Deciphering the Catalytic Mechanism of Virginiamycin B Lyase with Multiscale Methods and Molecular Dynamics Simulations

Due to the emergence of antibiotic resistance, the need to explore novel antibiotics and/or novel strategies to counter antibiotic resistance is of utmost importance. In this work, we explored the molecular and mechanistic details of the degradation of a streptogramin B antibiotic by virginiamycin B (Vgb) lyase of Staphylococcus aureus using classical molecular dynamics simulations and multiscale quantum mechanics/molecular mechanics methods. Our results were in line with available experimental kinetic information. Although we were able to identify a stepwise mechanism, in the wild-type enzyme, the intermediate is short-lived, showing a small barrier to decay to the product state. The impact of point mutations on the reaction was also assessed, showing not only the importance of active site residues to the reaction catalyzed by Vgb lyase but also of near positive and negative residues surrounding the active site. Using molecular dynamics simulations, we also predicted the most likely protonation state of the 3-hydroxypicolinic moiety of the antibiotic and the impact of mutants on antibiotic binding. All this information will expand our understanding of linearization reactions of cyclic antibiotics, which are crucial for the development of novel strategies that aim to tackle antibiotic resistance.

Analytical determination of virginiamycin drug residues in edible porcine tissues by LC-MS with confirmation by LC-MS/MS

A liquid chromatographic-mass spectrometric (LC-MS) method was developed and validated for the determination and confirmation of virginiamycin (VMY) M1 residues in porcine liver, kidney, and muscle tissues at concentrations > or =2 ng/g. Porcine liver, kidney, or muscle tissue is homogenized with methanol-acetonitrile. After centrifugation, the supernatant is diluted with phosphate buffer and cleaned up on a C18 solid-phase extraction cartridge. VMY in the eluate is partitioned into chloroform and the aqueous upper layer is removed by aspiration. After evaporating the chloroform in the residual mixture to dryness, the dried extract is reconstituted in mobile phase and VMY is quantified by LC-MS. Any samples eliciting quantifiable levels of VMY M1 (i.e., at concentrations > or =2 ng/g) are subjected to confirmatory analysis by LC-MSIMS. VMY S1, a minor component of the VMY complex, is monitored but not quantified or confirmed.

De novo formal synthesis of (-)-virginiamycin M2 via the asymmetric hydration of dienoates

A de novo approach to the formal total synthesis of the macrolide natural product (-)-virginiamycin M2 has been achieved via a convergent approach. The absolute and relative stereochemistry of the nonpeptide portion of (-)-virginiamycin M2 was introduced by two Sharpless asymmetric dihydroxylation reactions.

The conformation of acetylated virginiamycin M1 and virginiamycin M1 in explicit solvents

The three-dimensional structure of acetylated virginiamycin M(1) (acetylated VM1) in chloroform and in a water/acetonitrile mixture (83:17 v/v) have been established through 2D high resolution NMR experiments and molecular dynamics modeling and the results compared with the conformation of the antibiotic VM1 in the same and other solvents. The results indicated that acetylation of the C-14 OH group of VM1 caused it to rotate about 90 degrees from the position it assumed in non-acetylated VM1. The conformation of both VM1 and acetylated VM1 appear to flatten in moving from a nonpolar to polar solvent. However, the acetylated form has a more hydrophobic nature. The acetylated VM1 in chloroform and in water/acetonitrile solution had a similar configuration to that of VM1 bound to 50S ribosomes and to the Vat(D) active sites as previously determined by X-ray crystallography. Docking studies of VM1 to the 50S ribosomal binding site and the Vat(D) gave conformations very similar to those derived from X-ray crystallographic studies. The docking studies with acetylated VM1 suggested the possibility of a hydrogen bond from the acetyl carbonyl group oxygen of acetylated VM1 to the 2' hydroxyl group of ribose of adenosine 2538 at the ribosomal VM1 binding site. No hydrogen bonds between acetylated VM1 and the Vat(D) active sites were found; the loss of this binding interaction partly accounts for the release of the product from the active site.

Characterization of biosynthetic gene cluster for the production of virginiamycin M, a streptogramin type A antibiotic, in Streptomyces virginiae

Virginiamycin M (VM) of Streptomyces virginiae is a hybrid polyketide-peptide antibiotic with peptide antibiotic virginiamycin S (VS) as its synergistic counterpart. VM and VS belong to the Streptogramin family, which is characterized by strong synergistic antibacterial activity, and their water-soluble derivatives are a new therapeutic option for combating vancomycin-resistant Gram-positive bacteria. Here, the VM biosynthetic gene cluster was isolated from S. virginiae in the 62-kb region located in the vicinity of the regulatory island for virginiamycin production. Sequence analysis revealed that the region consists of 19 complete open reading frames (ORFs) and one C-terminally truncated ORF, encoding hybrid polyketide synthase (PKS)-nonribosomal peptide synthetase (NRPS), typical PKS, enzymes synthesizing precursors for VM, transporters for resistance, regulatory proteins, and auxiliary enzymes. The involvement of the cloned gene cluster in VM biosynthesis was confirmed by gene disruption of virA encoding a hybrid PKS-NRPS megasynthetase, which resulted in complete loss of VM production without any effect on VS production. To assemble the VM core structure, VirA, VirF, VirG, and VirH consisting, as a whole, of 24 domains in 8 PKS modules and 7 domains in 2 NRPS modules were predicted to act as an acyltransferase (AT)-less hybrid PKS-NRPS, whereas VirB, VirC, VirD, and VirE are likely to be essential for the incorporation of the methyl group into the VM framework by a HMG-CoA synthase-based reaction. Among several uncommon features of gene organization in the VM gene cluster, the lack of AT domain in every PKS module and the presence of a discrete AT encoded by virI are notable. AT-overexpression by an additional copy of virI driven by ermEp() resulted in 1.5-fold increase of VM production, suggesting that the amount of VirI is partly limiting VM biosynthesis.

Structures of MLSBK antibiotics bound to mutated large ribosomal subunits provide a structural explanation for resistance

Crystal structures of H. marismortui large ribosomal subunits containing the mutation G2099A (A2058 in E. coli) with erythromycin, azithromycin, clindamycin, virginiamycin S, and telithromycin bound explain why eubacterial ribosomes containing the mutation A2058G are resistant to them. Azithromycin binds almost identically to both G2099A and wild-type subunits, but the erythromycin affinity increases by more than 10(4)-fold, implying that desolvation of the N2 of G2099 accounts for the low wild-type affinity for macrolides. All macrolides bind similarly to the H. marismortui subunit, but their binding differs significantly from what has been reported in the D. radioidurans subunit. The synergy in the binding of streptogramins A and B appears to result from a reorientation of the base of A2103 (A2062, E. coli) that stacks between them. The structure of large subunit containing a three residue deletion mutant of L22 shows a change in the L22 structure and exit tunnel shape that illuminates its macrolide resistance phenotype.

Efficacy of Quinupristin-Dalfopristin against Methicillin-Resistant Staphylococcus aureus and Vancomycin-Insensitive S. aureus in a Model of Hematogenous Pulmonary Infection

BACKGROUND

Quinupristin-dalfopristin (Q-D) is a mixture of quinupristin and dalfopristin, which are semisynthetic antibiotics of streptogramin groups B and A, respectively.

METHODS

We compared the effect of Q-D to that of vancomycin (VCM) in murine models of hematogenous pulmonary infections caused by methicillin-resistant Staphylococcus aureus (MRSA) and VCM-insensitive S. aureus (VISA).

RESULTS

Treatment with Q-D resulted in a significant decrease in the number of viable bacteria in the lungs of mice in an MRSA infection model [Q-D 100 mg/kg, Q-D 10 mg/kg, VCM and control (mean +/- SEM): 2.99 +/- 0.44, 6.38 +/- 0.32, 5.75 +/- 0.43 and 8.40 +/- 0.14 log10 CFU/lung, respectively]. Compared with VCM, high-dose Q-D significantly reduced the number of bacteria detected in the VISA hematogenous infection model [Q-D 100 mg/kg, Q-D 10 mg/kg, VCM and control (mean +/- SEM): 5.17 +/- 0.52, 7.03 +/- 0.11, 7.10 +/- 0.49 and 7.18 +/- 0.36 log10 CFU/lung, respectively]. Histopathological examination confirmed the effect of Q-D.

CONCLUSION

Our results suggest that Q-D is potent and effective in the treatment of MRSA and VISA hematogenous pulmonary infections.

Structures of five antibiotics bound at the peptidyl transferase center of the large ribosomal subunit

Structures of anisomycin, chloramphenicol, sparsomycin, blasticidin S, and virginiamycin M bound to the large ribosomal subunit of Haloarcula marismortui have been determined at 3.0A resolution. Most of these antibiotics bind to sites that overlap those of either peptidyl-tRNA or aminoacyl-tRNA, consistent with their functioning as competitive inhibitors of peptide bond formation. Two hydrophobic crevices, one at the peptidyl transferase center and the other at the entrance to the peptide exit tunnel play roles in binding these antibiotics. Midway between these crevices, nucleotide A2103 of H.marismortui (2062 Escherichia coli) varies in its conformation and thereby contacts antibiotics bound at either crevice. The aromatic ring of anisomycin binds to the active-site hydrophobic crevice, as does the aromatic ring of puromycin, while the aromatic ring of chloramphenicol binds to the exit tunnel hydrophobic crevice. Sparsomycin contacts primarily a P-site bound substrate, but also extends into the active-site hydrophobic crevice. Virginiamycin M occupies portions of both the A and P-site, and induces a conformational change in the ribosome. Blasticidin S base-pairs with the P-loop and thereby mimics C74 and C75 of a P-site bound tRNA.

barS1, a gene for biosynthesis of a gamma-butyrolactone autoregulator, a microbial signaling molecule eliciting antibiotic production in Streptomyces species

From Streptomyces virginiae, in which production of streptogramin antibiotic virginiamycin M(1) and S is tightly regulated by a low-molecular-weight Streptomyces hormone called virginiae butanolide (VB), which is a member of the gamma-butyrolactone autoregulators, the hormone biosynthetic gene (barS1) was cloned and characterized by heterologous expression in Escherichia coli and by gene disruption in S. virginiae. The barS1 gene (a 774-bp open reading frame encoding a 257-amino-acid protein [M(r), 27,095]) is situated in the 10-kb regulator island surrounding the VB-specific receptor gene, barA. The deduced BarS1 protein is weakly homologous to beta-ketoacyl-acyl carrier protein/coenzyme A reductase and belongs to the superfamily of short-chain alcohol dehydrogenase. The function of the BarS1 protein in VB biosynthesis was confirmed by BarS1-dependent in vitro conversion of 6-dehydro-VB-A to VB-A, the last catalytic step in VB biosynthesis. Of the four possible enantiomeric products from racemic 6-dehydro-VB-A as a substrate, only the natural enantiomer of (2R,3R,6S)-VB-A was produced by the purified recombinant BarS1 (rBarS1), indicating that rBarS1 is the stereospecific reductase recognizing (3R)-isomer as a substrate and reducing it stereospecifically to the (6S) product. In the DeltabarS1 mutant created by homologous recombination, the production of VB as well as the production of virginiamycin was lost. The production of virginiamycin by the DeltabarS1 mutant was fully recovered by the external addition of VB to the culture, which indicates that the barS1 gene is essential in the biosynthesis of the autoregulator VBs in S. virginiae and that the failure of virginiamycin production was a result of the loss of VB production.

[Novel compounds active on staphylococci]

Research efforts to discover new compounds active against staphylococci are more than ever justified today. The incidence of methicillin-resistant staphylococci remains very high in hospitals, and the solution provided by glycopeptides is far from being satisfactory. These compounds exhibit mediocre pharmacokinetic and pharmacodynamic properties. Their ease and safety of use are poor. Finally, strains with diminished sensitivity to these antibiotics are beginning to appear. This article examines the opportunities offered by two new anti-staphylococcal agents: quinupristine-dalfopristine (Synercid) and linezolide (not marketed in France).

[Isolation of peptide antibiotic virginiamycin components and selection of their producer Streptomyces virginiae]

A method for chromatographic separation and quantitative determination of individual components of the antibiotic virginiamycin, produced by microbiological synthesis (Streptomyces virginiae strain 147), is described. The components, M1-2 and S1-5, were isolated from fermentation broth and identified by HPTLC and HPLC (the results obtained using the two methods correlate well with each other). Conditions of culturing of the producer and compositions of nutritive media were optimized. Using UV irradiation as a mutagenic factor, the producer was selected for increased level of synthesis of the antibiotic; this was achieved by inducing mutations that impart resistance to virginiamycin and meta-fluorophenylalanine, an analog of phenylalanine.

Structural basis for selectivity and toxicity of ribosomal antibiotics

Ribosomal antibiotics must discriminate between bacterial and eukaryotic ribosomes to various extents. Despite major differences in bacterial and eukaryotic ribosome structure, a single nucleotide or amino acid determines the selectivity of drugs affecting protein synthesis. Analysis of resistance mutations in bacteria allows the prediction of whether cytoplasmic or mitochondrial ribosomes in eukaryotic cells will be sensitive to the drug. This has important implications for drug specificity and toxicity. Together with recent data on the structure of ribosomal subunits these data provide the basis for development of new ribosomal antibiotics by rationale drug design.

A hybrid minimal principle for the crystallographic phase problem

Simulated annealing is used to solve the X-ray phase problem formulated as a minimization problem. The cost function consists of two parts, one represents the discrepancy between measured and calculated intensities while the other monitors the probability distribution of the triplets. From a random real-space structure at the start, the atoms are moved one by one to gradually reduce the cost function until the best structure emerges. Trial calculations for structures including hexadecaisoleucinomycin (HEXIL) are presented. Comparison of this method with other related methods is made.

A novel route to the vinyl sulfide nine-membered macrocycle moiety of Griseoviridin

The synthetic potentialities of cerium(III) chloride are demonstrated by the synthesis of a nine-membered ring heterocycle component of Griseoviridin (3) in optically active form. The key step involves the stereospecific formation of the alpha-carbalkoxy alkenyl sulfide moiety using a combination system of cerium(III) chloride heptahydrate and sodium iodide.

[The influence of steric crowding on the electrochemical reduction of amide groups in a pyridylcarboxamide seriesapplication to rote ction of amines in peptide synthesis]

To gain a better understanding of the effect exerted by the 3-hydroxypicolinoyl residue on the antibiotic activity of Pristinamycin IA, the C-N bond of picolinamide was cleaved electrochemically. A mechanistic study demonstrated that the presence of the peptidic macrolactone M markedly modified the expected cathodic behavior of pyridylcarboxamides. In order to assess the influence of steric crowding exerted by M on this original behavior, we look for models using two different approachs. First, tertiary pyridylcarboxamides were used to increase steric hindrance at the amide nitrogen position; second, M was opened by ammonolysis to decrease steric crowding at the amide nitrogen position. The electrochemical behavior of the selected compounds is presented in the first and the second parts of this study. Determination of pyridine nitrogen basicity in an N-substituted-3-methoxypicolinamide series is treated in the third part as a useful probe to evaluate the intensity of steric crowding at the amide nitrogen position. Finally, in the last part of this work, we propose the use of the picolinoyl residue (C6H4N-CO-ou Pic) as a protecting group for amines in peptide synthesis.

Streptogramins: a new class of antibiotics

Streptogramin antibiotics represent a unique class of antibacterials in the each member of the class consists of at least 2 structurally unrelated molecules: group a streptogramins (macrolactones) and group B streptogramins (cyclic hexadepsipeptides). Both group A and group B streptogramins inhibit protein synthesis at the ribosomal level, and they act synergistically against many isolates their combination generating bactericidal activities and reducing the possibility of emergencies of resistant strains. The mechanisms of acquired resistance to group B streptogramins remain unaffected by target modifications and active efflux. The pharmacokinetic parameters of group A and group B streptogramins in blood are quite similar. In addition, both the A and B group penetrate and accumulate in macrophages and in the bacterial gegetations of experimental endocarditis. Until recently, the complex and irregular composition of naturally occurring pristinamycin and virginiamycin, as well as the unavailability of soluble forms, have limited the clinical development of streptogramins. The synthesis of water soluble derivatives of pristinamycin IA and IIB has now allowed the development of injectable streptogramins with fixed compositions. This unique class of antibacterials will have a significant clinical impact in a world of increasing multidrug resistance affecting the Gram-positive cocci, especially staphylococci and pneumococci. The absence of cross-resistance to macrolides in many of these isolates and the rapid antibacterial killing against these species bright future for this class of antibiotics.

Quinupristin (RP 57669): a new tool to investigate ribosome-group B streptogramin interactions

Streptogramin antibiotics consist of two types of molecules, group A and group B. The group B molecule quinupristin (RP 57669) and the group A molecule dalfopristin (RP 54476) constitute the first water-soluble semisynthetic streptogramin, quinupristin/dalfopristin (RP 59500). When group B molecules bind to 50S subunits or to tightly coupled ribosomes, there is an increase in their fluorescence intensity, which is proportional to the concentration of the antibiotic-ribosome complex formed. We found here that the background fluorescence of unbound quinupristin is 10-fold lower than that of unbound virginiamycin S, a natural group B molecule often used experimentally. The association constants were found (i) to be similar for the binding of the two group B molecules to tightly coupled 70S ribosomes in the absence of the group A molecules (quinupristin: 3.5 x 10(7) M(-1); virginiamycin S: 2.8 x 10(7) M(-1)) and (ii) to similarly increase about 20-fold in the presence of the corresponding group A molecule (quinupristin + dalfopristin: 69 x 10(7) M(-1); virginiamycin S + virginiamycin M: 60 x 10(7) M(-1)). Similar results were obtained with 50S ribosomal subunits. Additionally, we provide evidence that the failure of the group B molecules to inhibit poly(Phe) synthesis is due to the displacement of the group B molecule during poly(Phe) polymerization on the ribosome, indicating that the artificial poly(Phe) peptide competes with the binding of the group B molecule.

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

Probing the substrate specificity of an enzyme catalyzing inactivation of streptogramin B antibiotics using LC-MS and LC-MS/MS

LC-MS and LC-MS/MS analyses indicated that an enzyme responsible for inactivating the antibiotic etamycin is specific for streptogramins and acts on both type B-I and B-II streptogramin subgroups. No enzymatic activity was detected for other cyclodepsipeptides such as surfactins and viscosin. It was demonstrated using analogs of etamycin that the picolinyl moiety is essential to obtain enzyme-generated ring-opened compounds. Because the picolinyl moiety is also essential for the biological activity of streptogramins, it is proposed that this residue is a distinctive topographic feature in the binding of this group of antibiotics to enzyme active sites.

Complete conversion of antibiotic precursor to pristinamycin IIA by overexpression of Streptomyces pristinaespiralis biosynthetic genes

A Streptomyces pristinaespiralis strain, which produces a streptogramin antibiotic pristinamycin II (PII) as a mixture of two biologically active molecules PIIB and PIIA, was genetically engineered to produce exclusively PIIA. The snaA,B genes, which encode a PIIA synthase that performs oxidation of the precursor (PIIB) to the final product (PIIA), were integrated in the chromosome of S. pristinaespiralis using an integrative derivative of the pSAM2 genetic element from Streptomyces ambofaciens. Integration was due to site-specific recombination at the attB site of S. pristinaespiralis, and no homologous recombination at the snaA,B locus was observed. The attB site of S. pristinaespiralis was sequenced and found to overlap the 3' end of a pro-tRNA gene. The integrants were stable in industrial conditions of pristinamycin production and showed no decrease in PII biosynthesis. Western blot analysis showed a constant production of the PIIA synthase in the overall fermentation process due to expression of the cloned snaA,B genes from the constitutive ermE promoter. This allows the complete conversion of the PIIB form into PIIA.

Identification and analysis of genes from Streptomyces pristinaespiralis encoding enzymes involved in the biosynthesis of the 4-dimethylamino-L-phenylalanine precursor of pristinamycin I

Four pap genes (papA, papB, papC, papM) were found by sequencing near to snbA, a Streptomyces pristinaespiralis gene which was previously shown to encode one of the pristinamycin I (PI) synthetases. Analysis of the homologies observed from the deduced amino acid sequences suggested that these four genes could be involved in the biosynthesis of the PI precursor 4-dimethylamino-L-phenylalanine (DMPAPA). This was first verified when disruption of papA in S. pristinaespiralis led to a PI- phenotype, which was reversed by the addition of DMPAPA into the culture medium. Further confirmation was obtained when papM was overexpressed in Escherichia coli and the corresponding protein purified to homogeneity. It catalysed the two successive N-methylation steps of 4-amino-L-phenylalanine leading to DMPAPA via 4-methylamino-L-phenylalanine. These results allowed us to assign a function to each of the four pap genes and to propose a biosynthetic pathway for DMPAPA.

Quinupristin-dalfopristin (RP 59500): an injectable streptogramin combination

The pharmacology, pharmacokinetics, activity, and potential clinical role of quinupristin-dalfopristin (RP 59500) are described. Quinupristin-dalfopristin is the first injectable formulation of the streptogramin antibiotics. Streptogramin drug products are each composed of two chemically distinct compounds, which when administered together act synergistically by inhibiting bacterial protein synthesis. Quinupristin-dalfopristin has shown activity in vitro against many strains of streptococci and staphylococci, including methicillin- and erythromycin-resistant strains of staphylococci. The combination is more active against Enterococcus faecium than Enterococcus faecalis. It has also shown activity in vitro against certain gram-negative organisms and anaerobes. The mean maximum blood concentration at the end of a one-hour infusion ranged from 0.95 mg/L for a 1.4-mg/kg dose to 24.2 mg/L for a 29.4-mg/kg dose; there was a linear correlation between dose and mean area under the concentration-time curve. Mean half-life ranged from 1.27 to 1.53 hours. The drug is under investigation in the United States in Phase III trials. Of 60 evaluable patients with documented bacteremia involving E. faecium resistant to vancomycin, 40 (67%) had a favorable clinical response. Of 11 patients with bacteremia caused by methicillin-resistant Staphylococcus aureus, 78% had a favorable response. The efficacy of quinupristin-dalfopristin in treating resistant infections has also been suggested by smaller studies and case reports. The drug may be useful in the prophylaxis of endocarditis. Adverse reactions are generally mild and transient. Quinupristin-dalfopristin may be useful in treating serious gram-positive infections, but more clinical study is needed.

Treatment of vancomycin-resistant enterococci, with a focus on quinupristin-dalfopristin

Enterococci are the second most common cause of hospital-acquired infections, and drug resistance among these organisms is a growing problem. Vancomycin-resistant enterococci (VRE) now account for 7.9% of the nosocomial enterococcal infections. There is no standard therapy for VRE. Although some agents have shown in vitro activity alone or in combination, including ciprofloxacin, doxycycline, novobiocin, teicoplanin, chloramphenicol, and rifampin, treatment options are limited to combinations of drugs with marginal efficacy against the pathogens. Quinupristin-dalfopristin is a new investigational agent with activity against gram-positive cocci, including VRE.

Activities of RPR 106972 (a new oral streptogramin), cefditoren (a new oral cephalosporin), two new oxazolidinones (U-100592 and U-100766), and other oral and parenteral agents against 203 penicillin-susceptible and -resistant pneumococci

Agar dilution was used to determine the MICs of RPR 106972 (a new oral streptogramin), cefditoren (a new oral cephalosporin), two new oxazolidinones (U-100592 and U-100766), and other oral and parenteral agents for 203 penicillin-susceptible and -resistant pneumococci. All pneumococci were inhibited by RPR 106972 at < or = 0.5 microgram/ml. Cefditoren was very active against all pneumococcal groups, with MICs of < or = 2.0 micrograms/ml. Amoxicillin with or without clavulanate was the next most active oral beta-lactam, followed by cefdinir, cefuroxime, cefpodoxime, and cefprozil. U-100592 and U-100766 were very active against all classes of pneumococci, with all MICs < or = 1.0 microgram/ml.

Streptogramins. A unique class of antibiotics

Streptogramin antibiotics represent a unique class of antibacterials in that each member of the class consists of at least 2 structurally unrelated molecules: group A streptogramins (macrolactones) and group B streptogramins (cyclic hexadepsipeptides). Both group A and group B streptogramins inhibit protein synthesis at the ribosomal level, and they act synergistically against many isolates, their combination generating bactericidal activities and reducing the possibility of emergence of resistant strains. The mechanisms of acquired resistance to group B streptogramins are similar to those induced by erythromycin, but group A streptogramins remain unaffected by target modifications and active efflux. The pharmacokinetic parameters of group A and group B streptogramins in blood are quite similar. In addition, both the A and B groups penetrate and accumulate in macrophages and in the bacterial vegetations of experimental endocarditis. There are important structural and biological differences between the streptogramins and the macrolides. The main differentiating features are the rapid anti-bacterial killing of streptogramins and the rarity of cross-resistance between the 2 groups of antibiotics.

Unusual transformation of the 3-hydroxy-picolinoyl residue of pristinamycin IA

Pristinamycin IA was modified in a two-step procedure to give original derivatives possessing a tricyclic nucleus (8a, 8b, 8c) or a substituted pyrrole ring (10a, 10b) in place of the natural exocyclic 3-hydroxy-picolinoyl residue. This transformation involved firstly preparation of pyridinium betaines 5 from pristinamycin IA and secondly a 1-3 dipolar cycloaddition between 5 and N-substituted maleimides or diethyl acetylenedicarboxylate. The compounds obtained were evaluated as antibacterial agents alone and in association with pristinamycin IIA.

Partial synthesis of five new analogues of the peptido-lactone Virginiamycine S1, modified in the fifth and/or sixth position ([Xxx5]-VS1 with Xxx = Ala, Asp, Asn and Lys and [Ala5,Gly6]-VS1)

We achieved the reconstruction of VS1-analogues containing a substitute for the fifth residue, gamma-oxo-Pip (Pip = pipecolic acid), starting from VS1-pentapeptide (VS5P;3) the latter being prepared by a two-step degradation process of the native antibiotic VS1 (1a). Protecting groups during the procedure were chosen in order to realize a minimal number of steps. Most of these gave excellent yields, including final cyclization between the fourth and fifth residue. In total, four analogues were synthesized with Ala, Asp, Asn and Lys (1b) replacing gamma-oxo-Pip. Among these, [Lys5(Tfa salt)]-VS1 is water-soluble, which is an important characteristic for eventual application of VS1 as a pharmaceutical agent. In the proposed reaction sequence, we made sure that residues 4 (MePhe) and 6 (Phg) became partially epimerised. We therefore obtained each time after cyclization a total of four epimers that have been separated by preparative TLC. The chiral identity of the final residues was realized by GC (Chirasil Val-III) on the total hydrolysates.

RP 59500: a proposed mechanism for its bactericidal activity

RP 59500 is a combination of RP 57669 and RP 54476, which are semisynthetic water soluble derivatives of pristinamycin IA (PIA) and pristinamycin IIA (PIIA), respectively. Like their precursors, these molecules are bacteristatic in their own right. In association, they exert bactericidal activity against a variety of Gram-positive bacteria. Experiments involving the binding of these antibiotics to the target bacterial ribosome showed that both the binding sites and the mechanism of action of the components of RP 59500 are identical to those of the parent molecules. By affinity-labelling with a structural analogue of RP 57669, it was demonstrated that L24, a protein of the 70S ribosomal subunit, was specifically labelled. Experiments using radioactive N-ethylmaleimide to label proteins possessing a thiol residue, indicated that proteins L24, L10 and L11 are not only close to each other in the ribosomal structure, but are also adjacent (if not actually part of) the channel through which newly synthesized proteins are extruded. We propose that the mechanism of action of these compounds is to close or narrow the extrusion channel of these proteins, which could lead to their accumulation on the ribosome. We cannot exclude, of course, the possibility that this accumulation disturbs peptidyl-tRNA hydrolase (PHT) activity, thereby depleting free tRNAs within the cell and inhibiting protein synthesis.

In-vitro and in-vivo synergic activity and fractional inhibitory concentration (FIC) of the components of a semisynthetic streptogramin, RP 59500

RP 59500 is a new semisynthetic injectable streptogramin antibiotic composed of two compounds which interact synergically, RP 57669 and RP 54476, derived from pristinamycin IA and pristinamycin IIB, respectively. The bacteristatic and bactericidal activities of RP 57669 and RP 54476 alone or combined in various proportions were tested by the chequerboard dilution technique. The fractional inhibitory concentration (FIC) index was determined for 14 Staphylococcus aureus isolates (including methicillin- and macrolide-resistant strains) and one culture collection strain. The FIC index was found to be much lower than 0.5, indicating the presence of synergy for all strains tested, whatever their resistance pattern. The ED50 of RP 57669 and RP 54476 in various combinations were also determined in three experimental murine models of septicaemia, caused by either S. aureus or Streptococcus pneumoniae, and a thigh abscess model caused by S. aureus. The combinations which performed best in the model of septicaemia were those in which the RP 57669: RP 54476 ratio ranged from 16:84 to 92:8, while those active against the thigh abscess model had ratios ranging from 8:92 to 84:16. That the drugs were active over a wide range of ratios suggests that synergy will be maintained even if one drug is cleared more rapidly than the other. The combination of 30:70, referred to as RP 59500, was selected for further studies, both in vitro and in various experimental models of infections.

Acid-base properties of pristinamycin IA and related compounds

The pKa values of acid-base equilibria involved in the protonation of pristinamycin IA and its model compound, N-isobutyl-3-hydroxypicolinamide, were determined by UV-visible absorption spectrometry and potentiometric titration. The equilibrium between dipolar ionic and uncharged neutral forms was investigated spectrometrically in methanol-water solutions. With pristinamycin IA, the dipolar ionic form predominated in aqueous solutions buffered to the isoelectric pH. The effects of structure on the ionization constants are briefly discussed.

Interaction between virginiamycin S and ribosomes is partly provided by a salt bridge with a Mg2+ ion

Type B streptogramins, such as virginiamycin S (VS), are cyclic hexadepsipeptides, inhibiting protein synthesis in prokaryotes. L-Thr connects a 3-hydroxypicolinyl residue (3-OH-Pic) to the peptide lactone ring. The fluorescence intensity of 3-OH-Pic is strongly increased by chelation to alkaline earth cations or binding to ribosomes. Similar behavior of the ribosome-VS complex and the VS-Mg chelate provides strong evidence for the presence of a VS-Mg chelate within the ribosomal binding site. Different models involving the ribosome binding of either members of the VS-Mg2+ chelate or both have been tested by fluorescence lifetime measurements, equilibrium titrations, and stopped-flow spectrofluorometry. Our data strongly suggest that (a) the interaction between VS and the ribosome is partly provided by a salt bridge between suitable acceptor atoms of the ribosome and the 3-OH-Pic residue, (b) Mg2+ can be exchanged by Mn2+ without dissociation of the ribosome-VS complex, (c) Mg2+ coordinates to the negative form of the 3-OH-Pic residue, probably via an interaction with the phenolate oxygen and the amide carboxyl group, and (d) the picolinyl residue is essential for the biological activity, as indicated by the lack of activity when the latter is replaced by a serine derivative.

A fluorescence lifetime study of virginiamycin S using multifrequency phase fluorometry

Using multifrequency phase fluorometry, fluorescence lifetimes have been assigned to the different protolytic forms of the antibiotic virginiamycin S. These lifetimes are 0.476 +/- 0.005 ns for the uncharged form, 1.28 +/- 0.2 and 7.4 +/- 0.2 ns for the zwitterionic form, 1.19 +/- 0.01 ns for the negatively charged form, and 1.9 +/- 0.1 ns for the double negatively charged form. The assignments are based on lifetime measurements as a function of pH, volume percent ethanol, and excitation wavelength. Excited-state proton transfer is taken into account. It is complete at pH values lower than 1, and no fluorescence of the fully protonated charged form is observed. At pH 8, an excited-state pK* increase is calculated, but proton association is too slow to cause excited-state proton transfer. The addition of divalent cations, at pH 9.4, increases the lifetime of the negatively charged form to a value dependent upon the specific nature of the cation (7.58 +/- 0.06 ns for Mg2+, 6.54 +/- 0.02 ns for Ca2+, and 3.74 +/- 0.05 ns for Ba2+). Monovalent cations do not influence the lifetimes, indicating that their binding to the macrocycle does not influence the fluorescent moiety. The model compound 3-hydroxypicolinamide shows an analogous behavior, but the retrieved lifetime can differ significantly.

Novel and potent gastrin and brain cholecystokinin antagonists from Streptomyces olivaceus. Taxonomy, fermentation, isolation, chemical conversions, and physico-chemical and biochemical properties

The discovery and physico-chemical characterization of three novel and minor virginiamycin M1 analogs as potent gastrin antagonists from a culture of a strain of Streptomyces olivaceus are described. These analogs are L-156,586, L-156,587 and L-156,588. They are, respectively, 15-dihydro-13,14-anhydro-, 13,14-anhydro- and 13-desoxy-analogs of virginiamycin M1. We also chemically converted virginiamycin M1 (via L-156,587) to L-156,586 and its unnatural epimer, L-156,906. These analogs are competitive and selective antagonists of gastrin and brain cholecystokinin binding at nanomolar concentrations. These are the most potent gastrin/brain cholecystokinin antagonists from natural products. The same compounds showed poor Gram-positive antibiotic activity versus virginiamycin M1. Structurally related Gram-positive antibiotics, griseoviridin and madumycin I, were inactive in gastrin and brain cholecystokinin binding at up to 100 microM.

Affinity labeling of the virginiamycin S binding site on bacterial ribosome

Virginiamycin S (VS, a type B synergimycin) inhibits peptide bond synthesis in vitro and in vivo. The attachment of virginiamycin S to the large ribosomal subunit (50S) is competitively inhibited by erythromycin (Ery, a macrolide) and enhanced by virginiamycin M (VM, a type A synergimycin). We have previously shown, by fluorescence energy transfer measurements, that virginiamycin S binds at the base of the central protuberance of 50S, the putative location of peptidyltransferase domain [Di Giambattista et al. (1986) Biochemistry 25, 3540-3547]. In the present work, the ribosomal protein components at the virginiamycin S binding site were affinity labeled by the N-hydroxysuccinimide ester derivative (HSE) of this antibiotic. Evidence has been provided for (a) the association constant of HSE-ribosome complex formation being similar to that of native virginiamycin S, (b) HSE binding to ribosomes being antagonized by erythromycin and enhanced by virginiamycin M, and (c) a specific linkage of HSE with a single region of 50S, with virtually no fixation to 30S. After dissociation of covalent ribosome-HSE complexes, the resulting ribosomal proteins have been fractionated by electrophoresis and blotted to nitrocellulose, and the HSE-binding proteins have been detected by an immunoenzymometric procedure. More than 80% of label was present within a double spot corresponding to proteins L18 and L22, whose Rfs were modified by the affinity-labeling reagent. It is concluded that these proteins are components of the peptidyltransferase domain of bacterial ribosomes, for which a topographical model, including the available literature data, is proposed.