Bacterial resistance development has become a very serious clinical problem for many classes of antibiotics. The 3-aryl-2-oxazolidinones are a relatively new class of synthetic antibacterial agents, having a new mechanism of action which involves very early inhibition of bacterial protein synthesis. We have prepared two potent, synthetic oxazolidinones, U-100592 and U-100766, which are currently in clinical development for the treatment of serious multidrug-resistant Gram-positive bacterial infections caused by strains of staphylococci, streptococci, and enterococci. The in vitro and in vivo (po and iv) activities of U-100592 and U-100766 against representative strains are similar to those of vancomycin. U-100592 and U-100766 demonstrate potent in vitro activity against Mycobacterium tuberculosis. A novel and practical asymmetric synthesis of (5S)-(acetamidomethyl)-2-oxazolidinones has been developed and is employed for the synthesis of U-100592 and U-100766. This involves the reaction of N-lithioarylcarbamates with (R)-glycidyl butyrate, resulting in excellent yields and high enantiomeric purity of the intermediate (R)-5-(hydroxymethyl)-2-oxazolidinones.

Oxazolidinones make up a relatively new class of antimicrobial agents which possess a unique mechanism of bacterial protein synthesis inhibition. U-100592 (S)-N-[[3-[3-fluoro-4-[4-(hydroxyacetyl)-1-piperazinyl]- phenyl]-2-oxo-5-oxazolidinyl]methyl]-acetamide and U-100766 (S)-N-[[3-[3-fluoro-4-(4-morpholinyl)phenyl]- 2-oxo-5-oxazolidinyl]methyl]-acetamide are novel oxazolidinone analogs from a directed chemical modification program. MICs were determined for a variety of bacterial clinical isolates; the respective MICs of U-100592 and U-100766 at which 90% of isolates are inhibited were as follows: methicillin-susceptible Staphylococcus aureus, 4 and 4 micrograms/ml; methicillin-resistant S. aureus, 4 and 4 micrograms/ml; methicillin-susceptible Staphylococcus epidermidis, 2 and 2 micrograms/ml; methicillin-resistant S. epidermidis, 1 and 2 micrograms/ml; Enterococcus faecalis, 2 and 4 micrograms/ml; Enterococcus faecium, 2 and 4 micrograms/ml; Streptococcus pyogenes, 1 and 2 micrograms/ml; Streptococcus pneumoniae, 0.50 and 1 microgram/ml; Corynebacterium spp., 0.50 and 0.50 micrograms/ml; Moraxella catarrhalis, 4 and 4 micrograms/ml; Listeria monocytogenes, 8 and 2 micrograms/ml; and Bacteroides fragilis, 16 and 4 micrograms/ml. Most strains of Mycobacterium tuberculosis and the gram-positive anaerobes were inhibited in the range of 0.50 to 2 micrograms/ml. Enterococcal strains resistant to vancomycin (VanA, VanB, and VanC resistance phenotypes), pneumococcal strains resistant to penicillin, and M. tuberculosis strains resistant to common antitubercular agents (isoniazid, streptomycin, rifampin, ethionamide, and ethambutol) were not cross-resistant to the oxazolidinones. The presence of 10, 20, and 40% pooled human serum did not affect the antibacterial activities of the oxazolidinones. Time-kill studies demonstrated a bacteriostatic effect of the analogs against staphylococci and enterococci but a bactericidal effect against streptococci. The spontaneous mutation frequencies of S. aureus ATCC 29213 were <3.8 x 10(-10) and <8 x 10(-11) for U-100592 and U-100766, respectively. Serial transfer of three staphylococcal and two enterococcal strains on drug gradient plates produced no evidence of rapid resistance development. Thus, these new oxazolidinone analogs demonstrated in vitro antibacterial activities against a variety of clinically important human pathogens.

TR-701 is the orally active prodrug of TR-700, a novel oxazolidinone that demonstrates four-to eightfoldgreater activity than linezolid (LZD) against Staphylococcus and Enterococcus spp. In this study evaluating the in vitro sensitivity of LZD-resistant isolates, TR-700 demonstrated 8-to 16-fold-greater potency than LZD against all strains tested, including methicillin-resistant Staphylococcus aureus (MRSA), strains of MRSA carrying the mobile cfr methyltransferase gene, and vancomycin-resistant enterococci. The MIC 90 for TR-700 against LZD-resistant S. aureus was 2 g/ml, demonstrating the utility of TR-700 against LZD-resistant strains. A model of TR-700 binding to 23S rRNA suggests that the increased potency of TR-700 is due to additional target site interactions and that TR-700 binding is less reliant on target residues associated with resistance to LZD.Oxazolidinone antibiotics are one of the newest classes of antibiotics developed within the past 30 years, with linezolid (LZD) representing the only marketed member of this class. In 2000, LZD (Zyvox) was granted approval for the treatment of infections associated with vancomycin-resistant Enterococcus faecium, nosocomial pneumonia, community-acquired pneumonia due to Streptococcus pneumoniae and methicillin-sensitive Staphylococcus aureus (MSSA), and complicated skin and skin structure infections, including cases due to methicillinresistant Staphylococcus aureus (MRSA) (1). Later approvals included pediatric use, pneumonia due to multidrugresistant S. pneumoniae, and treatment of diabetic foot infections, without osteomyelitis, caused by gram-positive bacteria. These approvals represent important milestones for the novel oxazolidinone class in the treatment of serious infections.Oxazolidinones have been shown to bind to the 50S ribosomal subunit and inhibit protein translation (31). A model of the binding of LZD to the 23S rRNA peptidyl transferase region has been previously published, based upon in vivo crosslinking experiments (18). This model predicts that LZD would specifically interfere with the binding of the amino acid portion of the aminoacyl tRNA to the ribosomal A site. The recent crystal structure of LZD bound to the 50S ribosomal subunit confirms these findings and suggests that the mechanism of inhibition involves competition with the incoming A site substrates (13). Mutations in the 23S rRNA central loop of domain V, the peptidyl transferase center (PTC), are associated with the development of LZD resistance.

A series of new nitrogen-carbon-linked (azolylphenyl)oxazolidinone antibacterial agents has been prepared in an effort to expand the spectrum of activity of this class of antibiotics to include Gram-negative organisms. Pyrrole, pyrazole, imidazole, triazole, and tetrazole moieties have been used to replace the morpholine ring of linezolid (2). These changes resulted in the preparation of compounds with good activity against the fastidious Gram-negative organisms Haemophilus influenzae and Moraxella catarrhalis. The unsubstituted pyrrolyl analogue 3 and the 1H-1,2,3-triazolyl analogue 6 have MICs against H. influenzae = 4 microgram/mL and M. catarrhalis = 2 microgram/mL. Various substituents were also placed on the azole moieties in order to study their effects on antibacterial activity in vitro and in vivo. Interesting differences in activity were observed for many analogues that cannot be rationalized solely on the basis of sterics and position/number of nitrogen atoms in the azole ring. Differences in activity rely strongly on subtle changes in the electronic character of the overall azole systems. Aldehyde, aldoxime, and cyano azoles generally led to dramatic improvements in activity against both Gram-positive and Gram-negative bacteria relative to unsubstituted counterparts. However, amide, ester, amino, hydroxy, alkoxy, and alkyl substituents resulted in no improvement or a loss in antibacterial activity. The placement of a cyano moiety on the azole often generates analogues with interesting antibacterial activity in vitro and in vivo. In particular, the 3-cyanopyrrole, 4-cyanopyrazole, and 4-cyano-1H-1,2,3-triazole congeners 28, 50, and 90 had S. aureus MICs

During the course of our investigations in the oxazolidinone antibacterial agent area, we have identified a subclass with especially potent in vitro activity against mycobacteria. The salient structural feature of these oxazolidinone analogues, 6 (U-100480), 7 (U-101603), and 8 (U-101244), is their appended thiomorpholine moiety. The rational design, synthesis, and evaluation of the in vitro antimycobacterial activity of these analogues is described. Potent activity against a screening strain of Mycobacterium tuberculosis was demonstrated by 6 and 7 (minimum inhibitory concentrations or MIC's < or = 0.125 micrograms/mL). Oxazolidinones 6 and 8 exhibit MIC90 values of 0.50 micrograms/mL or less against a panel of organisms consisting of five drug-sensitive and five multidrug-resistant strains of M. tuberculosis, with 6 being the most active congener. Potent in vitro activity against other mycobacterial species was also demonstrated by 6. For example, 6 exhibited excellent in vitro activity against multiple clinical isolates of Mycobacterium avium complex (MIC's = 0.5-4 micrograms/mL). Orally administered 6 displays in vivo efficacy against M. tuberculosis and M. avium similar to that of clinical comparators isoniazid and azithromycin, respectively. Consideration of these factors, along with a favorable pharmaco-kinetic and chronic toxicity profile in rats, suggests that 6 (U-100480) is a promising antimycobacterial agent.