Ampicillin
**Ampicillin** is a broad-spectrum, time-dependent, bactericidal aminopenicillin antibiotic. Key pharmacological features include: * **Spectrum of Activity:** Effective against many gram-positive organisms (including penicillin-sensitive enterococci like *E. faecium*), anaerobes (e.g., *Clostridium* spp.), and some gram-negative aerobes (*E. coli*, *Klebsiella*, *Haemophilus*, *Proteus mirabilis*). * **Resistance:** Like natural penicillins, it is highly susceptible to inactivation by **beta-lactamase-producing bacteria** (e.g., *Staphylococcus aureus*). It is ineffective against *Pseudomonas aeruginosa*, *Serratia*, *Enterobacter*, and atypical organisms (Mycoplasma, Rickettsia, fungi). * **Clinical Utility:** While oral ampicillin has largely been replaced by amoxicillin in small animals due to superior bioavailability, parenteral ampicillin remains a cornerstone therapy. * **Salt Forms:** * **Ampicillin Sodium:** Highly water-soluble, suitable for IV administration, providing rapid, high peak serum concentrations. * **Ampicillin Trihydrate:** A repository form for IM or SC use, providing slower absorption and lower peak serum levels (approximately half that of the sodium salt). It should not be used when high Minimum Inhibitory Concentrations (MICs) are required for systemic infections.
Mechanism: Ampicillin is a **beta-lactam antibiotic** that exerts its bactericidal effect by interfering with bacterial cell wall synthesis. * **Mechanism:** Ampicillin covalently binds to specific **Penicillin-Binding Proteins (PBPs)** (primarily transpeptidases) located inside the bacterial cell wall. * **Pathway:** Binding to PBPs โ inhibition of the final transpeptidation step of peptidoglycan cross-linking โ weakening of the cell wall structure โ activation of endogenous autolytic enzymes (autolysins) โ **osmotic lysis and bacterial cell death**. * **Pharmacodynamics:** Efficacy is **time-dependent**, meaning the free drug concentration must remain above the Minimum Inhibitory Concentration (MIC) for a significant portion of the dosing interval (typically >40-50% of the interval).
Dosing by species
- Gram-positive infections ยท 10-20 mg/kg PO twice daily; 5 mg/kg IM, SC twice daily; 5 mg/kg IV three times daily ยท PO/IM/SC/IV ยท q8h-q12h
- Gram-negative infections ยท 20-30 mg/kg PO three times daily; 10 mg/kg IM, SC three times daily; 10 mg/kg IV four times daily ยท PO/IM/SC/IV ยท q6h-q8h
- Susceptible UTI's ยท 12.5 mg/kg PO q12h for 3-7 days, 6.6 mg/kg IM or SC q12h for 3-7 days ยท PO/IM/SC ยท q12h ยท 3-7 days
- Susceptible soft tissue infections ยท 10-20 mg/kg PO, IM or SC q8h for 7 days ยท PO/IM/SC ยท q8h ยท 7 days
- Pneumonia, systemic ยท 22 mg/kg PO, IV or SC q8h for 7-14 days ยท PO/IV/SC ยท q8h ยท 7-14 days
- Meningitis, orthopedic infections ยท 22 mg/kg PO, IV, IM, SC q6-8h as long as necessary ยท PO/IV/IM/SC ยท q6-8h ยท As long as necessary
- Susceptible sepsis, bacteremia ยท 20-40 mg/kg IV, IM or SC q6-8h for as long as necessary ยท IV/IM/SC ยท q6-8h ยท As long as necessary
- Neonatal sepsis ยท 50 mg/kg IV or intraosseous q4-6h as long as necessary ยท IV/Intraosseous ยท q4-6h ยท As long as necessary
- Sepsis ยท 20-40 mg/kg IV q6-8h ยท IV ยท q6-8h
- Susceptible UTI's ยท 25 mg/kg PO q8h ยท PO ยท q8h
- To eliminate the leptospiremic phase of leptospirosis ยท 22 mg/kg q6-8h IV during the acute illness until patient is eating, then amoxicillin 22 mg/kg PO q8h ยท IV ยท q6-8h ยท During acute illness ยท Followed by oral amoxicillin
Routes of administration
Contraindications
- Patients with a known hypersensitivity to penicillins
- Oral administration in patients with septicemia, shock, or grave illness (due to delayed/diminished GI absorption)
- Oral or parenteral use in small hindgut fermenters (rabbits, guinea pigs, chinchillas, hamsters) due to risk of fatal clostridial enterotoxemia
Adverse effects
- Gastrointestinal upset (anorexia, vomiting, diarrhea)
- Antibiotic-associated diarrhea and superinfections (alteration of gut flora)
- Hypersensitivity reactions (rashes, fever, eosinophilia, anaphylaxis)
- Neurotoxicity (ataxia, seizures) at very high doses or prolonged use
- Elevated liver enzymes (rare)
- Tachypnea, dyspnea, edema, and tachycardia (reported in dogs)
Drug interactions
- Bacteriostatic Antimicrobials (e.g., chloramphenicol, macrolides, tetracyclines) ยท Potential in vitro antagonism; clinical significance is debated but concurrent use is traditionally discouraged.
- Methotrexate ยท Ampicillin may decrease the renal excretion of methotrexate, leading to increased levels and potential toxicity.
- Probenecid ยท Competitively blocks the tubular secretion of penicillins, increasing serum levels and prolonging half-life.
- Aminoglycosides (e.g., gentamicin, amikacin) ยท In vitro inactivation of aminoglycosides if mixed in the same syringe or fluid bag. May also occur in vivo in patients with severe renal failure.
- Tetracycline ยท Antagonism of bactericidal effect due to bacteriostatic action ยท moderate
- Erythromycin ยท Antagonism of bactericidal effect due to bacteriostatic action ยท moderate
- Chloramphenicol ยท Antagonism of bactericidal effect due to bacteriostatic action ยท moderate
- Aminoglycosides ยท In vitro inactivation if mixed in the same syringe; however, exhibits synergistic antimicrobial effects when used concurrently in vivo ยท major
Monitoring
- Clinical efficacy (resolution of infection signs)
- Adverse effects (GI signs, hypersensitivity reactions)
- Therapeutic drug monitoring is not routinely required due to the wide therapeutic index
Overdose
Acute oral penicillin overdoses are unlikely to cause significant problems other than **gastrointestinal distress** (vomiting, diarrhea, anorexia). In humans, very high dosages of parenteral penicillins, particularly in patients with underlying renal disease, have induced **CNS effects** (e.g., seizures, ataxia). Treatment is generally supportive and symptomatic.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.