Phenoxymethylpenicillin Other names: Penicillin V

Phenoxymethylpenicillin is a broad spectrum beta-lactam antibiotic with bactericidal action against Gram-positive bacteria and Gram-negative cocci. Its antimicrobial action is similar to that of benzyl penicillin.

Phenoxymethylpenicillin is inactivated by penicillinase and other beta-lactamases.

Phenoxymethylpenicillin binds to penicillin-binding proteins located on the inner membrane of the bacterial cell wall. Phenoxymethylpenicillin binds to and inactivates these proteins resulting in weakening of the bacterial cell wall and lysis.

Phenoxymethylpenicillin is usually active against the following organisms:

Gram-positive aerobes and anaerobes including:

Bacillus anthracis
Clostridium perfringens
Clostridium tetani
Corynebacterium diphtheriae
Erysipelothrix rhusiopathiae
Listeria monocytogenes
Peptostreptococcus spp.
Streptococcus agalactiae (Group B)
Streptococcus pneumoniae
Streptococcus pyogenes (Group A)

Gram-negative including:

Neisseria meningitidis
Neisseria gonorrhoeae

Absorption

Phenoxymethylpenicillin is stable under acidic conditions so it can be administered by oral route.

Phenoxymethylpenicillin is rapidly, but incompletely absorbed after oral administration and the absorption level is around 60%. The simultaneous administration of food slightly decreases the peak plasma concentration of phenoxymethylpenicillin, but does not appear to affect the extent of absorption. Peak plasma concentrations are reached in about 45 minutes. The peak plasma concentration increases approximately in proportion with increased doses. Peak serum concentrations of 3-6 jig per ml have been seen following dosage of 250 mg to 500 mg by mouth.

Distribution

Phenoxymethylpenicillin is widely distributed round the body tissues and fluids (volume of distribution about 0.2 1 kg-1 of body weight) and more readily penetrates inflamed tissues. It also diffuses across the placenta into foetal circulation and small amounts appear in the milk of nursing mothers. Eighty per cent is reported to be protein bound.

Biotransformation

Phenoxymethylpenicillin is partially metabolised to inactive penicilloic acid by hydrolysis of the lactam ring. This metabolism occurs in the liver.

Elimination

The plasma half-life of phenoxymethylpenicillin is about 45 minutes which may increase to four hours in renal failure.

Excretion is by tubular secretion into urine. About 40% of the dose is eliminated in the urine either as under unchanged or as penicilloic acid in the first 10 hours after oral administration. Small excretion occurs in bile.

Impaired absorption is seen in patients with coeliac disease.

There are no pre-clinical data.