Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

CHAPTER 8 ANTIBACTERIAL DRUGS
Antistaphylococcal penicillins
EXAMPLES
Cloxacillin, dicloxacillin, flucloxacillin, methicillin, oxacillin.
Antibacterial spectrum (Fig. 8.3)
● The spectrum of the antistaphylococcal penicillins is
similar to the natural narrow-spectrum penicillins
(although potency is less than that of penicillin G)
except that they are resistant to staphylococcal
β-lactamase.
● They have no activity against Gram-negative
bacteria.
● Unlike natural penicillins, they have little activity
against enterococci (Enterococcus faecalis and E.
faecium).
● Activity against anaerobes is variable; for example,
Clostridium are susceptible to cloxacillin but Bacteroides
are not.
Clinical applications
● Staphylococcal skin infections in dogs
● Surgical prophylaxis, especially for orthopedic
procedures
● Treatment of osteomyelitis, discospondylitis
Route of administration
Antistaphylococcal penicillins other than methicillin can
be given orally, although some inactivation by gastric
acid does occur. They are best given on an empty
stomach. Methicillin is rarely used in domestic animals.
Aminopenicillins
EXAMPLES
Amoxicillin, ampicillin, hetacillin.
Antibacterial spectrum (Fig. 8.4)
● The aminopenicillins were developed in the 1960s as
broad-spectrum penicillins. They are slightly less
active against Gram-positive and anaerobic bacteria
than penicillin G but have greater activity against
Gram-negative bacteria. Enhanced Gram-negative
activity is due to increased binding affinity to PBP1b
and PBP3 and enhanced ability to penetrate the outer
cell membrane of many Gram-negative species.
● Emergence of many resistant strains of Gramnegative
bacteria and increasing prevalence of Klebsiella
pneumoniae, Pseudomonas aeruginosa and
Enterobacter spp as pathogens (which are intrinsically
nonsusceptible to aminopenicillins) mean that
their classification as broad-spectrum antibacterial
drugs is now misleading.
● Aminopenicillins are as sensitive to hydrolysis by
Gram-positive or Gram-negative β-lactamases as
penicillin G and therefore are not active against
penicillinase-producing Staphylococcus or Gramnegative
bacteria that produce β-lactamases.
Clinical applications
● The relatively high prevalence of acquired resistance
has limited the role of nonpotentiated aminopenicillins
in small animal practice.
● They are used for soft tissue infections in dogs and
cats provided Staphylococcus is not suspected to be
involved.
● They are useful for treating cat abscesses.
● Treatment of uncomplicated urinary tract infections
can be successful as such high concentrations of drug
are achieved in urine but amoxicillin-clavulanate
might be a better choice.
● Aminopenicillins may also be useful in some enteric
infections.
● Amoxicillin in combination with metronidazole and
omeprazole has been used for treatment of
Helicobacter gastritis.
● Aminopenicillins should not be used for surgical
prophylaxis as Staphylococcus are common
pathogens.
Gram positive
aerobes
Gram negative
aerobes
Gram positive
aerobes
Gram negative
aerobes
Obligate
anaerobes
Penicillinaseproducing
Staphylococcus*
* Methicillin-resistant Staphylococcus aureus (MRSA) are resistant
Obligate
anaerobes
Penicillinaseproducing
Staphylococcus
Fig. 8.3 Antibacterial spectrum for antistaphylococcal
pencillins.
Fig. 8.4 Antibacterial spectrum for aminopenicillins
(nonpotentiated).
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