Antimicrobial Agents: Antibacterials and Antifungals

This chapter focuses on the classification and structural-activity relationship of antibiotics and antibacterial agents. The β-lactam family comprises four groups of molecules: penams, penems, cephems, and monocyclic β-lactams. To these should be added the β-lactamase inhibitors, some of the structures of which are featured in the four main groups. The azetidinone nucleus alone can be replaced, and monobactams, monocarbams, monophosphatams, and other heterocycles can be distinguished according to the substituents of the nitrogen atom. Based on studies by researchers, who demonstrated the transferability of resistance to certain antibiotics between Escherichia coli and Shigella spp., other researchers were able to demonstrate that shigellae inactivated chloramphenicol. The majority of aminoglycosides have similar basic structures and are divided into four groups: streptomycin derivatives, 2-deoxystreptamine derivatives, spectinomycin derivatives, and fortimicin derivatives. The macrolides are macrocyclic antibiotics characterized by a central lactone ring containing 12 to 16 members, few double bonds, and no nitrogen atom; one or two amino sugars and/or neutral sugars are attached to the lactone or aglycone ring. More than 400 compounds constitute the family of peptide antibiotics. Dihydrofolate reductase (DHFR) inhibitors used therapeutically are the result of studies relating to the specificity of this enzyme for bacterial, parasitic, or epithelial cells. The antibacterial compounds display good antibacterial activity and a lack of inhibitory activity against epithelial cell DHFR.

This chapter deals with the antibacterial agent that is only one molecule among a series of compounds prepared by chemists before becoming a drug. Preselection is a crucial stage in the life of a new drug. Once a molecule is synthesized, its in vitro activity against a panel of strains belonging to selected bacterial genera and species is determined. Activity against S. aureus producing penicillinase is added, as well as that against P. aeruginosa and Acinetobacter. Strains resistant to clindamycin and cephamycins must be included for gram-negative bacilli. Nondiscriminatory models are prepared during the preselection phase of a molecule. After determining the in vitro activity, it is essential to determine the in vivo activity parenterally and orally. In fact, the in vitro activity is not necessarily correlated with good in vivo activity. In these systemic infection models, activity is tested against gram-positive cocci, Enterobacteriaceae, and P. aeruginosa. The susceptibility of the bacterial strain considered responsible for the infection is determined in the clinical microbiology laboratory of the center in which the study is being conducted. However, semiautomatic devices related to the antibiotic sensitivity test are used by some microbiology laboratories. Determination of breakpoints is one of the major factors for an antibiotic. The mechanism of resistance is an important factor for determining breakpoints for given bacteria. The MIC at which these bacterial species are shown to be resistant is the cutoff and can be considered as the concentration under which the strain is considered susceptible.

Due to the importance of the cephems, it is essential to classify them so as to allow their optimal use. The cephems are characterized by an azetidinone ring, to which is attached an unsaturated hexacycle. Five classes of cephems may be distinguished in terms of the nature of the moiety: cephalosporins, cephamycins and cephabacins, oxa-1-cephems, carba-1-cephems, and miscellaneous. The dosage and dosing frequency of a cephalosporin are the result of a balance between its antibacterial activity and its pharmacokinetic profile. The molecules all have the same antibacterial spectrum, which principally includes gram-negative bacilli and gram-negative and gram-positive cocci, except for S. aureus. The carbacephem-type molecules are asymmetrical, and the substituents on the β-lactam moiety are in the cis position; the cis stereochemistry of the carbacephems is less stable thermodynamically than the trans stereochemistry. The antibacterial activity RWJ-54428 was designed to overcome methicillin resistance in methicillin-resistant S. aureus (MRSA) strains and ampicillin resistance in E. faecium by penicillin-binding proteins (PBP) alterations. In all species, total clearance was slower and the apparent elimination half-life was longer than that of RWJ- 54428. The antistaphylococcal activities of RWJ-333442 in comparison with RWJ-54428, vancomycin, and quinupristindalfopristin against S. aureus Smith were investigated in a sepsis model (Swiss-Webster mice) and were also investigated for S. aureus 076, a methicillin-resistant strain. A synergistic activity was shown to be species dependent in combination with streptomycin against E. faecalis.

Current research into carbapenems is directed towards the synthesis of molecules that are more stable against hydrolysis by dehydropeptidase I (DHP-I), the reduction of neurologic side effects, the improvement of pharmacokinetic characteristics, oral absorption, and the absence of cross-resistance with imipenem. The structure-activity relationships of the carbapenems are complex, as they derive from an equilibrium that is difficult to achieve between antibacterial activity, chemical stability, and stability against hydrolysis by renal DHP-I. The 1-β-aminoethyl derivatives are active in vitro but are unstable at high pH, as an intramolecular rearrangement is produced. The attachment of an amino acid or a small peptide prevents this rearrangement. In this series, the chloro-L-alanine derivative possesses good in vitro and in vivo activity against P. aeruginosa and Staphylococcus aureus. The carbapenems exhibit good activity against Neisseria gonorrhoeae and Neisseria meningitidis. The carbapenems possess remarkable activity against Enterobacteriaceae; in particular, they are active against strains producing plasmid-mediated broad-spectrum and extended-spectrum enzymes and chromosomally mediated enzymes. There are a number of mechanisms of resistance to carbapenems. These are based on single or multiple phenomena: crossing of the bacterial wall, modification of the target penicillin-binding proteins (PBPs), and hydrolysis by β-lactamases. The imipenemases are chromosomal metalloenzymes capable of hydrolyzing the carbapenems. The major problem with the carbapenems is their chemical instability in solution. The plasma pharmacokinetics of the carbapenems are complex because of their hydrolysis by renal DHP-I to various degrees, depending on the molecule, with major interindividual variability.

This chapter focuses on a few models of monocyclic β-lactams in which the structural changes are accompanied by changes to their antibacterial properties. The spectrum of activity of the monocyclic β-lactams: activity of all molecules against gram-negative cocci, Haemophilus influenzae, and Enterobacteriaceae; variable activity against Pseudomonas spp. and Acinetobacter baumannii depending on the compound; and lack of activity against gram-positive cocci and anaerobic bacteria. All of the monocyclic β-lactams inhibit Neisseria spp., M. catarrhalis, and H. influenzae at concentrations equal to or less than 0.25 µg/ml. The monocyclic β-lactams that have been the subject of killing kinetics studies have killing kinetics similar to those of the other β-lactams. The monocyclic β-lactams represent very weak substrates for the standard β-lactamases of gram-negative bacilli and for this reason are very stable against the hydrolytic activity of these enzymes. At subinhibitory concentrations, aztreonam and carumonam significantly increase the phagocytic activity and bactericidal activity of murine macrophage lines against different species of Enterobacteriaceae and Pseudomonas aeruginosa. The monocyclic β-lactams exhibit the general pharmacokinetic behavior of β-lactams, with similar parameters for the two types of compounds. The behavior of aztreonam, however, must be interpreted for a few sites where antibiotics diffuse poorly, such as the bronchial mucus, cerebrospinal fluid, prostate, and aqueous humor.

Bacterial peptidoglycans were first identified in the 1940s. The nucleotide precursors were isolated and characterized in 1949. Peptidoglycan biosynthesis has been investigated in many bacterial species. The glycan chains of peptidoglycan are small and repeated units composed of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc). The glmM gene was shown to be essential in Escherichia coli, but genetic data recently suggested that in Staphylococcus aureus there could be an alternative pathway for glucosamine-1-phosphate biosynthesis. The GlmU protein from E. coli is a bifunctional enzyme. Its C-terminal domain catalyzes acetylation of glucosamine-1-phosphate into GlcNAc-1-phosphate, whereas its N-terminal domain catalyzes uridylation to yield UDP-GlcNAc. Nucleoside antibiotics can be subdivided into different classes according to their chemical structures and/or their respective modes of action against MraY: the tunicamycin group, the ribosamino-uridine group, and finally the capuramycin group. Some of them possess activities against gram-positive bacteria, against Mycobacterium spp., and against Pseudomonas spp. This toxicity is explained by their high inhibitory activity upon glycoconjugate biosyntheses, such as of techoic acids, glycosaminoglycans, and glycoproteins. The major component, caprazamycin B, exhibits anti-M. tuberculosis activity and excellent activity against M. avium complex. The major chemical difference observed between liposidomycins and FR-900493 resides in the presence of an N-functionalized amino acid residue in the FR-900493 structure, instead of the diazepanone ring of liposidomycins. Lantibiotics such as mersacidin, are gene-encoded peptides that contain unusual thioether amino acids and/or 3-methyllanthionine.

The main modes of resistance to β-lactams are modification of cell targets (penicillin-binding proteins), impermeability of the cell wall, efflux, and production of inactivating enzymes, the β-lactamases. β-Lactamases are a complex family of enzymes, and currently more than 300 unique β-lactamases have been reported for gram-positive and gram-negative bacteria. Research on β-lactamases led to different classifications, one of which is the Ambler classification describing four classes: A, C, D (serine β-lactamases), and B (metalloprotease β-lactamases). β-Lactamase inhibitors can be divided globally into two groups: β-lactams and non-β-lactams. A number of compounds of clavulanic acid have been prepared to explore the structure-activity relationship. It was shown that (i) the lack of the C-4 carboxylic group does not eliminate the β-lactamase inhibitory activity and (ii) a catalytic group at position 2 enhances the inhibitory activity 4 to 10 times. Semisynthetic penicillanic derivatives such as halo penicillanic acid sulfones were designed to mimic the mechanism of enzyme inactivation observed with clavulanic acid. PS-5 is hydrolyzed by β-lactamases of B. cereus, Proteus vulgaris, C. freundii, and Streptomyces spp. Intensive research on β-lactamase inhibitors is needed: (i) enlarge the inhibitory spectrum to cover class C and class B enzymes, (ii) to overcome the rapid spread of ESBL, and (iii) to be prepared for an eventual reduction of activity of existing compounds, such as clavulanic acid.

This chapter gives an overview about aminocyclitol aminoglycosides. Aminoglycosides are rapidly bactericidal antibiotics, acting in particular on aerobic gram-negative bacilli, staphylococci, and gram-positive bacilli. The aminoglycosides are broad-spectrum antibiotics essentially active against gram-negative bacteria and staphylococci. A penicillin-aminoglycoside combination exerts synergistic and bactericidal activity on streptococci with low-level resistance to streptomycin. The antibiotics of the aminoglycoside family are bactericidal. Majority of studies relating to their mode of action have been undertaken with streptomycin. A number of studies have been conducted on the structure activity relationships of aminoglycosides. Certain bacteria may be naturally and consistently resistant to aminoglycosides. Resistance may be acquired by four different mechanisms: alteration of the target, interference with transport of the antibiotic, enzymatic inhibition of the antibiotic, and substitution of the target. The aminoglycosides can only be used parenterally, except for intestinal infections or indications for decontamination. Diffusion of the aminoglycosides is rapid. The apparent volume of distribution corresponds to 20 to 30% of body weight, a volume equivalent to that of the extracellular fluids. The pharmacokinetics of elimination of the aminoglycosides are dependent almost exclusively on renal function. The drug interactions observed with aminoglycosides are particularly detrimental when they are liable to exacerbate the specific toxic effects of aminoglycosides antibiotics: ototoxicity, nephrotoxicity, and curarizing effect. Aminoglycosides have both renal and cochleovestibular toxicity. Substantial progress has been made in terms of understanding the mechanisms of resistance and toxicity.

The natural macrolides of importance in human pathology are the 14- and 16-membered-ring macrolides; the semisynthetic derivatives of importance in human pathology are the 15-membered-ring macrolides (azalides). A characteristic of new molecules is the increased acid stability of the macrolides. This can be fully achieved with roxithromycin and azithromycin. The principal physicochemical properties of the macrolides are listed in this chapter. The natural antibacterial spectrum of the macrolides, irrespective of their structure, includes gram-positive bacteria; gram-negative cocci; fastidious gram-negative bacilli, such as Haemophilus influenzae, Moraxella catarrhalis, Pasteurella spp., and Bordetella spp.; anaerobic bacteria; intracellular bacteria; mycoplasmas and related species; spirochetes; bacteria responsible for gastrointestinal infections (V. cholerae and other Vibrio spp. and Campylobacter spp.); and H. pylori. Roxithromycin and azithromycin are very effective in experimental infection in rabbits with Treponema pallidum strain Nichols, whereas these molecules have cross-resistance with erythromycin A, as has been demonstrated with an erythromycin A-resistant strain of T. pallidum. The macrolides inhibit protein synthesis by binding to the 50S subunit of the bacterial ribosomes. The absorption and bioavailability of macrolides vary according to the molecule, partly explaining the difference in unit and dosage rhythm of the different macrolides. Many publications have shown that macrolides have nonantibiotic pharmacological effects. Some of these effects have been studied for their therapeutic potential (motilide) or are the subject of important basic research: anti-inflammatory or cardiological effects.

The streptogramins form a complex group of unique antibacterial agents. The streptogramins are composed of a mixture of two types of molecules: group A streptogramins and group B streptogramins. Each molecular complex contains both structures, but several compounds of each of the two groups may belong to the complex. The combinations of streptogramins plus aminoglycosides, rifampin, or β-lactams are synergistic. Both streptogramins A and B penetrate the bacterial cell by passive diffusion. Group A streptogramins act on the early stage of the protein synthesis, especially on the elongation phase of translation. Group B streptogramins exert inhibitory activity at the end of the process during extension of the peptide chain, causing premature detachment of the incomplete chain. A new mechanism of resistance was described in 1984: isolated resistance to factor A (pristinamycin II_A) and lincosamides (LSA phenotype) conferring low-level resistance to streptogramins. In the Protek study conducted from 2000 to 2001, it was shown that less than 0.02% of Streptococcus pneumoniae strains are resistant to dalfopristin-quinupristin. Each isolate had a 5-aminoacid tandem duplication in the L22 ribosomal protein gene (rplV), preventing synergistic ribosomal binding of the streptogramin combination. Staphylococcus aureus possesses a hydrolase on the same plasmid that opens the macrolactone ring of pristinamycin I_A and causes it to lose its antibacterial activity despite good ribosomal binding. The majority of these S. aureus strains have reduced susceptibility to lincosamides. This chapter also talks about dalfopristin-quinupristin (RP-59500), RPR 106972, and XRP 2868.

The aryl-oxazolidinones constitute a huge family composed of different chemical entities: antibacterials, agriculture derivatives, monoamine oxidase inhibitors, and antihistaminics. The antibacterial spectrum of DuP 105 and DuP 721 includes gram-positive cocci, gram-positive bacilli, anaerobic bacteria, and Mycobacterium tuberculosis. They are inactive against gram negative bacilli. Against common pathogens, including methicillin-resistant S. aureus (MRSA), ranbezolid displays in vitro activity similar to that of linezolid. Ranbezolid is active against gram-negative anaerobes. The oxazolidinones cannot cross the outer membrane of gram-negative bacilli, which explains their lack of activity against Enterobacteriaceae. A mutation in the 23S rRNA gene has been detected in linezolid- or eperezolid-resistant S. aureus and E.faecalis strains. New series of oxazolidinones have been synthesized with the intention of increasing the solubility in water so as to be able to prepare a pharmaceutical formulation for intravenous use, enlarge the antibacterial spectrum or enhance the antibacterial activity, or overcome linezolid resistance. A series of oxazolidinones has been synthesized with the aim of enhancing the activity against H. influenzae and M. catarrhalis. One of the concerns of the available oxazolidinones is the inhibition of monoamineoxidase (MAO), especially type A due to a structural similarity to MAO inhibitors such as toloxatone.

The use of tetracyclines is less widespread now than in the past due to the emergence of antibacterial resistance to these antibiotics and the discovery of more effective and selective molecules. The physicochemical properties of the tetracyclines may be modified by minimal structural variations. The antibacterial spectrum and activity of the tetracyclines include numerous gram-positive and gram-negative bacteria, anaerobes, rickettsiae, mycoplasmas, chlamydiae, Helicobacter pylori, and spirochetes. In the case of gram-negative bacteria, the first obstacle to be overcome is the bacterial outer membrane. Tetracyclines probably penetrate the periplasmic space via transmembrane proteins forming hydrophilic channels. The therapeutic use of tetracyclines is increasingly undermined by the frequent emergence of resistance. Hypersensitivity reactions to tetracyclines are rare but may include anaphylactic shock, urticaria, periorbicular edema, rashes, and morbilliform rashes. The hepatotoxicity of the tetracyclines, first described with chlortetracycline, is now known for all tetracyclines. The results of experimental studies in animals are contradictory with respect to the teratogenicity of tetracyclines: some authors have reported inhibited growth and skeletal deformities in the rat and chicken, whereas according to other authors this risk is nonexistent. Several authors have reported yellow or brown discoloration of the milk teeth in children born to mothers treated with tetracyclines (tetracycline and oxytetracycline) during the second and particularly the third trimesters of pregnancy. Majority of tetracyclines can cause a cerebral pseudotumor syndrome, characterized by a benign increase in intracranial pressure in children and adults. Tetracyclines are used in cases of nonspecific urethritis and urethral syndrome.

The majority of new fluoroquinolones have two ionizable functions: the 3-carboxyl group and a protonizable site at position 7, such as a 7-piperazinyl heterocycle. The majority of fluoroquinolones have good antistaphylococcal activity. Sitafloxacin, moxifloxacin, gatifloxacin, trovafloxacin, and gemifloxacin have excellent activity, while for grepafloxacin and gatifloxacin, the MIC50s are, respectively, 0.125 and 0.25 µg/ml. DC-756 is as active as sitafloxacin (DU-6859a), one of the most active fluoroquinolones against gram-positive cocci, against Staphylococcus aureus, Streptococcus pneumoniae, and Streptococcus pyogenes. KRQ 10099 is an analog of ofloxacin bearing a bicyclic nucleus instead of a piperazinyl moiety. KRQ 10018 is an 8-methyl-2-pyridone derivative. Against reference strains, KRQ 10018 is extremely active against gram-positive cocci, including ciprofloxacin-resistant S. aureus isolates and ofloxacin-resistant Staphylococcus epidermidis. The majority of fluoroquinolones are eliminated principally renally and/or biliarly. The major system is CYP1A2, the activity of which may be inhibited by the fluoroquinolones. In addition to its antiviral activity, zidovudine possesses a certain antibacterial activity against Enterobacteriaceae through its action on bacterial DNA. As a result of its activity on DNA, zidovudine might modify the activity of the fluoroquinolones. The currently available fluoroquinolones are not treatments for acute forms of malaria, but they may abolish a pauciparasitic form if administered simultaneously for another disease.

The majority of codrugs studied possess either a cephalosporin, a penem, or a carbapenem. Studies of the mechanisms of action of codrugs have been conducted principally with Ro-23-9424, Ro-24-4384 (carbamate), and Ro-24-8138 (tertiary amine). The activity of the fluoroquinolones is evaluated in vitro in terms of the inhibition of DNA gyrase activity and in vivo in the bacterium in terms of its action on the nucleoid (topoisomerase IV) using 4’,6’-diamidino-2-phenylindole. Studies have been conducted on E. coli TE-18, which is a strain derived from E. coli K-12. The selection of fleroxacin-resistant mutants from this strain is greater than that observed with fleroxacin alone or from another strain, such as E. coli JF 568. The 3’-esters are much less chemically stable than the other derivatives. The first molecule in which a cephalosporin and a quinolone were combined was Ro-23-5068. The most widely studied molecule has been Ro-23-9424, but Ro-24-6392 has also been widely studied in vitro and in terms of the mechanism of action. Ro-25-0534 is a combination of a catechol-type cephalosporin and ciprofloxacin, linked by a tertiary amine bond. This molecule is active against strains of Enterobacteriaceae which are cefotaxime resistant as a result of cephalosporinase production. Combinations of penems and fluoroquinolones have been synthesized with the various bonds described for the cephalosporins. The chapter also talks about carbapenems-fluoroquinolones, penicillins-Quinolones, monocyclic β-lactams/quinolones, and oxazolidinones-quinolones.

Peptide antibiotics differ structurally from conventional peptides in that they are cyclic and contain ester bonds and D-configured or unusual amino acids, unusual nuclei (such as saturated or unsaturated fatty acids), and heterocycles or amino or nonamino sugars. The peptide antibiotics can be divided into seven groups: I, linear peptides; II, cyclic peptides; III, glycopeptides; IV, lipoglycopeptides; V, lipopeptides; VI, thiazolopeptides; and VII, thiopeptides and chromopeptides. Polypeptide antibiotics induced during an infection in insects, such as cecropins or attacins, have been described in this class. It is highly effective against gram-negative bacilli like Escherichia coli and Pseudomonas aeruginosa, whereas gram-positive cocci, such as Staphylococcus aureus, are poorly inhibited. The majority of peptides of natural or synthetic origin cannot be used as therapeutic agents, as they are unstable in biological liquids, they are toxic, their antibacterial spectrum is limited, and they manifest only weak activity. The cyclic peptides are among the best-studied peptide antibiotics. The majority of molecules have two chlorine atoms on the triphenyl ether nucleus. The teichomycins are lipoglycopeptide antibiotics obtained from fermentation of an actinomycete, Actinoplanes teichomyceticus. In order of inhibitory activity, the pneumocandins may be classified as follows: B0 > A0, A4 > A2, B2 > C0. The bacitracin group consists of hexapeptide antibiotics with a substituted thiazolidine nucleus.

Rifampin was obtained by semisynthesis from the 3-formylrifamycin derivative. It is possible to separate two groups of molecules within the ansamycin family according to the aromatic nucleus: ansamycins of the naphthalene type and ansamycins of the benzene type. Chemical modifications of the ansa in general cause a reduction or even complete abolition of antibacterial activity. The chapter discusses the physiochemical properties and production of rifamycin. The activity of the ansamycins decreases in parallel over time in phosphate-buffered saline (PBS) medium or in 7 H10 medium with or without Tween 80. The majority of rifamycins inhibit the transcription of DNA to RNA. The use of rifampin as monotherapy is associated with the rapid appearance of resistant mutants among common pathogens, particularly Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, and Neisseria meningitidis. The ansamycins have weak activity against enteric gram-negative bacilli and non-fermenting gram-negative bacteria. Rifampin is one of the pivotal drugs in the treatment of tuberculosis. The ansamycins are extensively concentrated in phagocytic cells. Rifampin is one of the pivotal drugs in the treatment of tuberculosis. The ansamycins are extensively concentrated in phagocytic cells. Rifampin, rifabutin, and rifapentine are primarily antituberculosis and antileprosy agents. Staphylococcal infections are one of the indications of rifampin. The adverse effects of rifampin are dependent on treatment duration and dose.

This chapter summarizes information related mainly to the antibacterial activity of the 5-nitroimidazoles. Three 5-nitroimidazoles have been selected: metronidazole, ornidazole, and tinidazole. Metronidazole is the prototype and is therefore used most often as the representative molecule of the class; tinidazole is principally used as an antiprotozoal agent. The 5-nitroimidazoles are very active against the majority of gram-negative anaerobic bacteria and Clostridium. Anaerobic gram-positive cocci are more often susceptible to 5-nitroimidazoles, even if in the majority of studies 2 to 10% resistant strains are observed. Ornidazole levels measured in the cerebrospinal fluid (CSF) and brain represent at least 80% of levels in plasma; the same applies to concentrations in appendiceal fluid. The apparent elimination half-life and plasma clearance of ornidazole in nondialyzed subjects with renal insufficiency are similar to those in subjects with normal renal function. In patients with hepatic insufficiency, the peak concentrations in plasma occur later and the apparent elimination half-lives are prolonged. It is on the neurologic level that the most serious adverse effects are observed during high-dose and/or prolonged treatments. The three 5-nitroimidazole derivatives have very similar antibacterial properties; the pharmacokinetics are very similar, with a longer apparent elimination half-life for ornidazole than for metronidazole, possibly allowing the number of doses of ornidazole to be reduced. These 5-nitroimidazoles derivatives have excellent in vitro activity and well-established clinical efficacy in the treatment of anaerobic infections or in mixed infections when combined with antibiotics effective against aerobes.

Mupirocin is active against gram-positive cocci: it has moderate activity against gram-positive bacilli such as Listeria monocytogenes and Erysipelothrix rhusiopathiae; it is inactive against corynebacteria and Bacillus anthracis. It is inactive against Chlamydia spp. It is active against Staphylococcus aureus, whether the strains are susceptible or resistant to penicillin G, tetracyclines, erythromycin A, fusidic acid, lincomycin, chloramphenicol, or methicillin. Mupirocin has good activity against human and animal mycoplasmas. As mupirocin is highly serum protein bound, the in vitro activity is reduced in the presence of human serum. Mupirocin alters protein synthesis by blocking the incorporation of isoleucine into the peptide during synthesis. Since 1985, when mupirocin was introduced into clinical practice, two types of resistant strains have been described: those for which the MICs are between 8 and 256 µg/ml and those for which the MICs are 512 µg/ml. Extensive use of mupirocin has resulted in the emergence of resistant strains of S. aureus in the United Kingdom. Local application of mupirocin yields in situ concentrations of 20,000 mg/kg. Skin and skin structure infections are very common, but the use of topical antibiotics or other antibacterial agents is limited by a number of factors: the complexity of the anatomy of the skin, the bacterial ecology of the skin, and the nature of the microorganisms involved in these infections. Mupirocin eradicates methicillin-resistant strains of S. aureus colonizing the skin and produces an improvement in the healing of skin wounds.

The family of orthosomycins includes the everninomicins, which are oligosaccharide antibiotics containing two orthodiester bonds and a dichloroeverninic acid residue. The everninomicins are original antibiotics that belong structurally to the oligosaccharide antibacterial agents. They are characterized by an oligosaccharide structure with the presence of one or more orthodiester-type bonds. This type of bond is unusual in nature but has been described for a series of molecules grouped together in a family known as orthosomycins. Avilamycin is an orthosomycin complex extracted from Streptomyces viridochromogenes Tü 57 in 1965. An approach to the structure and activity of everninomicins has been undertaken by means of chemical modifications, particularly of evernimicin. The configuration of the two orthoester bonds at C-49 and C-16 is important for the antibacterial activity. Mechanisms of resistance to evernimicin are complex phenomena which include mainly mutations, methylation, and efflux. Recently it was shown that avilamycin A and evernimicin share common mechanisms of resistance. The rplP gene mutations have been also reported for enterococcal isolates resistant to avilamycin A. The steady-state apparent volume of distribution is on the order of 22 liters, indicating that evernimicin is distributed in the extravascular media. Evernimicin was thought to be an alternative in severe pneumococcal infections for which no other treatment is possible because of a multiresistant strain.

The recognition of Helicobacter pylori as an etiological agent of gastric peptic ulcer, gastric adenocarcinoma, and mucosaassociated lymphoid tissue lymphoma is one of the most important advances made in gastroenterology in the past 50 years. Eradication therapy is recommended for infected persons who develop peptic ulcer disease or gastric lymphoma or who are beginning long-term treatment with nonsteroidal anti-inflammatory drugs. Among the few conserved families of efflux systems associated with bacterial resistance to antibacterial agents, one is widespread among gram-negative bacilli: the RND family. In Tartu, Estonia, 56 patients suffering from H. pylori infections were enrolled for testing of the susceptibility of H. pylori clinical isolates to antibacterial agents. There are few data regarding the H. pyloriresistance rate in the United States. The frequency of primary clarithromycin and metronidazole resistance among patients enrolled in different geographical areas in the U.S.-based clinical trials between 1993 and 1999 was assessed by MIC determinations using E-test. The rate of resistance to amoxicillin and tetracycline in this study among H. pylori isolates was low. The marked decrease of metronidazole in gastric juice after repeated administration of omeprazole is explained by the elevation of gastric pH induced by omeprazole. Ciprofloxacin was administered as a single dose of 500mg in 15 patients undergoing biopsies. Ciprofloxacin levels ranged from 1.5 to 1,762 mg/kg. Optimal concentrations were obtained after 1 h of dosing and reached 500 mg/kg. The in vitro activities of various antibacterial agents against H. pylori were assessed, including linezolid, dalfopristin-quinupristin, mupirocin, aminoglycosides, and fosfomycin.

Paldimycin belongs to the group of antibiotics containing an isothiocyanate group. Three groups of molecules have been isolated: the proceomycins, senfolomycins A and B and the paulomycins. Proceomycin can be isolated from the fermentation broth of Streptomyces albolongus sp. nov. The physicochemical properties of Senfolomycins A and B are summarized in this chapter. They have good activity against Staphylococcus aureus and Enterococcus faecalis, but they are inactive against gram-negative bacilli. It has been possible to isolate paulomycins A and B, paldimycins A and B, and products 273a1α [A] and 273a1β [B]). The paulomycins differ from one another in the group at R₁, which is a 2-methylbutyryl for paulomycin A and an isobutyryl for paulomycin B. The paldimycins and the 273a2 compounds are produced by the addition of two and one N-acetyl-l-cysteine, respectively, to paulomycins A and B. Paldimycin exhibits optimal antibacterial activity when the pH of the culture medium is less than 7. Paldimycin has good activity against S. aureus and coagulase-negative staphylococci, irrespective of whether the strains are methicillin susceptible or resistant. Among the molecules that manifest good activity against methicillin-resistant strains of S. aureus, paldimycin is one of the most active. Paldimycin has moderate activity against Listeria monocytogenes, but it has good activity against Corynebacterium jeikeium. Paldimycin appears to be active against Chlamydia trachomatis.

Some fluoroquinolones have been demonstrated to be effective companions in standard regimens of tuberculosis (TB) therapy, but the emergence of M. tuberculosis isolates resistant to fluoroquinolones, all of them having a cross-resistance, has led to discontinuation of their use in monotherapy. In a study, Tosufloxacin, a 1,8-naphthyridone compound, was shown to be inactive against 98 isolates of M. tuberculosis in a nonspecified agar medium. In this study, it was shown that in vitro sparfloxacin was the most active compound against all M. tuberculosis isolates. The in vitro activities of balofloxacin, cadrofloxacin (CS-940), and other fluoroquinolones against 100 M. tuberculosis isolates were assessed using an egg-based Ogawa medium and 7H10 Middlebrook agar medium. Fluoroquinolones are widely distributed in tissue, including within cells, where M. tuberculosis resides. Moxifloxacin at 20 mg/kg daily allowed a 100% survival rate, in comparison with a 50% survival rate in control mice after challenge with a large inoculum of the virulent strain CSU 98. It has been shown in murine infection induced by M. tuberculosis H37RV that among the fluoroquinolones and aminoglycosides, sparfloxacin at 50 mg/kg and amikacin at 200 mg/kg are the most active compounds after isoniazid (25 mg/kg) in treating TB in mice. The most active compound is PD-161144-like gatifloxacin, which was reported to show good antimycobacterial activity.

Acquired resistance of Mycobacterium lepraeto the thioamides has been demonstrated in patients suffering from lepromatous leprosy treated with thioamides alone. Only the antibacterial treatment of leprosy, the principles on which it is based, the combinations with recommended antibiotics, and the results obtained to date in the field are discussed in this chapter. The whole argument behind modern chemotherapy of leprosy is based on the treatment of lepromatous or multibacillary leprosy, in the same way that the rationale for the chemotherapy of tuberculosis is based on the treatment of patients suffering from cavitary pulmonary tuberculosis. Dapsone, rifampin, and clofazimine therefore are automatically part of the initial phase of the chemotherapy of leprosy. From 109 at the beginning of treatment, the total number of viable M. leprae bacilli in a patient suffering from multibacillary leprosy therefore fell to 104! The majority of susceptible bacilli and mutants resistant to dapsone and clofazimine, which by definition are susceptible to rifampin, were eliminated. The first reason for continuing treatment beyond the first doses of rifampin is to prevent the risk of selection of rifampin-resistant bacilli. The second reason is that after very short-term multidrug therapy in patients suffering from Bamako multibacillary leprosy, it was found that the shorter the duration of antibiotic administration, the earlier relapses occurred. It is difficult to reduce the duration of the multidrug therapy recommended by the WHO. It is based on the combination of the three best-tolerated antibiotics, rifampin, dapsone, and clofazimine, of which only rifampin is highly bactericidal.

The use of mupirocin and fusidic acid in topical applications is jeopardized by the emergence of resistant bacteria. New medicinal chemical entities are needed to overcome this threat. Benzonaphthyridone molecules are one of the potential alternatives. From a new series of benzonaphthyridones, RP-60556A was selected for further preclinical investigation focused on its potential use as a topical antibacterial agent. RP-60556A is active against Staphylococcus aureus isolates with GlrA or GyrA mutations as well as against strains harboring an efflux mechanism of resistance (NorA). RP-60556A displays activity similar to those of fusidic acid and mupirocin against S. aureus strains susceptible to those antibacterials but remains active against mupirocin-and/or fusidic acid-resistant isolates. RP-60556A exhibits good in vitro activity against gram-positive anaerobes and against Bacteroides fragilis. This slight bactericidal activity remains against mupirocin-resistant S. aureus isolates. In vivo in a guinea pig model of cutaneous infection with S. aureus, a significant reduction of staphylococcal load per wound was shown in 24 h. The in vivo activity of a 2% topical formulation of RP-60556A in comparison with that of a 2% mupirocin formulation was investigated in a model of staphylococcal wound infection in guinea pigs. Antibacterials were applied 18 h after local bacterial challenge. RPR 203246 is slightly less active against S. aureus irrespective of the mechanism of resistance to pefloxacin.

Some mutilins, such as tiamulin and valnemulin, were introduced into veterinary medicine for the treatment of swine infections with Brachyspira hyodysenteriae (diarrhea) and Mycoplasma hyopneumoniae (pneumonia). Mutilins are semisynthetic derivatives having a tricyclic diterpenoid structure. They differ in their side chains. It was shown that the C-14 side chain is essential for antibacterial activity. The tricyclic diol mutilin did not inhibit protein synthesis. Pleuromutilins are mainly active against gram-positive bacteria and display moderate activity against fastidious gram-negative bacilli. Tiamulin MICs of >32, 4.0, and 0.06 to 0.5 µg/ml are observed for Bordetella bronchiseptica, Leptospira spp., and Mycoplasma spp., respectively. Tiamulin and valnemulin interact with adenines 2058 and 2059 and uracils 2506, 2584, and 2585. A new series of semisynthetic pleuromutilins was presented and one compound was selected for a preclinical investigation, SB264128. In pneumococcal infections, the burden reduction obtained with SB 264128 was significant (2.7 ± 1.0 log10 CFU/lung [mean plus/minus standard deviation]). In Haemophilus influenzae H 128 infection, bacterial counts in untreated rats were 5.3 plus/minus 0.7 log10 CFU/lung. SB 264128 exhibited a higher clearing activity (1.8 ± 0.2 log10 CFU/lung) than co-amoxiclav (3.5 ± 1.2 log10 CFU/lung).

Schematically, the current systemic antifungal agents can be divided into two categories: that of the antifungal antibiotics, which include griseofulvin and amphotericin B (AMB), and that of the chemical agents, which comprise all of the other molecules, flucytosine (5FC), the extremely rich series of azoles and, more recently, the allylamines and echinocandins. Given their value in the treatment of systemic mycoses, the authors discuss the three systemic azoles currently commercially available in parallel with voriconazole, which is currently available under an authorization for compassionate use, and conclude with the most recent triazoles under development. The azole compounds possess a relatively broad spectrum against numerous pathogenic or opportunistic fungal species, a spectrum that is exceeded only by that of AMB. Although all of these derivatives share good activity against some of these fungi (such as Candida albicans, Candida neoformans, and dimorphic fungi), major differences have been found in vitro and subsequently confirmed in animals and in clinical trials. Despite the considerable advances in antifungal therapy over the last few years, we are still far from possessing the ideal antifungal and progress is still necessary. Fungal infections remain a matter of concern, particularly in severe neutropenia (frequent inefficacy in disseminated mycoses) and AIDS (frequency of relapses and/or resistance in oroesophageal candidosis and cryptococcosis).

The chapter presents an overview of drug-drug interactions that occur during anti-infective treatments. The solubilization of antifungal azoles, indinavir, delavirdine, or even dapsone requires an acidic pH, so acid suppressants greatly reduce their bioavailability. Di- or trivalent cations lead to the formation of insoluble or weakly soluble chelates with FQ, tetracyclines, rifampin, and ethambutol. Any metabolizable drug that penetrates the enterocyte and gains access to its cytosol will be metabolized by enterocytic CYP, a new stage which may be the subject of an interaction either by competition between substrates or by modification of the metabolic capacities of the enterocyte following coprescription of an enzyme inducer or inhibitor. Pulmonary CYPs are induced by cigarette smoke, a means of eliminating more actively chemicals contained in the smoke. Nowadays, evaluation of potential drug-drug interactions is mandatory for the registration of new drugs by regulatory agencies. The participation of substrate metabolites in drug-drug interactions at both levels, drug transport proteins and CYP enzymes, could also lead to discrepancies. In the vast literature dedicated to metabolic drug interactions, CYP3A4 has received the greatest attention. In the vast literature dedicated to metabolic drug interactions, CYP3A4 has received the greatest attention. The question of a fixed margin versus a varying margin in noninferiority trials has been addressed statistically for anti-infective treatments, taking into account the variable underlying clinical success rate of the reference treatment.

The present knowledge of the impact of antimicrobial agents on the oropharyngeal and intestinal microflora and the optimal performance of such studies are discussed in this chapter. Disturbances in the oropharyngeal microflora are caused by administration of antimicrobial agents that reach high concentrations in oral tissues. Studies on the impact of antimicrobial agents on the normal microflora should be performed as comparative double-blind studies. Data on the impact of penicillins, cephalosporin, macrolides, ketolides, lincosamides, tetracyclines, nitroimidazoles and quinolones on the oropharyngeal microflora are summarized, as also the influence of monobactams, carbapenems, glycopeptides and streptogramins on the intestinal microflora. Knowledge about the interaction between antimicrobial agents and the normal microflora gives the clinician the possibility to choose agents associated with lesser degrees of ecological disturbances. Consequently, the risk of development of resistant strains and transfer of resistance elements between microorganisms is reduced. Further, consideration of the ecological consequences is also an important step to prevent distribution of resistant strains between patients in hospital settings. Individual pharmacokinetic variations as well as the character of composition and susceptibility of the normal microflora are variables that further influence the interactions.