Imtiaz Ibne Alam
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Therapeutic Classification of Penicillin with Examples

Penicillin is derived from a fungi named Penicillium. They are a group of Beta-lactam antibiotics, which are used in the treatment or prevention of bacterial infections originated by susceptible, particularly gram-positive, organisms. They are usually bactericidal in action when given in therapeutic doses.

Penicillins are effective in treating a variety of infections caused by susceptible bacteria like Clostridium, Neisseria, staphylococci, streptococci, and Listeria.

Therapeutic Uses
  • Urinary tract infections (UTIs).
  • Respiratory tract infections (RTIs).
  • Syphilis and gonorrhea.
  • Bacillary dysentery and typhoid.
  • Meningitis.
  • Infections in the bones and joints.
  • Infections of the skin and soft tissues.
  • Bronchitis and pneumonia.

Classification of Penicillin

Penicillins can be classified into various classes considering their origins, administration routes, antimicrobial spectrum, beta-lactamase resistance, and acid resistance.

The figure below shows the therapeutic classification of penicillins with examples based on above parameters.
therapeutic classification of penicillin
Figure 1. Therapeutic Classification of Penicillin | Adapted from Sahoo and Banik1

Important Classes of Penicillin

Natural Penicillins (Penicillin V, Penicillin G, Benzathine, Procaine Penicillin)

Picture of Streptococcus Pneumoniae
Streptococcus Pneumoniae
Source: Nathan Reading, CC-BY-2.0, via Flickr
The first novel agents introduced for clinical use in the penicillin family were antibiotics of the natural penicillin class. Natural penicillins are built on the original penicillin-G structure. The majority of gram (+) strains of staphylococci and streptococci as well as a few gram (-) bacteria are susceptible to antibiotics from this particular class of penicillin (for example, meningococcus). Penicillin V is regarded as the treatment of choice for streptococcal pharyngitis and is useful for anaerobic coverage in patients who have oral cavity infections.

Penicillinase-Resistant or Beta-Lactamase Resistant Penicillins (Oxacillin, Dicloxacillin, Cloxacillin, Methicillin, Nafcillin)

Picture of Staphylococcus Aureus
Staphylococcus Aureus
Source: Nathan Reading, CC-BY-2.0, via Flickr
Methicillin was the first drug of this class. Following its discovery, this class was gradually expanded to include dicloxacillin, oxacillin, nafcillin, and cloxacillin. In contrast to other penicillin classes, drugs of this class lack broad spectrum activity.

This class of penicillins are specifically designed to combat staphylococcal species as well as gram (+) cocci strains that produce penicillinase. For this reason, these penicillins are often called as anti-staphylococcal penicillins.

Aminopenicillins (Amoxicillin, Bacampicillin, Ampicillin) – Acid Stable Penicillins

The aminopenicillins were the first class of penicillins, which were discovered as active agents against several gram (-) bacteria (e.g. H. influenzae and E. coli). This class of penicillins have acid resistant properties, and for this reason they are available in oral dosage forms.

Orally administered drugs (amoxicillin, Bacampicillin and ampicillin) of this class are usually used in several mild infections, including sinusitis, tonsillitis, otitis media, bronchitis, bacterial diarrhea and UTIs. For the otitis media treatment, Amoxicillin is considered as the drug of choice.

Extended Spectrum Penicillins

Extended Spectrum Penicillins are also called as anti-pseudomonal penicillins. They include both acylaminopenicillins (mezlocillin, piperacillin, and azlocillin) and alpha-carboxypenicillins (ticarcillin and carbenicillin). The antibiotics of this class of penicillin have similar spectrum of antimicrobial activity as compared to aminopenicillins.

However, these penicillins have an additional anti-bacterial activity against many organisms (gram-negative) of the Enterobacteriaceae family, including several strains of Pseudomonas aeruginosa. The antibiotics of this class may be administered alone or in combination with Aminoglycosides. In addition, like the aminopenicillins, drugs of this class are also vulnerable to inactivation by beta-lactamases.

Medical Reference
  1. Sahoo BM, Banik BK. Therapeutic Potentials of β‐Lactam: A Scaffold for New Drug Development. Synthetic Approaches to Nonaromatic Nitrogen Heterocycles. 2020 Oct 29:59-88.