Determination of MIC Quality Control Ranges for ABT-733, a Novel Ketolide Antimicrobial Agent

ABT-773 is a novel ketolide that possesses potent antimicrobial activity against respiratory pathogens such as macrolide-susceptible and -resistant streptococci (90% minimal inhibitory concentrations [MIC₉₀s], 0.002 to 0.03 and 0.12 to 4 μg/ml, respectively), oxacillin-susceptible Staphylococcus aureus (MIC₉₀s, 0.03 to 0.06 μg/ml), Haemophilus influenzae (MIC₉₀s, 2 to 4 μg/ml), and Moraxella catarrhalis (MIC₉₀s, 0.12 to 0.25 μg/ml) (A. L. Barry, P. C. Fuchs, and S. D. Brown, Abstr. 5th Int. Conf. Macrolides, Azalides, Streptogramins, Ketolides, and Oxazolidinones, abstr. 2.12, p. 24, 2000; J. Dubois and C. St.-Pierre, Abstr. 5th Int. Conf. Macrolides, Azalides, Streptogramins, Ketolides, and Oxazolidinones, abstr. 2.15, p. 25, 2000). The less frequently isolated atypical pathogens Mycoplasma pneumoniae (MICs, ≤0.0005 μg/ml), Chlamydia pneumoniae (MIC₉₀, 0.015 μg/ml), and Legionella spp. (MIC₉₀s, 1 to 2 μg/ml) were also quite susceptible to ABT-773 (S. Strigl, P. M. Roblin, T. Reznick, and M. R. Hammerschlag, Abstr. 5th Int. Conf. Macrolides, Azalides, Streptogramins, Ketolides, and Oxazolidinones, abstr. 2.32, p. 29, 2000). The mechanism of ketolide action against these species appears to be protein synthesis inhibition with enhanced ribosomal binding as directly compared to that of macrolides (Z. Cao, R. Hammond, S. Pratt, A. Saiki, C. Lerner, R. Flamm, and P. Zhong, Abstr. 5th Int. Conf. Macrolides, Azalides, Streptogramins, Ketolides, and Oxazolidinones, abstr. 2.04, p. 22, 2000). The activity of ABT-773 has been generally superior to that of quinolones against some gram-positive species, making this series of compounds a potential candidate for the treatment of macrolide- or quinolone-resistant respiratory tract infections (Dubois St.-Pierre, 5th Int. Conf. Macrolides, Azalides, Streptogramins, Ketolides, and Oxazolidinones, abstr. 2.15).

Precise testing of ABT-773 must be ensured by the establishment of quality control (QC) parameters before human clinical trials can be widely conducted. The recently released tentative guidelines by the National Committee for Clinical Laboratory Standards (NCCLS) on QC development for the reference broth microdilution method were implemented for this investigation (1, 2). Briefly, each of at least seven qualifying medical center laboratories tested 10 replicates (daily) of all appropriate QC strains in each of three medium lots of Mueller-Hinton broth from the following manufacturers: AccuMed International, Inc. (Westlake, Ohio), BD Microbiologic Systems (Cockeysville, Md.), and Difco Laboratories (Sparks, Md.). Inter- and intralaboratory MIC results were then compared, and a proposed QC range was calculated (2). All broth microdilution trays were prepared with appropriate supplements by PML Microbiologics (Wilsonville, Oreg.). Quality assurance control antimicrobials having preestablished QC ranges were clarithromycin for S. aureus ATCC 29213, H. influenzae ATCC 49247, and Streptococcus pneumoniae ATCC 49619 and trovafloxacin for Enterococcus faecalis ATCC 29212 (Table 1).

TABLE 1.

Proposed QC ranges from 900 MIC determinations for ABT-773, a new ketolide, using the current NCCLS study design (2)

QC strain	Antimicrobial agenta	MIC (μg/ml)			% within proposed rangeb
		Mode	Range	Proposed rangeb
E. faecalis ATCC 29212	ABT-773	0.016	0.016–0.12	0.008–0.06 (0.008–0.03)	98.6 (92.1)
	Trovafloxacin	0.12	1–2	0.06–0.25	100.0
S. aureus ATCC 29213	ABT-773	0.06	0.016–0.12	0.016–0.12 (0.016–0.12)	100.0 (99.5)
	Clarithromycin	0.25	0.12–0.5	0.12–0.5	100.0
H. influenzae ATCC 49247	ABT-773	1	0.25–2	0.5–4 (1–4)	99.2 (96.8)
	Clarithromycin	4	4–16	4–16	100.0
S. pneumoniae ATCC 49619	ABT-773	0.008	0.004–0.016	0.004–0.016 (0.002–0.016)	100.0 (100.0)
	Clarithromycin	0.03	0.016–0.12	0.03–0.12	96.3

^aClarithromycin is the control agent. 

^bMIC QC ranges from the work of Brown et al. (5th Int. Conf. Macrolides, Azalides, Streptogramins, Ketolides, and Oxazolidinones, abstr. 2.07) are shown in parentheses. 

Table 1 lists the proposed QC ranges for ABT-773 and control agents based on application of the current NCCLS methodology (1). Nonfastidious QC strains were tested at seven participating medical centers (210 total tests), and fastidious QC strains were tested at eight medical centers (240 total tests). The proposed three-dilution MIC QC range for S. pneumoniae ATCC 49619 was the modal MIC (0.008 μg/ml) ± one log₂ dilution. All other QC strains tested followed a distribution of equal occurrences at two neighboring log₂ dilutions, resulting in a more broad, four-dilution QC range proposal. More than 98% of all participating laboratory test results were within the proposed QC ranges, and there was no significant variation between Mueller-Hinton-based lots observed with these recommended QC strains (data not shown). Overall, 99% of all control agent MICs were within NCCLS-approved QC ranges (Table 1).

The ABT-773 QC range results presented here are very similar to those reported by Brown et al. (S. D. Brown, A. L. Barry, and P. C. Fuchs, Abstr. 5th Int. Conf. Macrolides, Azalides, Streptogramins, Ketolides, and Oxazolidinones, abstr. 2.07, p. 23, 2000) using the same NCCLS trial design (2). For all QC strains except S. aureus ATCC 29213, the MIC QC ranges proposed by Brown et al. differ by only one log₂ dilution (e.g., E. faecalis ATCC 29212, 0.008 to 0.03 μg/ml [only 92.1% of results in range]; H. influenzae ATCC 49247, 1 to 4 μg/ml; S. pneumoniae ATCC 49619, 0.002 to 0.016 μg/ml). A combination of the results from these two large studies (Table 1) should aid the NCCLS in determining the most appropriate QC range needed for the in vitro susceptibility testing of ABT-773 in clinical trials and for routine clinical laboratory practice.

† The Quality Control Study Group includes C. Knapp, AccuMed, Westlake, Ohio; G. Hall, The Cleveland Clinic Foundation, Cleveland, Ohio; R. Rennie, University of Alberta Medical Center, Edmonton, Alberta, Canada; M. A. Pfaller, University of Iowa Health Care, Iowa City; A. Wanger, University of Texas—LBJ Medical Center, Houston; T. Haugen, Veterans Administration Medical Center, Iowa City, Iowa; D. Sewell, Veterans Administration Medical Center, Portland, Oreg.; and P. Murray, Washington University-Barnes, St. Louis, Mo.