1. University of Karbala College of veterinary medicine Second semester Pharmacology Lect. # 1 Antibacterial Drugs Part three Dr. Sattar K. Abdul-Hussain, Ph.D, DVM, DABT

2. TETRACYCLINES
The tetracyclines are polycyclic compounds that are amphoteric in nature.
Most of tetracyclines are prepared as the hydrochloride salt.
They chelate with cations such as Mg2+, Ca2+, Fe3+ to form insoluble complex.
They accumulate in growing teeth and bones.

3. TETRACYCLINES Mechanism of action:
Tetracyclines inhibit bacterial protein synthesis by binding to the 30S bacterial ribosome and preventing access of aminoacyl tRNA to the acceptor site on the mRNA-ribosome complex.
They block the addition of amino acids to the growing peptide chain (Figure 8).
They are bacteriostatic and broad spectrum.
Their antimicrobial spectrum includes gram positive and gram-negative aerobes and anaerobes, Rickettisiae, Spirochetes, Chlamydiae, Mycoplasma, and some protozoans such as Anaplasma spp. and Haemobartonella spp. (Figures 6 and 8)

4. Inhibition of bacterial protein synthesis by tetracyclines
Inhibition of bacterial protein synthesis by tetracyclines. Messenger (mRNA) attaches to the 30S subunit of bacterial ribosomal RNA. The P (peptidyl) site of the 50S ribosomal RNA subunit contains the nascent (the merging) polypeptide chain; normally, the aminoacyl tRNA charged with the next amino acid (aa) to be added to the chain moves into the A (acceptor) site, with complementary base pairing between the anticodon sequence of tRNA and the codon sequence of mRNA. Tetracyclines inhibit bacterial protein synthesis by binding to the 30S subunit and blocking tRNA binding to the A site (red X).

5. TETRACYCLINES
Therapeutic uses:
Large animals: in cattle, sheep, horses, and swine, tetracycline, chlortetracycline, and oxytetracycline are used in the treatment of local and systemic bacterial, chlamydial, rickettsial, and protozoal infections
Small animals: in dogs and cats, doxycycline, minocycline, and tetracycline are used in the treatment of respiratory and urinary tract infections. Also can be used as specific treatment for Borrelia (Lyme disease), Brucella, Haemobartonella, and Ehrlichia species infections. They are also effective in the treatment of psittacosis in birds.

6. TETRACYCLINES Administration:
Teteracylines are given orally or IV every 8-12 hours.
Oral therapeutic doses should be avoided in adult ruminants and used with caution in horses because of the danger of disrupting ruminal or colonic microflora, respectively.
Pharmacokinetics:
Oral absorption is fair for tetracyclines (range from 60-90%) of the given dose but not for chlortetracycline, which is only 35%.
Since divalent ions chelate with tetracyclines, milk, antacids, or iron salts should be avoided 3 hours before and after oral administration.
Teteracylines distribute well in all body tissues except the CNS.
Metabolism is minimal in domestic animals. Renal excretion by glomerular filtration is the major route of elimination for most tetracyclines.

7. TETRACYCLINES Adverse effects:
Nephrotoxicity: is common with tetracyclines except doxycycline and minocycline. Thus, they should be avoided if renal function is impaired.
Permanent teeth stain may occur in young animals due to the formation of a tetracycline-calcium phosphate complex in enamel and dentine.
Resistance to fungi, yeast, or bacteria may occur in the GI tract with prolonged administration. Oral tetracyclines should not be used with herbivores because of serious effects on ruminant digestion.
Antianabolic effect may develop with high doses due to binding to mitochondrial ribosomes.
Photosensitivity and hepatotoxicity can be developed with therapeutic use of tetracyclines.

8. TETRACYCLINES Resistance:
Resistance to tetracycline is widely spread because of the extensive use. Resistance may be developing due to decreased drug uptake or active transport of the tetracycline out of the bacterial cell.
Preslaughter withdrawal of oxytetracycline in food animals:
The Food Animal Residue Avoidance Databank (FARAD) recommends, in cattle, an extralabel withdrawal of 28 days for intrauterine treatment. It also recommends testing milk after intrauterine treatment, as there is inter-cow variability in the residue eliminations profiled in milk.
FARAD recommends an extralabel preslaughter withdrawal of 28 days in sheep and goats after IM or SC oxytetracycline administration. A milk withdrawal of 96 hours is recommended for sheep and goats.

9. CHLORAMPHENICOL
Chloramphenicol, an antibiotic produced by Streptomyces venezuelae, was introduced into clinical practice in 1948.
With the drug’s wide use, it became evident that chloramphenicol could cause serious and fatal blood disorders.
For this reason, chloramphenicol is now reserved for treatment for life-threatening infections such as meningitis, rickettsia infections in patients who cannot take safer alternatives because of resistance of allergies.
The compound is unique among natural compounds in that it contains a nitrobenzene moiety and is a derivative of dichloroacetic acid.

10. CHLORAMPHENICOL Mechanism of action:
Chloramphenicol and florfenicol inhibit protein synthesis in bacteria.
The drugs readily penetrated bacterial cell by facilitated diffusion. They inhibit protein synthesis by binding reversibly to the 50S ribosomal subunit.
Although binding of tRNA at the codon recognition site on the 30S ribosomal subunit is undisturbed, the drugs apparently prevent the binding of the amino acid-containing end of the aminoacyl tRNA to the accepter site of the 50S ribosome subunit.
The interaction between peptidyltransferase and its amino acid substrate cannot occur, and peptide bond formation inhibited.
Chloramphenicol and florfenicol are bacteriostatic and broad spectrum and are effective against most anaerobic bacteria (Figures 6 and 9).

11. Inhibition of bacterial protein synthesis by chloramphenicol
Inhibition of bacterial protein synthesis by chloramphenicol. Chloramphenicol binds to the 50S ribosomal subunit near the site of action of clindamycin and the macrolide antibiotics. These agents interfere with the binding of chloramphenicol and thus may interfere with each other’s if given at the same time.

12. CHLORAMPHENICOL Therapeutic uses:
Chloramphenicol is not recommended for use in food-producing animals because the danger of the residue—induced toxicity in humans. In dogs, cats, horses and birds for local and systemic infections, caused by anaerobes and Salmonella species.
Florfenicol is approved for use only in cattle for the treatment of bovine respiratory disease (BRD) caused by Pasturella species.
Pharmacokinetics:
Chloramphenicol: Is rapidly absorbed from the GI tract and distributed to all body tissues including the CNS. It is metabolized by liver to metabolites that are eliminated by kidneys.
Florfenicol: is absorbed orally in dogs and cats and from IM sites in cattle. It is like chloramphenicol, is widely distributed in all body tissues including CNS.
Administration:
In dogs, birds, and horses, chloramphenicol is administered orally, IM, IV, or SC every 6-8 hours.
Florfenicol is administered IM in cattle and repeated 48 hours later for a total of two doses of the slow-release preparation. In dogs and cats, it is administered IM or SC every 8 hours and every 12 hours, respectively.

13. CHLORAMPHENICOL
Adverse effects:
Anemia: it may occur in animals and humans. The anemia development is due to the inhibition of iron uptake by erythrocytes by chloramphenicol. The anemia development is dose-dependent. Another type of anemia may develop in humans by chloramphenicol, which is not dose related conditions. This type of anemia is aplastic anemia which often fatal and it is the reason for the drug’s ban in food-producing animals.
Anorexia and diarrhea with high or prolonged dosage
Florfenicol is not known to produce aplastic anemia admits use is permitted in beef cattle.

14. MACROLIDES
Introduction: They are another group of antibiotics that includes erythromycin, azithromycin, clarithromycin, tulathromycin, tylosin, and tilmicosin. They are compounds consisting of lactone rings to which are attached deoxy sugars. Mechanism of action:
They are bacteriostatic by inhibiting protein synthesis (Figure 6). They bind to the 50S ribosome to prevent translation of amino acids to the growing peptide chain.
Binding sited on the 50S ribosome overlap with binding sites of chloramphenicol and the lincosamides (especially clindamycin) and combination therapy should be avoided.
Their antimicrobial activity is primarily against gram-positive aerobes and anaerobes and Mycoplasma species.

15. Inhibition of bacterial protein synthesis by the macrolide antibiotics erythromycin, clarithromycin, and azithromycin. Macrolide antibiotics are bacteriostatic agents that inhibit protein synthesis by binding reversibly to the 50s bacterial ribosomal subunits. Erythromycin appears to inhibit the translocation step such that the nascent peptide chain temporarily residing at the A site of the transferase reaction fails to move to the P, or donor, site. Alternatively macrolides may bind and cause conformational change that terminates protein synthesis by indirectly interfering with transpeptidatioin and translocation.

16. MACROLIDES Therapeutic uses:
Erythromycin: In dogs, cats, and horses, it is an alternate to penicillin for infections caused by gram-positive aerobes and anaerobes.
Tylosin: it is used in cattle, sheep, and swine for the treatment of local and systemic infections caused by Mycoplasma and gram-positive bacteria. It is also added to feed as a growth promotant in these species.
Tilmicosin: is used in cattle for the treatment of respiratory disease. It has potentially fatal toxic effects in horses and humans.
Azithromycin: It is used as alternative for erythromycin for R. equi pneumonia in foals.
Tulathromycin: it is used for the treatment of bovine and swine respiratory diseases.
Clarithromycin: is used in dogs and cats for the treatment of mycobacterial infections including canine leproid granuloma, feline leprosy, and for Helicobacter species in cats and ferrets, and for R. equi in foals.

17. MACROLIDES Administration:
Erythromycin: is administered orally or IM three tines a day to dogs, cats and foals and IM once a day in cattle, sheep, and swine.
Tylosin: is administered SC to cattle every 72 hours
Tulathromycin: it can be given IM to cattle as treatment for respiratory infections.
Azithromycin: is administered orally once a day to dogs, cats, and foals
Clarithromycin: is administered orally twice a day to dogs, cats, ferrets, and foals
Resistance:
Bacterial resistance to macrolide antibiotics may be chromosomal or plasmid mediated and is due to decreased drug binding by the 50S ribosome. Active efflux, or enzymatic inactivation by resistant bacteria may occur as mechanisms of resistances.
Adverse effects:
Drugs belong to this group of antibiotics have relatively few adverse effects. However, mild GI tract disturbances with oral doses and pain and irritation with IM injection sites may occur.

18. Introduction: Mechanism of action: LINCOSAMIDES
This group of antibiotics includes: (lincomycine, clindamycin, and pirlimycin). They are derivative of a sulfur-containing octose with an amino acid-like side chain and are highly lipid soluble.
Mechanism of action:
Lincoasmides bind to the bacterial 50S subunit of bacterial ribosomes and suppresses protein synthesis (Figures 9 and 10).
Since this is the same site for binding of chloramphenicol and the macrolides, combined therapy should be avoided. Binding one of these antibiotics to the ribosome may inhibit the interaction of the others.
Lincomycin and clindamycin are bacteriostatic and are active against gram-positive aerobes and anaerobes, Toxoplasma species, Neospora canis, and Mycoplasma species.

19. LINCOSAMIDES
Therapeutic uses: Clindamycin is used in dogs and cats for periodontal disease, osteomyelitis, dermatitis, and deep soft tissue infections caused by gram-positive organisms. Prilimycin is prepared and used for the treatment of bovine mastitis. Pharmacokinetics: Oral absorption is 50% for lincomycin and 90% for clindamycin. Distribution is wide to include all body tissues with low penetration to CNS. They are metabolized by the liver microsomal enzymes into metabolites that are excreted in urine, bile, and feces. Administration: In dogs and cats, clindamycin is administered orally or IM twice a day. Pirlimucin is given by intramammary infusion.

20. LINCOSAMIDES
Resistance: Altered drug binding by bacterial ribosomes is the usual form of resistance. Cross-resistance between lincosamides and macrolides is common. Adverse effects: They are contraindicated in horses, rabbits, hamsters, and guinea pigs because they may produce a severe or fatal diarrhea due to altered GI flora.

21. MISCELLANEOUS ANTIBACTERIAL DRUGS
Aminocyclitols: They bind to the 30S ribosome and inhibit protein synthesis. Spectinomycin is used in dogs, cats, horses, calves, and poultry to treat enteric and respiratory infections
Metronidazole:
It is used in veterinary medicine to treat infection caused by bacteria and protozoa by producing cytotoxic metabolites, which disrupt the DNA (Figure 6).
Its use in food-producing animal is prohibited because the potential carcinogenicity.
It is used in dogs, cats, and horses for the treatment of severe infections.
It is also used in treatment of protozoal infections such as giardiasis and trichomoniasis in dogs.
Rifampin: It inhibits DNA-dependent RNA polymerase, which prevents initiation of tRNA synthesis (Figure 6). It is effective against intracellular infections.
Tiamulin: It binds to the 50S bacterial ribosome to inhibit protein synthesis (Figure 6). Its mechanism of action and antibacterial spectrum are similar to macrolides. It is active against gram-positive cocci, mycoplasma, Spirochetes, and some gram-negative pathogens.

22. MISCELLANEOUS ANTIBACTERIAL DRUGS
5. Vancomycin: It blocks the second step of bacterial cell wall synthesis by inhibiting polymer release from the cell membrane (Figure 6). It is bactericidal for gram-positive organisms. It is reserved antibiotic that administered IV over min every 6-8 hours for methicillin- resistant staphylococcal infections of bone and soft tissue in dogs and cats.
6. Bacitracin: It inhibits the second step of cell wall synthesis (Figure 6). It is bactericidal for gram-positive bacteria and Spirochetes. It is used as an ointment for topical infection in combination with polymixin B and/or neomycin. It is also added to poultry rations for the prevention and treatment of clostridial enteritis and as a growth promotant.
7. Polymyxin B: It interacts with phospholipids in the bacterial cell membrane to produce a detergent-like effect and membrane disruption (Figure 6). It is used topically to treat gram- negative bacterial infection of the skin, eye, and ear in all species. It is not absorbed orally and it is too nephrotoxic for parenteral use.
8. Nitrofurans: They reduced by the bacteria to reactive intermediates that inhibit nucleic acid synthesis (Figure 6). They produce DNA fragmentation and may also block mRNA translation. They are broad spectrum and bacteriostatic. Nitrofurazone is used topically as antibacterial ointment, powder, and water-soluble wound dressings in all species.

23. MISCELLANEOUS ANTIBACTERIAL DRUGS
9. Novobiocin: It is a coumarin antibiotic. It blocks binding of ATP to DNA gyrase to inhibit supercoiling of bacterial DNA (Figure 6). It is bacteriostatic for gram-positive cocci. It is used for wound treatment and the treatment of mastitis.
10. Streptogramins: Virginiamycin is used for poultry. It binds to the 50S ribosome to inhibit protein synthesis (Figure 6). It is administered as a medicated feed additive in broiler chickens as growth promotant. It is also used in as a feed additive in cattle to increase feed efficiency and to reduce the incidence of liver abscesses.
11. Ionophore antibiotics: They are polyether antibiotics derived from Streptomyces used primarily in poultry for feed efficiency and anticoccidial activity. They include monensin, lasalocid, laidlomycin, salinomycin and narasin. They act as alkali metal ionophores by complexing with sodium in the cell membrane to reduce passive extracellular transport of potassium and intracellular influx of H+, which kills bacteria and coccidian by lowering intracellular pH. They are used as premixes or medicated feed for growth promotion and control of coccidiosis in cattle and broiler chickens.

24. Pharmacokinetics: Adverse effects: CARBAPENEMS – Cont’d
Oral administration is strongly not recommended because of acid hydrolysis and poor absorption.
Imipenem is given IV and is eliminated by renal filtration and metabolism in the renal tubules.
Adverse effects:
May include anorexia, vomiting, and diarrhea;
CNS toxicity including seizures and tremors; and
Hypersensitivity reactions including pruritis, fever, and rarely, anaphylaxis.

25. MONOBACTAMS Therapeutic uses: Pharmacokinetics:
Monobactams are similar to penicillin with -lactam ring in which the adjacent thiazolidine ring has been replaced. Aztreonem is an example of this group. Like other -lactam agents, it binds to penicillin binding proteins present in gram-negative aerobic bacteria and disrupts cell wall synthesis (Figure 8-6).
Therapeutic uses:
It may be used as reserve antibiotic in veterinary medicine to treat severe gram-negative infections. In humans it used to replace aminoglycosides, which are more toxic when used with macrolides and lincosamides.
Pharmacokinetics:
Aztreonem when given parenterally, it will distribute in body tissue in a manner similar to that of penicillin G.
It has a good penetration to CSF and is excreted by the kidneys.

26. AMINOGLYCOSIDES Mechanism of action:
The aminoglycoside antibiotics are bactericidal.
Bacterial killing is concentration dependent
A post-antibiotic effect, that is, residual bactericidal activity persisting after the serum concentration has fallen below the minimum inhibitory concentration (MIC), also is characteristic of aminoglycoside antibiotic.
These properties may be account for the efficacy of high-dose, extended-interval dosing regimens of aminoglycosides.
Aminoglycosides diffuse through aqueous channels formed by porin proteins in the outer membrane of gram-negative bacteria to enter the periplasmic space.

27. Mechanism of action-Cont’d
Transport of aminoglycosides across the cytoplasmic (inner) membrane depends on:
Electron transport which is required to maintain the membrane electrical potential (interior negative) to drive permeation of these antibiotics. This phase of transport has been termed energy-dependent phase I (EDP1).
This phase is rate limiting and can be blocked or inhibited by:
Divalent cations such as Ca2+ and Mg2+,
Hyperosmolarity
A high reduction in pH, and
Aerobic conditions
The last two conditions impair the ability of the bacteria to maintain the membrane potential, which is the driving force for transport. Thus, the antimicrobial activity of aminoglycosides is reduced markedly in the anaerobic environment of an abscess, in hyperosmolar acidic urine, and in other conditions that limit EPD1.
Once inside the cell, aminoglycosides binds to 30S ribosomal fragment which interfere or inhibit the rate of protein synthesis by causing misreading and premature termination of mRNA translation which results in the synthesis of abnormal proteins (Figures 6 and 7)

28. Mechanism of action-Cont’d

29. Therapeutic uses
They are used in treatment of gram-negative infection in all species. Examples of aminoglycosides are:
Streptomycin and dihydrostreptomycin: these are the oldest groups of this class of antibiotics. Their use has declined due to development of new broader spectrum aminoglycosides (e.g., gentamicin and amikacin).
Neomycin: it is used orally to treat enteric infections and topically for skin, ear, and eye infections.
Gentamicin and amikacin: they are broad-spectrum aminoglycosides. They are used in all species for the treatment of susceptible infections of the skin, respiratory tract, ear, eye, urinary tract, and septicemia.
Kanamycin: its antimicrobial activity is similar to that of gentamicin except it is not effective against Pseudomonas spp. It is currently used in veterinary medicine only as and oral preparation combined with bismuth subcarbonate and aluminum magnesium silicate for the treatment of bacterial enteritis in dogs and for symptomatic relief of the associated diarrhea.

30. AMINOGLYCOSIDES – Cont’d
Pharmacokinetics:
Aminoglycosides are not well absorbed from GI tract because their high polarity in aqueous environment.
They minimally distributed to the extracellular fluid, CNS and ocular tissue.
They have the potential to accumulate in renal cortex and otic endolyph, which may lead to predisposition of these tissues to toxicity.
Aminoglycoside are unchanged in the urine by glomerular filtration.
They are not recommended for use in producing animal due to prolonged residency in kidneys.
Administration:
For systemic infections, aminoglycoside are administered IM or SC. For enteric infections, an oral dose twice a day may be used.
Resistance to aminoglycosides: Bacteria may be resistant to aminoglycosides because:
Failure of the antibiotic to penetrate intracellularly,
Inactivation of the drug by microbial enzymes, or
Low affinity of the drug for the bacterial ribosome
Inactivation of aminoglycosides by bacteria enzymes is the most common form of resistance. The numerous amino and hydroxyl side groups are sites of attack by acetylases, phosphorylases, and adenylases.

31. AMINOGLYCOSIDES – Cont’d
Adverse effects:
Aminoglyco sides are relatively more toxic than other classes of antimicrobials. However toxicity is reversible if the treatment is stopped at early stages of development of toxicity. Since high residues of aminoglycosides tend to accumulate in kidneys, nephrotoxicity may develop. So, in animals with decreased renal function, aminoglycosides should be avoided as first choice of treatment or in combination with other ototoxic, or nephrotoxic drugs such as furosemide or amphotericin B.
Ototoxicity: it may be due to progressive damage to cochlear sensory cells and/or vestibular cells of the inner ear result in in deafness and ataxia, respectively.
Nephrotoxicity: it is due to damage of the membranes of proximal tubular cells result in a loss of brush border enzymes, impaired absorption, proteinuria, and decreased glomerular filtration rate.
Neuromuscular blockade: it relatively rare toxicity associated with use of aminoglycosides. It is believed due to the prejunctional blockade of acetylcholine release and decreased postsynaptic sensitivity to acetylcholine