Antistaphylococcal Penicillins
Agents: methicillin, cloxacillin, dicloxacillin, nafcillin, oxacillin
It did not take long for Staphylococcus spp. to become resistant to penicillin. Within a few years of penicillin becoming widely available, staphylococcal strains began to produce beta-lactamases, rendering penicillin useless in these infections. The basic structure of penicillin was modified to resist these destructive enzymes, leading to the antistaphylococcal penicillins. This modification gave these drugs activity against staphylococci that produce penicillinases (beta-lactamases active against penicillins), but did not add to the poor Gram-negative activity of the natural penicillins.
Spectrum
Good: MSSA, streptococci
POOH Gram-negative rods, enterococci, anaerobes, MRSA
Adverse Effects
Similar to those for other beta-lactams, with a possibly higher incidence of acute interstitial nephritis
(AIN).
Important Facts
- Antistaphylococcal penicillins have a short half-life and must be dosed frequently. This presents a problem, because they cause phlebitis. Does your patient have phlebitis? Try a first-generation cephalosporin instead.
- Most are eliminated from the body in large part by the liver and do not need to be adjusted in cases of renal dysfunction.
- These drugs are interchangeable therapeutically. Therefore, Staphylococcus aureus that is susceptible to methicillin (which is no longer used) is susceptible to oxacillin, nafcillin, and the rest. That is, MSSA = OSSA = NSSA, etc.
What They're Good For
Infections caused by MSSA, such as endocarditis and skin and soft tissue infections.
Don't Forget!
Beta-lactams kill staphylococci more quickly than vancomycin, so patients with MSSA infections who lack serious beta-lactam allergies should be switched to beta-lactams, such as antistaphylococcal penicillins.