Beta-Lactams

Beta-lactams include a wide variety of antibiotics that seem to exist only to confuse students and clinicians. Penicillins, cephalosporin's, and carbapenems are all beta-lactams. Monobactams (aztreonam) are structurally similar, but they lack one of the two rings that other beta-lactams have and have little to no cross-allergenicity with other beta-lactams. To make matters more confusing, not all beta-lactams end in -cillin or -penem or start with ceph-.

We believe the best approach to keeping beta-lactams straight is to group them into classes and learn the characteristics of each class. If you work in a hospital, you will likely have only one or two drugs of each class to worry about. Fortunately, all beta-lactams have a few things in common.

• All beta-lactams can cause hypersensitivity re-actions, ranging from mild rashes to drug fever to acute interstitial nephritis (AIN) to anaphylaxis. There is some cross-sensitivity among classes, but there is no way to predict exactly how often that will occur. Studies on the matter differ greatly in their conclusions.

• Seizures can result from very high doses of any beta-lactam. Some are more notorious for this adverse effect than others. Accumulation to toxic levels can occur when the dose of a beta-lactam is not properly adjusted for a patient’s renal function. Did you check your patient’s renal function? 

• All beta-lactams share a mechanism of action— inhibition of transpeptidases (that is, penicillin-binding proteins) in the bacterial cell wall.  Thus, giving two beta-lactams in combination for the same infection is generally not useful. There are a few exceptions to this rule, but not many.

• All beta-lactams lack activity against atypical organisms such as  Mycoplasma pneumoniae and Chlamydophila pneumoniae. Add another drug to your regimen if you are concerned about these bugs, as in cases of community-acquired pneumonia.

• Nearly all currently available beta-lactams lack activity against MRSA. Add vancomycin or another agent if this bug is suspected. Among the available beta-lactams, only the recently approved cephalosporin ceftaroline has anti-MRSA activity.

Once you know the similarities among beta-lactams, it is easier to learn the differences among them.

References

Gallagher ,J.C. and MacDougall ,c. (2012). Antibiotics Simplified. Second Edition. Jones & Bartlett Learning, LLC.