Penicillins

Penicillins are one of the largest and oldest classes of antimicrobial agents. Since the development of the natural penicillins in the 1930s, further penicillin development has been directed by the need  to combat increasing antimicrobial resistance.  Classes of penicillins with expanded Gram-negative spectra overcome the shortfalls of natural penicillins, and they can be grouped fairly easily by spectrum of activity.

Penicillins have several things in common:

• Penicillins have very short half-lives (˂  2 hours) and must be dosed multiple times per day. The half-lives of most of them are pro-longed in the presence of renal dysfunction.

• Like other beta-lactams, penicillins can cause hypersensitivity reactions. If a patient has a true hypersensitivity reaction to a penicillin, other penicillins should be avoided, even if they are from different subclasses of penicillins. If the reaction is not severe, cephalosporins or carbapenems may be useful.

• Many penicillins are relatively poorly absorbed,  even those available as oral formulations. This can lead to diarrhea when oral therapy is needed. Pay attention to the dosing of oral versus intravenous (IV) penicillins—often, a con-version from IV to oral therapy means there will be a substantial decrease in the amount of active drug in the body.

Many penicillins were developed after the natural penicillins became available. Until re-searchers developed beta-lactamase inhibitors,  development primarily focused on either improved activity against staphylococci (MSSA) or GNRs  (Figure 1).

Figure 1:  Penicillin Drug Development

References

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