Class of antibacterial agents

Several subclassifications of the penicillins, based mainly on differences in antibacterial spectra, are recognized.

Narrow-spectrum β-Lactamase-sensitive Penicillins:

This group includes naturally occurring penicillin G (benzylpenicillin) in its various pharmaceutical forms and a few biosynthetic acid-stable penicillins intended for oral use (penicillin V [ phenoxymethyl-penicillin] and phenethicillin). Penicillins in this class are active against many gram-positive and a limited number of gram-negative bacteria, but they are susceptible to β-lactamase (penicillinase) hydrolysis.

Narrow-spectrum β-Lactamase-resistant Penicillins:

This group, through substitution on the penicillin nucleus (6-aminopenicillanic acid [6-APA]), is refractory to a greater or lesser degree to the effects of various β-lactamase enzymes produced by resistant gram-positive organisms, particularly Staphylococcus aureus . However, penicillins in this class are not as active against many gram-positive bacteria as penicillin G and are inactive against almost all gram-negative bacteria. Acid-stable members of this group (that are used PO) include isoxazolyl penicillins, such as oxacillin, cloxacillin, dicloxacillin, and flucloxacillin. Methicillin and nafcillin are available as parenteral preparations. Temocillin is a semisynthetic penicillin that is β-lactamase stable but also active against nearly all isolates of gram-negative bacteria exceptPseudomonas spp .

Broad-spectrum β-Lactamase-sensitive Penicillins:

Penicillins in this class are derived semisynthetically and are active against many gram-positive and gram-negative bacteria. However, they are readily destroyed by the β-lactamases (produced by many bacteria). Many members of the group are acid stable and are administered either PO or parenterally. Of those used in veterinary medicine, aminopenicillins, eg, ampicillin and amoxicillin, are the best known. Several ampicillin precursors that are more completely absorbed from the GI tract also belong to this class (eg, hetacillin, pivampicillin, talampicillin). Mecillinam is less active than ampicillin against gram-positive bacteria but is highly active against many intestinal organisms (except Proteus spp ) that do not produce β-lactamases.

Broad-spectrum β-Lactamase-sensitive Penicillins with Extended Spectra:

Several semisynthetic broad-spectrum penicillins are also active against Pseudomonas aeruginosa , certainProteus spp , and even strains of Klebsiella , Shigella , and Enterobacter spp in certain cases. Examples of this class include carboxypenicillins (carbenicillin, its acid-stable indanyl ester, and ticarcillin), ureido-penicillins (azlocillin and mezlocillin), and piperazine penicillins (piperacillin).

β-Lactamase-protected Broad-spectrum Penicillins (Potentiated Penicillins):

Several naturally occurring and semisynthetic compounds can inhibit many of the β-lactamase enzymes produced by penicillin-resistant bacteria. When used in combination with broad-spectrum penicillins, there is a notable synergistic effect because the active penicillin is protected from enzymatic hydrolysis—and thus is fully active against a wide variety of previously resistant bacteria. Examples of this chemotherapeutic approach include clavulanate-potentiated amoxicillin and ticarcillin as well as sulbactam-potentiated ampicillin.

Carbapenems:

Imipenem is one of the most active drugs against a wide variety of bacteria. It is derived from a compound produced by Streptomyces cattleya . Aztreonam is a related (monobactam) compound.