AMOXICILLIN

Amoxicillin is used in almost all species for several bacterial infections including lower urinary tract infections, soft tissue infections, and pneumonia.

Amoxicillin is a semi synthetic beta lactam antibiotic (penicillin group). It is susceptible to beta lactamase enzyme so combined with beta lactamase inhibitors like clavulanic acid (called potentiated penicillins).

MECHANISM OF ACTION

Amoxicillin inhibits bacterial cell wall synthesis. Amoxicillin generally has a narrow spectrum of activity that includes streptococci, non-betalactamase producing staphylococci, and other gram-positive cocci and bacilli. Many Staphylococcus strains are resistant because of beta-lactamase production. Most enteric gram-negative Enterobacteriaceae bacilli are resistant. Susceptible gram-negative bacteria include some species of Proteus, Pasteurella multocida, and Histophilus spp. Resistance among other gram-negative bacteria is common.

In dogs, the peak concentration, half-life, volume of distribution (VD/F), and clearance (CL/F) are 11 mcg/mL, 1.3 hours, 0.72 L/kg, and 6.5 mL/kg/min, respectively. In cats, these values are 12 mcg/mL, 1.4 hours, 1.05 L/kg, and 7.8 mL/kg/min, respectively. Amoxicillin oral absorption in small animals is higher than ampicillin (two times higher in some animals). Amoxicillin oral absorption in adult horses is less than 10% and is not recommended. In pigs, oral absorption is approximately 40% with a half-life after oral administration of 45 minutes.

CLINICAL USES

Amoxicillin is used for a variety of infections in all species, including lower UTIs, soft tissue infections, and pneumonia. It is generally more effective for infections caused by gram-positive bacteria. Because of a short half-life, frequent administration is needed for treating gram-negative infections.

SIDE EFFECTS

Amoxicillin is usually well tolerated. Allergic reactions are possible. Diarrhea and vomiting are common with oral doses. Oral administration to horses or cattle can cause diarrhea or enteritis.

CONTRAINDICATIONS

Use cautiously in animals allergic to penicillin like drugs.

DRUG INTERACTIONS

Do not mix with other drugs in compounded formulations.

PRECAUTIONS

Dose requirements are based on the approved label dose, or the dose used to determine susceptibility testing interpretive categories. Amoxicillin is not a preferred choice for gram-negative infections caused by Enterobacteriaceae bacteria because most are clinically resistant.

MONITORING

For testing for susceptibility, the CLSI recommends using ampicillin to test for amoxicillin susceptibility. The CLSI breakpoint for susceptible organisms is ≤ 0.25 mcg/mL for staphylococci, streptococci, and gram-negative bacilli. For uncomplicated canine lower urinary tract pathogens, use a breakpoint of ≤ 8 mcg/mL (this breakpoint also can be applied to cats for uncomplicated infections). For cattle pathogens, use a breakpoint of ≤ 0.25 mcg/mL. For equine respiratory pathogens (streptococci), use a break point of ≤ 0.25 mcg/mL. At this breakpoint for susceptibility (0.25 mcg/mL), gram-negative Enterobacteriaceae bacteria test resistant, and testing of these isolates is unnecessary.

FORMULATIONS

Injectables:

100 and 250 mg/mL amoxicillin trihydrate for injection are available.

Tablets:

Amoxicillin is available in 50 mg, 100 mg, 150 mg, 200 mg, 400 mg, 500 mg, and 875 mg tablets and 250 mg and 500 mg capsules (human preparations). Chewable tablets are available in 125 mg, 200 mg, 250 mg, and 400 mg sizes. Amoxicillin trihydrate is available in 50 mg, 100 mg, 200 mg, and 400-mg tablet form.

Syrup/Suspensions:

40 mg/mL and 50 mg/mL amoxicillin oral suspensions are available.

Other formulations:

No other formulation available.

STORAGE

Amoxicillin should be stored in a tightly sealed container at room temperature. Oral liquid suspensions are stable for 14 days once prepared. Other formulations should be protected from moisture. Optimum stability is at pH 5.8–6.5. Above this pH, hydrolysis occurs.

DOSE RATE

Dose rates for various animals are listed below. Adjust the dose rates according to the pathophysiology and condition of each case.

Dogs

22 mg/kg q12h PO, IV, SC, IM
For gram-positive infections: 10 mg/kg PO, IM, SC twice daily for at least 2 days after symptoms subside.
For Gram-negative infections: 20 mg/kg PO three times daily or IM, SC twice daily for at least 2 days after symptoms subside.
For susceptible UTIs: 10–20 mg/kg PO q12h for 5–7 days.
For susceptible systemic infections (bacteremia/sepsis): 22–30 mg/kg IV, IM, SC q8h for 7 days.
For susceptible orthopedic infections: 22–30 mg/kg IV, IM, SC, or PO q6–8h for 7–10 days.
For Lyme disease: 22 mg/kg PO q12h for 21–28 days.

Cats

12.5 mg/kg q12h PO, IV, IM, SC
For gram-positive infections: 10 mg/kg PO, IM, SC twice daily for at least 2 days after symptoms subside.
For gram-negative infections: 20 mg/kg PO three times daily or IM, SC twice daily for at least 2 days after symptoms subside.
For susceptible UTI’s and soft tissue infections: 50 mg (total dose per cat) or 11–22 mg/kg PO once daily for 5–7 days.
For sepsis: 10–20 mg/kg IV, SC, or PO q12h for as long as necessary.
C. perfringens, bacterial overgrowth (GI): 22 mg/kg PO once daily for 5 days.
C. perfringens enterotoxicosis: 11–22 mg/kg PO two to three times daily for 7 days.
For treating H. pylori infections using triple therapy: amoxicillin 20 mg/kg PO twice daily for 14 days; metronidazole 10–15 mg/kg PO twice daily; clarithromycin 7.5 mg/kg PO twice daily.

Calves

Non-ruminating: 10–22 mg/kg q8–12h PO

Cattles

6.6–22 mg/kg q8–12h IV, SC, IM q12h
For respiratory infections: 11 mg/kg IM or SC q12h

Horses

For respiratory infections: 20–30 mg/kg PO q6h
Foals: Amoxicillin Sodium: 15–30 mg/kg IV or IM q6–8h

Birds

100 mg/kg q8h, IM, SC, PO
Ratites: 15–22 mg/kg PO twice daily; in drinking water: 66 mg/litre for 3–5 days.

Reptiles

For all species: 22 mg/kg PO q12–24h; not very useful unless used in combination with aminoglycosides.

ADMINISTRATION

Amoxicillin typically administered via intravenous (IV), intramuscular (IM), subcutaneous (SC) and per oral (PO) routes. It is essential to check the label for specific preparation instructions since certain formulations may be designated for specific routes of administration.

ORAL BIOAVAILABILITY

Oral absorption in horses is less than 10%, and it is not suitable for treatment of adult horses. However, oral absorption in foals is 36%–43%. Oral absorption in small animals is 50%–60%. Oral doses in large animals are not well absorbed, and amoxicillin is not recommended by this route.

PROTEIN BINDING

Protein binding for Amoxicillin is around18%.

ELIMINATION HALF LIFE

The elimination half-life of Amoxicillin is approximately 6 hours.

METABOLISM

Metabolism of Amoxicillin takes place in the Liver.

EXCRETION

Excretion of Amoxicillin takes place in the Kidneys.

ANTIDOTE

No specific antidote.

NOTES

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