Cephalosporins
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Mechanism of Action
Cephalosporins are beta-lactams just like penicillins and work by disrupting the peptidoglycan in the cell wall of bacteria. As such they are bacteriocidal and time dependent killers.
Spectrum of Activity
Unlike penicillins it is hard to make broad generalisations about the spectrum of activity of the cephalosporin sub-groups. The two common points are that they are all stable to Staphylcococal beta lactamase and are effective against most obligate anaerobes.
First Generation Cephalosporins - Cefalexin, Cefalonium, Cephapirin
- Active against gram positive organisms
- Active aginst many gram negatives. Pseudomonas species are resistant.
Second Generation Cephalosporins - Cefuroxime
- Very broad spectrum with very high activity against enterobacteriaeceae.
- Struggle against the most difficult gram negative organisms
Third Generation Cephalosporins - Cefoperazone, Ceftiofur
- Very active against gram negatives, especially the hardier species, including Pseudomonas
- Less active against gram positives compared to other cephalosporins; Staphylococci and enterococci can be resistant.
Fourth Generation Cephalosporins - Cefquinome
- Very broad spectrum, only the very hardiest of gram negatives aren't susceptible. It is still active against Pseudomonas.
Pharmacokinetic Considerations
They are organic acids, hydrophilic and will ionise at physiological pH. They generally have poor oral bioavailability as they unstable in acid environments. They have a limited volume of distribution (0.2-0.3l/kg), this means the drug is mainly confined to plasma and interstitial space. As the are hydrophilic they can't enter cells and won't cross the blood brain barrier, unless it is damaged. They have short half-lives of about 0.5 - 1.2 hours. They are readily excreted by the kidneys, via tubular secretion in the proximal convoluted tubule. This results in high concentrations of the drug in urine.
Exceptions are:
- Cefalexin which is stable in acid and so suitable for oral dosing.
- Ceforperazone is excreted in bile rather than in urine.
Side Effects and Contraindications
- Like Penicillins they can cause fatal colitis in hamsters and guinea-pigs.
- They are often painful when given intramuscularly.
- Very high doses has been known to result in nephrotoxicity.
- They can result in bleeding problems because of interference with the formation of vitamin K in the gut (esp Moxalactam).
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