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Pharmacological particulars
Pharmacotherapeutic group: Antibacterials for systemic use. Penicillins with extended spectrum.
ATCvet code: QJ01CA04
Pharmacodynamic properties
General properties
Amoxicillin is a β-lactam antibiotic and its structure contains the β-lactam ring and thiazolidine ring common to all penicillins. β-lactam antibiotics prevent the bacterial cell wall from forming by interfering with the final stage of peptidoglycan synthesis. They inhibit the activity of transpeptidase enzymes, which catalyse cross-linkage of the glycopeptide polymer units that form the cell wall. They exert a bactericidal action but cause lysis of growing cells only. β-lactam antibiotics can be referred to as a time-dependent antibiotic.
Antimicrobial spectrum
Amoxicillin is a broad spectrum antibiotic and generally active against some Gram-negative and most Gram-positive bacteria (Germ-vet 2007) e.g. penicillin sensitive Pasteurella spp., Proteus spp., Streptococcus spp., E. coli, and Gram-positive cocci.
Amoxicillin is acid-resistant but is not resistant to the action of β-lactamases, which can hydrolyse the molecules causing the β-lactam ring structure to open, causing inactivity of the antibiotic.
Most Gram-negative bacteria are intrinsically resistant to many β-lactam drugs. This is partly due to the mechanism of action of the drug and the structure of the membrane of the bacteria.
Acquired resistance to β-lactam drugs in clinical isolates may be due to β-lactamase activity specified by plasmids or to mutational changes in chromosomal loci. In some strains a single step mutation may be responsible for resistance, whereas in others resistance may be due to several mutations.
Acquired resistance prevalence may be high in E coli.
Pharmacokinetic properties
Amoxicillin is well absorbed after oral administration. In dogs, the systemic bioavailability is 60-70%. Amoxicillin has a relatively small apparent distribution volume, low plasma-protein binding (34% in dogs) and a short elimination half-life period due to active tubular excretion by the kidneys.
After absorption, highest concentrations are found in the kidneys (urine) and bile, followed by the liver, lungs, heart and spleen.
Distribution of amoxicillin into cerebrospinal fluid is low unless the meninges are inflamed.