Bactericidal activity of quinolones against Streptococcus pneumoniae by time-kill methodology

The increase in Streptococcus pneumoniae strains resistant to the drugs traditionally used in treatment has made the search for alternative agents necessary. We studied the bactericidal activity of ciprofloxacin, ofloxacin, levofloxacin, sparfloxacin, and trovafloxacin against six penicillin-resistant Streptococcus pneumoniae strains, using time-kill methodology. Our results indicate that trovafloxacin and sparfloxacin had greater bactericidal activity than ciprofloxacin, ofloxacin, and levofloxacin against all the strains tested, since these two quinolones showed bactericidal activity against all six strains at concentrations of not more than 4 mg/l. Ciprofloxacin and levofloxacin did not have bactericidal activity, in the range of concentrations of antibiotics used, against any of the strains studied; when such activity did exist, the concentration of antibiotic used was higher. Received: June 29, 1998 / Accepted: December 3, 1998     

Bactericidal activities of DU-6859a and clinafloxacin (CI-960) against staphylococci.

Time killing curves were calculated at concentrations of 2 and 8 times the MICs of DU-6859a and clinafloxacin against six isolates of Staphylococcus aureus. Both quinolones produced a decrease in the log10 CFU per milliliter of > or = 3 within 3 h at 2 and 8 times the MIC for the ciprofloxacin-susceptible isolates and at 8 times the MIC for the ciprofloxacin-resistant isolates; however, only 8 times the MIC of DU-6859a consistently prevented regrowth of all isolates after 24 h of incubation.     

Synergic activity, for anaerobes, of trovafloxacin with clindamycin or metronidazole: chequerboard and time-kill methods

Chequerboard titrations were used to test the activity of trovafloxacin, alone and in combination with clindamycin or metronidazole, against 156 Gram-positive or Gram-negative anaerobes, including 47 Bacteroides fragilis group, 36 Prevotella spp., 26 fusobacteria, 21 peptostreptococci and 26 clostridia. MIC50/MIC90 values (mg/L) of each drug alone against all 156 strains were: trovafloxacin, 0.5/1; clindamycin, 0.25/2; metronidazole, 1/2. Synergy (FIC indices 0. 5-2.0); no antagonism (FIC indices >4.0) was seen. In addition, synergy was tested by time-kill methodology for each of the above combinations against 12 Gram-positive or Gram-negative strains. Results indicated that synergy (defined as a >/= 2 log(10) decrease in cfu/mL at 48 h compared with the more active drug alone) was found between trovafloxacin at or below the MIC and both clindamycin and metronidazole at or below the MIC in one strain each of Bacteroides fragilis, Bacteroides thetaiotaomicron, Prevotella intermedia, Fusobacterium varium, Peptostreptococcus asaccharolyticus and Clostridium bifermentans. Synergy between trovafloxacin (相似文献   

Antianaerobic activity of the ketolide RU 64004 compared to activities of four macrolides, five beta-lactams, clindamycin, and metronidazole.

Agar dilution methodology (with added Oxyrase in the case of the macrolide group to allow incubation without added CO2) was used to compare the activity of RU 64004, a new ketolide, with the activities of erythromycin, azithromycin, clarithromycin, roxithromycin, clindamycin, amoxicillin with and without clavulanate, piperacillin with and without tazobactam, metronidazole, and imipenem against 379 anaerobes. Overall, RU 64004 yielded an MIC at which 50% of the isolates are inhibited (MIC50) of 1.0 microg/ml and an MIC90 of 16.0 microg/ml. In comparison, MIC50s and MIC90s of erythromycin, azithromycin, clarithromycin, and roxithromycin were 2.0 to 8.0 and >64.0 microg/ml, respectively. MICs of macrolides, including RU 64004, were higher for Bacteroides ovatus, Fusobacterium varium, Fusobacterium mortiferum, and Clostridium difficile than for the other species. RU 64004 was more active against gram-positive rods and cocci, Prevotella and Porphyromonas spp., and fusobacteria other than F. mortiferum and F. varium than against the Bacteroides fragilis group. Overall MIC50s and MIC90s (in micrograms per milliliter), respectively, of other compounds were as follows: clindamycin, 1.0 and 16.0; amoxicillin, 4.0 and 64.0; amoxicillin-clavulanate, 0.5 and 4.0; piperacillin, 8.0 and >64.0; piperacillin-tazobactam, 1.0 and 16.0; metronidazole, 1.0 and 4.0; and imipenem, 0.25 and 1.0.     

In vitro activity of DU-6859a against anaerobic bacteria.

The activity of a new quinolone agent, DU-6859a, against 330 strains of anaerobic bacteria was determined by using the National Committee for Clinical Laboratory Standards-approved Wadsworth brucella laked blood agar method; the activity of DU-6859a was compared with those of amoxicillin-clavulanate (2:1), chloramphenicol, ciprofloxacin, clindamycin, fleroxacin, imipenem, lomefloxacin, metronidazole, sparfloxacin, and temafloxacin. DU-6859a and chloramphenicol inhibited all of the isolates at concentrations of 1 and 16 micrograms/ml, respectively; amoxicillin-clavulanate, imipenem, and metronidazole inhibited > or = 94% of the isolates at their respective breakpoints (8, 8, and 16 micrograms/ml). MICs of DU-6859a at which 90% of the strains were susceptible were 1 to 5 twofold dilutions lower than those of the other quinolones for every group of organisms. MICs of DU-6859a at which 90% of the strains were susceptible (total numbers of strains tested are in parentheses) were < or = 0.25 micrograms/ml for Bacteroides fragilis (57), other B. fragilis group species (84), Bilophila wadsworthia (15), Clostridium species (27) (including C. difficile, C. perfringens, and C. ramosum), Fusobacterium nucleatum (16), Fusobacterium mortiferum-F. varium group species (10), Peptostreptococcus species (20), non-spore-forming gram-positive rods (20), and Prevotella species (25).     

Antimicrobial activity of DU-6859, a new potent fluoroquinolone, against clinical isolates.

DU-6859, (-)-7-[(7S)-amino-5-azaspiro(2,4)heptan-5-yl]-8-chloro-6- fluoro-1-[(1R,2R)-cis-2-fluoro-1-cyclopropyl]-1,4-dihydro-4-oxoquinol one-3- carboxylic acid, is a new fluoroquinolone with antibacterial activity which is significantly better than those of currently available quinolones. The MICs for 90% of methicillin-susceptible and -resistant Staphylococcus aureus and Staphylococcus epidermidis clinical isolates (MIC90s) were 0.1, 3.13, 0.1, and 0.39 microgram/ml, respectively. MIC50s of DU-6859 against quinolone-resistant, methicillin-resistant S. aureus were 8-, 32-, 64-, and 128-fold lower than those of tosufloxacin and sparfloxacin, ofloxacin and fleroxacin, ciprofloxacin, and lomefloxacin, respectively. DU-6859 inhibited the growth of all strains of Streptococcus pneumoniae and Streptococcus pyogenes at 0.1 and 0.2 microgram/ml, respectively, and was more active against enterococci than the other quinolones tested. Although the activity of DU-6859 against Pseudomonas aeruginosa was roughly comparable to that of ciprofloxacin at the MIC50 level, it was fourfold more active than ciprofloxacin at the MIC90 level. DU-6859 was also more active against other glucose-nonfermenting bacteria, Haemophilus influenzae, Moraxella catarrhalis, and Neisseria gonorrhoeae, than the other drugs tested. Strains of Bacteroides fragilis and Peptostreptococcus spp. were susceptible to DU-6859; MIC90s were 0.39 and 0.2 microgram/ml, respectively. DU-6859 generally showed activities twofold or greater than those of ciprofloxacin and the other drugs against almost all members of the family Enterobacteriaceae. The action of DU-6859 against the clinical isolates was bactericidal at concentrations near the MICs. DU-6859 activity was not affected by different media, pH, inoculum size, or human serum but was decreased in human urine.     

Antipneumococcal activity of BAY 12-8039, a new quinolone, compared with activities of three other quinolones and four oral beta-lactams.

Activities of BAY 12-8039 against 205 pneumococci were tested by agar dilution. MICs (in micrograms per milliliter) at which 50 and 90% of the isolates are inhibited (MIC50s and MIC90s, respectively) were 0.125 and 0.25 (BAY 12-8039), 2.0 and 4.0 (ciprofloxacin and ofloxacin), and 0.25 and 0.5 (sparfloxacin). Beta-lactam MIC50s and MIC90s for penicillin-susceptible, -intermediate, and -resistant strains, in that order, were 0.016 and 0.03, 0.25 and 2.0, and 2.0 and 4.0 (amoxicillin); 0.03 and 0.06, 0.25 and 4.0, and 4.0 and 8.0 (ampicillin); 0.03 and 0.06, 0.5 and 4.0, and 4.0 and 8.0 (cefuroxime); and 0.03 and 0.125, 0.25 and 2.0, and 4.0 and 8.0 (cefpodoxime). At two times their MICs after 24 h, BAY 12-8039, ciprofloxacin, ampicillin, and cefuroxime were uniformly bactericidal (99.9% killing) against 12 strains; other compounds were bactericidal at four times their MICs.     

MIC and time-kill study of activities of DU-6859a, ciprofloxacin, levofloxacin, sparfloxacin, cefotaxime, imipenem, and vancomycin against nine penicillin-susceptible and -resistant pneumococci.

MIC and time-kill methods were used to test the activities of DU-6859a, ciprofloxacin, levofloxacin, sparfloxacin, cefotaxime, imipenem, and vancomycin against nine penicillin-susceptible, -intermediate, and -resistant pneumococci. The MIC of penicillin for penicillin-susceptible strains was 0.016 micrograms/ml, those for intermediate strains were 0.25 to 1.0 microgram/ml, and those for resistant strains were 2.0 to 4.0 micrograms/ml. Of the four quinolones tested, DU-6859a had the lowest MIC (0.064 micrograms/ml), followed by sparfloxacin (0.25 to 0.5 micrograms/ml) and levofloxacin and ciprofloxacin (both 1.0 to 4.0 micrograms/ml). Vancomycin inhibited all strains at MICs of 0.25 to 0.5 micrograms/ml. The MICs of imipenem and cefotaxime for penicillin-susceptible, -intermediate, and -resistant strains were 0.004 to 0.008, 0.008 to 0.032, and 0.25 micrograms/ml and 0.016, 0.125 to 0.5, and 2.0 micrograms/ml, respectively. DU-6859a was bactericidal at eight times the MICs (0.5 micrograms/ml) for seven of the nine strains after 4 h and bactericidal for all nine strains after 6 h at eight times the MICs and after 12 h at two times the MICs. By comparison, sparfloxacin, the next most active quinolone, was uniformly bactericidal at two times the MICs only after 24 h, with little activity after 2 h. Levofloxacin and ciprofloxacin were bactericidal against all strains after 12 h at eight times the MICs and against all strains at 24 h at four times the MICs. Imipenem was bactericidal against all strains, at concentrations exceeding the MICs, after 24 h. Cefotaxime was also uniformly bactericidal only after 24 h of incubation at two times the MICs. Vancomycin, despite having uniformly low MICs for all strains irrespective of their penicillin susceptibility, was uniformly bactericidal only at two times the MICs after 24 h.     

Comparative serum bactericidal activity against test anaerobes in volunteers receiving imipenem, clindamycin, latamoxef and metronidazole

Ten healthy volunteers received on separate days the following regimens: imipenem 500 mg, clindamycin 600 mg, latamoxef 1 g, and metronidazole 500 mg. The antibiotics were given intravenously as an infusion over 15 min. Blood samples were obtained before and 30 min, 1 and 6 h after the start of the infusion. Serum bacteriostatic and bactericidal activities were measured against the following strains of strict anaerobes: two strains of Bacteroides fragilis, one strain each of B. vulgatus, B. thetaiotaomicron, B. oralis, Fusobacterium symbiosum, Eubacterium lentum, Clostridium perfringens, and Peptostreptococcus magnus. Sera from patients receiving clindamycin showed the highest inhibitory and bactericidal activities except against B. thetaiotaomicron and F. symbiosum. Imipenem had similar inhibitory and bactericidal activity to that shown by latamoxef. Metronidazole had a moderate activity against all strains although the activity persisted for 6 h. Latamoxef was the most active antibiotic against the test strain of C. perfringens.     

In vitro activity of DU-6859a, a new fluorocyclopropyl quinolone.

DU-6859a was tested against 844 recent clinical isolates (most from bacteremias) by using reference MIC determination procedures. The activity of DU-6859a against members of the family Enterobacteriaceae was comparable to that of ciprofloxacin (range of MICs for 90% of isolates [MIC90], < or = 0.015 to 1 microgram/ml), and the highest MICs were observed among Serratia marcescens and Providencia rettgeri isolates. The DU-6859a MIC90 for Pseudomonas aeruginosa and Xanthomonas maltophilia was 0.5 microgram/ml. Pneumococci (MIC90, 0.06 microgram/ml), Haemophilus influenzae (MIC90, < or = 0.004 microgram/ml), Moraxella catarrhalis (MIC90, < or = 0.015 microgram/ml), and pathogenic neisseriae (MIC90, 0.015 to 0.03 microgram/ml) were very susceptible to DU-6859a. All staphylococci had DU-6859a MICs of < or = 1 microgram/ml, including oxacillin- and ciprofloxacin-resistant strains. DU-6859a was very active against isolates resistant to ceftazidime (MIC90, < or = 0.12 microgram/ml), ciprofloxacin (MIC90, < or = 8 micrograms/ml), and gentamicin (MIC90, < or = 1 microgram/ml).     

Pharmacodynamics of metronidazole determined by a time-kill assay for Trichomonas vaginalis.

The pharmacodynamic effects of metronidazole on Trichomonas vaginalis have been poorly characterized. The present in vitro study was performed to characterize the relationship between killing of trichomonads and metronidazole exposure (metronidazole concentration and time of exposure). Five laboratory strains and five recent clinical isolates of T. vaginalis were studied. The minimum lethal concentrations (MLCs) of metronidazole for the strains ranged from 0.8 to 25 micrograms/ml under anaerobic conditions. Metronidazole exhibited concentration-dependent killing against T. vaginalis at concentrations ranging from 0.1 to > 10 times the MLC. The endpoint measurement, the kill rate constant, which was derived from the reduction in the logarithm of the colony count divided by exposure time, compared with the kill rate constant for the growth control was not affected by the time of assessment between 2 and 24 h. The kill rate constant-versus-metronidazole exposure curves were similar when concentration was expressed as a multiple of the MLC. There were no apparent differences between the clinical isolates and laboratory strains. These data suggest that peak metronidazole concentration and/or area under the plasma concentration-versus-time curve are the important pharmacodynamic parameters to be optimized.     

In vitro antibacterial activity of DU-6859a, a new fluoroquinolone.

The in vitro antibacterial activity of DU-6859a, a new fluoroquinolone, against a wide variety of clinical isolates was evaluated and compared with those of tosufloxacin, ofloxacin, ciprofloxacin, and sparfloxacin. DU-6859a showed potent broad-spectrum activity against gram-positive, gram-negative, and anaerobic bacteria, and its activity was greater than those of the control quinolones. By comparison of MICs at which 90% of strains are inhibited, DU-6859a had potent activity against bacteria resistant to the control quinolones. The time-killing curves of quinolones showed that the number of viable cells decreased rapidly during 2 to 4 of incubation, and regrowth was not seen even after 8 h incubation. At a concentration of four times the MIC, the frequencies of appearance of spontaneous mutants of Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa resistant to DU-6859a were < or = 4.0 x 10(-9) to 1.9 x 10(-8). The 50% inhibitory concentrations of DU-6859a were 0.86 and 1.05 micrograms/ml for the supercoiling activities of DNA gyrases isolated from E. coli and P. aeruginosa, respectively. The rank order of the 50% inhibitory concentrations observed for both DNA gyrases roughly paralleled the MICs.     

Pharmacokinetics and serum bactericidal activities of quinolones in combination with clindamycin, metronidazole, and ornidazole.

To enhance the antimicrobial spectrum of the quinolones against anaerobic organisms and gram-positive bacteria, we investigated in two studies the parenteral combinations of ciprofloxacin (200 mg) and ofloxacin (200 mg) with metronidazole (500 mg) or clindamycin (600 mg) and the oral combinations of enoxacin (400 mg) and fleroxacin (400 mg) with metronidazole (400 mg), clindamycin (300 mg), or ornidazole (500 mg) (only with fleroxacin). The pharmacokinetics and serum bactericidal activities (SBAs) against 5 aerobic and 2 anaerobic species (total, 58 strains) were determined in two groups of 10 healthy volunteers by using a randomized crossover study design. The additions of metronidazole, clindamycin, and ornidazole did not affect the pharmacokinetics of the quinolones. The combination of clindamycin with ciprofloxacin, ofloxacin, and, to a lesser extent, fleroxacin resulted in an increase of the SBA against gram-positive strains (mean peak titers): Staphylococcus aureus, ciprofloxacin alone, 1:5.5; ciprofloxacin-clindamycin, 1:19.9; ofloxacin alone, 1:3.6; ofloxacin-clindamycin, 1:17.5; fleroxacin alone, 1:4.3; fleroxacin-clindamycin, 1:8.1; Streptococcus pneumoniae (fleroxacin and enoxacin were not tested), ciprofloxacin alone, 1:2.0; ciprofloxacin-clindamycin, 1:53; ofloxacin alone, 1:2.6; and ofloxacin-clindamycin, 1:49.2. The high SBA of quinolones against gram-negative bacteria was not affected by the combinations; however, relatively low activities against Pseudomonas aeruginosa were detected. In general, against anaerobic bacteria, low bactericidal activities were determined in both studies (mean peak titers ranged from 1:2.1 to 1:3.1; mean trough titers range from 1:2.0 to 1:2.9). In clinical settings with severe mixed infections, a parenteral therapy consisting of modern quinolones together with clindamycin or imidazole derivatives seems to be active and offers no obvious interactions.     

Comparative in vitro activity of DU-6859a, a new fluoroquinolone agent, against gram-positive cocci.

The in vitro activity of DU-6859a (DU), a new fluoroquinolone agent, was evaluated against 233 gram-positive cocci and was compared with those of ciprofloxacin, vancomycin, nafcillin, and ampicillin. The MICs of DU for 90% of the staphylococci tested were < or = 0.06 microgram/ml. All of the groups A and B and viridans group streptococci were inhibited by < or = 0.125 microgram of DU per ml, which was 32-fold more active than ciprofloxacin. On the basis of MICs for 90% of the strains tested, DU was 32- and 16-fold more active than ciprofloxacin against Enterococcus faecalis and Enterococcus faecium, respectively. The bactericidal activity of DU was demonstrated by time-kill techniques against all ciprofloxacin-susceptible enterococci. DU shows promise for the treatment of infections with gram-positive cocci and warrants further evaluation by in vitro and in vivo studies.     

Bactericidal activity of bacteriophage endolysin HY-133 against Staphylococcus aureus in comparison to other antibiotics as determined by minimum bactericidal concentrations and time-kill analysis

Methicillin-resistant Staphylococcus aureus (MRSA) decolonization is expensive and time consuming, and new agents are necessary due to increasing resistance rates. The administration of bacteriophages or particularly their endolysins may offer an alternative treatment strategy and could provide a solution to overcome the selection pressure due to classical antibiotics. Here, the bactericidal activity was characterized for the recombinant chimeric bacteriophage endolysin HY-133 in comparison to other antimicrobials. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) were determined for 2 reference strains, 24 clinical MRSA and methicillin-susceptible S. aureus (MSSA) isolates, as well as 6 isolates with high-level mupirocin resistance. Additionally, HY-133 activity against bacteria in stationary or exponential growth phase was compared in 12 isolates. Time-kill curves were performed with 2 representative isolates to investigate the pharmacodynamics until 48-h incubation time. All experiments were performed in comparison to daptomycin and mupirocin. The MIC_50/90 and MBC_50/90 values were in the range 0.12–0.5?mg/L for all 3 growth conditions comparable to daptomycin with 0.5/0.5?mg/L, respectively. The MBC was almost always equal the MIC and without considerable differences between MSSA and MRSA. Time-kill curves revealed a rapid bactericidal effect of HY-133 within the first 2 h, similar to daptomycin. Even with low concentrations, the recombinant endolysin HY-133 was highly active against all tested MSSA and MRSA isolates including mupirocin-resistant isolates. The application of this alternative agent may offer a future strategy for MRSA/MSSA decolonization and, potentially, for treatment purposes.     

In vitro and in vivo antimycobacterial activities of a new quinolone, DU-6859a.

A new fluoroquinolone, DU-6859a, was studied for its in vitro and in vivo antimycobacterial activities. MIC determination by the agar dilution method with 7H11 medium revealed that DU-6859a had MICs at which 90% of M. kansasii (0.78 microgram/ml), M. marinum (1.56 micrograms/ml), M. scrofulaceum (1.56 micrograms/ml), M. fortuitum (0.39 microgram/ml), M. chelonae subsp. abscessus (6.25 micrograms/ml), and M. chelonae subsp. chelonae (1.56 micrograms/ml) were inhibited were 4 to 32 times lower than those of ofloxacin and sparfloxacin. The MICs of DU-6859a at which 90% of M. tuberculosis (0.2 microgram/ml) and M. avium-M. intracellulare complex (12.5 micrograms/ml each) were inhibited were comparable to those of sparfloxacin but were four- to eightfold lower than those of ofloxacin. Thus, DU-6859a possessed more potent in vitro activity than sparfloxacin and ofloxacin against most mycobacterial species. DU-6859a exerted significant efficacy against infections caused by M. intracellulare and M. chelonae subsp. abscessus induced in mice when it was given at a dose of 1 mg per mouse (ca. 50 mg/kg of body weight) in terms of reducing the frequency of occurrence and the degree of gross pulmonary or renal lesions and bacterial loads in the lungs, spleens, or kidneys. The efficacy of DU-6859a was greater than that of ofloxacin and was more pronounced against M. chelonae infections than against M. intracellulare infections.     

Bactericidal activity of quinupristin-dalfopristin against Staphylococcus aureus: clindamycin susceptibility as a surrogate indicator

Of 516 Staphylococcus aureus strains tested, 97.1% were susceptible to quinupristin-dalfopristin, which was bactericidal for 22 (56%) of the 39 strains tested, comparable to vancomycin. All 17 clindamycin and macrolide-resistant strains were inhibited but not killed by quinupristin-dalfopristin, whereas all 22 clindamycin-susceptible strains (5 were macrolide resistant) were killed.     

Bactericidal activity of deptomycin (LY146032) compared with those of ciprofloxacin, vancomycin, and ampicillin against enterococci as determined by kill-kinetic studies.

This study used kill-kinetic methods to provide data on the bactericidal activity of subinhibitory (1/2 X MIC), inhibitory (1 x MIC), and suprainhibitory (4X, 6X, and 8X MIC) concentrations of deptomycin (LY146032) against strains of enterococci compared with those of ciprofloxacin, vancomycin, and ampicillin. Deptomycin was the most active agent tested, as determined by broth microdilution methods, with all strains being inhibited at concentrations less than or equal to 2 micrograms/ml. The kill-kinetic demonstrated that deptomycin had greater activity at all concentrations tested than the other cell wall-active agents; regrowth was seen, however, at lower concentrations. At higher concentrations (6X and 8X MIC), all agents tested demonstrated the same or less bactericidal activity than at 4X MIC, presumably due to the Eagle effect. Nevertheless, these results suggest that further evaluation of deptomycin as a therapeutic agent for serious enterococcal infections is warranted.     

Comparative in vitro activities of DU-6859a, levofloxacin, ofloxacin, sparfloxacin, and ciprofloxacin against 387 aerobic and anaerobic bite wound isolates.

The activities of DU-6859a, levofloxacin, ofloxacin, sparfloxacin, and ciprofloxacin against bite wound isolates were determined by the agar dilution method. DU-6859a was the most active compound (MICs, < or = 0.125 microg/ml) against all Pasteurella species, Staphylococcus aureus, and streptococci; anaerobes were susceptible to < or = 0.5 microg/ml, except fusobacteria, which were susceptible to < or = 2 microg/ml. Against aerobes, levofloxacin was more active than ofloxacin (MIC at which 90% of isolates are inhibited [MIC90], < or = 1.0 microg/ml for both) and sparfloxacin and ciprofloxacin were also active (MIC90s, < or = 0.25 and < 1 microg/ml, respectively).     

In vitro and in vivo antifungal activities of DU-6859a, a fluoroquinolone, in combination with amphotericin B and fluconazole against pathogenic fungi.

DU-6859a is an investigational fluoroquinolone agent with potent bactericidal activity, but by itself it has no antifungal activity. When combined with amphotericin B (AmB), however, DU-6859a clearly enhanced the in vitro antifungal activity of AmB against Candida albicans, Candida tropicalis, Candida krusei, Candida glabrata, and Cryptococcus neoformans in microdilution checkerboard studies. Positive interactions of DU-6859a with AmB against Aspergillus fumigatus were dependent on the medium used; yeast nitrogen base supplemented with amino acids, ammonium sulfate, and 1% glucose was better for demonstrating synergism, while in RPMI 1640 medium, unexpected antagonism between the drugs occurred against three of the strains tested. In combination with fluconazole (Flu), DU-6859a increased the activity of Flu against C. albicans both in synthetic amino acid medium fungal and in supplemented yeast nitrogen base. An in vitro time-kill study revealed that DU-6859a combined with AmB significantly suppressed the regrowth of C. albicans compared with the suppression brought about by AmB used alone in a concentration-dependent fashion. Furthermore, in a model of C. albicans infection in mice, the fungal load in infected kidneys was significantly less in mice given the combination treatment of DU-6859a plus either AmB or Flu, and thus, the combination treatment resulted in prolonged survival of infected mice compared with treatment with either antifungal alone. The prolonged survival in mice given the combined treatment was also observed in mice with A. fumigatus infection, indicating that DU-6859a potentiated the actions of the antifungal agents in vivo as well as in vitro.