Comparative In Vitro Activities of Investigational Peptide Deformylase Inhibitor NVP LBM-415 and Other Agents against Human Mycoplasmas and Ureaplasmas

Abstract

Peptide deformylase inhibitor LBM-415 and seven other drugs were tested against Mycoplasma pneumoniae (100 isolates), Mycoplasma hominis (20 isolates), Mycoplasma fermentans (10 isolates), and Ureaplasma species (50 isolates). LBM-415 was active against M. pneumoniae (MICs, ≤0.008 μg/ml). It showed no activity against M. hominis and M. fermentans and modest activity against Ureaplasma spp.

NVP LBM-415 (formerly PDF-713) is an investigational peptide deformylase (PDF) inhibitor that has in vitro activity against major respiratory tract pathogens, including Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis (1, 9). This drug also has activity against many other gram-positive bacterial pathogens, including staphylococci and enterococci (4, 9).

Mycoplasma pneumoniae is a significant cause of upper and lower respiratory tract infections in persons of all age groups (13, 14). Mycoplasma hominis and Ureaplasma species are associated with a variety of conditions involving the urogenital tract and sometimes cause extragenital infections in adults and infants, especially in immunocompromised hosts of all ages (13). Mycoplasma fermentans may also behave as an opportunistic pathogen in some settings (13). Antimicrobial treatment, usually rendered empirically, is often necessary to treat infections due to these organisms. We performed the present study to evaluate the in vitro inhibitory and bactericidal activities of NVP LBM-415 against clinical isolates of human mycoplasmas and ureaplasmas in comparison to antimicrobial agents that are known to have in vitro activity against these organisms.

The microorganisms tested included the following: 100 M. pneumoniae isolates collected from a broad geographic area over a 10-year period from the respiratory tract of individuals with proven respiratory disease; 10 M. fermentans isolates from the mycoplasma collection at the National Institutes of Health; 20 M. hominis isolates derived from clinical specimens of the urogenital tract or wound cultures; and 50 Ureaplasma isolates, including both U. urealyticum and U. parvum derived from adult urogenital or neonatal respiratory cultures. Drugs tested in addition to LBM-415 included azithromycin, clarithromycin, doxycycline, levofloxacin, gatifloxacin, moxifloxacin, and clindamycin. Clindamycin was tested only against M. hominis and M. fermentans because it is not recommended for treatment of M. pneumoniae (3) and has no activity against Ureaplasma spp. (13). Azithromycin and clarithromycin were tested only against M. pneumoniae and Ureaplasma spp. since M. hominis is always resistant to macrolides and M. fermentans is frequently resistant to these agents as well (5, 13). For the determination of MICs, antimicrobial powders were dissolved and diluted according to the manufacturers' instructions on the day of assay. Serial twofold dilutions of drugs were made in 10 B broth for Ureaplasma and SP4 broth for Mycoplasma spp. (13). MICs were then determined as described previously (12). Five M. pneumoniae and two Ureaplasma isolates were randomly chosen for further testing to determine minimal bactericidal concentrations. Time-kill assays were also performed on these same M. pneumoniae isolates, using procedures described previously (12).

A summary of MICs obtained is shown in Table 1. NVP LBM-415 demonstrated excellent in vitro activity against all 100 isolates of M. pneumoniae. All isolates were inhibited by LBM-415 at ≤0.008 μg/ml, and the MIC₉₀ (0.001 μg/ml) was the lowest of any drug tested except azithromycin. There was also activity in vitro against ureaplasmas, with MICs ranging from 0.5 to 8.0 μg/ml, but LBM-415 was the least potent agent tested against these organisms in terms of MIC₅₀ and MIC₉₀. One Ureaplasma isolate was resistant to doxycycline (MIC = 8 μg/ml), and its corresponding MIC for LBM-415 was 4 μg/ml. LBM-415 had no appreciable activity in vitro against M. hominis or M. fermentans, as evidenced by all MICs for these mycoplasmas exceeding 32 μg/ml, with most of them exceeding 256 μg/ml.

TABLE 1.

MIC summary for LBM-415 tested against mycoplasmas and ureaplasmas

Organism and drug (n)	MIC (μg/ml)a
	Range	50%	90%
M. pneumoniae (100)
LBM-415	0.000063-0.008	0.0005	0.001
Azithromycin	0.000063-0.001	0.000125	0.00025
Clarithromycin	0.000125-0.008	0.002	0.004
Doxycycline	0.016-0.5	0.25	0.25
Levofloxacin	0.016-2.0	1.0	1.0
Gatifloxacin	0.0005-0.5	0.125	0.125
Moxifloxacin	0.032-0.25	0.063	0.125
M. hominis (20)
LBM-415	32->256	>256	>256
Clindamycin	0.008-1.0	0.063	0.031
Doxycyclineb	0.016-8.0	2.0	8.0
Levofloxacin	0.125	0.25	0.5
Gatifloxacin	0.016-0.125	0.063	0.125
Moxifloxacin	0.016-0.063	0.031	0.063
M. fermentans (10)
LBM-415	128->256	>256	>256
Clindamycin	0.008-0.125	0.063	0.125
Doxycycline	0.063-0.5	0.5	0.5
Levofloxacin	0.063-0.25	0.125	0.25
Gatifloxacin	0.008-0.016	0.016	0.016
Moxifloxacin	0.016-0.031	0.016	0.031
Ureaplasma species (50)
LBM-415	0.5-8.0	2.0	8.0
Azithromycin	0.125-4.0	1.0	2.0
Clarithromycin	0.016-0.5	0.031	0.125
Doxycyclinec	0.031-8.0	0.063	0.25
Levofloxacin	0.25-2.0	0.5	1.0
Gatifloxacin	0.25-2.0	0.5	1.0
Moxifloxacin	0.125-1.0	0.25	0.5

^a50% and 90%, MIC₅₀ and MIC₉₀, respectively.

^bSix isolates were resistant to doxycycline (MICs = 4 to 8 μg/ml).

^cOne isolate was resistant to doxycycline (MIC = 8 μg/ml).

NVP LBM-415 activity against M. pneumoniae and Ureaplasma species was bacteriostatic. Minimal bactericidal concentrations for this agent were ≥8 times the corresponding MIC for both Ureaplasma isolates and all five M. pneumoniae isolates tested. Bacteriostatic effect was also evident by the time-kill assay. This observation is consistent with what has been reported for this drug against other bacterial pathogens such as Staphylococcus aureus and Escherichia coli (2, 4).

Even though genes encoding PDF have been identified in all eubacterial human pathogens for which the entire genomic sequence is known, recent evidence suggests that peptide deformylation is not essential under all circumstances in every eubacterial species (11). PDF genes have been described in M. pneumoniae, Mycoplasma genitalium, and Ureaplasma spp. (7, 8), but some mycoplasmas of animal origin such as Mycoplasma hyopneumoniae do not encode the gene for PDF (10). Whether the lack of inhibitory activity of NVP LBM-415 against M. fermentans and M. hominis is due to a lack of PDF in these species, whether it may be present but nonessential, or whether an alternative mechanism of resistance is operative is not known, since their complete genomic content has not been determined. It should be noted that M. hyopneumoniae is a member of the M. hominis taxonomic group within the class Mollicutes, as is M. fermentans (15). Since these organisms are somewhat closely related, it is plausible that none of them encodes the gene for PDF.

Our MIC data indicated that M. pneumoniae is susceptible to LBM-415 at very low concentrations rivaling those of the macrolides, which are known to be the most potent drugs against this organism. However, its inhibition of Ureaplasma spp. was considerably less. Whether these higher MICs for Ureaplasma spp. were a direct result of less potent inhibition or possibly other factors, such as the effect of the medium composition and low pH (pH 6.0) necessary to cultivate ureaplasmas in vitro, is not known.

NVP LBM-415 is an interesting agent that merits further study for potential use in treatment of respiratory tract infections caused by M. pneumoniae and other bacterial pathogens for which it has known inhibitory properties. Resistance mechanisms in peptide deformylase inhibitors have been described, so it is important to consider the potential impact of this in their clinical developments (6).