In vitro antibacterial activities of PD 138312 and PD 140248, new fluoronaphthyridines with outstanding gram-positive potency

Potent 6-desfluoro-8-methylquinolones as new lead compounds in antibacterial chemotherapy

In a furtherance of our SAR study on the C-6 position of quinolone antibacterials, a series of 6-desfluoro-8-methylquinolones were synthesized and evaluated for their in vitro antimicrobial activity. As a result of this study, compounds with strong activity against Gram-positive bacteria, including ciprofloxacin-resistant and methicillin-resistant Staphylococcus aureus, were identified. The best Gram-positive antibacterial activity was exhibited by piperidinyl derivative 6c, which was 17 times more potent than ciprofloxacin and displayed extremely high activity against Streptococcus pneumoniae with an MIC value of <0.016 microg/mL. Thus, we have shown that substituent combinations in the quinolone ring, excluding the C-6 fluorine atom, might produce powerful antibacterial agents.

Studies on 6-aminoquinolones: synthesis and antibacterial evaluation of 6-amino-8-methylquinolones

The 6-aminoquinolone had previously been identified as a new class of quinolone antibacterial agents. To continue our structure-activity relationship (SAR) study in this series, novel 6-amino-8-methylquinolone derivatives have now been synthesized and evaluated for in vitro antibacterial activity. We have shown that the coupled presence of a methyl group at the C-8 position with an amino group at C-6 is effective for enhancing antibacterial activity, particularly against Gram-positive bacteria. The SARs associated with the N-1, C-6, and C-7 are discussed. The 1,2,3,4-tetrahydroisoquinolinyl derivative 19v showed the highest antibacterial activity with MIC values on Gram-positive bacteria superior to that of ciprofloxacin, especially against Staphylococcus aureus strains, including those strains which are methicillin-and ciprofloxacin-resistant.

In vivo therapeutic efficacies of PD 138312 and PD 140248, two novel fluoronaphthyridines with outstanding gram-positive potency

PD 138312 and PD 140248 are novel broad-spectrum 7-pyrrolidinyl fluoronaphthyridines with a cyclopropyl or a difluorophenyl substitution at the 1 positions, respectively. They have been demonstrated to have excellent in vitro activity against gram-positive organisms. These compounds were evaluated for their in vivo potencies against acute systemic infections in mice and in a mouse pneumococcal pneumonia model. They were very effective by both the oral and subcutaneous routes of administration. Most remarkable were their comparative median protective values against methicillin-resistant Staphylococcus aureus, Streptococcus pneumoniae, and Streptococcus pyogenes. In general, these compounds were 28- to 100-fold more active than ciprofloxacin against these clinically significant organisms when the drugs were given orally and 10- to 38-fold more active when the drugs were given parenterally. Average ratios of drug concentrations in mice after drug administration by the oral route to that after administration by the subcutaneous route indicate 34 to 44% greater bioavailabilities of PD 138312 and PD 140248 compared with that of ciprofloxacin. In a multidose pneumococcal mouse pneumonia model these new quinolones were extremely effective, with median curative doses of 2 to 2.8 mg/kg of body weight per dose. Ciprofloxacin was ineffective (median curative dose, >100 mg/kg per dose) in this model. Comparative pharmacokinetic studies in mice revealed a relative superiority of PD 140248. Peak levels of PD 140248 in blood after the administration of a single oral 50-mg/kg dose were twice those of PD 138312 and ciprofloxacin, with PD 140248 having a substantially longer half-life. These results indicate that PD 138312 and PD 140248 have excellent therapeutic potential against clinically important gram-positive pathogens when the drugs are administered both orally and parenterally.

Current perspectives on glycopeptide resistance

In the last 5 years, clinical isolates of gram-positive bacteria with intrinsic or acquired resistance to glycopeptide antibiotics have been encountered increasingly. In many of these isolates, resistance arises from an alteration of the antibiotic target site, with the terminal D-alanyl-D-alanine moiety of peptidoglycan precursors being replaced by groups that do not bind glycopeptides. Although the criteria for defining resistance have been revised frequently, the reliable detection of low-level glycopeptide resistance remains problematic and is influenced by the method chosen. Glycopeptide-resistant enterococci have emerged as a particular problem in hospitals, where in addition to sporadic cases, clusters of infections with evidence of interpatient spread have occurred. Studies using molecular typing methods have implicated colonization of patients, staff carriage, and environmental contamination in the dissemination of these bacteria. Choice of antimicrobial therapy for infections caused by glycopeptide-resistant bacteria may be complicated by resistance to other antibiotics. Severe therapeutic difficulties are being encountered among patients infected with enterococci, with some infections being untreatable with currently available antibiotics.

In vitro and in vivo activities of clinafloxacin, CI-990 (PD 131112), and PD 138312 versus enterococci

Certain new fluoroquinolones have high activity against enterococci. Against Enterococcus faecalis (n = 18), MICs at which 90% of the isolates were inhibited were as follows (in micrograms per milliliter): clinafloxacin, 0.125; CI-990, 0.5; and PD 138312, 0.25 (compared with 1 microgram/ml for ciprofloxacin and 2 micrograms/ml for ofloxacin). Strains producing beta-lactamase or that were vancomycin resistant or resistant to high-level gentamicin were not quinolone cross-resistant. The drugs were bactericidal and were unaffected by 50% human serum. Oral efficacies (in milligrams per kilogram of body weight for 50% protective doses) in lethal mouse infections with quinolone-susceptible strains were 4.3 to 24 for clinafloxacin, 7.2 to 39 for CI-990, 7.2 to 76 for PD 138312, and 41 to > 100 for ciprofloxacin; when the drugs were given subcutaneously, the order was similar and values ranged from 1.1 to 12.5. Clinafloxacin, CI-990, and PD 138312 may have therapeutic potential in systemic enterococcal infections in humans.

In vitro activity of fluoroquinolones against gram-positive bacteria

This paper reviews the in vitro activities of several newer fluoroquinolone antimicrobials that exhibit enhanced potency against Gram-positive bacteria. Several of these agents demonstrate 10-fold greater activity than older members of this class against Staphylococcus aureus and inhibit [minimum inhibitory concentration (MIC90) values < or = 2 mg/L] many isolates resistant to ciprofloxacin or ofloxacin. Markedly enhanced activity is also noted against Streptococcus pneumoniae, 90% of isolates being inhibited at concentrations 10- to 100-fold lower than those of the older agents. Enterococci also exhibit greater susceptibility to several of the newer fluoroquinolones, although relative cross-resistance with the earlier drugs is noted. As determined by dilution techniques, the new fluoroquinolones generally demonstrate bactericidal activity at concentrations at or near their MIC values. The activities of the new compounds described here are decreased at low pH, but are not affected by the addition of up to 50% human serum to the test medium. Resistance is rarely detected (frequency < 10(-9)) when high density bacterial suspensions are plated in the presence of 4 times the MIC of these compounds. However, colonies displaying relative resistance to the new agents can be selected by serial passage in incremental antimicrobial concentrations.