The muraymycins, a family of nucleoside-lipopeptide antibiotics, were purified from the extract of Streptomyces sp. LL-AA896. The antibiotics were purified by chromatographic methods and characterized by NMR spectroscopy, degradation studies, and mass spectrometry. The structures of 19 compounds were established. The muraymycins constitute a new antibiotic family whose core structure contains a glycosylated uronic acid derivative joined by an aminopropane group to a hexahydro-2-imino-4-pyrimidylglycyl residue (epicapreomycidine) containing dipeptide that is further extended by a urea-valine moiety. Members of this family show broad-spectrum in vitro antimicrobial activity against a variety of clinical isolates (MIC 2 to >64 mug/mL). The muraymycins inhibited peptidoglycan biosynthesis. The fatty acid substituent and the presence or absence of the amino sugar play important roles in biological activity. One of the most active compounds, muraymycin A1, demonstrated protection in vivo against Staphylococcus aureus infection in mice (ED50 1.1 mg/kg).

The lack of antagonism seen with tigecycline combinations in both chequerboard and time-kill kinetic studies is an encouraging outcome, suggesting that tigecycline may prove to be effective in combination therapy as well as in monotherapy.

The 9-t-butylglycylamido derivative of minocycline (TBG-MINO) is a recently synthesized member of a novel group of antibiotics, the glycylcyclines. This new derivative, like the first glycylcyclines, theN,N-dimethylglycylamido derivative of minocycline and 6-demethyl-6-deoxytetracycline, possesses activity against bacterial isolates containing the two major determinants responsible for tetracycline resistance: ribosomal protection and active efflux. The in vitro activities of TBG-MINO and the comparative agents were evaluated against strains with characterized tetracycline resistance as well as a spectrum of recent clinical aerobic and anaerobic gram-positive and gram-negative bacteria. TBG-MINO, with an MIC range of 0.25 to 0.5 μg/ml, showed good activity against strains expressing tet(M) (ribosomal protection), tet(A), tet(B),tet(C), tet(D), and tet(K) (efflux resistance determinants). TBG-MINO exhibited similar activity against methicillin-resistant Staphylococcus aureus (MRSA), penicillin-resistant streptococci, and vancomycin-resistant enterococci (MICs at which 90% of strains are inhibited, ≤0.5 μg/ml). TBG-MINO exhibited activity against a wide diversity of gram-negative aerobic and anaerobic bacteria, most of which were less susceptible to tetracycline and minocycline. The in vivo protective effects of TBG-MINO were examined against acute lethal infections in mice caused by Escherichia coli, S. aureus, andStreptococcus pneumoniae isolates. TBG-MINO, administered intravenously, demonstrated efficacy against infections caused byS. aureus including MRSA strains and strains containingtet(K) or tet(M) resistance determinants (median effective doses [ED50s], 0.79 to 2.3 mg/kg of body weight). TBG-MINO demonstrated efficacy against infections caused by tetracycline-sensitive E. coli strains as well asE. coli strains containing either tet(M) or the efflux determinant tet(A), tet(B), ortet(C) (ED50s, 1.5 to 3.5 mg/kg). Overall, TBG-MINO shows antibacterial activity against a wide spectrum of gram-positive and gram-negative aerobic and anaerobic bacteria including strains resistant to other chemotherapeutic agents. The in vivo protective effects, especially against infections caused by resistant bacteria, corresponded with the in vitro activity of TBG-MINO.

Tigecycline, an expanded-broad-spectrum glycylcycline antibiotic is not affected by the classical tetracycline resistance determinants found in Staphylococcus aureus. The in vitro selection of mutants with reduced susceptibility to tigecycline was evaluated for two methicillin-resistant S. aureus strains by serial passage in increasing concentrations of tigecycline. Both strains showed a stepwise elevation in tigecycline MIC over a period of 16 days, resulting in an increase in tigecycline MIC of 16-and 32-fold for N315 and Mu3, respectively. Transcriptional profiling revealed that both mutants exhibited over 100-fold increased expression of a gene cluster, mepRAB (multidrug export protein), encoding a MarR-like transcriptional regulator (mepR), a novel MATE family efflux pump (mepA), and a hypothetical protein of unknown function (mepB). Sequencing of the mepR gene in the mutant strains identified changes that presumably inactivated the MepR protein, which suggested that MepR functions as a repressor of mepA. Overexpression of mepA in a wild-type background caused a decrease in susceptibility to tigecycline and other substrates for MATE-type efflux pumps, although it was not sufficient to confer high-level resistance to tigecycline. Complementation of the mepR defect by overexpressing a wild-type mepR gene reduced mepA transcription and lowered the tigecycline MIC in the mutants. Transcription of tet(M) also increased by over 40-fold in the Mu3 mutant. This was attributed to a deletion in the promoter region of the gene that removed a stem-loop responsible for transcriptional attenuation. However, overexpression of the tet(M) transcript in a tigecycline-susceptible strain was not enough to significantly increase the MIC of tigecycline. These results suggest that the overexpression of mepA but not tet(M) may contribute to decreased susceptibility of tigecycline in S. aureus.Staphylococcus aureus is an important human pathogen causing infections that range in severity from superficial skin abscesses to more serious invasive diseases (32). Methicillin-resistant S. aureus (MRSA) is a problem in hospitals worldwide (1). The transfer of vancomycin resistance from Enterococcus species into S. aureus (8) and the emerging problem of MRSA infections in the community setting (27) are of particular concern. Tigecycline, a novel glycylcycline antibiotic, exhibits good antimicrobial activity against a broad spectrum of gram-positive and gram-negative pathogens including MRSA and S. aureus strains with intermediate and high levels of resistance to vancomycin (7,23).

Tigecycline (GAR-936) and daptomycin are potent antibacterial compounds in advanced stages of clinical trials. These novel agents target multiply resistant pathogenic bacteria. Daptomycin is principally active against gram-positive bacteria, while tigecycline has broad-spectrum activity. When tested by the standard protocols of the National Committee for Clinical Laboratory Standards in Mueller-Hinton broth II, tigecycline was more active than daptomycin (MICs at which 90% of isolates tested are inhibited, 0.12 to 1 and 0.5 to 16 g/ml, respectively) against staphylococcal, enterococcal, and streptococcal pathogens. Tigecycline (GAR-936), a glycylcycline (36), and daptomycin, a lipopeptide (1), are novel antibacterial compounds undergoing clinical development. Tigecycline is a broad-spectrum, protein-inhibiting, antibacterial agent possessing activity against strains resistant to other chemotherapeutic agents (14,29). Daptomycin, a cell wall-inhibiting antibiotic with a spectrum of activity limited to gram-positive bacteria, has also been demonstrated to have activity against resistant bacteria (34). Early clinical trials with daptomycin were discontinued due to less-than-desired outcomes (32) including unwanted side effects on skeletal muscle. However, new dosage regimens (27) have allowed daptomycin to progress into clinical trials (37). These antibacterial agents offer new alternatives for the treatment of infections caused by clinically relevant pathogens for which limited therapeutic options exist.The rise in the incidence of methicillin-resistant Staphylococcus aureus (MRSA) strains (28) and the emergence of strains with intermediate glycopeptide resistance (38) have emphasized the lack of therapeutic alternatives. Recently, a collection of glycopeptide-intermediate S. aureus (GISA) strains with reduced susceptibilities to the glycopeptide antibiotics (vancomycin and teicoplanin) has been assembled by the Network on Antibiotic Resistance in Staphylococcus aureus (NARSA). That study was undertaken to evaluate the in vitro activities of tigecycline, daptomycin, and comparative antibiotics against these GISA and other drug-resistant gram-positive isolates by the standard methodology of the National Committee for Clinical Laboratory Standards (NCCLS) (26). The activity of daptomycin was determined in both Mueller-Hinton broth II (MHB II) and Mueller-Hinton broth supplemented with 50 mg of calcium per liter. In addition, the effects of calcium concentration and the culture medium on the activities of the antibiotics were determined for the GISA, MRSA, and methicillin-susceptible S. aureus (MSSA) isolates, as daptomycin is a calcium-dependent antibiotic. The supplemental calcium concentrations (25, 50, and 75 mg/liter) recommended by other investigators (34) were used for these studies. MATERIALS AND METHODSOrganisms. Routine clinical isolates were collected from various medical centers in the United States and Canada between 1990 and 1999. Identification of each culture was performed by conventional methodologie...

N,N-Dimethylglycylamido (DMG) derivatives of minocycline and 6-demethyl-6-deoxytetracycline are new semisynthetic tetracyclines referred to as the-glycylcyclines. The in vitro activities of the glycylcyclines were evaluated in comparison with those of minocycline and tetracycline against strains carrying characterized tetracycline resistance determinants and against 995 recent clinical isolates obtained from geographically distinct medical centers in North America. The glycylcyclines were active against tetracycline-resistant strains carrying efflux [tet(A), tet(B), tet(C), and tet(D) in Escherichia coli and tet(K) in Staphylococcus aureus] and ribosomal protection [tet(M) in S. aureus, Enterococcusfaecalis, and E. coli)] resistance determinants. Potent activity (MIC for 90% of strains, c0.5 ,ug/ml) was obtained with the glycylcyclines against methicillinsusceptible and methicillin-resistant S. aureus, E. faecalis, Enterococcus faecium, and various streptococcal species. The glycylcyclines exhibited good activity against a wide diversity of gram-negative aerobic and anaerobic bacteria, most of which were less susceptible to minocycline and tetracycline. The activities of the glycylcyclines against most organisms tested were comparable to each other. The in vivo efficacies of the glycylcyclines against acute lethal infections in mice when dosed intravenously were reflective of their in vitro activities. The glycylclines had efficacies comparable to that of minocycline against infections with methicillin-susceptible and methicillin-resistant S. aureus strains, a strain carrying tet(K), and a tetracyclinesusceptible E. coli strain but exceeded the effectiveness of minocycline against infections with resistant isolates, including strains harboring tet(M) or tet(B). Levels of DMG-6-demethyl-6-deoxytetracycline in serum were higher and more sustained than those of DMG-minocycline or minocycline. Our results show that the glycylcyclines have potent in vitro activities against a wide spectrum of gram-positive and gram-negative, aerobic and anaerobic bacteria, including many resistant strains. On the basis of their in vitro and in vivo activities, the glycylcyclines represent a significant advance to the tetracycline class of antibiotics and have good potential value for clinical efficacy.The tetracyclines, first isolated at Lederle Laboratories in 1945 from a strain of Streptomyces aureofaciens, represented a significant advance in the treatment of many infections (11). The activity of the tetracyclines against a wide variety of gram-positive and gram-negative aerobic and anaerobic bacteria, mycoplasmas, and rickettsiae and their efficacy against both intracellular and extracellular pathogens permitted their widespread use (12). Through modifications of the fermentation conditions and semisynthetic synthesis, several analogs, such as minocycline (MINO) and doxycycline, which exhibited improved antimicrobial activity and more favorable pharmacokinetic properties over those of the early tetracyclines were prepared (20, 27...

The in vitro activity of tigecycline was evaluated against 4913 baseline pathogens isolated from 1986 patients enrolled in 4 pivotal phase 3 clinical trials. The trials, which were conducted in 38 countries worldwide, involved patients with complicated skin and skin-structure infections or complicated intra-abdominal infections. Tigecycline was active against the most prevalent pathogens for each infection type, including gram-positive and gram-negative strains of both aerobic and anaerobic bacteria (MICs, < or =2 microg/mL for most pathogens). The spectrum of activity of tigecycline included important pathogens, such as Staphylococcus aureus (including methicillin-resistant S. aureus), Streptococcus pyogenes, Escherichia coli, Klebsiella pneumoniae, and Bacteroides fragilis. A few genera, such as Pseudomonas aeruginosa and members of the tribe Proteeae, were generally less susceptible to tigecycline than were other gram-negative pathogens. The susceptibility of the pathogens to tigecycline was similar for isolates obtained from patients enrolled in the studies of complicated skin and skin-structure infection or of complicated intra-abdominal infection. For most pathogens, the susceptibility to tigecycline was similar across all geographic regions. The excellent expanded broad-spectrum activity of tigecycline demonstrated in vitro against clinical isolates confirmed its potential utility for pathogens associated with complicated skin and skin-structure infections or complicated intra-abdominal infections.

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