盤尼西林 G / 青黴素 G
**盤尼西林 G (Penicillin G)**（苄青黴素）是一種窄效的天然β-內醯胺類抗生素，提取自*Penicillium chrysogenum*。它對大多數**革蘭氏陽性需氧菌**（如鏈球菌、不產青黴素酶的葡萄球菌）、部分革蘭氏陰性菌（如巴斯德桿菌）以及許多**厭氧菌**（如梭狀芽孢桿菌）具有高度療效。 > **臨床要點：** 盤尼西林 G 的鹽類形式決定了其給藥途徑與藥代動力學特徵： * **水性鹽類（鈉鹽或鉀鹽）：** 專供靜脈 (IV) 或肌肉 (IM) 注射。能迅速達到高血清峰值濃度，但半衰期極短，需要頻繁給藥（例如每 4-6 小時一次）。 * **長效儲庫型鹽類（普魯卡因和苄星）：** 配製為懸浮液，**僅限肌肉 (IM) 或皮下 (SC) 注射**（嚴禁靜脈注射）。普魯卡因 (Procaine) 可提供持續 12-24 小時的儲庫效應，而苄星 (Benzathine) 則提供可維持數天的極低且延長的藥物濃度。 盤尼西林 G 容易被許多葡萄球菌和革蘭氏陰性腸道菌產生的 β-內醯胺酶（青黴素酶）分解而失效。
作用機制: 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.
各物種劑量
- 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.
給藥途徑
禁忌症
- 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)
不良反應
- 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
藥物相互作用
- 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.
監測
- Clinical efficacy (resolution of infection signs)
- Signs of toxicity or hypersensitivity (anaphylaxis, rash)
- Electrolytes (if using high doses of Na or K salts IV)
過量
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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