Penicillin V Mechanism of Action

Pharmacology: Phenoxymethylpenicillin is an antimicrobial acid derived from the growth of certain strains of Penicillum notatum. Its action may be bacteriostatic or bactericidal, depending on its concentration against most Gram-positive bacteria and Gram-negative cocci and against some spirochetes and actinomycetes. In adequately high concentrations at the site of action, it has a bactericidal effect caused by inhibition of the synthesis of the bacterial cell wall. It is more resistant to inactivation by gastric acid secretions and more slowly inactivated by penicillinase thus it is slightly more active than benzyl-pencinillin against some staphylococci, but is slightly less active against streptococci and much less active against Gram-negative micro-organisms including gonococci. Though it is more slowly inactivated by penicillinase, resistance of microorganisms to it is usually natural and is associated with insensitive organisms and those which produce penicillinase. Resistance is more common among staphylococci.
Phenoxymethylpenicillin is more stable in gastric acid secretions and is more completely absorbed than benzylpenicillin from G.I.T. following oral administration. Absorption is usually rapid, but peak serum concentrations may be variable. Peak serum concentrations of about 0.7μg per ml have been observed following a dose of 125 mg and 3 to 5 μg per ml following 250 to 500 mg. Absorption of phenoxymethylpenicillin appears to be reduced in subjects with celiac disease and appears to be more rapid in fasting state. Its half-life is about 30 minutes. Phenoxymethylpenicillin is widely distributed throughout the body and enters pleural, pericardial, ascitic and synovial fluids and also the cerebrospinal fluid when the meninges are inflamed.
It crosses the placenta and is also secreted in milk. About 50 to 80% of phenoxymethylpenicillin is bound to plasma proteins in the blood. Phenoxymethylpenicillin may be metabolised in the liver by hydroxylation. Excretion of phenoxymethylpenicillin is mainly renal where 20 to 35% of an oral dose is excreted in the urine in 24 hours. This renal excretion is delayed by probenecid. It is also excreted in small amounts in bile.