Penicillin G
**Penicillin G** (benzylpenicillin) is a narrow-spectrum, natural beta-lactam antibiotic derived from *Penicillium chrysogenum*. It is highly effective against most **Gram-positive aerobes** (e.g., *Streptococcus* spp., non-penicillinase-producing *Staphylococcus*), some Gram-negative bacteria (e.g., *Pasteurella*), and many **anaerobes** (e.g., *Clostridium* spp.). > **Clinical Pearl:** The salt form of Penicillin G dictates its route of administration and pharmacokinetic profile: * **Aqueous salts (Sodium or Potassium):** Formulated for IV or IM use. They provide rapid, high peak serum concentrations but have a very short half-life, requiring frequent dosing (e.g., q4-6h). * **Repository salts (Procaine and Benzathine):** Formulated as suspensions for **IM or SC use ONLY** (never IV). Procaine provides a depot effect lasting 12-24 hours, while Benzathine provides very low, prolonged concentrations lasting several days. Penicillin G is susceptible to degradation by beta-lactamase (penicillinase) enzymes produced by many *Staphylococci* and Gram-negative enteric bacteria.
กลไกการออกฤทธิ์: Penicillin G is a **time-dependent bactericidal** antibiotic. * It covalently binds to **Penicillin-Binding Proteins (PBPs)** located on the inner membrane of the bacterial cell wall. * This binding → **inhibits the transpeptidation enzyme** responsible for cross-linking peptidoglycan strands. * Failure of cell wall synthesis → activation of autolytic enzymes within the cell wall → **osmotic lysis** and bacterial death. Because it targets cell wall synthesis, it is most effective against actively dividing bacteria.
ขนาดยาตามชนิดสัตว์
- Soft tissue, systemic infections · 40,000 Units/kg · PO · q6-8h · for as long as necessary · Penicillin G potassium
- Soft tissue infections · 20,000 Units/kg · IM, SC · q12h · for as long as necessary · Penicillin G procaine
- Orthopedic infections · 20,000-40,000 Units/kg · IM · q8h · for as long as necessary · Penicillin G procaine
- Resistant organisms (Actinomyces) · 50,000-100,000 Units/kg · IM, SC · q12h · for as long as necessary · Penicillin G procaine
- Susceptible infections · 50,000 Units/kg · IM · q5 days · Penicillin G benzathine
- Susceptible infections · 20,000-40,000 Units/kg · IM · once a day to twice daily · Procaine Pen G
- Susceptible infections · 20,000 Units/kg · SC, IM or IV · q4h · Sodium or potassium Pen G
- Susceptible infections · 40,000 Units/kg · PO · three times daily · Sodium or potassium Pen G
- Susceptible infections · 25,000 Units/kg · IM or SC · once per day · Penicillin G procaine. For moderately susceptible bacteria give twice daily.
- Clostridial abomasitis and enteritis in calves · 10,000-20,000 Units/kg · PO · q12-24h · for 1-4 days · Procaine Penicillin G. Oral preferred to provide activity in intestinal lumen.
วิธีการให้ยา
ข้อห้ามใช้
- Known hypersensitivity to penicillins or cephalosporins
- Intravenous administration of Procaine or Benzathine suspension formulations
- Oral administration in horses and hindgut fermenters (rabbits, guinea pigs) due to risk of fatal dysbiosis (unless specifically indicated, e.g., calves with clostridial enteritis)
อาการไม่พึงประสงค์
- Hypersensitivity reactions (anaphylaxis, urticaria, rash)
- Gastrointestinal upset (anorexia, vomiting, diarrhea) with oral use
- Procaine toxicity (CNS excitement, seizures) in small birds and horses if inadvertently given IV
- Pain or tissue reaction at IM injection sites
อันตรกิริยาระหว่างยา
- Bacteriostatic Antibiotics (e.g., Tetracyclines, Macrolides) · May antagonize the bactericidal activity of penicillins, which require actively dividing cells to be effective.
- Aminoglycosides (e.g., Amikacin, Gentamicin) · In vivo synergy against certain bacteria; however, physically incompatible if mixed in the same syringe or IV line (inactivation of the aminoglycoside).
- Probenecid · Competitively inhibits renal tubular secretion of penicillins, significantly prolonging their half-life and increasing serum concentrations.
การติดตาม
- Clinical efficacy (resolution of infection signs)
- Signs of toxicity or hypersensitivity (anaphylaxis, rash)
- Electrolytes (if using high doses of Na or K salts IV)
การได้รับยาเกินขนาด
Because penicillins usually have minimal toxicity associated with their use, monitoring for efficacy is usually all that is required unless toxic signs develop. > **Clinical Pearl:** Massive overdoses, particularly of aqueous salts given rapidly IV, can lead to neuromuscular hypersensitivity, seizures, or electrolyte imbalances (hyperkalemia with potassium salt, hypernatremia with sodium salt). Treatment is supportive.
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