Core pharmacology

Class Group

ANTIBIOTIC:
PENICILLIN

Legacy Cicm Level

Level 1

Introduction

Narrow spectrum, semisynthetic anti-staphylococcal penicillin

Indications Uses

useful for treating staphlococci which are resistant to benpen due to beta-lactamase activity
. It is well absorbed from the gut but is given IV if the infection is serious. It should not be used if the organism is sensitive to benpen as
benpen is more bacteriocidal.

Presentation

PO/IV. Tablet/Powder for reconstitution

Mechanism of action

Beta lactam ring binds to penicillin binding protein (transpeptidase) and
prevents crosslinking of bacterial peptidoglycan
inhibits cell wall synthesis

Bacteria eventually lyse due to ongoing activity of cell wall autolytic enzymes (autolysins and murein hydrolases) while cell wall assembly is arrested

Less bactericidal but stable to staph beta-lactamases

Antimicrobial Spectrum

narrow spectrum but is
useful for treating staphlococci which are resistant to benpen due to beta-lactamase activity
It should not be used if the organism is sensitive to benpen as benpen is more bactericidal

Resistance Mechanism

MRSA develop or acquire the gene mecA which synthesizes an additional penicillin binding protein that enables it to continue cell wall synthesis in the presence of a beta lactam drug

Adverse Effects Toxicity

Up to 10% of the population have allergies to penicillins. Due to the high
percentage excreted renally unchanged dose adjustment is required in low urine
output states. Severe cholestatic hepatitis has been reported idiosyncratically.

Absorption

bioavailabilty 50–70%
routes of administration PO, IV or IM
doses Usually given QID or TDS in up to 2g/day

Distribution

penetration into CSF occurs with inflamed meninges only

Protein binding

up to 95% PB

Volume of distribution

Vd ~0.28 L/kg

Metabolism

Hepatic- active metab

Excretion

excretion Urine (50-65% as unchanged drug)

Half-life

half life 0.75–1 hours, prolonged in anuria/renal impair