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Antimicrobial Agents and Chemotherapy, July 2000, p. 1894-1899, Vol. 44, No. 7
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Antipneumococcal Activity of ABT-773 Compared to
Those of 10 Other Agents
Todd A.
Davies,1
Lois M.
Ednie,1
Dianne M.
Hoellman,1
Glenn A.
Pankuch,1
Michael R.
Jacobs,2 and
Peter C.
Appelbaum1,*
Department of Pathology, Hershey Medical Center, Hershey,
Pennsylvania 17033,1 and Department of
Pathology, Case Western Reserve University, Cleveland, Ohio
441062
Received 9 March 2000/Returned for modification 11 April
2000/Accepted 27 April 2000
 |
ABSTRACT |
MICs, time-kills, and postantibiotic effects (PAEs) of ABT-773 (a
new ketolide) and 10 other agents were determined against 226 pneumococci. Against 78 ermB- and 44 mefE-containing strains, ABT-773 MICs at which 50% of the
isolates tested were inhibited (MIC50s) and
MIC90s were 0.016 to 0.03 and 0.125 µg/ml, respectively. Clindamycin was active only against macrolide-resistant strains containing mefE (MIC50, 0.06 µg/ml;
MIC90, 0.125 µg/ml). Activities of pristinamycin
(MIC90, 0.5 µg/ml) and vancomycin (MIC90,
0.25 µg/ml) were unaffected by macrolide or penicillin resistance, while
-lactam MICs rose with those of penicillin G. Against 19 strains with L4 ribosomal protein mutations and two strains with mutations in domain V of 23S rRNA, ABT-773 MICs were 0.03 to 0.25 µg/ml, while macrolide and azalide MICs were all
16.0 µg/ml. ABT-773 was bactericidal at twice the MIC after 24 h for 8 of 12 strains (including three strains with erythromycin MICs greater than or
equal to 64.0 µg/ml). Kill kinetics of erythromycin, azithromycin, clarithromycin, and roxithromycin against macrolide-susceptible strains
were slower than those of ABT-773. ABT-773 had longer PAEs than
macrolides, azithromycin, clindamycin, or
-lactams, including
against ermB-containing strains. ABT-773, therefore, shows
promising in vitro activity against macrolide-susceptible as well as
-resistant pneumococci.
 |
INTRODUCTION |
The incidence of pneumococci
resistance to penicillin G and other
-lactam and non-
-lactam
compounds has increased worldwide at an alarming rate, including in the
United States. Major foci of infections currently include South Africa,
Spain, Central and Eastern Europe, and parts of Asia (1, 8, 9,
13). The problem is exacerbated by the tendency of these strains
to spread from country to country and from continent to continent
(18, 19). In the United States, a recent survey has shown an
increase in resistance to penicillin from <5% before 1989 (including
<0.02% of isolates for which the MIC of penicillin is
2.0 µg/ml)
to 6.6% in 1991 to 1992 (with penicillin MICs
2.0 µg/ml for 1.3% of isolates) (3). In another, more recent, survey, 23.6%
(360) of 1,527 clinically significant pneumococcal isolates were not susceptible to penicillin (6). It is also important to note the high rates of isolation of penicillin-intermediate and -resistant pneumococci (approximately 30%) in middle ear fluids from patients with refractory otitis media, compared to pneumococci isolated from
other sites (2).
Pneumococcal strains with intermediate and, especially, full resistance
to penicillin G are often resistant to erythromycin (7, 12,
15). In the United States, Breiman and coworkers in 1991 to 1992 demonstrated erythromycin resistance rates of 3.7 and 2.2% in patients
1 to 2 and
4 years of age, respectively (3). A recent
study by Doern and coworkers (6) has documented erythromycin
resistance rates of 19 to 20% and 49% in penicillin-intermediate and
-resistant strains, respectively (6). In Europe,
erythromycin resistance rates are generally higher. For example, 27.5%
of all pneumococci studied in France in 1992 (63% of
penicillin-resistant strains) were erythromycin resistant
(11).
Macrolide resistance in pneumococci is predominantly mediated by two
mechanisms: (i) strains containing the ermB gene, coding for
a ribosomal methylase, are resistant to 14-membered macrolides such as
erythromycin, clarithromycin, and roxithromycin; 15-membered azalides
such as azithromycin; and 16-membered macrolides such as josamycin and
spiramycin, as well as the lincosamide clindamycin; (ii) strains
containing the mefE gene, coding for an efflux pump, are
resistant to 14-membered macrolides and azalides but are susceptible to
16-membered macrolides as well as clindamycin. Although clarithromycin generally yields MICs for pneumococci which are one or two dilutions lower than those of other macrolides, erythromycin-resistant
pneumococci are resistant in vitro to all other existing macrolides
(7, 31). However, recent work indicates that achievable
levels of clarithromycin may be achievable in
mefE-containing strains, in which macrolide MICs are lower
(usually 1.0 to 16.0 µg/ml) than is the case with
ermB-containing strains (
64.0 µg/ml) (D. Shortridge, G. Doern, J. Beyer, A. Brueggemann, and R. K. Flamm,
Abstr. 36th Infect. Dis. Soc. Am. Annu. Meet., abstr. 225F, 1998).
Up to now, the only members of the macrolide lincosamide streptogramin
group which is consistently active against all pneumococci, irrespective of their penicillin- or erythromycin-susceptibility status, have been pristinamycin and quinupristin/dalfopristin, two
parenteral streptogramins, and the ketolide group. HMR 3647 (telithromycin), the first ketolide to be developed, has been shown to
be very active against pneumococci irrespective of their macrolide or
penicillin susceptibility status (22, 25-27).
ABT-773 is a recently developed ketolide (4) (A. M. Nilius, M. Bui, L. Almer, D. Hensey, J. Beyer, Z. Ma, Y. S. Orr,
and R. Flamm, Abstr. 9th Eur. Congr. Clin. Microbiol. Infect. Dis., abstr. P-177, 1999; Z. Ma, R. F. Clark, and Y. Or, Abstr. 39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 2133, 1999; Z. Cao, R. Hammond, S. Pratt, A. Saiki, C. Lerner, and P. Zhong, Abstr.
39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 2135, 1999).
The present study examined (i) the susceptibility by agar dilution of
226 penicillin- and erythromycin-susceptible and -resistant pneumococci
to ABT-773, compared to erythromycin, azithromycin, clarithromycin,
roxithromycin, clindamycin, pristinamycin, amoxicillin, ceftriaxone,
imipenem, and vancomycin; (ii) the activity of the above compounds
against six erythromycin-susceptible and six erythromycin-resistant pneumococci by time-kill methodology; and (iii) the postantibiotic effect of the above drugs against eight pneumococcal strains, including
three ermB-containing strains.
 |
MATERIALS AND METHODS |
Bacteria.
For agar dilution MICs, pneumococci comprised 83 macrolide-susceptible (MIC,
0.25 µg/ml) and 143 macrolide-resistant
(MIC,
0.5 µg/ml) strains. Of these, 55 were penicillin susceptible, 85 were penicillin intermediate, and 86 were penicillin resistant. Of
the macrolide-resistant strains (which included intermediate strains by
NCCLS classification), 75 carried the ermB and 44 carried the mefE gene; three strains carried both erm and
mef in spite of repeated subcultures in an attempt to
separate the two genes from a possible heterogenous population (these
were included in the erm-containing group for data analysis,
because they exhibited the Erm phenotype). In addition, 19 strains with
the L4 ribosomal protein mutation and two with mutations in domain V of
the 23S rRNA (A. Tait-Kamradt, T. Davies, M. Jacobs, P. Appelbaum, and J. Sutcliffe, Abstr. 39th Intersci. Conf. Antimicrob. Agents
Chemother., abstr. 842, 1999) were tested to determine the MIC of
macrolides. For time-kill studies, six macrolide-susceptible and six
macrolide-resistant strains (three carrying erm, three
carrying mef) were tested, while for postantibiotic effect
(PAE) studies, five macrolide-susceptible and three macrolide-resistant
strains were studied.
Antimicrobials and MIC testing.
ABT-773 powder was obtained
from Abbott Laboratories (Abbot Park, Ill.); other antimicrobials were
obtained from their respective manufacturers. Agar dilution methodology
was performed on 226 strains (12, 13) by using
Mueller-Hinton agar (BBL Microbiology Systems, Cockeysville, Md.)
supplemented with 5% sheep blood. Broth MICs for eight strains tested
by PAE were performed according to NCCLS recommendations by using
cation-adjusted Mueller-Hinton broth with 5% lysed defibrinated horse
blood. Standard quality control strains, including Streptococcus
pneumoniae ATCC 49619, were included in each run of agar and broth
dilution MICs. All incubation took place in air (20).
Time-kill testing.
For time-kill studies, glass tubes
containing 5 ml of cation-adjusted Mueller-Hinton broth (Difco) plus
5% lysed horse blood with doubling antibiotic concentrations were
inoculated with 5 × 105 to 5 × 106
CFU of bacteria per ml and were incubated at 35°C in a shaking water
bath. Antibiotic concentrations were chosen to comprise two doubling
concentrations above and one concentration below the agar dilution MIC.
Growth controls with inoculum but no antibiotic were included with each
experiment (22, 25).
Lysed horse blood was prepared as described previously (22,
25). The initial bacterial inoculum was prepared by making a
0.5-McFarland-standard suspension in Mueller-Hinton broth containing 5% lysed horse blood from an 18-h culture on a sheep blood agar plate.
Dilutions required to obtain the correct final inoculum (5 × 105 to 5 × 106 CFU/ml) were determined by
prior viability studies using each strain (22, 25).
To inoculate each tube of serially diluted antibiotic, 50 µl of
diluted inoculum was delivered by pipette beneath the surface
of the
broth. Tubes were then vortexed and plated for viability
counts within
10 min (approximately 0.2 h). The original inoculum
was determined
by using the untreated growth control. Only tubes
containing an initial
inoculum within the range of 5 × 10
5 to 5 × 10
6 CFU/ml were
acceptable.
Viability counts of antibiotic-containing suspensions and controls
without drug were performed by plating 1:10 dilutions of
0.1-ml
aliquots from each tube in sterile Mueller-Hinton broth
onto 5% sheep
blood agar plates (BBL). The number of dilutions
required varied with
the initial inoculum on plates containing
undiluted inoculum and
depended upon the original MIC of the antibiotic
for the strain.
Recovery plates were incubated for up to 72 h.
Colony counts were
performed on plates yielding 30 to 300 colonies.
The lower limit of
sensitivity of colony counts was 300 CFU/ml
(
22-25).
Time-kill assays were analyzed by determining the number of strains
which yielded a

log
10 CFU per ml of 1, 2, and 3, at 0,
3, 6, 12, and 24 h, compared to counts at 0 h. Antimicrobials
were considered bactericidal at the lowest concentration that
reduced
the original inoculum by

3 log
10 CFU/ml (99.9%) at each
of the time periods and were considered bacteriostatic if the
inoculum
was reduced by 0 to 3 log
10 CFU/ml. With the sensitivity
threshold and inocula used in these studies, no problems were
encountered in delineating 99.9% killing, when present. The problem
of
bacterial carryover was addressed as described previously (
22,
25). For macrolide time-kill testing, only strains with MICs

4.0 µg/ml were
tested.
PAE testing.
The PAE was determined by the viable plate
count method (5, 29, 30) using Mueller-Hinton broth (MHB)
supplemented with 5% lysed horse blood. The PAE was induced by
exposure to 10 times the MIC for 1 h except in the case of
pristinamycin where, owing to rapid bactericidal activity, 1 times the
MIC was used. Additionally, the one quinolone-resistant strain was
exposed at quinolone concentrations 5 times the MIC. Tubes containing 5 ml of broth with antibiotic were inoculated with approximately 5 × 106 CFU/ml. Growth controls with inoculum but no
antibiotic were included with each experiment. Tubes were placed in a
shaking water bath at 35°C for 1 h. At the end of the exposure
period, cultures were diluted 1:1,000 to remove antibiotic. A control containing bacteria preexposed to antibiotic at a concentration of 0.01 times the MIC was also prepared (29, 30).
Viability counts were determined before exposure and immediately after
dilution (0 h) and then every 2 h until tube turbidity
reached 1 McFarland standard. Inocula were prepared by suspending
growth from an
overnight blood agar plate in broth. The broth
was incubated at 35°C
for 2 to 4 h in a shaking water bath until
turbidity matched 1 McFarland standard and was checked for viability
by plate counts
(
29,
30).
The PAE was defined as PAE = T

C (where T is time required for
viability counts of an antibiotic-exposed culture to increase
by 1 log
10 above counts immediately after dilution and C is
corresponding
time for growth control [
5]). For each
experiment, viability
counts (log
10 CFU per milliliter)
were plotted against time, and
the results were expressed as the mean
of two separate assays
± standard
deviation.
Characterization of macrolide-resistant strains.
Macrolide-resistant strains were screened by the
erythromycin-clindamycin double disk method (7). PCR primers
were purchased from Sigma/Genosys Biotechnologies (The Woodlands,
Tex.). Primers and PCR conditions for ermB and
mefE have been described previously (31). Primers
and PCR conditions for L4 ribosomal protein in S. pneumoniae
have been described (A. Tait-Kamradt et al., 39th ICAAC, abstr. 842; A. Tait-Kamradt, T. Davies, M. Cronan, M. Jacobs, P. Appelbaum, and J. Sutcliffe, submitted for publication). To locate mutants in 23S rRNA,
the genes were initially amplified from total genomic DNA as three
overlapping contigs by using primers and PCR conditions described
previously (A. Tait-Kamradt et al., 39th ICAAC, abstr. 842). The
peptidyl transferase region from each allele of 23S rRNA was amplified
by using unique primers beyond the 3' end of each 23S rRNA as described
previously (A. Tait-Kamradt et al., 39th ICAAC, abstr. 842). PCR
products were purified by using QIAquick PCR Purification Kit (QIAGEN,
Valencia, Calif.) and were sequenced by using an Applied Biosystems
model 373 DNA sequencer. Sequence comparisons were performed by using Vector NTI sequence analysis software (InforMax, Inc., North Bethesda, Md.).
 |
RESULTS |
Results of agar dilution testing with 205 strains (excluding those
with the L4 and 23S rRNA mutations) classified by penicillin susceptibility are presented in Table 1.
The ketolide ABT-773 had the lowest MICs of all macrolides and azalides
tested, irrespective of penicillin MIC. For other macrolides and
azalides, MICs rose with those of penicillin; however, owing to the
inclusion of several macrolide-resistant but penicillin-susceptible
strains from our collection, macrolide MICs were lower for
penicillin-intermediate than for penicillin-susceptible organisms. This
is currently not the case in the United States, where less than 5% of
penicillin-susceptible pneumococci are macrolide resistant
(6). All strains were susceptible to pristinamycin and
vancomycin, while
-lactam MICs rose with those of penicillin G.
Results of agar dilution testing with the above 205 strains
classified by erythromycin susceptibility are presented in Table 2. ABT-773 was very active irrespective
of erythromycin susceptibility. ABT-773 MICs for erm- and
mef-containing strains did not differ significantly. Against
83 erythromycin-susceptible and 122 erythromycin-resistant pneumococci
(carrying erm and mef), the agar dilution MIC at
which 50% of the isolates tested were inhibited
(MIC50)/MIC90s in micrograms per milliliter)
were as follows: ABT-773,
0.008/0.016, 0.03/0.125 (erm and
mef); erythromycin, 0.03/0.06, >64.0/>64.0
(erm), 2.0/8.0 (mef); azithromycin, 0.06/0.125,
>64.0/>64.0 (erm), 2.0/8.0 (mef); clarithromycin, 0.03/0.06, >64.0/>64.0 (erm), 1.0/8.0
(mef); roxithromycin, 0.25/0.25, >64.0/>64.0
(erm), 8.0/32.0 (mef); and clindamycin, 0.06/0.06, >64.0/>64.0 (erm), 0.06/0.125 (mef).
Pristinamycin and vancomycin MICs remained unchanged irrespective of
macrolide susceptibility, and amoxicillin, ceftriaxone, and imipenem
MICs rose with those of penicillin G.
MICs of ABT-773 were compared to those of macrolides and azalides
against 21 ermB- and mefE-negative
macrolide-resistant strains, comprising 19 strains with L4 ribosomal
protein mutations (69GTG71
TPS) and 2 with a
23S rRNA mutation (A2059
G). ABT-773 MICs ranged between 0.03 and
0.25 µg/ml, compared to 16.0 to >64.0 µg/ml for erythromycin,
azithromycin, clarithromycin, and roxithromycin. Clindamycin MICs
ranged between 0.06 and 0.125 µg/ml for strains containing the L4
mutation but were higher (0.5 to 2.0 µg/ml) for 23S rRNA strains.
Time-kills for six macrolide-susceptible, three ermB- and
three mefE-carrying strains, for which MICs were similar to
those presented in Tables 1 and 2, are shown in Table
3. MICs were based upon agar dilution and
macrodilution, which were all within 1 dilution of each other. The
ketolide ABT-773 was bactericidal at 2 times the MIC for 8 of 12 strains (including those highly resistant to macrolides [MICs greater
than or equal to 64 µg/ml]). After 12 h, ABT-773 gave 90%
killing of 10 of 12 strains at 2 times the MIC. Kill kinetics of the
macrolides erythromycin, clarithromycin, and roxithromycin, and the
azalide azithromycin, against macrolide-susceptible strains were slower
than those of ABT-773. ABT-773 was bacteriostatic against all four
strains which did not give 99.9% killing after 24 h (data not
shown). Clindamycin, for all strains except those for which the MIC of
the antibiotic was greater than or equal to 64 µg/ml, was
bactericidal for eight of nine strains tested after 24 h at 2 times the MIC. Pristinamycin was rapidly bactericidal, showing
significant killing after 3 h, with bactericidal activity against
all 12 strains after 24 h at 2 times the MIC.
-Lactams (amoxicillin, ceftriaxone, and imipenem) started to show significant killing after 12 h; after 24 h at 2 times the MIC,
bactericidal activity was seen against 11 strains. Vancomycin gave
rapid kill kinetics at early time periods, with bactericidal activity
against all 12 strains tested at 2 times the MIC.
Microdilution MICs as well as PAEs for the eight strains tested are
presented in Table 4. Pristinamycin PAEs
(MICs, 0.125 to 0.5 µg/ml) are not given due to rapid bactericidal
activity (25). PAEs of macrolides and clindamycin were only
tested against five strains with MICs less than or equal to 0.25 µg/ml. Macrolide PAEs were longer than those of
-lactams. ABT-773
had longer PAEs strain for strain (individual data not shown) than
macrolides, azalides, or clindamycin. Additionally, the three strains
for which macrolide, azalide, and clindamycin MICs were greater than 64.0 µg/ml yielded mean ABT-773 PAEs of 5.6 h (range, 3.7 to
7.5 h), 1.7 h (range, 1.4 to 2.0 h), and 2.9 h
(range 2.4 to 3.4 h) (Table 4).
 |
DISCUSSION |
ABT-773 is a new ketolide (4) (A. M. Nilius et
al., Abstr. 9th Eur. Congr. Clin. Microbiol. Infect. Dis., abstr.
P-177; Z. Ma et al., 39th ICAAC, abstr. 2133; Z. Cao et al., 39th
ICAAC, abstr. 2135) which, in preliminary studies, has been reported to
be more potent in vitro than the macrolides against Haemophilus influenzae, Moraxella catarrhalis,
Legionella spp., Neisseria gonorrhoeae, and
Listeria monocytogenes. ABT-773 was also more potent against
macrolide-susceptible strains of S. pneumoniae, Streptococcus pyogenes, Staphylococcus aureus,
Staphylococcus epidermidis, enterococci, Helicobacter
pylori, and Mycobacterium avium complex, and also
against Corynebacterium spp., Mycoplasma pneumoniae, Chlamydia trachomatis, Borrelia
burgdorferi, and Toxoplasma gondii. ABT-773 had potent
activity against macrolide-resistant streptococci and enterococci
irrespective of their macrolide resistance mechanism but had little
detectable activity against constitutively macrolide-resistant
staphylococci and macrolide-resistant H. pylori and M. avium complexes (4) (D. Shortridge, N. C. Ramer,
J. Beyer, Z. Ma, Y. Or, and R. K. Flamm, Abstr. 39th Intersci.
Conf. Antimicrob. Agents Chemother., abstr. 2136, 1999; M. M. Neuhauser, J. L. Prause, R. Jung, N. Boyea, J. M. Hackleman,
L. H. Danziger, and S. L. Pendland, Abstr. 39th Intersci.
Conf. Antimicrob. Agents Chemother., abstr. 2139, 1999; F. Goldstein,
M. D. Kitzis, M. Miegi, and J. F. Acar, Abstr. 39th Intersci.
Conf. Antimicrob. Agents Chemother., abstr. 2142, 1999; A. L. Barry, P. C. Fuchs, and S. D. Brown, Abstr. 39th Intersci.
Conf. Antimicrob. Agents Chemother., abstr. 2144, 1999; S. L. Pendland, J. L. Prause, M. M. Neuhauser, N. Boyea, J. M. Hackleman, and L. H. Danziger, Abstr. 39th Intersci. Conf.
Antimicrob. Agents Chemother., abstr. 2145, 1999; R. Jung, D. H. Li, S. L. Pendland, and L. H. Danziger, Abstr. 39th Intersci.
Conf. Antimicrob. Agents Chemother., abstr. 2146, 1999; A. A. Khan, F. G. Araujo, J. C. Craft, and J. S. Remington, Abstr. 39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 2147, 1999). ABT-773 has been shown to be effective against S. pneumoniae in an experimental rat lung model (J. Meulbroek, M. Mitten, K. W. Mollison, P. Ewing, J. Alder, A. M. Nilius,
R. K. Flamm, Z. Ma, and Y. Or, Abstr. 39th Intersci. Conf.
Antimicrob. Agents Chemother., abstr. 2151, 1999).
Results of this study indicate that ABT-773 was very active against
macrolide-susceptible and -resistant pneumococci. MICs against
macrolide-resistant pneumococci carrying erm did not differ significantly from those seen in mef-carrying strains.
Similar results have been reported by others for ABT-773 and by us as well as other workers for telithromycin (4, 26).
Additionally, ABT-773 gave low MICs against strains with the L4 and 23S
rRNA mutations. The latter mechanism has been recognized only recently, and is present in strains isolated from Central and Eastern Europe (P. C. Appelbaum, unpublished data). A few strains with the 23S rRNA mutation have also been found in clinical specimens (J. A. Sutcliffe, personal communication). The significance of these new
resistance mechanisms is currently unknown.
Results for other macrolides, azalides, and other compounds are similar
to those reported previously (7, 15-17, 21, 23, 24, 27, 28,
32), with lower macrolide MICs against strains carrying
mef than against those carrying erm. In the
United States, both mechanisms of macrolide resistance are commonly
encountered in clinical specimens (D. Shortridge et al., Abstr. 36th
Infect. Dis. Soc. Am. Annu. Meet., abstr. 225F).
Long macrolide PAEs have previously been compared to those of other
compounds (10, 14, 29, 30). In the present study, ABT-773
gave the longest PAEs of all drugs tested, including macrolide and
nonmacrolide compounds.
Results of the present study document low ABT-773 MICs against
macrolide-susceptible and -resistant pneumococci. Although MICs were
slightly higher against strains containing erm and
mef, MICs were significantly lower than those of other
macrolides and azalides. If results of pharmacokinetic, pharmacodynamic
(area under the concentration-time curve and MIC), and animal studies are promising, this compound is of potential use for therapy of macrolide-susceptible and -resistant pneumococci.
 |
ACKNOWLEDGMENTS |
This study was supported by a grant from Abbott Laboratories,
Chicago, Ill.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Pathology, Hershey Medical Center, P.O. Box 850, Hershey, PA 17033. Phone: (717) 531-5113. Fax: (717) 531-7953. E-mail:
pappelbaum{at}psghs.edu.
 |
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Antimicrobial Agents and Chemotherapy, July 2000, p. 1894-1899, Vol. 44, No. 7
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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