Pharmacotherapeutic group: Beta-lactam antibacterials, penicillins.
ATCvet code: QJ01CE02
Pharmacodynamic properties
Phenoxymethylpenicillin is a narrow-spectrum penicillin with activity mainly against gram-positive bacteria.
Phenoxymethylpenicillin, as all penicillins, exerts a bactericidal action, on bacteria during the stage of active multiplication. It forms an irreversible binding to penicillin-binding proteins (PBPs), enzymes that facilitate the formation of cross-links of peptidoglycan chains in the synthesis of the bacterial cell wall. This results in abnormal cell growth and cytolysis of the cell.
Phenoxymethylpenicillin is an acid-stable derivative of benzylpenicillin and has a largely comparable spectrum of activity.
Development of resistance is mainly based on the formation of beta-lactamase, an enzyme that breaks open the beta-lactam ring rendering the antibiotic ineffective. Cross-resistance exists between phenoxymethylpenicillin and other beta-lactam antibiotics.
Minimum Inhibitory Concentrations (MICs) of phenoxymethylpenicillin were determined against Clostridium perfringens isolates from clinical cases of necrotic enteritis in chickens during 1998 and 1999. The MIC for C. perfringens isolated from faeces, liver and caecum samples were <0.01-0.05 μg/ml.
Pharmacokinetic properties
The most important advantage of phenoxymethylpenicillin in comparison with penicillin G is that it is more stable in an acid environment and it is therefore better absorbed from the gastrointestinal tract.
Following oral use, phenoxymethylpenicillin for the most part escapes decomposition by gastric juices, as it is stable at a low pH.
Phenoxymethylpenicillin is well distributed over most of the tissues, leading to a high concentration in the kidneys and the liver. Phenoxymethylpenicillin is partially decomposed in the gastrointestinal tract. A small portion of the absorbed amount is metabolised in the body. For the most part, phenoxymethylpenicillin is excreted in unaltered active form in urine and faeces.
Following a single administration of the product in chickens at a dose of 15 mg of phenoxymethylpenicillin potassium/kg body weight by oral gavage, maximum plasma concentrations of 0.40 + 0.15 mg/l were achieved within 1.7 + 1.0 hours after administration. Phenoxymethylpenicillin is well absorbed and has an absolute bioavailability of 69%.