in vivo effects of a combination of a beta-lactam (ceftazidime) and a beta-lactamase inhibitor (dicloxacillin) to synergistically kill a ceftazidime-resistant variant, Pseudomonas aeruginosa PA-48, which overproduces type Id cephalosporinase constitutively. In vitro, dicloxacillin plus ceftazidime exerted bactericidal synergy at approximately 10(5) CFU/ml of inoculum (but not at approximately 10(7)-CFU inoculum), whereas other beta-lactamase inhibitors (sulbactam, clavulanic acid) showed no enhanced killing of PA-48 when combined with ceftazidime at clinically achievable levels for each agent. Dicloxacillin was a potent competitive inhibitor of the extracted Id cephalosporinase from strain PA-48 in short-term comixture studies (less than 10 min [Ki = 2 nM]); in contrast, longer-term comixture studies (90 min) indicated that dicloxacillin functions as a competitive substrate for the enzyme. Growth of PA-48 cells in the presence of dicloxacillin (12.5 to 100 micrograms/ml) had no significant effect on the production rates or functional activity of the Id enzyme. In experimental aortic valve endocarditis due to the ceftazidime-resistant variant (PA-48), rabbits received either no therapy, ceftazidime (25 mg/kg intramuscularly, every 4 h), or ceftazidime plus dicloxacillin (200 mg/kg intramuscularly, every 4 h). The combination regimen reduced mean bacterial densities of PA-48 within cardiac vegetations significantly below those in the other groups at both days 3 and 6 of treatment (P less than 0.005). However, mean vegetation bacterial densities remained greater than 6 log10 CFU/g in the combined treatment group. This modest in vivo synergistic effect (as compared to striking in vitro synergy at approximately 10(5)-CFU inoculum) most likely reflects the high densities of PA-48 achieved in vivo within cardiac vegetations (greater than 8 log10 CFU/g).

Characterization of impermeability variants of Pseudomonas aeruginosa isolated during unsuccessful therapy of experimental endocarditis

We characterized five amikacin-resistant variants of Pseudomonas aeruginosa isolated from aortic valve vegetations during unsuccessful therapy of experimental endocarditis. These organisms were cross resistant to other aminoglycosides. No aminoglycoside-modifying enzymes were produced by these strains. However, all five variants demonstrated significant defects in permeability and intracellular uptake of [3H]amikacin when compared with the amikacin-susceptible parental strain (0 to 26% of that of the parental strain; mean, approximately 15%). The permeability defects were unstable in vitro, with normalization after serial passage in antibiotic-free media. The variants grew as nonpigmented, small-colony types, with in vitro generation times approximately 1.5 to 2 times longer than that of the parental strain (30 to 40 versus 20 min, respectively). Two impermeability variants were compared with the parental strain for ability to induce experimental endocarditis in rabbits with aortic catheters. Both variants were virulent in vivo; however, mean bacterial densities in vegetations were approximately 2.5 log10 CFU/g lower in animals challenged with the variants than in animals challenged with the parental strain, probably reflecting a slower in vivo growth rate.

Efficacy of ciprofloxacin in experimental aortic valve endocarditis caused by a multiply beta-lactam-resistant variant of Pseudomonas aeruginosa stably derepressed for beta-lactamase production

The emergence of multi-beta-lactam resistance is a limiting factor in treating invasive Pseudomonas infections with newer cephalosporins. The in vivo efficacy of ciprofloxacin, a new carboxy-quinolone, was evaluated in experimental aortic valve endocarditis caused by a strain of Pseudomonas aeruginosa which is stably derepressed for beta-lactamase production and is resistant to ceftazidime and multiple other beta-lactam agents. A total of 51 catheterized rabbits with aortic catheters in place were infected with this strain and then received no therapy (controls), ceftazidime (75 mg/kg per day), or ciprofloxacin (80 mg/kg per day). Ciprofloxacin sterilized all blood cultures and significantly lowered vegetation densities of P. aeruginosa by day 2 of treatment versus controls (P less than 0.0005) and animals receiving ceftazidime (P less than 0.0005). This beneficial effect of ciprofloxacin was also noted on therapy days 6 and 11. Ciprofloxacin rendered most vegetations (85%) culture negative over the 11-day treatment period and achieved bacteriologic cure in 73% of animals (P less than 0.0005 versus other therapy groups). Ciprofloxacin prevented bacteriologic relapse at 6 days posttherapy. No ciprofloxacin resistance was detected among Pseudomonas isolates from cardiac vegetations. Ciprofloxacin warrants further evaluation in vivo versus multi-drug-resistant gram-negative bacillary infections.

Efficacy of amikacin and ceftazidime in experimental aortic valve endocarditis due to Pseudomonas aeruginosa

The in vivo efficacies of amikacin, ceftazidime, and their combination were evaluated in experimental aortic valve endocarditis due to Pseudomonas aeruginosa. Eighty catheterized rabbits were infected with a P. aeruginosa strain susceptible to both amikacin and ceftazidime and then received no therapy (controls), amikacin (15 mg/kg per day), ceftazidime (100 mg/kg per day), or amikacin-ceftazidime. Amikacin-ceftazidime significantly lowered vegetation titers of P. aeruginosa at day 7 of therapy versus other regimens (P less than 0.0005). However, by day 14 of therapy, vegetation titers in animals receiving amikacin or ceftazidime regimens or both were not different from those of untreated controls; this was associated with in vivo development of amikacin resistance in most infected vegetations (79%), a phenomenon not seen at day 7 of therapy. Amikacin resistance was unstable in vivo, being undetectable in vegetations examined 5 days after treatment with amikacin had been completed. In contrast, ceftazidime resistance (first noted at day 7 of therapy in 12% of vegetations) persisted after termination of treatment with this agent. These in vivo observations on loss of amikacin resistance and persistence of ceftazidime resistance were mirrored during in vitro passage studies of amikacin- or ceftazidime-resistant P. aeruginosa strains isolated from cardiac vegetations. Amikacin resistance was no longer detectable by passage 5 in antibiotic-free media; however, ceftazidime resistance was stable despite 15 such passages. In vivo development of aminoglycoside-beta-lactam resistances was associated with poor bacteriologic efficacy in this model.