Canis ISSN: 2398-2942

Therapeutics: antimicrobial drug

Synonym(s): Antibiotic

Contributor(s): Juan Escala, Maggie Fisher, Linda Horspool, Lauren Trepanier

Antimicrobial agents which inhibit bacterial wall synthesis

The beta-lactam family of antibiotics

1. Penicillins

Spectrum of activity

  • Narrow spectrum: natural penicillins (penicillin G Benzylpenicillin , penicillin V Phenoxymethylpenicillin ) and penethamate (diethylaminoethylester of benzylpenicillin, prodrug):
    • Gram-positive bacteria, anaerobes, fastidious Gram-negative bacteria, eg Haemophilus spp, Pasteurella spp. NOT Enterobacteriaceae.
    • Beta-lactamase susceptible.
  • Anti-staphlococcal: isoxazolyl penicillins (cloxacillin, methicillin, oxacillin):
    • Gram-positive bacteria only. Less active than penicillin G.
      Stable to beta-lactamases.
  • Broad spectrum: aminopenicillins (ampicillin Ampicillin ('Amifipen'), amoxicillin Amoxicillin ('Clamoxyl') , hetacillin, pivampicillin):
    • Gram-positive bacteria (less active than penicillin G), Gram-negative bacteria including E. coliP. mirabilis, Salmonella spp. NOT Klebsiella spp. and Pseudomonas aeruginosa.
    • Beta-lactamase susceptible (resistance).
  • Extended spectrum: carboxypenicillins (carbenicillin, carfecillin, ticarcillin Ticarcillin , ticarcillin plus clavulanic acid), ureidopenicillins (azlocillin, piperacillin, mezlocillin):
    • Broader spectrum than aminopenicillins for Enterobacteriaceae.
      Active against P. aeruginosa. Klebsiella spp are resistant.
    • Beta-lactamase susceptible → developing Gram-negative resistance.
  • Gram-negative: amidinopenicillin:
    • Gram-negative only (binds to penicillin-binding protein (PBP) 2).
    • Unstable to beta-lactamases.
    • Also produced as metabolite of pivmecillinam.
    • Rarely used for dogs and cats.

Pharmacokinetics

  • Inorganic acids (pKa 2.7).
  • Unstable in gastric acid - EXCEPT: penicillin V, isoxazoyl penicillins (not methicillin (meticillin)), aminopenicillins, carfecillin.
  • Confined to extracellular fluids and some transcellular fluids, (joint, pleural, bile).
  • Cross membranes poorly.
  • Renal elimination. Short half-life in all species, (0.5-1.2 hours).
  • Formulations to slow absorption and prolong duration of action → reduce peak blood levels obtained.
  • Amoiycillin better absorbed orally than ampicillin.
  • Hetacillin and pivamicillin broken down in body → ampicillin.
  • Carfecillin broken down by gastrointestinal mucosa → carbenicillin.

Toxicity

  • Generally very safe.
  • Can see acute anaphylaxis or mild hypersensitivity.
  • If contain procaine, DO NOT administer IV.
  • Can cause fatal Clostridium difficile, colitis in guinea pigs and chinchillas.

2. Cephalosporins

Spectrum of activity

  • Stable to staphylococcal beta-lactamase.
  • Variable instability to Gram-negative beta-lactamases.
  • In general, as activity against Gram-negative spectrum is increased, activity against Gram-positive spectrum is reduced.
  • Oral cephalosporins: eg cephalexin Cefalexin ('Rilexine', 'Ceporex'), cefadroxil ('Cefa-cure'), cephadrine, cefachlor:
    • Gram-positive and many Gram-negative bacteria.
      NOT indole positive Proteus spp and Pseudomonas spp.
    • Parenteral cephalosporins : variable activity against Gram-negative bacteria, high activity against Gram-positive bacteria:
    • Group I: eg cefacetrile, cephalonium, cephaloridine, cephalothin, cephazolin. Pseudomonas spp resistant.
    • Group II: eg ceftiofur ('Excenel', 'Naxcel'), cefuroxime, cefamandole, cefotetan, cefovecin ('Convenia') link:Cefovecin. High activity against Enterobactericeae.
    • Group III:eg cefoperazone, cefsulodin, ceftazidine, cefquinone ('Cobactan', 'Cephaguard'). High activity against Pseudomonas spp.
    • Group IV(cephamycins): eg cefoxitin, moxolactam, cefmetazole.None licenced for veterinary use.Good for Bacteroides spp; NOT Pseudomonas spp except for moxolactam.

Pharmacokinetics

  • Similar to penicillins except cefovecin (effective for up to 14 days, depending on the pathogen).

Toxicity

  • Generally very safe.
  • Can have acute anaphylaxis or mild hypersensitivity.
  • Painful on IM injection.
  • Nephrotoxicity at high doses, especially cephaloridine.
  • Some cause bleeding problems due to interference with vitamin K in gut, especially moxolactam.

Beta-lactamase inhibitors

1. Clavulanic acid

  • With amoxycillin to give 'Synulox' or 'Clavamox'  Clavulanate.
  • Inhibits all beta-lactamases except cephalosporinases.
  • Little antibacterial activity alone, but broadens spectrum of other penicillins.
  • Pharmacokinetics - match penicillin family.

2. Sulbactam

  • Synthetic enzyme inhibitor.
  • Broader spectrum than clavulanic acid. Less potent.

3. Carbapenems

  • Human medicine, eg imipenem.
  • Not approved for veterinary use.

Spectrum of activity

  • Almost all clinically important aerobes and anaerobes (Gram-positive and Gram-negative).
  • Some resistance starting to develop.

Pharmacokinetics

  • Injectable only.
  • Wider distribution in body than penicillins.
  • Renal elimination - hydrolyzed by tubular brush border enzyme.
  • Given with inhibitor of renal enzyme (cilastatin) → high urine concentrations of active drug.

Other cell wall antibacterials

1. Glycopeptides

  • Vancomycin, teicoplanin.
  • Bactericidal.
  • Human medicine.
  • Not approved for veterinary use. Use, if at all, should be restricted to life-threatening infections where no other agent is available.

Spectrum of activity

  • Gram-positive bacteria only.
  • Excellent against staphylococci.
  • Resistance rare.

Pharmakokinetics

  • No oral absorption.
  • Poor tissue penetration.
  • Half-life in dog: vancomycin - 2 hours; teicoplanin - much longer.
  • Renal excretion.

Toxicity

  • Highly irritant to tissues, (give vancomycin by IV administration only).
  • Ototoxic in man.
  • Nephrotoxic.
  • Teicoplanin given IM.

2. Bacitracin

  • Bactericidal  Bacitracin.
  • Indication: topical treatment of superficial skin and mucosal infections, (with polymixin B to give broad spectrum); growth promoter in cattle and pigs; Clostridium perfringens enteritis.

Spectrum of activity

  • Gram-positive bacteria only.
  • Resistance is rare.

Pharmacokinetics

  • Not absorbed orally.

Toxicity

  • Highly toxic parenterally - nephrotoxic.

Antimicrobial agents which target the cell membrane

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Drugs which inhibit protein synthesis

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Antifungal drugs

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Further Reading

Publications

Refereed papers

  • Recent references from VetMed Resource and PubMed.
  • Adams V J, Campbell J R, Waldner C L, Dowling P M & Shmon C L (2005) Evaluation of client compliance with short-term administration of antimicrobials to dogs. JAVMA 226, 567-574 PubMed.
  • Grave K & Tanem H (1999) Compliance with short-term oral antibacterial drug treatment in dogs. JSAP 40(4), 158-162.
  • Barter L S, Watson A D & Maddison J E (1996) Owner compliance with short term antimicrobial medication in dogs. Aust Vet J 74, 227-280 PubMed.


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