Synthesis and biological evaluation of benzo[b]naphthyridones, a series of new topical antibacterial agents.

We describe here the synthesis and biological evaluation of a series of benzo[b]naphthyridones, a new family of tricyclic antibacterial compounds that have a gram-positive spectrum of activity. RP60556A, one of the most potent of these compounds, is bactericidal against multiresistant cocci, especially multiresistant Staphylococcus aureus strains. Its physico-chemical and biological properties make it particularly suitable for topical antibacterial use.

Synthesis and in vitro antibacterial activity of catechol-spiramycin conjugates.

The first synthesis of siderophore conjugates of two macrolide antibiotics, spiramycin 1 and neospiramycin 2, which are unable to penetrate the outer membrane of gram-negative bacteria are described. These novel conjugates were prepared by regioselective acylation of a hydroxyl function of 1 and 2 with a dihydroxybenzoic Fe(III) complexing ligand linked via a carboxyl group containing spacer to the macrolide antibiotics. The preliminary biological evaluation of these novel conjugates under standard and iron depleted conditions has shown that their antibacterial activity was comparable to that of spiramycin 1 and neospiramycin 2.

Recent developments in streptogramin research.

The streptogramins are a class of antibiotics remarkable for their antibacterial activity and their unique mechanism of action. These antibiotics are produced naturally, but the therapeutic use of the natural compounds is limited because they do not dissolve in water. New semisynthetic derivatives, in particular the injectable streptogramin quinupristin/dalfopristin, offer promise for treating the rising number of infections that are caused by multiply resistant bacteria. The streptogramins consist of two structurally unrelated compounds, group A and group B. The group A compounds are polyunsaturated macrolactones: the group B compounds are cyclic hexadepsipeptides. Modifications of the group B components have been mainly performed on the 3-hydroxypicolinoyl, the 4-dimethylaminophenylalanine and the 4-oxo pipecolinic residues. Semi-synthesis on this third residue led to the water-soluble derivative quinupristin. Water-soluble group A derivatives were obtained by Michael addition of aminothiols to the dehydroproline ring of pristinamycin IIA. Followed by oxidation of the intermediate sulfide into the sulfone derivatives (i.e., dalfopristin). Water-soluble derivatives (both group A and group B) can now be obtained at the industrial scale. Modified group B compounds are now also being produced by mutasynthesis, via disruption of the papA gene. Mutasynthesis has proved particularly useful for producing PIB, the group B component of the oral streptogramin RPR 106972. The streptogramins inhibit bacterial growth by disrupting the translation of mRNA into protein. Both the group A and group B compounds bind to the peptidyltransferase domain of the bacterial ribosome. The group A compounds interfere with the elongation of the polypeptide chain by preventing the binding of aa-tRNA to the ribosome and the formation of peptide bonds, while the B compounds stimulate the dissociation of the peptidyl-tRNA and may also interfere with the release of the completed polypeptide by blocking its access to the channel through which it normally leaves the ribosome. The synergy between the group A and group B compounds appears to result from an enhanced affinity of the group B compounds for the ribosome. Apparently, the group A compound induces a conformational change such that B compound binds with greater affinity. The natural streptogramins are produced as mixtures of the group A and B compounds, the combination of which is a more potent antibacterial agent than either type of compound alone. Whereas the type A or type B compound alone has, in vitro and in animal models of infection, a moderate bacteriostatic activity, the combination of the two has strong bacteriostatic activity and often bactericidal activity. Minimal inhibitory concentrations of quinupristin/dalfopristin range from 0.20 to 1 mg/l for Streptococcus pneumonae, from 0.25 to 2 mg/l for Staphylococcus aureus and from 0.50 to 4 for Enterococcus faecium, the principal target organisms of this drug. Quinupristin/dalfopristin also has activity against mycoplasmas, Neisseria gonorrhoeae, Haemophilus influenz, Legionella spp. and Moraxella catarrhalis. Bacteria develop resistance to the streptogramms by ribosomal modification, by producing inactivating enzymes, or by causing an efflux of the antibiotic. Dimethylation of an adenine residue in rRNA, a reaction that is catalyzed by a methylase encoded by the erm gene class, affects the binding of group B compounds (as well as the macrolides and lincosamides; hence, MLSB resistance), but group A and B compounds usually maintain their synergy and their bactericidal effect against MLSB-resistant strains. erm genes are widespread both geographically and throughout numerous bacterial genera. Several types of enzymes (acetyltransferases, hydrolases) have been identified that inactivate the group A or the group B compounds. Genes involved in streptogramin efflux have so far been found only in staphylococci, particularly in coagulase-negative species

Antistaphylococcal activities of quinupristin/dalfopristin in vitro across platelet-fibrin matrices and in experimental endocarditis.

In-vitro and in-vivo efficacies of quinupristin/dalfopristin and vancomycin against methicillin-resistant Staphylococcus aureus (MRSA) responsible for endocarditis have been compared. The following parameters were investigated: MIC, activity across a platelet-fibrin matrix simulating cardiac vegetations, killing of bacteria on the cardiac vegetations resulting from experimental aortic valve endocarditis in an animal model, and concentrations of antibiotics in the serum and vegetations of infected rabbits. The same bacterial strain was used for all experiments. The MICs of quinupristin/dalfopristin and vancomycin were 0.25 and 1 mg/L, respectively. When tested for their ability to penetrate platelet-fibrin matrices, both drugs were bactericidal against the MRSA strain (> or = 2 log10 cfu/mL decrease in 2 h). Both drugs significantly reduced the bacterial counts in vegetations in infected rabbits. Within 12 h of intravenous administration of 20 mg/kg, concentrations of quinupristin/dalfopristin decreased from 5.3 to < 0.10 mg/L in serum and from 12.9 to < 1 mg/kg in the valve vegetations. Although the concentrations of vancomycin in the serum and the infected tissue were higher than those of quinupristin/dalfopristin, the latter combination was equally effective, perhaps because its bactericidal activity is rapid and because it can more easily penetrate the cardiac vegetations.

In-vitro bactericidal activity of quinupristin/dalfopristin against adherent Staphylococcus aureus.

A simple in-vitro assay was developed to test antibiotics for bactericidal activity against Staphylococcus aureus adhering to an artificial biomaterial. The activity of the semisynthetic injectable streptogramin quinupristin/dalfopristin combination was evaluated with this assay and compared with that of vancomycin. Fibronectin-coated nylon membranes were used to mimic implanted biomaterial. To obtain adherent bacteria, 400 microL of a suspension of S. aureus strain Wood (5 x 10(6) cfu/mL) were placed for 1.5 h at 37 degrees C in contact with the membranes. After elimination of non-adherent bacteria by successive rinses, membranes were exposed to antibacterial agents at concentrations of 5, 10 and 50 x MIC. Drug-free controls were also prepared. After various incubation times, bacteria on both treated and control membranes were removed by sonication and subcultured on to agar medium. After incubation, the number of cfu on treated membranes was compared with that on t0 control membranes. The criterion for bactericidal activity was a 99.9% reduction of the t0 control inoculum. Under these experimental conditions, both quinupristin/dalfopristin and vancomycin, at concentrations achievable in blood (6.25 and 20 mg/L, respectively), demonstrated a potent bactericidal activity against S. aureus adhering to a support. The bactericidal effect of quinupristin/dalfopristin was more rapid than that of vancomycin. Quinupristin/dalfopristin may therefore be a useful alternative to vancomycin in the treatment of implant-associated S. aureus infections.

Bactericidal activity and kinetics of RP 59500 in a mouse model of Staphylococcus aureus septicaemia.

The bactericidal activity of RP 59500, a semisynthetic streptogramin, was compared with that of vancomycin against Staphylococcus aureus. This activity was evaluated in vitro by the kill curve method and in vivo using a model of mouse septicaemia. In vitro, RP 59500 (MIC = 0.12 mg/L) was more rapidly bactericidal against S. aureus IP 8203 than was vancomycin (MIC = 1 mg/L). In vivo, RP 59500 (120 mg/kg) was bactericidal against staphylococci in the blood of infected mice 1 h after administration, an effect which lasted for up to 7 h, whereas vancomycin at the same dose was bactericidal only 4 h after administration. The serum concentrations of vancomycin were higher than those of RP 59500 for at least 4 h after administration, and the Cmax and AUC of vancomycin were 4.8 and 8.3 times higher, respectively, than those of RP 59500 (Cmax = 13.2 mg/L; AUC0(-1) = 15.2 mg/L/h), although the agents had similar elimination half-lives (about 0.5 h). RP 59500 was also administered to mice as a fractionated dose (30 mg/kg x 4, 40 mg/kg x 3, 60 mg/kg x 2). The onset of its bactericidal effect was delayed by fractionating the dose, but the suppression of bacteria in the blood was prolonged.

Post-antibiotic effects of RP 59500 with Staphylococcus aureus.

The post-antibiotic effect (PAE) of the semisynthetic streptogramin RP 59500 was evaluated with four clinical isolates of Staphylococcus aureus (two strains susceptible to methicillin and to the macrolide-lincosamide-streptogramin B (MLSB) group and two strains resistant to these drugs). The PAE was defined as the time required for either the viable counts to increase by one log10 or for bacteria to regain their maximal rate of growth. Similar results were obtained regardless of which definition was used. For each experiment, the time of exposure of the bacteria to the drug ranged from 15-80 min. At a concentration of 0.5 x MIC and an exposure time of 80 min, RP 59500 produced a PAE in three of the four strains examined. At higher concentrations (1, 2 and 4 x MIC), a PAE was observed with all four strains. When considered as the time required for bacterial growth to return to its maximal rate, the PAE lasted for at least 7 h with the two methicillin-susceptible strains when they were exposed for 80 min to RP 59500 at 4 x MIC, compared with 5 h for the two methicillin-resistant strains and the same exposure conditions. The concentration of antibiotic was found to be a more important parameter in determining the PAE than the time of exposure.