PHARMACEUTICAL MICROBIOLOGY - V. A. Galynkin - 2015

PART II. ANTIMICROBIAL AGENTS

CHAPTER 8. ANTIBIOTICS AND SYNTHETIC CHEMOTHERAPEUTIC DRUGS

8.1 Antibiotics

General Concepts

The term "antibiotic" was first coined in 1889 by Paul Vuillemin to describe the antagonistic Interactions Between Plants and animals. In 1942, Waksman defined an antibiotic as "a chemical substance produced by microorganisms which has the capacity to inhibit the growth of and even to destroy other microorganisms in dilute solutions." These are secondary metabolites necessary for their producers to survive under conditions of competition with other microorganisms.

In addition to antimicrobial activity, they may exhibit other pharmacological properties: acting as immunosuppressants, Enzyme Inhibitors, antitumor and cytotoxic agents, insecticides, and herbicides. To date, approximately 20,000 antibiotic substances have been described; however, no more than 160 antibiotics are used in clinical practice, of which 30% are natural products, 30% are semisynthetic, and the remainder are synthetic. Chemical synthesis is often more economically viable for producing analogs of natural compounds than Biosynthesis. Nevertheless, biosynthesis remains indispensable for obtaining natural products, precursors of semisynthetic antibiotics, and Enzymes required for the enzymatic transformation of natural or synthetic compounds. The continuous emergence of antibiotic-resistant microorganisms necessitates the discovery of new producers and novel antimicrobial drugs.

The activity of antibiotics, i.e., their ability to suppress the growth of a microbial population, is determined by various Methods (see Chapter 9).

Based on their spectrum of activity, antibiotics are classified into antiviral, antibacterial, antifungal, and antiprotozoal agents. Some of them have a broad spectrum of activity, while others are active only against specific groups of microorganisms, such as solely against Gram-positive Bacteria. Antibiotics are highly diverse in their Structure and MECHANISM OF ACTION.

8.1.1 β-Lactam antibiotics

Penicillins. The general STRUCTURE OF THE penicillin molecule is shown in Fig. 43. Both natural and Semisynthetic penicillins exist. Natural penicillins (benzylpenicillin G, phenoxymethylpenicillin, or penicillin V) are synthesized during the Fermentation of Penicillium notatum and P. chrysogenum. Semisynthetic penicillins are derivatives of the core penicillin molecule, 6-aminopenicillanic acid (6-APA), in which the side chain is represented by various radicals introduced via acylation of 6-APA. They differ in their activity and other properties (Table 19). Thus, unlike natural benzylpenicillin, many semisynthetic penicillins are resistant to the acidic environment of The Stomach and can be administered orally. Benzylpenicillin derivatives such as benzathine, benethamine, and procaine release penicillin slowly, maintaining its high concentration in the Blood for a prolonged period, and therefore do not require repeated administration to the patient. Carbenicillin and ampicillin esters undergo Enzymatic Hydrolysis after absorption from the intestinal mucosa, establishing a high level of active antibiotic in the blood.

Class="center">Fig. 43. General structure of the penicillin molecule. The arrow indicates the bond hydrolyzed by β-lactamase.

Unlike benzylpenicillin, some semisynthetic penicillins are resistant to microbial β-lactamase (penicillinase), an enzyme that inactivates penicillin by cleaving the β-lactam ring. There are agents active against both Gram-positive and Gram-negative microorganisms, including Pseudomonas aeruginosa (a bacterium highly resistant to many antibacterial agents).

Table 19. Penicillins

* - carbenicillin esters;

** — ampicillin esters;

*** — substituted ampicillins;

**** — 6-β-amidinopenicillins; •• — inactive against Gram-positive bacteria.

Mecillinam, 6-β-amidinopenicillanic acids, and their ester pivmecillinam have an unusual

spectrum of antibacterial activity: they are not active against Gram-positive microorganisms like classical penicillins, but exclusively against Gram-negative ones.

8.1.2 Cephalosporins

Cephalosporins (cephems), similar to penicillins, belong to the group of β-lactam antibiotics. They have a broader spectrum of activity compared to penicillins and are more resistant to β-lactamases. Individuals with a penicillin allergy are generally not sensitive to cephems. The producer of the natural antibiotic cephalosporin C is the fungus Acremonium chrysogenum, originally named Cephalosporium acremonium. Numerous semisynthetic preparations have been derived from natural cephalosporin (Fig. 44).

Fig. 44. Structure of cephalosporins.

Clavams differ from penicillins (Fig. 45) in the replacement of sulfur in the thiazolidine ring of penicillin with oxygen in the oxazolidine ring of the clavam, and the absence of a side chain at position 6. Clavulanic acid (Fig. 45, A), produced by Streptomyces clavuligerus, exhibits low antibacterial activity, yet it is a potent inhibitor of staphylococcal β-lactamase and most types of β-lactamases produced by Gram-negative bacteria, particularly those with "penicillinase" rather than "cephalosporinase" activity. In clinical practice, Augmentin is used—a combination drug of clavulanic acid and amoxicillin, which is a broad-spectrum semisynthetic penicillin that is sensitive to β-lactamase.

Fig. 45. Clavams: A — clavulanic acid; B — latamoxef; C — carbapenems; D — olivanic acids; E — thienamycin.

In 1-oxacephems, the sulfur atom in the dihydrothiazine ring of cephalosporin is replaced by oxygen. Such a molecule is susceptible to β-lactamases; however, the Introduction of a 7-α-methoxy group (as in cefoxitin) makes the compound more stable. Latamoxef (Fig. 45, B), a broad-spectrum antibiotic resistant to β-lactamases, also belongs to this group.

Carbapenems (Fig. 45, C) are analogues of penicillins or clavams in which the sulfur atom (in penicillins) or oxygen atom (in clavams) is replaced by carbon. These include olivanic acids (Fig. 45, D), produced by Streptomyces olivaceus, which are broad-spectrum antibiotics and potent β-lactamase inhibitors, as well as thienamycin (Fig. 45, E), which possesses similar properties but is unstable. Its derivative, N-formimidoylthienamycin, lacks this drawback.

Nocardicins (A–D) are produced by Nocardia sp., with nocardicin A being the most active broad-spectrum antibiotic (Fig. 46, A).

Fig. 46. A — Nocardicin A; B — aztreonam; C — sulfonpenicillanic acid (Na salt); D — bromopenicillanic acid (Na salt).

Monobactams are monocyclic β-lactams lacking a thiazolidine ring, synthesized by various bacterial strains. Among the antibiotics in this group, aztreonam (Fig. 46, B) has proven to be the most effective; it acts against many Gram-negative bacteria and is resistant to β-lactamases, though it lacks activity against Bacteroides and Gram-negative anaerobes.

Other β-lactams. Penicillanic acid derivatives, such as sulfo- and bromopenicillanic acids (Fig. 46, C, D, E), act as inhibitors of certain types of β-lactamases.

8.1.3 Tetracyclines

Tetracyclines are produced by certain species of streptomycetes and are also obtained as semisynthetic preparations (Fig. 47). For instance, chlortetracycline is formed during the fermentation of B. aureofaciens, from which tetracycline and clomocycline are derived via catalytic hydrogenation. Chemical modification of tetracycline yields minocycline. Oxytetracycline is produced by S. rimosus and serves as the source for methacycline, the hydrogenation of which yields doxycycline. A mutant of aureofaciens produces methylchlortetracycline. Thiatetracyclines, such as thiatetracycline, contain a sulfur atom at position 6 of the molecule and are active against tetracycline-resistant bacteria. Currently, natural tetracyclines are practically unused, having been superseded by semisynthetic preparations (doxycycline and minocycline).

Fig. 47. Tetracyclines: 1 — oxytetracycline; 2 — chlortetracycline; 3 — tetracycline; 4 — demethylchlortetracycline; 5 — doxycycline; 6 — methacycline; 7 — clomocycline; 8 — minocycline; 9 — thiatetracycline.

Tetracyclines are broad-spectrum antibiotics. Resistance to them develops relatively slowly. However, cross-resistance does occur, meaning that a microorganism resistant to one tetracycline is also resistant to all members of this group.

The Use of tetracyclines may result in dysbiosis, including The Development of candidiasis.

8.1.4 Rifamycins

Rifamycins are produced as an antibiotic complex by Streptomyces mediterranei, with rifamycin B being the most promising among them, serving as the basis for the creation of the semisynthetic analogue rifampicin (Fig. 48).

Fig. 48. Rifampicin.

Rifampicin is a broad-spectrum antibiotic. Resistance develops quite rapidly, which is why its use is recommended to be restricted, primarily to mycobacterial infections. The drug is poorly absorbed when taken orally, and is therefore administered parenterally or topically.

8.1.5 Aminoglycoside Antibiotics

Aminoglycoside antibiotics represent a large class of compounds produced by certain actinomycetes and bacilli. Aminoglycosides contain aminosugars in their structure: deoxystreptamine in the molecules of neomycin, framycetin, gentamicin, kanamycin, tobramycin, amikacin, netilmicin, sisomicin, and streptidine in the molecules of streptomycin and dihydrostreptomycin. Aminocyclitol streptomycin lacks a carbohydrate component (Fig. 49). These are broad-spectrum antibiotics. Streptomycin and kanamycin are effective against tuberculosis. Aminoglycosides show weak activity against streptococci and are ineffective against enterococci, Providencia, Bacteroides, and other anaerobes.

Fig. 49. Aminoglycoside antibiotics.

The modification of aminoglycoside antibiotics yields agents resistant to the inactivating enzymes of resistant microbial strains: 3-deoxykanamycin (tobramycin), amikacin—which features a substituted aminobutyryl group at the 2-deoxystreptamine amino group—and N-methylsisomicin (netilmicin).

8.1.6 Macrolides

Macrolide molecules contain large lactone rings linked to amino sugars via glycosidic bonds. The most important representatives of this group are erythromycin, oleandomycin, triacetyloleandomycin, and spiramycin. Erythromycin (Fig. 50), produced by Streptomyces erythreus, is active against Gram-positive bacteria, Neisseria, Haemophilus influenzae, and Legionella pneumophila, but lacks activity against enterobacteria. Its potency increases as pH rises to 8.5; erythromycin thiocyanate is more stable than the free base in the acidic gastric environment and is administered orally, persisting longer in the bloodstream and penetrating Tissues more effectively than other forms; in vivo, it is hydrolyzed to the free base.

Fig. 50. Erythromycin.

Staphylococcus aureus is less sensitive to erythromycin than pneumococci or hemolytic streptococci and rapidly develops resistant variants. Therefore, this antibiotic should not be used for prolonged periods.

Oleandomycin (produced by S. antibioticus), its ester (triacetyloleandomycin), and spiramycin share the same spectrum of antibacterial activity as erythromycin, though they are less potent. While resistance develops slowly in clinical practice, cross-resistance across the entire antibiotic group does occur.

8.1.7 Polypeptide antibiotics

Polypeptide antibiotics represent a rather diverse group of compounds synthesized by bacilli and actinomycetes. They include bacitracin, which is active against Gram-positive and Gram-negative cocci (though not other bacteria); polymyxins, active against many Gram-negative bacteria except Serratia marcescens and Proteus spp.; and capreomycin and viomycin, which act against Mycobacterium tuberculosis.

Due to its high toxicity, bacitracin is approved for use only in cases of absolute necessity.

Polymyxins exhibit nephrotoxicity, though polymyxins B and E (colistin) are less toxic. Sodium colistimethate is used for parenteral administration. Sodium sulfomixin—a mixture of sulfomethylated polymyxin B and sodium bisulfite—is less toxic than polymyxin B sulfate while retaining its activity.

Cationic Peptides represent a novel class of antibiotics derived from both natural sources and chemical synthesis. In nature, they are ubiquitous, found in bacteria, Fungi, plants, animals, and humans.

In humans, cationic peptides (defensins) comprise a group of peptides containing 23 to 35 Amino Acids; they are present in leukocytes and other tissues, exhibit activity against bacteria, Protozoa, fungi, and Viruses, and possess chemotactic properties.

In plants, these are thionins, which are produced in response to infection and are active against certain bacteria and fungi.

In bacteria, they function as bacteriocins: colicin in E. coli, nisin in Lactococcus lactis, subtillin in B. subtilis, etc. While less potent compared to Other Antibiotics, they remain active against strains resistant to conventional drugs, induce rapid microbial death, and can provoke the development of resistance.

Cationic peptides neutralize the negative charge of bacterial lipopolysaccharides and bind to them, blocking their interaction with host Cells and thereby preventing the development of endotoxic Shock in patients.

Cationic peptides have low immunogenicity as they share common Antigens with the macroorganism. They are degraded by intestinal peptidases and thus do not persist in the environment.

Nisin is obtained through the fermentation of L. lactis or a recombinant strain of B. subtilis and is used for food preservation and the Treatment of bovine mastitis.

8.1.8 Other Antibacterial Antibiotics

The antibiotics described below (Fig. 51) cannot be classified into any of the previously discussed groups.

Fig. 51. Antibiotics: A — chloramphenicol; B — fusidic acid; C — lincomycin; D — clindamycin.

Chloramphenicol (levomycetin), synthesized by S. venezuelae, is a broad-spectrum antibiotic that exerts solely a bacteriostatic effect and is active against spirochetes, rickettsiae, and chlamydiae. Because it induces aplastic anemia in some patients, its use is recommended only when alternative treatments are unavailable. Its fluorinated derivatives are active against resistant microorganisms.

Fusidic acid, isolated from the culture of the fungus Fusidium coccineum and utilized in the form of its sodium salt, is active against Gram-positive bacteria, particularly staphylococci, although streptococci remain relatively resistant. While it is effective against multiresistant staphylococcal strains, resistant bacterial variants may also emerge against this agent.

Lincomycins. Lincomycin (produced by S. lincolniensis) and its synthetic analogue clindamycin share a similar spectrum of activity with macrolides, although they are ineffective against E. coli. The use of clindamycin carries the risk of overgrowth in the gut of Clostridium difficile, the CAUSATIVE AGENT OF diarrheal infections. Cross-resistance between lincomycins and erythromycin is possible.

8.1.9 Antifungal antibiotics

Unlike the wide variety of antibacterial antibiotics, there are only a few antifungal antibiotics available for systemic use (Fig. 52).

Fig. 52. Antifungal antibiotics: A — griseofulvin; B — nystatin; C — amphotericin B (R=H) and its methyl ester (R=CH₃).

Griseofulvin is produced by Penicillium griseofulvum, is active against dermatophytes, and has no effect on Candida spp. and bacteria. It is administered orally in tablet form. It penetrates into the deep layers of the Skin and Hair keratin, making it effective in the treatment of dermatophytoses.

Polyenes. Polyene antibiotics are produced by streptomycetes and are characterized by a large lactone ring and a hydrophobic region consisting of 4 to 7 conjugated double bonds in their structure. The most important among them are amphotericin B and nystatin.

Nystatin is active against Candida spp. and is used for oral and gastrointestinal candidiasis, as it is poorly absorbed from the gut and ineffective in Other forms of candidiasis.

Amphotericin B is effective against systemic mycoses. Since it is poorly absorbed from the gastrointestinal tract, it is administered intravenously under strict medical supervision due to its nephrotoxicity. Amphotericin methyl ester is less toxic while retaining its antifungal activity.

Echinocandins are a new class of antibiotics represented by numerous fermentation products of Aspergillus nidulans and A. rugulosus and their semi-synthetic analogues. They are active against fungi of the genera Candida, Aspergillus, and Pneumocystis. This family consists

of cyclic lipopeptides that differ in their side chains (fatty acid residues) attached to the hexapeptide ring. They are expected to find clinical application in the near future.

8.1.10 Synthetic chemotherapeutic agents

Sulfonamides — a large group of chemotherapeutic agents (Fig. 53) that act against streptococci, E. coli, Proteus mirabilis, and are typically inactive against Pseudomonas aeruginosa, indole-positive Proteus strains, and Klebsiella spp. Resistant strains emerge in Staphylococcus aureus, Haemophilus influenzae, Neisseria gonorrhoeae, N. meningitidis, E. coli, Proteus mirabilis, and others.

Fig. 53. Sulfonamides.

Sulfonamides vary in their ability to be absorbed from the gastrointestinal tract; for instance, sulfadimidine and sulfadiazine are absorbed rapidly, whereas succinylsulfathiazole and phthalylsulfathiazole are absorbed very poorly.

They are used in the treatment of various infectious diseases caused by Gram-positive and Gram-negative microorganisms.

Diaminopyrimidine derivatives (Fig. 54) comprise a group of substances with antineoplastic (methotrexate), antiprotozoal (pyrimethamine), and antibacterial (trimethoprim, tetroxoprim) activity. The latter possess a broad spectrum of activity, although resistant bacterial strains tend to emerge.

Fig. 54. Diaminobenzylpyrimidines.

Isonicotinic acid derivatives (isoniazid, ethionamide, prothionamide) as well as para-aminosalicylic acid (Fig. 55) are effective against mycobacteria and are used in the Treatment of tuberculosis. Ethionamide is less active than isoniazid, but it acts against isoniazid-resistant bacterial strains. Prothionamide is similar in activity to ethionamide but is better tolerated by patients.

Fig. 55. Para-aminosalicylic acid (A); isonicotinic acid derivatives: isoniazid (B), ethionamide (C), prothionamide (D), and pyrazinamide (E).

Pyrazinccarboxylic acid amide — pyrazinamide (Fig. 55) acts against tubercle bacilli that are resistant to other antituberculosis drugs.

Nitrofuran compounds. Several hundred nitrofuran compounds have been synthesized, but only a few possess therapeutic properties. Of key importance is the presence of the -CH=N- group at C-2 and the nitro group at C-5, whereas the presence of -CH=CH- at C-2 is of lesser significance. Biological activity is lost if (a) the nitro ring is reduced, (b) the -CH=N- bond is hydrolyzed, or (c) the -CH=CH- bond is oxidized.

Nitrofurans exhibit a broad spectrum of antimicrobial activity. Furazolidone is highly effective against enterobacteria and is used to treat bacterial diarrhea and gastrointestinal disorders. Nitrofurantoin is prescribed for Urinary Tract infections because the drug concentrates in the urine, showing the highest activity at acidic pH levels. Nitrofurazone is primarily used topically for treating wounds and Burns, as well as certain types of ear infections.

Quinolones. Nalidixic acid was used for some time to treat urinary tract infections. Subsequently, other, more potent 4-quinolone derivatives were synthesized, including ciprofloxacin, norfloxacin, oxolinic acid, ofloxacin, cinoxacin, etc. (Fig. 56).

Fig. 56. Quinolones: A — norfloxacin, B — nalidixic acid, C — cinoxacin.

Nalidixic acid is active exclusively against Gram-negative bacteria, whereas newer derivatives are also effective against staphylococci, though not streptococci. Norfloxacin has a broad spectrum of activity and concentrates in the urine when administered orally, making it a promising agent for the treatment of urinary tract infections.

Imidazole derivatives (Fig. 57). Metronidazole inhibits the growth of pathogenic protozoa such as Trichomonas vaginalis and Entamoeba histolyticum. When administered orally, it is highly effective against Urogenital infections caused by T. vaginalis. It is active against anaerobic bacteria and facultative anaerobes under anaerobic (but not aerobic) conditions. It is administered orally or in suppository form.

Fig. 57. Imidazole derivatives: A — metronidazole, B — clotrimazole, C — miconazole, D — econazole, E — ketoconazole.

Clotrimazole, miconazole, econazole, and ketoconazole are active against pathogenic fungi and certain bacteria, and are used topically. No development of resistance has been observed either in vitro or in vivo.

5-Fluorocytosine (Fig. 58 displays its highest activity against Yeasts, specifically Candida and Cryptococcus species.

Fig. 58. 5-Fluorocytosine.