Resistant Penicillin - ANTISTAPHYLOCOCCAL PENICILLINS or PENICILLINASE**
They include:
- Methicillin.
- Cloxacillin, Dicloxacillin, Flucloxacillin.
- Nafcillin: It is more active than methicillin and cloxacillin.
- Spectrum: Staphylcoccus spp only
- Cons: Narrow spectrum, Only one Indication
- Drugs: Methicillin Cloxacillin, Dicloxacillin, Flucloxacillin, Nafcillin.
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, but it did not add to the poor gram-negative activity of the natural penicillins.
The only indication is infection by B-lactamase producing staph. They have low activity against other gram-positive and inactive against gram-negative bacteria.
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Most antistaphylococcal penicillins are eliminated from the body in large part by the liver and do not need to be adjusted in cases of renal dysfunction.
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These drugs are interchangeable therapeutically. Therefore, S aureus that is susceptible to methicillin (which is no longer used) is susceptible to oxacillin, nafcillin, and the rest.
- Beta-lactams kill staphylococci more quickly than vancomycin, so patients with MRSA infections who lack serious beta-lactam allergies should be switched to beta-lactams, such as antistaphylococcal  penicillins or first-generation cephalosporins. This has been shown to be an important difference in serious infections.