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Antimicrobial Agents and Chemotherapy, April 2000, p. 1062-1066, Vol. 44, No. 4
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
In Vitro Activities of Daptomycin, Vancomycin, Linezolid, and
Quinupristin-Dalfopristin against Staphylococci and Enterococci,
Including Vancomycin- Intermediate and -Resistant Strains
Michael J.
Rybak,*
Ellie
Hershberger,
Tabitha
Moldovan, and
Richard G.
Grucz
The Anti-Infective Research Laboratory,
Department of Pharmacy Services, Detroit Receiving
Hospital/University Health Center, College of Pharmacy and Allied
Health Professions, and Division of Infectious Diseases, Department
of Internal Medicine, School of Medicine, Wayne State University,
Detroit, Michigan 48201
Received 26 March 1999/Returned for modification 1 August
1999/Accepted 19 December 1999
 |
ABSTRACT |
The in vitro activity of daptomycin was compared with those of
vancomycin, linezolid, and quinupristin-dalfopristin against a variety
(n = 203) of gram-positive bacteria, including
methicillin-resistant Staphylococcus aureus and S. epidermidis (MRSA and MRSE, respectively), vancomycin-resistant
enterococci (VRE), and vancomycin-intermediate S. aureus
(VISA). Overall, daptomycin was more active against all organisms
tested, except Enterococcus faecium and VISA, against which
its activity was similar to that of quinupristin-dalfopristin. In
time-kill studies with MRSA, MRSE, VRE, and VISA, daptomycin demonstrated greater bactericidal activity than all other drugs tested,
killing
3 log CFU/ml by 8 h. Daptomycin may be a potential alternative drug therapy for multidrug-resistant gram-positive organisms and warrants further investigation.
 |
TEXT |
Daptomycin is a cyclic polypeptide
derived from Streptomyces roseosporus and representing a
class of antimicrobial agents known as the peptolides (acid lipopeptide
antibiotics) (3). The spectrum of activity is similar to
those of vancomycin and teicoplanin, with activity against a wide
variety of aerobic and anaerobic gram-positive bacteria, including
methicillin-resistant Staphylococcus aureus (MRSA).
Daptomycin is also active against vancomycin-resistant gram-positive
bacteria, including enterococci (4, 5, 7). The mechanism of
action differs from that of the glycopeptides, involving the disruption
of amino acid transport by the cell membrane and alterations of the
cytoplasmic membrane potential (1-3). The bactericidal
activity of daptomycin is concentration dependent and is influenced by
pH and ionized calcium concentrations (9). Like teicoplanin,
daptomycin is highly protein bound (94%), and its in vitro activity is
altered in the presence of serum or albumin. However, even in the
presence of serum, the activity of daptomycin is superior to that of
vancomycin or teicoplanin (6, 11). In early clinical trials,
daptomycin was efficacious in patients with skin and skin structure
infections and bacteremia at dosages of 2 to 3 mg/kg of body weight
every 12 h (investigator's brochure, Eli Lilly & Co.). However,
clinical trials were suspended when treatment failures were noted in
patients with S. aureus endocarditis (Eli Lilly & Co.,
personal communication). Possible reasons for failure included the high
degree of protein binding and the degree of penetration into cardiac
vegetations (9). Experimental animal endocarditis studies
replicating clinical trial dosages and achievable concentrations
demonstrated that slightly higher dosage regimens of daptomycin were as
effective as or more effective than vancomycin in reducing vegetative
bacterial densities (8). Recently, renewed interest in
daptomycin has occurred secondarily to the need for new agents with
activity against vancomycin-resistant organisms, including enterococci. Our objectives were to reevaluate the bactericidal activity of daptomycin compared to those of vancomycin, the oxazolidinone linezolid, and the streptogramin quinupristin-dalfopristin against various gram-positive organisms.
(This work was presented in part at the 38th Interscience Conference on
Antimicrobial Agents and Chemotherapy.)
Bacterial strains.
The 203 gram-positive organisms used
consisted of the following: 100 S. aureus isolates, 50 methicillin-susceptible S. aureus (MSSA), 50 MRSA, and 3 vancomycin-intermediate-susceptible S. aureus (VISA). The 50 coagulase-negative staphylococci consisted of 25 methicillin-susceptible S. epidermidis (MSSE) isolates
and 25 methicillin-resistant S. epidermidis (MRSE). The 50 enterococci consisted of 25 Enterococcus faecalis isolates
and 25 E. faecium (20 of these isolates were resistant to
vancomycin). MICs were also determined by use of a standard reference
strain of S. aureus (ATCC 25923) with all antibiotics at
baseline and during each susceptibility test for quality assurance purposes.
Antibiotics.
Daptomycin (Eli Lilly & Co. lot no. RS0113) was
supplied by Cubist Pharmaceuticals, Inc., Cambridge, Mass. Vancomycin
susceptibility-grade powder was purchased commercially (Sigma Chemical
Co., St. Louis, Mo.). Linezolid susceptibility-grade powder (lot
5014-TJF-490 A) was supplied by Pharmacia and Upjohn, Inc., (Kalamazoo,
Mich.), and quinupristin-dalfopristin susceptibility-grade powder (lot 9609410) was supplied by Rhone-Poulenc Rorer (Collegeville, Pa.).
MIC and MBC determinations.
The MICs and minimal bactericidal
concentrations (MBCs) were determined for each isolate in duplicate by
a microdilution technique with Mueller-Hinton broth (Difco
Laboratories, Detroit, Mich.) supplemented with calcium (25 mg/liter)
and magnesium (12.5 mg/liter) (SMHB) for vancomycin, linezolid, and
quinupristin-dalfopristin. Mueller-Hinton broth supplemented with
calcium at 75 mg/liter (physiological ionized Ca2+
concentration) and magnesium at 12.5 mg/liter (SMHB-PCA) was always
used for microdilution susceptibility testing of daptomycin. The MIC
and MBC of daptomycin were also determined with a subset of each
organism (n = 22) and a 50:50 mixture of pooled human serum and SMHB-PCA. Susceptibility testing for each drug was performed in quadruplicate according to the guidelines of the National Committee for Clinical Laboratory Standards (10).
Killing curves.
The bactericidal activities of daptomycin,
vancomycin, linezolid, and quinupristin-dalfopristin were compared by
use of killing curve analyses with four representative clinical
isolates (R499, an MRSA; MU50, a VISA; R227, an MRSE; and R588, a
vancomycin-resistant enterococcus [VRE] [E. faecalis]).
Three to five colonies from overnight growth on tryptic soy agar at
35°C were added to normal saline and adjusted as necessary to produce
a 0.5 McFarland standard suspension of organisms. This suspension was
diluted appropriately (1:10) with SMHB (SMHB-PCA for daptomycin) to
achieve an inoculum of 107 CFU/ml. A 0.2-ml suspension of
an organism was added to 1.7 ml of SMHB (final inoculum,
106 CFU/ml) with a 0.1-ml stock solution of each antibiotic
at concentrations one and four times the MICs for the respective
organism in a 24-well tissue culture plate (final volume, 2.0 ml per
well). Culture wells were incubated at 35°C with constant shaking for
24 h. Samples (0.1 ml) were removed at 0, 8, and 24 h;
appropriately diluted with cold 0.9% sodium chloride to reduce
antibiotic carryover; plated onto tryptic soy agar (Difco) with an
Autoplate Spiral Diluter (model 3000; Spiral Bioscience, Frederick,
Md.); and incubated at 35°C for 24 h. The limit of detection for
this method is 250 CFU/plate, corresponding to 2.5 log10
CFU/ml. Growth control wells for each organism were prepared without
antibiotic and run in parallel to the antibiotic test wells. Killing
curves were also determined with daptomycin at a concentration four
times the MIC in SMHB-PCA and SMHB-PCA with pooled human serum (50:50)
against the four representative isolates mentioned above. All time-kill curve experiments were performed in triplicate.
The drug activities are reported in Table
1. Against MSSA and MRSA, daptomycin
activity was approximately 8- to 30-fold greater than those of all
other agents. The activity of daptomycin against MSSE and MRSE was
equal to that of quinupristin-dalfopristin and 2- to 16-fold greater
than those of vancomycin and linezolid. Against VISA,
quinupristin-dalfopristin was the most active agent, followed by
daptomycin, linezolid, and vancomycin. The activity of daptomycin
against both vancomycin-sensitive and -resistant E. faecalis
was greater than those of all other agents tested. The activity of
daptomycin against both vancomycin-sensitive and -resistant E. faecium was equal to that of quinupristin-dalfopristin and greater
than those of all other agents tested. On average, the MICs of
daptomycin in the presence of 50% serum against MSSA (n = 4), MRSA (n = 4), MSSE (n = 4),
MRSE (n = 4), E. faecium (n = 5),
and VISA (n = 1) increased one- to eightfold, while
serum had no effect on the MICs of linezolid,
quinupristin-dalfopristin, or vancomycin (data not shown). In time-kill
studies, at the MIC, killing was minimal and regrowth occurred for all
agents (data not shown). At concentrations four times the MIC,
daptomycin and vancomycin achieved 99.9% killing of MRSA in 8 h,
which was greater than the killing seen with linezolid and
quinupristin-dalfopristin (P, <0.05) (Fig.
1). Against VRE at 8 and 24 h,
daptomycin had greater activity than linezolid and
quinupristin-dalfopristin (P, <0.05). Against MRSE,
daptomycin, vancomycin, and quinupristin-dalfopristin had greater
activity than linezolid early (P, <0.05); however, no
differences were observed between the regimens at 24 h. All antibiotics tested achieved 99.9% killing of the VISA isolate by
24 h. The addition of pooled human serum to SMHB-PCA significantly (P, <0.05) decreased the activity of daptomycin. However,
killing activity was significantly improved when the daptomycin
concentration was adjusted according to the MIC obtained in SMHB-PCA
plus pooled human serum; data are shown for two of the isolates (Fig.
2).
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TABLE 1.
Activities of daptomycin (D), vancomycin (V), linezolid
(L), and quinupristin-dalfopristin (Q-D) against various
bacterial strains
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FIG. 1.
Time-kill experiments performed at four times the MIC
against MRSA R499 (A), VISA 992 (B), VRE R588 (E. faecalis)
(C), and MRSE R227 (D). Results are means ± standard deviations.
GC, growth control; D, daptomycin; V, vancomycin; L, linezolid; Q-D,
quinupristin-dalfopristin. MICs of D, V, L, and Q-D, respectively, were
as follows: for R499, 0.125, 0.5, 2.0, and 0.25; for 992, 0.5, 8.0, 2.0, and 0.25; for R588, 1.0, 128.0, 4.0, and 2.0; and for R277, 0.25, 1.0, 4.0, and 0.06.
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FIG. 2.
(A) Time-kill experiments performed with daptomycin in
broth and in broth plus pooled human serum (PHS) at four times the MIC
against VRE 588. A superscript "A" indicates the MIC determined in
SMHB-PCA (MIC, 1.0 µg/ml). A superscript "B" indicates the MIC
determined in SMHB-PCA plus PHS (MIC, 2.0 µg/ml). (B) Time-kill
experiments performed with daptomycin in broth and in broth plus PHS at
four times the MIC against VISA 992. A superscript "A" indicates
the MIC determined in SMHB-PCA (MIC, 0.5 µg/ml). A superscript
"B" indicates the MIC determined in SMHB-PCA plus PHS (MIC, 1.0 µg/ml).
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|
Daptomycin represents a unique semisynthetic antimicrobial agent with
activity against a broad range of gram-positive pathogens,
including
VRE and vancomycin-resistant staphylococci. Unlike those
of many other
agents, the activity of daptomycin is bactericidal
for staphylococci
and enterococci and is concentration dependent.
The activity of
daptomycin in the presence of serum is decreased
because of its high
degree of protein binding; however, its activity
is improved when
concentrations are equal to or greater than four
times the MIC
(achieved clinically). This conclusion was supported
by the killing
curves determined in the presence of serum for
VRE but to a lesser
extent for the VISA isolate. Our susceptibility
data are similar to
those reported by others previously for gram-positive
organisms,
including more recent data against newer pathogens,
such as VRE and
VISA (N. V. Jacobus, B. Goldin, L. McDermott,
and D. R. Snydman, Abstr. 38th Intersci. Conf. Antimicrob. Agents
Chemother., p.
F-113, 1998; N. V. Jacobus, L. McDermott, J. R.
Lonks,
J. M. Boyce, and D. R. Snydman, Abstr. 38th Intersci. Conf.
Antimicrob. Agents Chemother., p. F-112, 1998). The primary mechanism
of action of daptomycin relies heavily on ionized calcium content.
The
difference in the activities of daptomycin in SMHB (
9)
and
in SMHB-PCA is notable and should be considered when one is
determining
MICs of this drug. Although early studies with daptomycin
were not
successful at low doses for patients with endocarditis,
based upon the
long serum half-life of 8.5 h and the dose-dependent
postantibiotic effect of 1 to 6 h, it is likely that this drug
could achieve success if given once daily. Newly proposed dosing
for
daptomycin at 4 and 6 mg/kg/day, administered once daily,
should
achieve serum concentrations necessary to improve the activity
of the
drug for a variety of gram-positive
organisms.
 |
ACKNOWLEDGMENTS |
This study was supported by a grant from Cubist Pharmaceuticals.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: The
Anti-Infective Research Laboratory, Department of Pharmacy Services,
Detroit Receiving Hospital/University Health Center, 4201 St. Antoine,
Detroit, MI 48201. Phone: (313) 745-4554. Fax: (313) 993-2522. E-mail: mrybak{at}dmc.org.
 |
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Antimicrobial Agents and Chemotherapy, April 2000, p. 1062-1066, Vol. 44, No. 4
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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