ANTIMICROBIAL AGENTS AND CHEMOTHERAPY, Sept. 1990, 0066-4804/90/091839-04$02.00/0 Copyright © 1990, American Society for Microbiology
p. 1839-1842
Vol. 34, No. 9
In Vitro Activity of Dirithromycin (LY 237216) Compared with Activities of Other Macrolide Antibiotics KWOK-WOON YUt AND HAROLD C. NEU* Departments of Medicine and Pharmacology, College of Physicians & Surgeons, Columbia University, 630 West 168th Street, New York, New York 10032 Received 30 April 1990/Accepted 12 June 1990 Dirithromycin inhibited Streptococcus pyogenes, Streptococcus pneumoniae, and other hemolytic streptococci at concentrations of s0.03 to 0.12 ,ug/ml, with 90% inhibition at 0.12 pg/ml, which is comparable to results using erythromycin. Group A streptococci, listeriae, and enterococci resistant to erythromycin were resistant to dirithromycin. Erythromycin-susceptible staphylococci were inhibited by 0.5 ,g/ml, but for erythromycinresistant isolates MICs were 28 ,ug/ml. For Haemophilus influenzae, MICs were .8 ,ug/ml, two- to fourfold greater than for erythromycin. The activity of dirithromycin against staphylococci and streptococci was not decreased by the addition of human serum.
Although macrolide antibiotics have been available since the 1950s, there has been great interest in the clinical use of erythromycin and in new compounds (5, 9). There have been attempts to synthesize macrolides with improved pharmacokinetics and tolerability, particularly by adults, and to look for agents which have increased antibacterial activity (5, 6). We wished to examine the in vitro activity of dirithromycin (LY 237216), a compound which is an oxazime derivative of erythromycin. Dirithromycin is metabolized in humans to erythromyclamine. This metabolite has an extremely long half-life of greater than 24 h (6), which would allow once daily administration of dirithromycin. Dirithromycin was a gift of Lilly Research Laboratories, Indianapolis, Ind.; clarithromycin and erythromycin were obtained from Abbott Laboratories, Abbott Park, Ill.; roxithromycin was obtained from Roussel UCLAF, Paris, France; and azithromycin was a gift from Pfizer Laboratories, Groton, Conn. Organisms used in this study were isolates obtained from patients seen at The Presbyterian Hospital in New York City in the last two years, except for methicillin-resistant Staphylococcus aureus isolates which came from multiple institutions throughout New York City. Only a single isolate from a patient was tested. MICs were determined by using Mueller-Hinton agar for staphylococci and aerobic gram-negative species. Activity against streptococcal species was determined with MuellerHinton agar supplemented with 5% sheep blood. Activity against Haemophilus and Neisseria species was determined with chocolatized Mueller-Hinton agar to which IsoVitaleX (BBL Microbiological Systems, Cockeysville, Md.) was added. Plates were incubated in 5% CO2. Activity against Bacteroides spp. and other anaerobic species was determined by using Mueller-Hinton agar supplemented with 5% laked sheep blood and vitamin K. An inoculum of 104 CFU was used for all agar tests according to the National Committee for Clinical Laboratory Standards guidelines (7). Incubation of all tests was at 35°C for 18 to 20 h, except for anaerobic organisms, which were incubated for 48 h. Broth * Corresponding author. t Present address: Department of Medicine, Veterans General Hospital, Taipei, Taiwan.
dilution was performed with an inoculum of 5 x 105 CFU/ml in a volume of 1 ml. MBCs were determined by subculturing 0.01-ml samples to antibiotic-free blood agar plates from tubes which did not show visible growth. The MBC was defined as the lowest concentration that produced 99.9% reduction in CFU from the control organisms according to the method of Pearson et al. (8). Controls of Staphylococcus aureus ATCC 29213 and Streptococcus (Enterococcus) faecalis ATCC 29212 were included in every run. The effect of the combination of dirithromycin and gentamicin was determined by an agar checkerboard method, and dirithromycin was combined at fixed concentrations of 0.5 and 4 ,ug/ml with cefotaxime, ceftazidime, and piperacillin, present in twofold increasing concentrations, and tested against aerobic gram-negative bacteria. The effect of human serum on the activity of dirithromycin was determined by using heat-inactivated normal human serum. Tubes (1 ml) contained 50% serum and medium. Todd-Hewitt broth was used for Streptococcus pyogenes and Streptococcus pneumoniae. Mueller-Hinton broth was used for Staphylococcus aureus. Dirithromycin was inactive, with an MIC of >32 ,ug/ml, against all of the gram-negative enteric species (10 isolates each) of Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Enterobacter cloacae, Citrobacter freundii, Shigella species, Salmonella species, Yersinia enterocolitica, Aeromonas spp., Serratia marcescens, Morganella morganii, Acinetobacter spp., and Pseudomonas aeruginosa. The activity of dirithromycin in comparison to the activities of other macrolides is shown in Table 1. Dirithromycin inhibited Streptococcus pyogenes at concentrations comparable with the concentrations of erythromycin and with the other macrolides. It did not inhibit five isolates of Streptococcus pyogenes resistant to erythromycin (data not shown). Dirithromycin had activity comparable to that of erythromycin against streptococci of groups B, C, and G and against Streptococcus pneumoniae. Dirithromycin usually was twofold less active than erythromycin against individual isolates of Enterococcus faecalis, and erythromycin-resistant E. faecalis isolates were not inhibited by dirithromycin. The six Enterococcus faecium isolates tested were resistant to both erythromycin and dirithromycin, with MICs of 16 to >32 ,ug/ml. Dirithromycin had activity against Listeria monocytogenes similar to the activities of the other macrolides. 1839
1840
NOTES
ANTIMICROB. AGENTS CHEMOTHER.
TABLE 1. Comparative in vitro activity of dirithromycin
Organism (no. of isolates)
MIC
Antibiotic
Range
(jg/ml)a 50%
90%
Streptococcus pyogenes (46)
Dirithromycin Erythromycin Roxithromycih Azithromycin Clarithromycin
p. 1839-1842
Vol. 34, No. 9
In Vitro Activity of Dirithromycin (LY 237216) Compared with Activities of Other Macrolide Antibiotics KWOK-WOON YUt AND HAROLD C. NEU* Departments of Medicine and Pharmacology, College of Physicians & Surgeons, Columbia University, 630 West 168th Street, New York, New York 10032 Received 30 April 1990/Accepted 12 June 1990 Dirithromycin inhibited Streptococcus pyogenes, Streptococcus pneumoniae, and other hemolytic streptococci at concentrations of s0.03 to 0.12 ,ug/ml, with 90% inhibition at 0.12 pg/ml, which is comparable to results using erythromycin. Group A streptococci, listeriae, and enterococci resistant to erythromycin were resistant to dirithromycin. Erythromycin-susceptible staphylococci were inhibited by 0.5 ,g/ml, but for erythromycinresistant isolates MICs were 28 ,ug/ml. For Haemophilus influenzae, MICs were .8 ,ug/ml, two- to fourfold greater than for erythromycin. The activity of dirithromycin against staphylococci and streptococci was not decreased by the addition of human serum.
Although macrolide antibiotics have been available since the 1950s, there has been great interest in the clinical use of erythromycin and in new compounds (5, 9). There have been attempts to synthesize macrolides with improved pharmacokinetics and tolerability, particularly by adults, and to look for agents which have increased antibacterial activity (5, 6). We wished to examine the in vitro activity of dirithromycin (LY 237216), a compound which is an oxazime derivative of erythromycin. Dirithromycin is metabolized in humans to erythromyclamine. This metabolite has an extremely long half-life of greater than 24 h (6), which would allow once daily administration of dirithromycin. Dirithromycin was a gift of Lilly Research Laboratories, Indianapolis, Ind.; clarithromycin and erythromycin were obtained from Abbott Laboratories, Abbott Park, Ill.; roxithromycin was obtained from Roussel UCLAF, Paris, France; and azithromycin was a gift from Pfizer Laboratories, Groton, Conn. Organisms used in this study were isolates obtained from patients seen at The Presbyterian Hospital in New York City in the last two years, except for methicillin-resistant Staphylococcus aureus isolates which came from multiple institutions throughout New York City. Only a single isolate from a patient was tested. MICs were determined by using Mueller-Hinton agar for staphylococci and aerobic gram-negative species. Activity against streptococcal species was determined with MuellerHinton agar supplemented with 5% sheep blood. Activity against Haemophilus and Neisseria species was determined with chocolatized Mueller-Hinton agar to which IsoVitaleX (BBL Microbiological Systems, Cockeysville, Md.) was added. Plates were incubated in 5% CO2. Activity against Bacteroides spp. and other anaerobic species was determined by using Mueller-Hinton agar supplemented with 5% laked sheep blood and vitamin K. An inoculum of 104 CFU was used for all agar tests according to the National Committee for Clinical Laboratory Standards guidelines (7). Incubation of all tests was at 35°C for 18 to 20 h, except for anaerobic organisms, which were incubated for 48 h. Broth * Corresponding author. t Present address: Department of Medicine, Veterans General Hospital, Taipei, Taiwan.
dilution was performed with an inoculum of 5 x 105 CFU/ml in a volume of 1 ml. MBCs were determined by subculturing 0.01-ml samples to antibiotic-free blood agar plates from tubes which did not show visible growth. The MBC was defined as the lowest concentration that produced 99.9% reduction in CFU from the control organisms according to the method of Pearson et al. (8). Controls of Staphylococcus aureus ATCC 29213 and Streptococcus (Enterococcus) faecalis ATCC 29212 were included in every run. The effect of the combination of dirithromycin and gentamicin was determined by an agar checkerboard method, and dirithromycin was combined at fixed concentrations of 0.5 and 4 ,ug/ml with cefotaxime, ceftazidime, and piperacillin, present in twofold increasing concentrations, and tested against aerobic gram-negative bacteria. The effect of human serum on the activity of dirithromycin was determined by using heat-inactivated normal human serum. Tubes (1 ml) contained 50% serum and medium. Todd-Hewitt broth was used for Streptococcus pyogenes and Streptococcus pneumoniae. Mueller-Hinton broth was used for Staphylococcus aureus. Dirithromycin was inactive, with an MIC of >32 ,ug/ml, against all of the gram-negative enteric species (10 isolates each) of Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Enterobacter cloacae, Citrobacter freundii, Shigella species, Salmonella species, Yersinia enterocolitica, Aeromonas spp., Serratia marcescens, Morganella morganii, Acinetobacter spp., and Pseudomonas aeruginosa. The activity of dirithromycin in comparison to the activities of other macrolides is shown in Table 1. Dirithromycin inhibited Streptococcus pyogenes at concentrations comparable with the concentrations of erythromycin and with the other macrolides. It did not inhibit five isolates of Streptococcus pyogenes resistant to erythromycin (data not shown). Dirithromycin had activity comparable to that of erythromycin against streptococci of groups B, C, and G and against Streptococcus pneumoniae. Dirithromycin usually was twofold less active than erythromycin against individual isolates of Enterococcus faecalis, and erythromycin-resistant E. faecalis isolates were not inhibited by dirithromycin. The six Enterococcus faecium isolates tested were resistant to both erythromycin and dirithromycin, with MICs of 16 to >32 ,ug/ml. Dirithromycin had activity against Listeria monocytogenes similar to the activities of the other macrolides. 1839
1840
NOTES
ANTIMICROB. AGENTS CHEMOTHER.
TABLE 1. Comparative in vitro activity of dirithromycin
Organism (no. of isolates)
MIC
Antibiotic
Range
(jg/ml)a 50%
90%
Streptococcus pyogenes (46)
Dirithromycin Erythromycin Roxithromycih Azithromycin Clarithromycin
